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GIFT OF 
MICHAEL REESE 




A SYSTEMATIC HANDBOOK 



OF 



VOLUMETRIC ANALYSIS. 



SYSTEMATIC HANDBOOK 



OF 



VOLUMETRIC ANALYSIS; 



OR, 



THE QUANTITATIVE ESTIMATION 

OF CHEMICAL SUBSTANCES BY MEASURE, APPLIED TO 
LIQUIDS, SOLIDS, AND GASES, 



ADAPTED TO THE REQUIREMENTS OF PURE CHEMICAL RESEARCH, 
PATHOLOGICAL CHEMISTRY, PHARMACY. METALLURGY, MANUFACTURING 

CHEMISTRY, PHOTOGRAPHY, ETC., AND FOR THE VALUATION 
OF SUBSTANCES USED IN COMMERCE, AGRICULTURE, AND THE ARTS. 



BY 

FRANCIS SUTTON, F.I.C., F.C.S., 

PUBLIC ANALYST FOR THE COUNTY OF NORFOLK: 

LATE ^lEMBER OF OtTUNCIL OF THE SOCIETY OF PUBLIC ANALYSTS ; 

LATE MEMBER OF COUNCIL OF THE PHARMACEUTICAL SOCIETY OF GREAT BRITAIN; 

CO I! RESPONDING MEMBER OF THE IMPERIAL PHARMACEUTICAL SOC. OF ST. PETERSBURG!! ; 

CORRESPONDING MEMBER OF THE AUSTRIAN APOTHEKER VEREIN, VIENNA; 

CONSULTING CHEMIST TO THE NORFOLK CHAMBER OF AGRICULTURE; 

ETC., ETC. 



OF THE 

DIVERSITY 

OF 




SEVENTH EDITION, ENLARGED AND IMPROVED. 



LONDON: 



J. & A. CHURCHILL, 7 GREAT MARLBOROUGH STREET 

(Removed from 11 Neiv Burlington Street), 
1896. 

[All rights reserrej,'] 




OWING to the large edition of this work issued at the end of 
1890, a rather longer period than usual has occurred 
between successive issues. The book, however, has been 
out of print for nearly a year owing to pressure of other 
matters, and the time required for investigation of new 
processes or modifications of old ones. 

It will be seen that considerable alterations and additions 
have been made in various sections, so as to bring the 
work up to date as closely as possible. 

The sections thus altered, "<aiict> 'pthers entirely new, 
comprise chiefly the articles on Calibration of Instruments, 
the Kjeldahl process, Boric Acid, Hydrofluoric Acid and 
Fluorides, Arsenic, Chromium, Copper, Cyanogen and 
Cyanides, Iron, Lead, Manganese, Mercury, Nickel, 
Phosphoric Acid, Sugar, Sulphur and its compounds, 

Tannin, Zinc, Oils and Fats, and Urine. 

- 
As respects the volumetric method as applied to many 

organic substances, and the action of modern indicators 
in such work, nothing has been attempted, partly because 
the results hitherto obtained have not been altogether 
satisfactory, but mainly because this subject comes 
specially within the scope of my friend A. H. Allen's 
well-known work on Organic Analysis, and it cannot in 
my opinion be left in better hands. 

My thanks are especially due to Mr. W. B. Giles, F.I.C., 
for his original article on the estimation of Hydrofluoric 
Acid, and for the benefit of his long practical experience 
in the examination of Sulphur Compounds and Phosphoric 

Acid. 



VI PREFACE. 

Mr. J. W. Westmoreland has also rendered great 
service in the articles on Copper, Iron, and Manganese. 

Dr. James Edmunds has also favoured me with 
suggestions on Urinary analysis, which I believe to be of 
considerable practical value. 

I have availed myself in some instances of the excellent 
abstracts of original papers now being published in the 
Analyst, which reflect great credit upon the present 
management in this department. 

My son, W. L. Sutton, A.I.C., has rendered me help in 
the general revision of the book and the correction of 
proof sheets. 

This labour has hitherto been taken, in the five previous 
editions, by my friend W. Thorp, B.Sc., who would 
willingly have continued his kind services, but the delay 
in preparation of the book has necessitated extra rapidity 
in printing and revision. 

The nomenclature of chemical substances is mainly the 
same as in previous editions, and inasmuch as the book is 
largely used by many persons who are practical workers, and 
not advanced theoretical chemists, I have continued the 
use of such terms as sodic bicarbonate in place of sodium 
hydrogen carbonate, and similar modern terms. 

The aim throughout the whole series of editions has 
been to make the book a guide to practical workers, and to 
condense the descriptions of processes as much as is 
possible, without the sacrifice of accuracy or clearness. 
Notwithstanding, the present edition will be enlarged by 
more than thirty pages. 

FEANCIS SUTTON. 

NORWICH, 
August, 1896. 




CONTENTS 



PART I. 

Sect. Page 

1. GENERAL PRINCIPLES . . . . .1 

2. The Balance ..... 5 

3. Volumetric Analysis without "Weights . . .5 

4. Volumetric Analysis without Burettes ... 6 

5. The Burette . . . . . .7 

6. The Pipette ... 15 

7. The Measuring Flasks . . . . .15 

8. The Correct Reading of Graduated Instruments . . 17 

9. Calibration of Graduated Apparatus . . .19 

10. The Weights and Measures to be adopted in Volumetric Analysis 23 

11. Preparation of Normal Solutions in General . . 27 

12. Direct and Indirect Processes of Analysis . . .31 

PAET II. 

13. ALKALIMETRY ..... 33 

14. Indicators used in Saturation Analyses . . .33 

15. Normal Alkaline and Acid Solutions ... 44 

16. Correction of Abnormal Solutions . . . .51 
Table for the Systematic Analysis of Acids, Alkalies, and Alkaline 

Earths ..... 54 

17. Titration of Alkaline Salts . . . .55 

18. Titration of Alkaline Earths .... 69 

19. Ammonia . . . . . .72 

20. ACIDIMETEY ..... 88 

21. Acetic Acid . . . . . .89 

22. Boric Acid and Borates .... 92 

23. Carbonic Acid . . . . . .93 

24. Citric Acid ..... 103 

25. Formic Acid ...... 104 

26. Hydrofluoric Acid ..... 105 

27. Oxalic Acid . . . . . .109 

28. Phosphoric Acid . . . . .109 

29. Sulphuric Anhydride ..... Ill 

30. Tartaric Acid . . . . .112 

31. Estimation of Combined Acids in Neutral Salts . .114 

32. Extension of Alkalimetric Methods 117 



Vlll CONTENTS. 

PART III. 

Sect. Page 

33. ANALYSIS BY OXIDATION OR REDUCTION . . . 120 

34. Permanganic Acid and Ferrous Oxide . . . 121 

35. Titration of Ferric Salts by Permanganate . vC . 124 

36. Calculation of Permanganate Analyses . . . 125 

37. Chromic Acid and Ferrous Oxide . . ... . 126 

38. Iodine and Thiosulphate . . . .128 

39. Analysis of Substances by Distillation with Hydrochloric Acid . 132 

40. Arsenious Acid and Iodine - . 136 



PART IV. 

41. ANALYSIS BY PRECIPITATION . . , . 138 

42. Indirect Analyses by Silver and Potassic Chromate . . 140 

43. Silver and Thiocyanic Acid .... ... 142 

44. Precision in Colour Reactions 143 



PART V. 

45. Alumina ...... 145 

46. Antimony . . . . . 147 

47. Arsenic . . . . . . 149 

48. Barium . . . . . .154 

49. Bismuth ...... 154 

50. Bromine . . . . . .156 

51. Cadmium ...... 159 

52. Calcium . . . . . .160 

53. Cerium ...... 162 

54. Chlorine . . . . . .162 

55. Chlorine Gas and Bleach .... 164 
Chlorates, lodates, and Bromates .... 166 

56. Chromium ...... 167 

57. Cobalt . . . . . .173 

58. Copper . ... 175 

59. Cyanogen . . . . . . 189 

60. Ferro- and Ferri-C3 r anides .... 195 
Thiocyanates or Sulphocyanides .... 197 

61. Gold . 198 

62. Iodine ...... 199 

63. Ferrous Iron ..... 206 

64. Ferric Iron ...... 210 

65. Iron Ores . . , . . .214 

66. Lead ..... . . 222 

67. Manganese. ..... 226 

68. Mercury . . . . . .238 

69. Nickel 243 



CONTENTS. IX 



Sect. 

70. Nitrogen as Nitrates and Nitrites. . . . 245 

71. Oxygen and Hydrogen Peroxide . 269 

72. Phosphoric Acid and Phosphates . . . 284 

73. Silver ..... 297 

74. Sugar .... -305 

75. Sulphur, Sulphides, and Sulphites 

76. Sulphuric Acids and Sulphates . . 325 

77. Sulphuretted Hydrogen .... 329 

78. Tannic Acid . . . . . - 331 

79. Tin . . . 339 

80. Uranium 

81. Vanadium .... 341 

82. Zinc . ... 342 

83. Oils and Pats 

84. Glycerin . -363 

85. Phenol (Carbolic Acid) . . 366 

86. Carbon Bisulphide . 367 

APPENDIX TO PART V. 

Arsenic and Arsenic Acid 

Boric Acid in Milk 

Mixtures of Chlorides, Hypochlorites, and Chlorates 372 

Chloric and Nitric Acids . . 373 



PART VI. 

87. Analysis of Urine ..... 377 

88. Analysis of Potable Waters and Sewage . . . 398 

89. Analytical Processes for Water . . 405 

90. Interpretation of Results of Water Analysis . . 444 

91. Water Analysis without Gas Apparatus . . 455 

92. Reagents and Processes employed .... 463 

93. Oxygen Dissolved in Water .... 474 
Table for Calculations and Logarithms . . . 476 



PART VII. 

94. Volumetric Analysis of Gases and Construction of Apparatus 480 

95. Gases Estimated Directly and Indirectly . . . 494 

96. H} r drochloric, Hydrobromic, and Hydriodic Acids . 494 

97. Analysis of Air, Carbonic Anhydride, SH 2 , and SO 2 . . 496 

98. Indirect Determinations .... 502 

99. Improvements in Gas Apparatus .... 517 

100. Simpler Methods of Gas Analysis . . .. 547 

101. The Nitrometer, Gasvolumeter, and Gravivolumeter . 557568 



Names of Elementary Substances occurring- in Volumetric 
Methods, with their Symbols and Atomic Weights. 



Name. 


Symbol. 


Exact Atomic 
Weight 
as found by 
the latest 
researches. 


Atomic Weight 
adopted in 
this Edition. 


Aluminium 


Al 


27-3 


27-3 


Antimony 
Arsenic 


Sb 

As 


119-6 
74-9 


120-0 
75-0 


Barium 


Ba 


136-8 


136-8 


Bismuth . 


Bi 


208-0 


208-0 


Bromine 


Br 


79-75 


80-0 


Cadmium . 


Cd 


111-6 


111-6 


Calcium 


Ca 


39-9 


40-0 


Carbon 


C 


11-97 


12-0 


Cerium 


Ce 


141-2 


141'2 


Chlorine . 


Cl 


35-37 


35-37 


Chromium 


Cr 


52-4 


52-4 


Cobalt 


Co 


58-6 


59-0 


Copper 
Gold 


Cu 
Au 


63-18 
196-2 


63-0 
196-5 


Hydrogen 
Iodine 


H 

I 


1-0 
126-86 


1-0 
127-0 


Iron . 


Fe 


55-88 


56-0 


Lead 


Pb 


206-4 


206-4 


Magnesium 
Manganese 
Mercury 
Molybdenum 
Nickel 


Mg 
Mn 
Hg 
Mo 

Ni 


23-94 
55-0 
199-8 
95-8 
58-6 


24-0 
55-0 
200-0 
95-8 
59-0 


Nitrogen . 
Oxygen 
Phosphorus 
Platinum 


N 

P 
Pt 


14-01 
15-96 
30-96 
194-3 


14-0 
16-0 
31-0 
194-3 


Potassium 


K 


39-04 


39-0 


Silver . 
Sodium 


Ag 

Na 


107-66 
22-99 


107-66 
23-0 


Strontium 


Sr 


87-2 


87-2 


Sulphur . 
Tin . 


S 
Sn 


31-98 
117-8 


32-0 
118-0 


Tungsten . 
Uranium 


w 

Ur 


184-0 
239-8 


184-0 
240-0 


Vanadium 


Va 


51-2 


51-2 


Zinc . 


Zn 


64-9 


65-0 



[XI] 



Abbreviations and Explanations. 

The formulae are constructed on the basis H=l. = 16 
H 2 = 18. 

The normal temperature for the preparation and use of standard 
solutions is 16 C., or about 60 Fahr. 

c.c. denotes cubic centimeter. 

gm. gram = 15-43235 grains English. 

grn. grain. 

dm. decem= 10 fluid grains at 16 C. 

1 liter=1000 c.c. at 16 C. 

1 c.c. = 1 gm. distilled water at 16 C. 

1 dm. = 10 grn. 

Distilled water is to be used in all the processes, unless other- 
wise expressed. 

Normal Solutions are those which contain one gram atom of 
reagent (taken as monobasic), or an equivalent in some active 
constituent (e.r/. oxygen) in the liter (see page 28). 

Decinormal Solutions are one-tenth of that strength = T ^-. 
Centinormal, one hundredth = -$. 

Empirical Standard Solutions are those which contain no 
exact atomic proportion of reagent, but are constructed generally so 
that 1 c.c. = 0'01 gm. (one centigram) of the substance sought. 

A Titrated Solution (from the French word titre, title or 
power) denotes a solution whose strength or chemical power has 
been accurately found by experiment. 

When a chemical substance or solution is directed to be titrated, 
the meaning is, that it is to be quantitatively tested for the amount 
of pure substance it contains by the help of standard or titrated 
solutions. The term is used in preference to tested or analyzed, 
because these expressions may relate equally to qualitative and 
quantitative examinations, whereas titrations can only apply to 
quantitative examination. 

J. C. S. denotes Journal of the Chemical Society (Transactions 
only). 

/. S. C. I. Journal of the Society of Chemical Industry. 
Z. a. (7. Zeitschrift fiir Analytische Chemie. 
C. N. Chemical News. 

Other book-references are given in full. 



EREATA AND ADDENDA. 

Page 25. Line 15 from top, read 760 in.m. in place of 0'76 m.m. 

Page 139. Line 3 from bottom, read " nitrates " in place of " hydrates." 

Page 149. Line 3 from bottom, omit the words " arsenic obtained as 
sulphide/' and substitute " arsenical material." 



OF THE , 

P"KIVERSITY 



VOLUMETRIC ANALYSIS 

OF 

LIQUIDS AND SOLIDS, 



PART I. 

GENERAL PRINCIPLES. 

1. QUANTITATIVE analysis by weight, or gravimetric analysis, 
consists in separating out the constituents of any compound, either 
in a pure state or in the form of some new substance of known 
composition, and accurately weighing the products. Such opera- 
tions are frequently very complicated, and occupy a long time, 
besides requiring in many cases elaborate apparatus, and the exercise 
of much care and experimental knowledge. Volumetric processes 
on the other hand, are, as a rule, quickly performed ; in most cases 
are susceptible of extreme accuracy, and need much simpler 
apparatus. The leading principle of the method consists in sub- 
mitting the substance to be estimated to certain characteristic 
reactions, employing for such reactions solutions of known 
strength, and from the volume of solution necessary for the pro- 
duction of such reaction, determining the weight of the substance 
to be estimated by aid of the known laws of chemical equivalence. 

Volumetric analysis, or quantitative chemical analysis by measure, 
in the case of liquids and solids, consequently depends upon the 
following conditions for its successful practice : 

1. A solution of the reagent or test, the chemical power of 
which is accurately known, called the " standard solution." 

2. A graduated vessel from which portions of it may be 
accurately delivered, called the " burette." 

3. The decomposition produced by the test solution with any 
given substance must either in itself or by an indicator be such, 
that its termination is unmistakable to the eye, and thereby the 
quantity of the substance with which it has combined accurately 
calculated. 



VOLUMETRIC ANALYSIS. 1. 

Suppose, for instance, that it is desirable to know the quantity of 
pure silver contained in a shilling. The coin is first dissolved in 
nitric acid, by which means a bluish solution, containing silver, 
copper, and probably other metals, is obtained. It is a known fact 
that chlorine combines with silver in the presence of other metals 
to form silver chloride, which is insoluble in nitric acid. The pro- 
portions in which the combination takes place are 35*37 of chlorine 
to every 107*66 of silver; consequently, if a standard solution of 
pure sodic chloride is prepared by dissolving in water such a weight 
of the salt as will be equivalent to 35*37 grains of chlorine ( = 58 -37 
grains XaCl) and diluting to the measure of 1000 grains; every 
single grain measure of this solution will combine with 0*10766 grain 
of pure silver to form silver chloride, which is precipitated to the 
bottom of the vessel in which the mixture is made. In the process 
of adding the salt solution to the silver, drop by drop, a point is at 
last reached when the precipitate ceases to form. Here the process 
must stop. On looking carefully at the graduated vessel from 
which the standard solution has been used, the operator sees at 
once the number of grain measures which has been necessary to 
produce the complete decomposition. For example, suppose the 
quantity used was 520 grain measures ; all that is necessary to be 
done is to multiply 520 by the coefficient for each grain measure, 
viz. 0*10766, which shows the amount of pure silver present to be 
55*98 grains. 

This method of determining the quantity of silver in any given 
solution occupies scarcely a quarter of an hour, whereas the estimation 
by weighing could not be done in half a day, and even then not so 
accurately as by the volumetric method. It must be understood 
that there are certain necessary precautions in conducting the above 
process which have not been described; those will be found in their 
proper place; but from this example it will at once be seen that the 
saving of time and trouble, as compared with the older methods of 
analysis, is immense ; besides which, in the majority of instances 
in which it can be applied, it is equally accurate, and in many cases 
much more so. 

The only conditions on which the volumetric system of analysis 
are to be carried on successfully are, that great care is taken with 
respect to the graduation of the measuring instruments, and their 
agreement with each other, the strength and purity of the standard 
solutions, and the absence of other matters which would interfere 
with the accurate estimation of the particular substance sought. 

The fundamental distinction between gravimetric and volumetric 
analysis is, that in the former method, the substance to be 
estimated must be completely isolated in the purest possible state 
or combination, necessitating in many instances very patient and 
discriminating labour ; whereas, in volumetric processes, such com- 
plete separation is very seldom required, the processes being so 
contrived as to admit of the presence of half a dozen or more 



$ 1. GENERAL PRINCIPLES. 3 

other substances which have no effect upon the particular chemical 
reaction required. 

The process just described for instance, the estimation of silver 
in coin, is a case in point. The alloy consists of silver and copper, 
with small proportions of lead, antimony-, tin, gold, etc. None of 
these things affect the amount of salt solution which is chemically 
required to precipitate the silver, whereas, if the metal had to be 
determined by weight it would be necessary to first filter the nitric 
acid solution to free it from insoluble tin, gold, etc. ; then 
precipitate with a slight excess of sodic chloride; then to bring the 
precipitate upon a filter, and wash repeatedly with pure water until 
every trace of copper, sodic chloride, etc., is removed. The pure 
silver chloride is then carefully dried, ignited separately from the 
filter, and weighed ; the filter burnt, residue as reduced metallic 
silver and filter ash allowed for, and thus finally the amount of 
silver is found by the balance with ordinary weights. 

On the other hand the volumetric process has been purely 
chemical, the burette or measuring instrument has taken the place 
of the balance, and theoretical or atomic weights have supplanted 
ordinary weights. 

The end of the operation in this method of analysis is in all 
cases made apparent to the eye. In alkalimetry it is the change 
of colour produced in litmus, turmeric, or other sensitive colouring 
matter. The formation of a permanent precipitate, as in the 
estimation of cyanogen. A precipitate ceasing to form, as in 
chlorine and silver determination. The appearance of a distinct 
colour, as in iron analysis by permanganate solution, and so on. 

I have adopted the classification of methods used by Mohr and 
others, namely : 

1. Where the determination of the substance is effected by 
saturation with another substance of opposite properties generally, 
understood to include acids and alkalies, or alkaline earths. 

2. Where the determination of a substance is effected by a 
reducing or oxidizing agent of known power, including most 
metals, with their oxides and salts ; the principal oxidizing agents 
being potassic permanganate, potassic bichromate, and iodine; and 
the corresponding reducing agents, ferrous and stannous compounds, 
:and sodic thiosulphate. 

3. Where the determination of a substance is effected by 
precipitating it in some insoluble and definite combination, an 
example of which occurs in the estimation of silver described 
above. 

This classification does not rigidly include all the volumetric 
processes that may be used, but it divides them into convenient 
sections for describing the peculiarity of the reagents used, and 
their preparation. If strictly followed out, it would in some cases 
necessitate the registration of the body to be estimated under two 
or three heads. Copper, for instance, can be determined residually 

B 2 



4 VOLUMETRIC ANALYSIS. 1. 

by potassic permanganate ; it can also be determined by precipitation 
with sodic sulphide. The estimation of the same metal by potassic 
cyanide, on the other hand, would not come under any of the 
heads. 

It will be found, therefore, that liberties have been taken with 
the arrangement ; and for convenient reference all analytical pro- 
cesses applicable to a given body are included under its name. 

It may be a matter of surprise to some that several distinct 
volumetric methods for one and the same substance are given; 
but a little consideration will show that in many instances greater 
convenience, and also accuracy, may be gained in this way. The 
operator may not have one particular reagent at command, or he 
may have to deal with such a mixture of substance as to preclude 
the use of some one method; whereas another may be quite 
free from such objection. The choice in such cases of course 
requires judgment, and it is of the greatest importance that the 
operator shall be acquainted with the qualitative composition of the 
matters with which he is dealing, and that he should ask himself 
at every step why such and such a thing is done. 

It will be apparent from the foregoing description of the 
volumetric system, that it may be successfully used in many 
instances by those who have never been thoroughly trained as 
analytical chemists ; but we can never look for the scientific 
development of the system in such hands as these. 

In the preparation of this work an endeavour has been made to 
describe all the operations and chemical reactions as simply as 
possible, purposely avoiding abstruse mathematical expressions,, 
which, though they may be more consonant with the modern study 
of chemical science, are hardly adapted to the technical operator. 



NIVERSIT 

2. INSTRUMENTS. 



THE IKSTBUMENTS AND APPAKATUS. 

THE BALANCE. 

2. STRICTLY speaking, it is necessary to have two balances in 
order to carry out the volumetric system completely ; one to carry 
about a kilogram in each pan, and turn when loaded with 
about five milligrams. This instrument is used for graduating 
flasks, or for testing them, and for weighing large amounts of pure 
reagents for standard solutions. The second balance should be 
light and delicate, and to carry about fifty grams, and turn easily 
and quickly when loaded with one or two-tenths of a milligram. 
This instrument serves for weighing small quantities of substances 
to be tested, many of which are hygroscopic, and need to be 
weighed quickly and with great accuracy ; it also serves for testing 
the accuracy of pipettes and burettes. 

For all technical purposes, however, a moderate-sized balance 
of medium delicacy is quite sufficient, especially if rather large 
quantities of substances are weighed and brought into solution- 
then further subdivided by means of measuring flasks and pipettes. 

The operator also requires, besides the balance and the graduated 
instruments a few beakers, porcelain basins, flasks, funnels, stirring 
rods, etc., as in gravimetric analysis ; above all he must be 
practically familiar with proper methods of filtration, washing of 
precipitates, and the application of heat. 

VOLUMETRIC ANALYSIS WITHOUT WEIGHTS. 

3. THIS is more a matter of curiosity than of value; but, 
nevertheless, one can imagine circumstances in which it might be 
useful. In carrying it out, it is necessary only to have (1) a 
correct balance, (2) a pure specimen of substance to use as a weight, 
(3) an accurate burette filled with the appropriate solution. It is 
not necessary that the strength of this should be known ; but the 
state of concentration should be such as to permit the necessary 
reaction to occur under the most favourable circumstances. 

If a perfectly pure specimen of substance, say calcic carbonate, 
be put into one scale of the balance, and be counterpoised with an 
impure specimen of the same substance, and both titrated with the 
same, acid, and the number of c.c. used for the pure substance be 
called 100, the number of c.c. used for the impure substance w r ill 
correspond to the percentage of pure calcic carbonate in the specimen 
examined. 

The application of the process is, of course, limited to the use of 
such substances as are to be had pure, and whose weight is not 
variable by exposure; but where even a pure substance of one kind 
cannot be had as a weight, one of another kind may be used as a 
substitute, and the required result obtained by calculation. For 



6 VOLUMETRIC ANALYSIS. 4. 

instance, it is required to ascertain the purity of a specimen of sodic 
carbonate, and only pure calcic carbonate is at hand to use as a 
weight; equal weights of the two are taken, and the impure 
specimen titrated with acid. To arrive at the required answer, it 
is necessary to find a coefficient or factor by which to convert the 
number of c.c. required by the sodic carbonate, weighed on the 
calcic, into that which should be required if weighed on the sodic, 
basis. A consideration of the relative molecular weights of the 
two bodies will give the factor thus 

Calcic carbonate 100 

= TTf ; , . w . = 

bodic carbonate lOb 

If, therefore, the c.c. used are multiplied by this number, the 
percentage of pure sodic carbonate will be obtained. The method 
may be extended to a number of substances, 011 this principle, with 
the exercise of a little ingenuity. 

L. de Koningh has communicated to me a similar method 
devised by himself and Peacock, in which the same end is 
attained without the aid of a pure substance as standard, thus : 
Say a specimen of impure common salt is to be examined, a 
moderate portion is put on the balance and counterpoised with 
silver nitrate; the latter is then dissolved up to 100 c.c. and placed 
in a burette. The salt is dissolved in water, a few drops of 
chromate added and titrated with the silver solution, of which 
10 c.c. is required; the salt is therefore equal to 10 per cent, 
of its weight of silver nitrate, then 

16-96 : 58-37 : : 10 = 344 % XaCl 

Or, in the case of an impure soda ash, an equal weight of oxalic- 
acid is taken and made up to 100 c.c. ; the soda requires, say, 
50 c.c. for saturation, or 50 per cent., then 

126 : 106 : : 50 = 42 % Na 2 CO :{ 

It may happen that, in some cases, more than one portion of the 
reagent is required to decompose the substance tested, and to 
provide against this two or more lots should be weighed in the 
first instance. 

VOLUMETRIC ANALYSIS WITHOUT BURETTES OR 
OTHER GRADUATED INSTRUMENTS. 

4. THIS operation consists in weighing the standard solutions 
on the balance instead of measuring them. The influence of 
variation in temperature is, of course, here of no consequence. The 
chief requisite is a delicate flask, fitted with a tube and blowing- 
ball, as in the burette fig. 7, or an instrument known as 
Schuster's alkalimeter may be used. A special burette has been 
devised for this purpose by Casamajor (C. N. xxxv. 98). The 



5. 



INSTRUMENTS. 



method is capable of very accurate results, if care be taken in 
preparing the standard solutions and avoiding any loss in pouring 
the liquid from the vessel in which it is weighed. It occupies 
much more time than the usual processes of volumetric analysis, 
but at great extremes of temperature it is far more accurate. 

THE BURETTE. 

5. THIS instrument is used for the delivery of an accurately 
measured quantity of any particular standard solution. It invari- 
ably consists of a long glass tube of even bore, throughout the 





Fig. l. Pig. 2. 

length of which are engraved, by means of hydrofluoric acid, 
certain divisions corresponding to a known volume of fluid. 



8 



VOLUMETRIC ANALYSIS. 



5. 



It may be obtained in a great many forms, under the names of their 
respective inventors, such as Mohr, Gay Lussac, Links, etc., 
but as some of these possess a decided superiority over others, it is 
not quite a matter of indifference which is used, and therefore a 
slight description of them may not be out of place here. The 
burette, with india-rubber tube and clip, contrived by Mohr, is 
shown in figs. 1 and 2, and with stop-cock in fig. 3. "This latter 
form of instrument is now made and sold at such a moderate price 
that it has largely displaced the original. 





Fig. 3. 



Fig. 4. 



The advantages possessed by Mohr's burette are, that its fixed 
upright position enables the operator at once to read off the volume 
of solution used for any analysis. The quantity of fluid to be 
delivered can be regulated to the greatest nicety ; and the instru- 
ment not being held in the hand, there is no chance of increasing 
the bulk of the fluid by the heat of the body, and thus leading to 
incorrect measurement, as is the case with Bin ks' or Gay Lussac's 



5. 



INSTRUMENTS. 



9 



burette.. The principal disadvantage, however, of these two latter 
forms is, that a correct reading can only be obtained by placing 
them in an upright position, and allowing the fluid to find its perfect 
level. The preference should, therefore, unhesitatingly be given 
to Mohr's burette. The tap burette may be used not only for 
solutions affected by the rubber tube, but for all other solutions, 
and may also be arranged so as to deliver the liquid in drops, 
leaving both the hands of the operator disengaged. A new 




Pig. 5. 



arrangement is shown in fig. 4, the tap being placed obliquely 
through the spit, so as to avoid its dropping out of place ; the 
floats shown are very small thermometers. Owing to the action of 
caustic alkalies upon glass, tap burettes do not answer well for 
strong solutions of potash or soda, unless emptied and washed 
immediately after use. Two convenient forms of stand for Mohr's 
burettes are shown in figs. 5 and 6 ; in the latter, the arms carrying 



10 



VOLUMETRIC ANALYSIS. 



the burettes revolve. A very good modification of this burette, as 
usually made, is to have the top funnel-shaped, which not only 
admits of easier filling, but the burette may be slung in a stand by 
the funnel without other support, so as to be tilted from the 
vertical when titrating hot solutions. When not in use the dust 
may be kept out by a greased glass plate. 

Special care should always be taken with Molir's form of 
burette to fill the delivery point of the instrument and the 
intervening rubber tube with the liquid, before commencing a 
titration. This is easily done by filling the burette well above the 
mark, then rapidly opening the clip wide to expel the air 
bubbles when this is done the excess of liquid may be quietly 
run out to the mark. In the tap burette the air space is smaller 
than with the rubber tube, but the same method should be 
invariably adopted. 

We are indebted to Mohr for another form of instrument to 
avoid the contact of permanganate and india-rubber, viz., the foot 
burette, with elastic ball, shown in fig. 7. 

The flow of liquid from the 
exit tube ca^n be regulated to 
a great nicety by pressure 
upon the ball, which should 
be large, and have two open- 
ings, one cemented to the 
tube with marine glue, and 
the other at the side, over 
which the thumb is placed 
when pressed, and on the 
removal of which it refills 
itself with air. 

G a v L u s s a c ' s burette , 
supported in a wooden foot, 
may be used instead of the 
above form, by inserting a 
good fitting cork into the 
open end, through which a 
small tube bent at right 
angles is passed. If the 
burette is held in the right 
hand, slightly inclined to- 
wards the beaker or flask 
into which the fluid is to be 
measured, and the mouth 
applied to the tube, any 
portion of the solution may 
be emptied out by the pressure 
of the breath, and the disadvantage of holding the instrument in 
a horizontal position, to the great danger of spilling the contents, 




INSTRUMENTS. 



11 



is avoided ; at the same time the beaker or flask can be held 
in the left hand and shaken so as to mix the fluids, and by 
this means the end of the operation be more accurately determined 
(see fig. 8). 

There is an arrangement of Mohr's burette which is extremely 
serviceable, when a series of titrations of the same character have 
to be made, such as in alkali works, assay offices, etc. It consists 
in having a ~J~ piece of glass tube inserted between the lower 
end of the burette and the spring clip, communicating with 





Fi-. 9. Pig. 10. 

a reservoir of the standard solution, placed above so that the 
burette may be filled by a syphon, as often as emptied, and in so 
gradual a manner that no air bubbles occur, as in the case of filling 
it with a funnel, or pouring in liquid from a bottle ; beside which, 
this plan prevents evaporation or dust in the standard solution 
either in the burette or reservoir. 

Figs. 9 and 11 show this arrangement in detail. Connections 



12 



VOLUMETRIC ANALYSIS. 



5. 



of this kind may now be had with glass stop-cocks, either of the 
simple form or the patent two-way cock, made by Greiner and 
Fried richs, and supplied by most apparatus dealers (fig. 10). 

It sometimes happens that a solution requires titration at a hot or 
even boiling temperature, such as the estimation of sugar by copper 




-Fig. 11. 



rig. 12. 



solution: here the ordinary arrangement of Mohr's burette will 
not be available, since the steam rising from the liquid heats the 
burette and alters the volume of fluid. This may be avoided either 
by using a special burette, in which the lower end is extended at a 
right angle with a stop-cock, or by attaching to an ordinary burette 



5. INSTRUMENTS. 13 

a much longer piece of india-rubber tube, so that the burette 
stands at the side of the capsule or beaker being heated, and the 
elastic tube is brought over its edge ; the pinch-cock is fixed 
midway ; no heat can then reach the body of fluid in the burette,, 
since there can be no conduction past the pinch-cock, or a burette- 
with funnel neck described on p. 10 may be used. 

Gay Lussac's burette is shown in figs. 8 and 12. By using it 
in the following manner, its natural disadvantages may be overcome 
to a great extent. Having fixed the burette into the foot securely,, 
and filled it, take it up by the foot, and resting the uppeu end upon 
the edge of the beaker containing the solution to be titrated, drop 
the test fluid from the burette, meanwhile stirring the contents, 
of the beaker with a glass rod ; by a slight elevation or depression, 
the flow of test liquid is regulated until the end of the operation is 
secured, thus avoiding the annoyances which arise from alternately 
placing the instrument in an upright and horizontal position. 




Pig. 13. 

B inks' burette is well known, and need not be described; it 
is the least recommendable of all forms, except for very rough 
estimations. 

It is convenient to have burettes graduated to contain from 
30 to 50 c.c. in y 1 ^ c.c., and 100 or 110 c.c. in 4 or -J c.c. 

The pinch-cock generally used in Mohr's burette is shown in 
fig. 1. These are made of brass and are now generally nickel-plated 
to prevent corrosion ; another form is made of one piece of steel 
wire, as devised by Hart; the wire is softened by heating and 
coiled round, as shown in fig. 13. When the proper shape has 
been attained, the clip is hardened and tempered so as to convert it. 
into a spring. . 



VOLUMETRIC ANALYSIS. 



Another pinch-cock is shown in fig. 13. It may be made of 
hard wood, horn, or preferably, of flat glass rod. The levers 
should be long. A small piece of cork, of the same thickness 
as the elastic tube of the burette when pressed close, should be 
fastened at the angles of the levers as shown in the engraving. 



50 CC 




10CC 



Pig. 14. 



Pig. 15. 



The use of any kind of pinch-cock may be avoided, and a very 
ilelicate action obtained, by simply inserting a not too tightly fitting 
piece of solid glass rod into the elastic tube^ between the end of the 
burette and the spit ; a firm squeeze being given by the finger and 
thumb to the elastic tube surrounding the rod, a small canal is 
opened, and thus the liquid escapes, and of course can be controlled 
by the operator at will (see fig. 14). 




6. INSTRUMENTS. 

THE PIPETTE. 

6. THE pipettes used in volumetric work are of two kinds, 
viz., those which deliver one certain quantity only, and those which 
are graduated on the stem, so as to deliver various quantities at the 
discretion of the analyst. In the former kind, or whole pipette, 
the graduation should be that in which the fluid runs out by its 
own weight, but the last few drops empty themselves slowly ; 
if, however, the lower end of the pipette be touched against the 
moistened edge of the beaker or the surface of the fluid into 
which it is emptied, the flow is hastened considerably, and in 
graduating the pipette, it is preferable to adopt this plan. 

In both the whole and graduated pipettes, the upper end is 
narrowed to about -J inch, so that the pressure of the finger is 
sufficient to arrest the flow at any point. 

Pipettes are invariably filled by sucking the upper end with the 
mouth, unless the liquid is volatile or highly poisonous, in which 
case it is best to use some other kind of measurement. Beginners 
invariably find a difficulty in quickly filling the pipette 
above the mark, and stopping the fluid at the exact 
point. Practice with pure water is the only method of 
overcoming this. 

Fig. 15 shows two whole pipettes, one of small and the 
other of large capacity, and also a graduated pipette of 
medium size. It must be borne in mind that the pipette 
graduated throughout the stem is not a reliable in- 
strument for accurate titration, owing to the difficulty of 
stopping the flow of liquid at any given point, and 
reading off the exact measurement. Its chief use is in 
the approximate estimation of the strength of any 
standard solution in the course of preparation. 

Fig. 16 shows a very useful form of pipette for 
measuring strong acids or alkalies, etc., the bulb prevent- 
ing the entrance of any liquid into the mouth. Pig. 16. 



THE MEASURING- FLASKS. 

7. THESE indispensable instruments are made of various 
capacities ; they serve to mix up standard solutions to a given 
volume, and also for the subdivision of the substance to be 
tested by means of the pipettes. They should be as narrow 
in the neck as is compatible with pouring in and out, and the 
graduation line should fall just below the middle of the neck, 
so to allow room for shaking up the fluid. Convenient sizes 
are 100, 200, 250, 300, 500, and 1000 c.c., all graduated to 
contain the respective quantities. If required to deliver these 
volumes they must have a second higher mark in the neck, 
obtained by weighing into the wetted and drained flasks the 



16 



VOLUMETRIC ANALYSIS. 



respective number of grams of distilled water at 16 C. 
flask is shown in fig;. 17. 



A liter 





Pig. 17. 



Fisr. 18. 



W. B. Giles lias described a modified flask (C. N. Ixix. 99) 
shown in fig. 18. It is handy in making up standard solutions 
where the reagent cannot be weighed in an absolutely pure state, 
for instance, sulphuric acid, ammonic thiocyanate, or uranic salts. 
Such a quantity, hoAvever, is taken as will give a solution about 
a ninth or tenth too strong, and the measure is made up to 1100 c.c. 
The real strength is then taken by two titrations on 25 or 30 c.c. 
with a known standard, so that its exact working strength is 
known ; the remainder of the 100 c.c. is then removed down to the 
1000 c.c. mark, and a slight calculation will show how much water 
has to be added to the 1000 c.c. to make a correct solution. If 
only a liter is made up, an unknown volume is left in the flask, 
and it must be transferred to a measuring cylinder, where, owing 
to the large diameter of the vessel, the graduation can never be so 
accurate as in the narrow neck of . the flask. Should the solution 
prove to be only about a tenth too strong, the necessary dilution 
may be made in the flask itself; but if stronger than this, the 
flask must be emptied into the store bottle and rinsed out with the 
measured quantity of water required, which is then drained into 
the store bottle, and the whole carefully mixed. 



INSTRUMENTS. 



17 



Besides the measuring flasks 
it is necessary to have graduated 
vessels of cylindrical form, 
for the purpose of preparing 
standard solutions, etc. 

Fig. 19 shows a stoppered 
cylinder for this purpose, 
generally called a test mixer. 
Wide-mouthed open cylinders, 
with spouts, are also used of 
various sizes and graduated 
like fig. 19. 

ON THE CORRECT 

READING- OF GRADUATED 

INSTRUMENTS. 

8. THE surface of liquids 
contained in narrow tubes is 
always curved, in consequence 
of the capillary attraction 
exerted by the sides of the 
tube, and consequently there 
is a difficulty in obtaining a 
distinct level in the fluid to 
be measured. If, however, 
the lowest point of the curve 
is made to coincide with the 
graduation mark, a correct 
proportional reading is always 
obtained, hence this method of 
reading is the most satisfac- 
tory (see fig. 20). 

The eye may be assisted 
materially in reading the 
divisions on a graduated 
tube by using a piece of 
white paper or opal glass 
held at an angle of 30 or 
40 from the burette and 
near the surface of the 
liquid, or a small card, the 
lower half of which is 
blackened, the upper re- 
maining white. If the line 
of division between the 
black and white be held 
about an eighth of an inch below the surface of the liquid, 




Fig. 20. 



18 



VOLUMETRIC ANALYSIS. 



and the eye brought on a level with it, the meniscus then can be 
seen by transmitted light, bounded below by a sharply defined 
black line. A card of this kind, sliding up and down a support,, 
is of great use in verifying the graduation of the burettes or 
pipettes with a cathetometer. Another good method 
is to use a piece of mirror, upon which are gummed 
two strips of black paper, half an inch apart ; apply it 
in contact with the burette so that the eye can be 
reflected in the open space. The operator may consult 
with advantage the directions for calibration on the 
opposite page, and details of graduating and verifying* 
measuring instruments for the analysis of gases as- 
described in Part 7. In taking the readings of 
burettes, pipettes, and flasks, the graduation mark 
should coincide as nearly as possible with the level of 
the operator's eye. 




Pig. 21. 



v 



Erdmann's Float. This useful little instrument 
to accompany Mohr's burette, gives the most accurate 
reading that can be obtained j one of its forms is 
shown in fig. 21, another, containing a thermometer, 
is shown in fig. 4. The latest form is shown in fig. 22, 
where the ring-mark is made within the bulb, as indeed 
it is best to be in all -cases. A special form for use with 
dark-coloured solutions like iodine, permanganate, &c., is 
to have two bulbs with the ring-mark in the upper bulb, 
and the instrument is so weighted that the upper bulb 
stands out of the liquid, and of course may then be read 
off as easily as if the liquid were transparent. The 
instrument consists essentially of an elongated glass tube, 
K rather smaller in diameter than the burette itself, and 
Pio- 22 weighted at the lower end with a globule of mercury. 
The actual height of the liquid in the burette is not- 
regarded, because if the operator begins with the line on 
the float, opposite the graduation mark on the burette, 
the same proportional division is always maintained. 

It is essential that the float should move up and down 
in the burette without wavering, and the line upon it 



should always be parallel to the 
burette. 



graduations of the 




Filter for ascertaining: the end re-action in certain pro- 
cesses. This is shown in fig. 23, and the instrument is 
known as Be ale's filter. It serves well for taking a few 
drops of clear solution from any liquid in which a pre- ^f* 
cipitate will not settle readily. To use it, a piece of filter 
paper is tied over the lower end, arid over that a piece of fine muslin, 
to keep the paper from being broken. When dipped into a muddy 



9. INSTRUMENTS. 19 

mixture, the clear fluid rises and may be poured out of the little 
spout for testing. If the process in hand is not completed, the 
contents are washed hack to the bulk, and -the operation repeated 
as often as may be required. 



THE CALIBRATION OF GRADUATED APPARATUS. 

9. IT is obvious that in the practice of volumetric analysis 
the absolute correctness of the graduations of the vessels used to 
a given standard is not necessary, so long as they agree with one 
another. In the present day there* are many makers of in- 
struments, some using the liter of 1000 grams of distilled water 
at 4 C., others at 15'5 C., and again at 17'5 C. Under these 
circumstances it is conceivable that operators may purchase, from 
time to time, a mixture of instruments of a heterogeneous 
character. The German Imperial Standard Commission have, 
I believe, now made it legal only to use for official purposes the 
liter and its divisions, containing 1000 grams of pure water at 
4 C. (p. 23). These instruments for use in that country are all 
stamped in the same way as commercial measures are stamped by 
law in this country. If, then, instruments are sent abroad, they 
Avill not agree with the bulk of those hitherto used. On this 
account, as well as for general accuracy, it is necessary to calibrate 
or measure the divisions upon the various instruments by 
actual experiment, carried on in a room kept at the temperature 
of 16 C. 

Flasks. The shortest way to get at the true contents of a liter 
flask, or to correct it for a given temperature by making a fresh 
mark, is to weigh the contents by substitution, which is done as 
follows : 

The flask is cleaned and dried, by first rinsing with alcohol, then 
ether, and the latter blown out with a bellows or driven off by 
warming, when cool it is placed on a sufficiently large and 
sensitive balance, together with a kilogram weight, side by side 
a shallow metal tray is placed on the other pan, and sufficient shot 
added to exactly balance the flask and weight ; both the latter are 
then removed, leaving the shot on the other pan. The flask is 
then placed level, and distilled water at 16 C. poured in up to 
the mark ; the moisture in the neck is removed after a few 
minutes by filter paper and the flask placed on the empty pan, if 
the two pans are in equilibrium the mark is correct, if not, water 
must be added or removed, with a small pipette, and the mark 
altered. Smaller flasks are calibrated in the same way. 

To calibrate a flask for delivering an exact liter or less, some 
water is poured into the empty flask, which is drained for half 
a minute, and weighed with its stopper ; it is then filled to the 
neck with pure water, and closed by the glass or rubber stopper, 

c 2 



20 VOLUMETRIC ANALYSIS. 9. 

to prevent evaporation, and water added or removed as before. 
A nick is then made with a diamond, or sharp file, opposite the 
lowest part of the meniscus, which may be extended to a proper 
mark after the flask is emptied. Such a flask, when correctly 
marked, will deliver the volume required at the given temperature, 
after the contents have been poured out and drained for half 
a minute. 

Burettes. After firmly fixing in its stand, filling with pure 
water at 16 C., and getting rid of the air bubbles in the tap or 
spit, the exact level at the mark is made preferably with aij 
ErdmaAn float; successive quantities of 5 or 10 c.c. are then run 
into a small dry tared beaker and rapidly weighed. If great 
accuracy is required a closed vessel ought to be employed, but this 
necessitates the drying after each weighing ; a very small beaker 
can be easily wiped dry, and rapid weighings made without any 
sensible loss of accuracy. If the weighings have shown reasonable 
accuracy, say within a milligram or so for each c.c., it will be 
sufficiently correct ; if otherwise, a table must be constructed, 
showing the correct contents at any given point. 

An excellent method of calibrating tap burettes is described by 
Carnegie (C. N. Ixiv. 42), which saves the labour involved in 
the separate weighings just described, but does not give the weight 
contents. A small column of CS 2 , saturated with water, and 
tinted with iodine, is used to measure the spaces between the 
graduation marks of the instrument. The burette is connected by 
rubber tube with a reservoir of water like that used for mercury 
in gas apparatus, and by the pressure of the water in this reservoir 
5 c.c. or so of the CS 2 may be moved from the bottom upwards, 
throughout the whole length of the instrument, so as to compare 
portions of the scale throughout. It is essential that the measure- 
ment takes place from the bottom, which is done by allowing 
water to flow in up to the lower mark of the burette, then gently 
running in the portion of CS 2 from a long fine pipette ; when 
settled, and the meniscus observed, a cautious opening of the tap 
will allow of the movement of the column, through the various 
divisions, up to the top. 

Pipettes. With the instrument made to deliver one quantity 
only it is generally sufficient to fill it by suction above the mark, 
then gently release the pressure of the finger, until the exact mark 
is reached. The contents are then run into a dry tared beaker, 
drained for half a minute in contact with the sides of the beaker, 
and the beaker quickly weighed. If not fairly correct, trials must 
be made by placing a thin strip of gummed paper on the stem, 
and marking the height of each trial until the correct weight is 
found, when a permanent mark may be made. 

Graduated pipettes are best calibrated by filling them above the 



9. 



PRESERVATION OF SOLUTIONS. 



21 



mark, fixing them in a stand like a burette, closing the top with 
a stout piece of rubber tube, clamped with a strong clip, then, 
after adjusting the level, drawing off in quantities of 5 c.c. or so, 
and weighing in the same way as directed for burettes. 

Cylinders. The only method of calibrating these vessels is to 
measure into them repeatedly various volumes of water, from 
delivery pipettes of proved accuracy, taking precautions as to level, 
meniscus, and the proper drainage of the pipette after each 
delivery. 

Preservation of Solutions. There are test solutions which, in 
consequence of their proneness to decomposition, cannot be kept 
at any particular strength for a length of time ; consequently they 
must be titrated on every occasion before being used. Stannous 
chloride and sulphurous acids are examples of such solutions. 
Special vessels have been devised for keeping solutions liable to 
alter in strength by access of air, as shown in figs. 24 and 25. 




Pig. 25. 



22 



VOLUMETRIC ANALYSIS. 



9. 



Fig. 24 is especially applicable to caustic alkaline solutions, the 
tube passing through the caoutchouc stopper being filled with dry 
soda-lime, resting on cotton wool. 

Fig. 25, designed by Mohr, is a considerable improvement 
upon this, since it allows of the burette being filled with the 
solution from the store bottle quietly, and without any access of 
air whatever. The vessel can be used for caustic alkalies, baryta, 
stannous chloride, permanganate, and sulphurous acids, or any other 
liquid liable to undergo change by absorbing oxygen. The corks 
are dried and soaked in melted paraffins ; or, still better, may be 
substituted by caoutchouc stoppers ; and a thin layer of rectified 
paraffin oil is poured on the top of the solution, where, of course, 
owing to its low specific gravity, it always floats, placing an 
impermeable division between the air and the solution ; and as 
this body (which should always be as pure as possible) is not 
affected by these reagents in their diluted state, this form offers 
great advantages. Solutions not affected chemically by contact 
with air should nevertheless be kept in bottles, the corks or stoppers 
of which are perfectly closed, and tied over with india-rubber or 
bladder to prevent evaporation, and should further be always 
shaken before use, in case they are not quite full. The influence 
of bright light upon some solutions is very detrimental to their 
chemical stability ; hence it is advisable to preserve some solutions 
not in immediate use in the dark, and 
at a temperature not exceeding 15 or 
16 C. 

The apparatus devised by J. C. 
Chorley, and shown in fig 26, will be 
found useful for preserving and delivering 
known volumes of such solutions as 
alcoholic potash, which are liable to 
contamination by exposure to air. The 
wash bottle inserted in the cork of the 
large store bottle contains a solution of 
caustic soda, and serves to wash all air 
entering the large bottle. By means of 
the three-way stop-cock at the bottom of 
the apparatus the solution is allowed to 
fill the pipette and overflow into its upper 
chamber, the excess being caught in the 
small side bulb and reservoir ; this solution 
serves to wash all air entering the pipette 
when the stop-cock is turned to deliver 
the solution, which is run off to a mark 
just above the tap. When full, the side 
reservoir may be emptied by withdrawing 
the small ground stopper. Fig. 26. 





' 



10. WEIGHTS AND MEASURES. 2o 

ON THE SYSTEM OF WEIGHTS AND MEASURES 
TO BE ADOPTED IN VOLUMETRIC ANALYSIS. 

10. IT is mucli to be regretted that the decimal system of 
weights and measures used on the Continent is not universally 
adopted, for scientific and general purposes, throughout the civilized 
world. Its great advantage is its uniformity throughout. The 
unit of weight is the gram ( = 15-43235 grains troy), and a gram 
of distilled water at 4 C., or 39 Fahr., measures exactly a cubic 
centimeter. The kilogram contains 1000 grams, the liter 1000 
cubic centimeters. 

It may not be out of place here to give a short description of the 
origin of the French decimal system, now used exclusively for 
scientific purposes in that country, and also in Prussia, Austria, 
Holland, Sweden, Denmark, Belgium, and Spain. 

The commission appointed in France for the purpose of instituting 
a decimal system of weights and measures, founded their standard 
on the length of the meridian arc between the pole and equator, 
the ten-millionth part of which' was called the metre ( = 39*3710 
English inches), although the accuracy of this measurement has 
been disputed. It would have been preferable, as since proposed, 
that the length of a pendulum vibrating exactly 86,400 times in 
twenty-four hours, or one second for each vibration, equivalent to 
39 '1372 English inches, should have been taken as the standard 
juttrvj in which case it would have been much easier to verify the 
standard in case it should be damaged or destroyed. However, the 
actual mid-re in use is equal to 39 '371 inches, and from this standard 
its multiples and subdivisions all proceed decimally ; its one-tenth 
part being the decimetre, one-hundredth the centimetre, and one- 
thousandth the millimetre. 

In accordance with tins, a cube of distilled water at its greatest 
density, viz., 4 C., or 39 Fahr., whose side measures one decimeter, 
has exactly the weight of one kilogram, or 1000 grams, and occupies 
the volume of one liter, or 1000 cubic centimeters. 

This simple relationship between liquids and solids is of great 
value in a system of volumetric analysis, and even for ordinary 
analysis by weight ; for technical purposes it is equally as applicable 
as the grain system, the results being invariably tabulated in 
percentages. 

With these brief explanations, therefore, I have only to state 
that the French decimal system will be mainly used throughout 
this treatise ; but at the same time, those who may desire to adhere 
to the ordinary grain weights, can do so without interfering with 
the accuracy of the processes described. 

As has been before stated, the true cubic centimeter contains 
one gram of distilled water at its greatest density, viz., 4 C., 
or 39 Fahr. ; but as this is a degree of temperature at which it 
is impossible to work for more than a month or two in the year, it 
is better to take the temperature of 16 C., or about 60 Fahr., as 



24 VOLUMETRIC ANALYSIS. W. 

the standard ; because in winter most laboratories or rooms have 
furnaces or other means of warmth, and in summer the same 
localities ought not, under ordinary circumstances, to have a much 
higher degree of heat than 16 C. In order, therefore, that the 
graduation of instruments on the metrical system may be as- 
uniform as possible with our own fluid measures, the cubic- 
centimeter should contain one gram of distilled water at 16 C, 
The true c.c. (i.e. = 1 gm. at 4 C., or 39 Fahr.) contains only 
0-999 gm. (strictly 0-998981) at that temperature; but for con- 
venience of working, and for uniformity with our own standards- 
of volume, it is better to make the c.c. contain one gram at 16 C. 
The real difference is one-thousandth part. The operator, there- 
fore, supposing he desires to graduate his own measuring flasks, 
must weigh into them 250, 500, or 1000 grams of distilled water 
at 16 C., or 60 Fahr. 

Fresenius and others have advocated the use of the strict liter 
by the graduation of instruments, so that they shall contain 
999 gm. at 16 C. Mohr, on the contrary, uses a 1000 gm., at 
the temperature of 17 '5, the real difference being T2 c.c. in the 
liter, or about one eight-hundredth part. 

It will be seen above that I have advocated a middle course on 
two grounds: (1) That in testing instruments it is much easier 
to verify them by means of round numbers, such as 5 or 10 gm. 
(2) That there are many thousands of instruments already in use- 
varying between the two extremes ; and as these cannot well be 
annihilated, the adoption of a mean will -give a less probable amount 
of error between the respective instruments ; and, moreover, the 
difference between the liter at 4 and 16 being one-thousandth 
part, it is easy to correct the measurement for the exact liter. 

It matters not which plan is followed, if all the instruments in 
a particular set coincide with each other ; but it would be 
manifestly wrong to use one of Mohr's burettes with one of 
Fresenius' measuring flasks. Operators can, however, without 
much difficulty re-mark their measuring flasks to agree with their 
smaller graduated instruments, if they are found to differ to any 
material extent. 

Variations of Temperature. In the preparation of standard 
solutions, one thing must especially be borne in mind ; namely, 
f that saline substances on being dissolved in water have a consider- 
able effect upon the volume of the resulting liquid. The same is 
also the case in mixing solutions of various salts or acids with each 
other (see Gerlach, " Specifische Gewichte der Salzosungen ; " 
also Gerlach, " Sp. Gewichte von wasserigen Losungen," Z. a C. 
viii. 245). 

In the case of strong solutions, the condensation in volume is as 
a rule considerable : and, therefore, in preparing such solutions for 
volumetric analysis, or in diluting such solutions to a given volume 



10. 



INFLUENCE OF TEMPERATURE. 



25 



for the purpose of removing aliquot portions subsequently for 
examination, sufficient time must be given for liquids to assume 
their constant volume at the standard temperature. If the strength 
of a standard solution i-s known for one temperature, the strength 
corresponding to another temperature can only be calculated if the 
rate of expansion by heat of the liquid is known. The variation 
cannot be estimated by the known rule of expansion in distilled 
water; for Gerlacli has shown that even weak solutions of acids 
and salts expand far more than water for certain increments of 
temperature. The rate of expansion for pure water is known, 
and may be used for the purpose of verifying the graduation of 
instruments, where extreme accuracy is required. The following 
short table furnishes the data for correction. 

The weight of 1000 c.c. of water at t C., when determined by 
means of brass weights in air of t C., and at 0'76 m.ni. pressure, 
is equal to 1000 x gm. 

Slight variations of atmospheric pressure may be entirely 
disregarded. 



t* 


10 


11 


12 


13 


14 


15 16 


17 


18 


19 




X 


1-34 


1-43 1-52 


1'63 


176 


1-89 2-04 


2'2 


2-37 


2-55 




t 


20 


21 


22 


23 


24 


25 26 


27 


28 


29 


30 


X 


274 


2-95 


3-17 


3-39 


3'63 


3-88 4-13 


4-39 


4-67 


4-94 


5-24 



x is the quantity to be subtracted from 1000 to obtain the 
weight of 1000 c.c. of water at the temperature i. Thus at 20 
2-74 must be deducted from 1000 = 997*26. 

Bearing the foregoing remarks in mind, therefore, the safest plan 
in the operations of volumetric analysis, so far as measurement is \ 
concerned, is to use solutions as dilute as possible. Absolute! 
accuracy in estimating the strength of standard solutions can only 
be secured by weight, the ratio of the weight of the solution to the 
weight of active substance in it being independent of temperature. 

Casamajor (C. N. xxxv. 160) has made use of the 
data given by Matthiessen in his researches on the expansion 
of glass, water, and mercury, to construct a table of corrections to 
be used in case of using any weak standard solution at a different 
temperature to that at which it was originally standardized. 

The expansion of water is different at different temperatures ; 
the expansion of glass is known to be constant for all temperatures 
up to 100. The correction of volume, therefore, in glass burettes, 
must be the known expansion of each c.c. of water for every 1 C., 
less the known expansion of glass for the same temperature. 

It is not necessary here to reproduce the entire paper of 
Casamaj or, but the results are shortly given in the following table. 



* 

UNIVERSITY 



26 VOLUMETRIC ANALYSIS. 10. 

The normal temperature is 15 C. ; and the figures given are the 
relative contractions below, and expansions above, 15 C. 

DC?. C. Deg. C. 

7 _ -Q00612 24 + '001686 

8 _ -000590 25 + '001919 

9 _ -000550 26 + '002159 
10 _ -000492 ' 27 + '002405 



1 1 -000420 

12 '000334 



28 4- '002657 

29 + -002913 



13 '000236 30 + "003179 

14 '000124 31 + '003453 

15 Normal 32 + "003739 

16 + '000147 33 + -004035 

17 + -000305 34 + -004342 

18 + -000473 35 + '004660 

19 + '000652 36 + '0049S7 

20 + -000841 37 + "005323 

21 + -001039 38 + -005667 

22 + -001246 39 + '006040 

23 + '001462 40 + '000382 

By means of these numbers it is easy to calculate the volume of 
liquid at 15 C. corresponding to any volume observed at any 
temperature. If 35 c.c. of solution has been used at 37 C., the 
table shows that 1 c.c. of water in passing from 15 to 37 is 
increased to 1 -005323 c.c. ; therefore, by dividing 35 c.c. by 1 "005323 
is obtained the quotient 34'819 c.c., which represents the volume 
at 15 corresponding to 35 c.c. at 37 ; or the operation can be 
simplified by obtaining the factor, thus : 

1^5323 = 0-991705 

A table can thus be easily constructed which would show the 
factor for each degree of temperature. 

These corrections are useless for concentrated solutions, such as 
normal alkalies or acids ; with great variations of temperature 
these solutions should be used by weight. 

Instruments graduated on the Grain System. Burettes, pipettes, 
and flasks may also be graduated in grains, in which case it is best 
to take 10,000 grains as the standard of measurement. In order 
to lessen the number of figures used in the grain system, so far 
as liquid measures are concerned, I propose that ten fluid grains be 
called a decem, or for shortness dm. ; this term corresponds to the 
cubic centimeter, bearing the same proportion to the 10,000 grain 
measure as the cubic centimeter does to the liter, namely, the 
one-thousandth part. The use of a term like this will serve to 
prevent the number of figures, which are unavoidably introduced 
by the use of a small unit like the grain. 

Its utility is principally apparent in the analysis for percentages, 
particulars of which will be found hereafter. 



11. NORMAL SOLUTIONS. 27 

The 1000 grain burette or pipette will therefore contain 100 
tlecems, the 10,000 gr. measure 1000 dm., and so on. 

The capacities of the various instruments graduated on the grain 
system may be as follows : 

Flasks : 10,000, 5000, 2500, and 1000 grs. = 1000, 500, 250, and 
100 dm. Burettes : 300 grs. in 1-gr. divisions, for very delicate 
purposes = 30 dm. in y^j-GOO grs. in 2-gr. divisions, or i dm.; 
1100 grs. in 5-gr. divisions, or J dm. ; 1100 grs. in 10-gr. divisions, 
or 1 dm. The burettes are graduated above the 500 or 1000 grs. 
in order to allow of analysis for percentages by the residual method. 
Whole pipettes to deliver 10, 20, 50, 100, 200, 500, and 1000 grs., 
graduated ditto, 100 grs. in y 1 ^ dm. ; 500 grs. in ?, dm. ; 1000 grs. 
in 1 dm. 

Those who may desire to use the decimal systems constructed on 
the gallon measure = 70,000 grains, will bear in mind that the 
"septem"of Griffin, or the "decimillem" of Acland are each 
equal to 7 grs. ; and therefore bear the same relation to the 
pound = 7000 grs., as the cubic centimeter does to the liter, or the 
decem to the 10,000 grs. An entirely different set of tables for 
calculations, etc., is required for these systems ; but the analyst 
may readily construct them when once the principles contained in 
this treatise are understood. 

VOLUMETRIC ANALYSIS BASED ON THE SYSTEM OF 
CHEMICAL EQUIVALENCE AND THE PREPARATION 
OF NORMAL TITRATED SOLUTIONS. 

11. WHEX analysis by measure first came into use, the test 
solutions were generally prepared so that each substance to be tested 
had its own special reagent ; and the strength of the standard 
solution was so calculated as to give the result in percentages. 
Consequently, in alkalimetry, a distinct standard acid was used for 
soda, another for potash, a third for ammonia, and so on, necessi- 
tating a great variety of standard solutions. 

Griffin and Ure appear to have been the first to suggest the use 
of standard test solutions based on the atomic system; and folio wing- 
in their steps Mohr has worked out and verified many methods of 
analysis, which arc of great value to all who concern themselves 
with scientific and especially technical chemistry. .Not only has 
Mohr done this, but in addition to it, he has enriched his 
processes with so many original investigations, and improved the 
necessary apparatus to such an extent, that he may with justice 
be called the father of the volumetric system. 

Normal Solutions. It is of great importance that no misconcep- 
tion should exist as to what is meant by a normal solution ; but it 
does unfortunately occur, as may be seen by reference to the 
chemical journals, also to Muir's translations, of Fleischer's 
book (see Allen, C. N. xl. 239, also Analyst, xiii. 181). 



VOLUMETRIC ANALYSIS. 11. 

Normal solutions as originally devised are prepared so that one 
liter at 16 C. shall contain the hydrogen equivalent of the active 
reagent weighed in grams (H=l). Seminormal, quintinormal, 
clecinormal, and centinormal solutions are also required, and may 
be shortly designated as J f- -~~ and y^ solutions.* 

In the case of univalent substances, such as silver, iodine, 
hydrochloric acid, sodium, etc., the equivalent and the atomic 
(or in the case of salts, molecular) weights are identical ; thus, a 
normal solution of hydrochloric acid must contain 36*37 grams of 
the acid in a liter of fluid, and sodic hydrate 40 grams. In the 
case of bivalent substances, such as lead, calcium, oxalic acid, 
sulphurous acid, carbonates, etc., the equivalent is one half of the 
atomic (or in the case of salts, molecular) weight ; thus, a normal 
solution of oxalic acid would be made by dissolving 63 grams of 
the crystallized acid in distilled water, and diluting the liquid to 
the measure of one liter. 

Further, in the case of trivalent substances, such as phosphoric 
acid, a normal solution of sodic phosphate would be made by 
weighing -f-= 119*3 grams of the salt, dissolving in distilled 
water, and diluting to the measure of one liter. 

One important point, however, must not be forgotten, namely, 
that in preparing solutions for volumetric analysis the value of a 
reagent as expressed by its equivalent hydrogen-weight must not 
always be regarded, but rather its particular reaction in any given 
analysis ; for instance, tin is a quadrivalent metal, but when 
using stannous chloride as a reducing agent in the analysis of 

* It is much to be regretted that the word "normal," originally based on the 
equivalent system, should now be appropriated by those who advocate the use of 
solutions based on molecular weights, because it not only leads to confusion between 
the two systems, but to utter confusion between the advocates of the change them- 
selves. In Fleischer's German edition of his Maasanalyse the molecular system 
is advocated, but, as the old atomic weights are used, the solutions are really, in the 
main, of the same strength as those based on the equivalent system. Pattiusoii 
Muir, however, in his translation, has thought proper to use modern atomic weights, 
and the curious result is that one is directed to prepare a normal solution of caustic 
potash, with 39 '1 grams K to the liter, while a normal potassic carbonate is to contain 
138'2 grams K-CO a , or 78'2 grams K, in the same volume of solutions. Again, Muter, 
in his Manual of Analytical Chemistry, defines a normal solution as having one molecular 
weight of the reagent in grams per liter; then follows the glaring inconsistency, 
among others, of directing that a decinormal solution of iodine should contain 12 '7 
grams of I per liter, whereas, if it was strictly made according to the original definition, 
it shotild contain 25'4 grams in the liter. Menschutkin's Analytical Chemistry, 
translated by Locke, recently published by Macmillan & Co., unfortunately adopts 
the molecular system. 

If the unit H be adopted as the basis or standard, everything is simplified, and 
actual normal solutions may be made and used; but, on the molecular system, this 
is, in many cases, not only unadvisable but impossible, besides leading to ridiculous 
inconsistencies. As Allen points out in the reference above, it is, to say the least 
of it, highly inconvenient that the nomenclature of a standard solution should be 
capable of two interpretations. I have given the term systematic to this handbook, 
and I maintain that the equivalent system used is the only systematic and consistent 
one ; it was adopted originally by M o h r , followed by Freseuius, and continued 
by Classen in the new edition of Mohr's Titrirmethode. Allen himself has 
unhesitatingly preferred to tise it in his Organic Analysis, and these, together with 
this treatise, being all text-books having a wide circulation, ought to settle definitely 
the meaning of the term normal as applied to systematic standard solutions. Anyhow, 
it is to be hoped that those who communicate processes to* the chemical journals, or 
abstractors of foreign articles for publication, will take care to distinguish between 
the conflicting systems. 



11. NORMAL SOLUTIONS. 29 

iron, the half, and not the fourth, of its molecular weight is 
required, as is shown by the equation Fe 2 Cl + Sn C1 2 = 2 Fe Cl 2 
+ Sn Cl 4 . 

In the same manner with a solution of potassie permanganate 
Mn KO 4 when used as an oxidizing agent, it is the available oxygen 
which has to be taken into account, and hence in constructing a 
normal solution one-fifth of its molecular weight =J^ = 31*6 grams 
must be contained in the liter. 

Other instances of a like kind occur, the details of which will 
be given in the proper place. 

A further illustration may be given in order to show the method 
of calculating the results of this kind of analysis. 

Each c.c. of ^ silver solution will contain Yy-Jo-Q- of the atomic 
weight of silver = 0*010766 gin., and will exactly precipitate 
T _i__ of the atomic weight of chlorine = '003537 gm. from any 
solution of a chloride. 

In the case of normal oxalic acid each c.c. will contain ^Vo of 
the molecular weight of the acid = 0*063 gm., and will neutralize 
__!__ of the molecular weight of sodic monocarbonate = 0*053 gm., 
or will combine with o-^Vo f the atomic weight of a dyad metal 
such as lead = 0;1032 gm., or will exactly saturate T oVo- f the 
molecular weight of sodic hydrate = 0*040 gm., arid so on. 

Where the 1000 grain measure is used as the standard in place 
of the liter, 63 grains of oxalic acid would be used for the normal 
.solution;- but as 1000 grains is too small a quantity to make, it is 
better to weigh 630 grains, and make up the solution to 10,000 
grain measures = 1000 dm. The solution would then have exactly 
the same strength as if prepared on the liter system, as it is pro- 
portionately the same in chemical power ; and either solution may 
be used indiscriminately for instruments graduated on either scale, 
bearing in mind that the substance to be tested with a c.c. burette" 
must be weighed on the gram system, and vice versa, unless it be 
desired to calculate one system of weights into the other. 

The great convenience of this equivalent system is, that the 
numbers used as coefficients for calculation in any analysis are 
familiar, and the solutions agree with each other, volume for 
volume. We have hitherto, however, looked only at one side of 
its advantages. For technical purposes the plan allows the use of 
all solutions of systematic strength, and simply varies the amount 
.of substance tested according to its equivalent weight. 

Thus, the normal solutions say, are- 
Crystallized oxalic acid =63 gm. per liter 
Sulphuric acid =49 gm. 
Hydrochloric acid -=36.37. gm. 
Nitric acid =63 gm. 
Anhydrous sodic carbonate =53 gm. 
Sodic hydrate =40 gm. 
Ammonia = 17 gm. 



30 VOLUMETRIC ANALYSIS. 11. 

100 c.c of any one of these normal acids should exactly neutralize 
100 c.c. of any of the normal alkalies, or the corresponding amount 
of pure substance which the 100 c.c. contain. In commerce we 
continually meet with substances used in manufactures which, are 
not pure, and it is necessary to know how much pure substance 
they contain. 

Let us take, for instance, refined soda ash (sodic carbonate). If 
it were absolutely pure, 5 '3 gm. of it should require exactly 100 c.c. 
of any normal acid to saturate it. If we therefore weigh that 
quantity, dissolve it in water, and deliver into the mixture the 
normal acid from a burette, the number of c.c. required to saturate 
it will show the percentage of pure sodic carbonate in the sample. 
Suppose 90 c.c. are required 90 %. 

Again a manufacturer buys common oil of vitriol, and requires 
to know the exact percentage of pure hydrated acid in it; 4'9 grams 
are weighed, diluted with water, and normal alkali delivered in 
from a burette till saturated; the number of c.c. used will be the 
percentage of real acid. Suppose 58'5 c.c. are required = 58'5 %. 

On the grain system, in the same way, 53 grains of the sample of 
soda ash would require 90 dm. of normal- acid, also equal to 90 %. 

Or, suppose the analyst desires to know the equivalent percentage 
of dry caustic soda, free and combined, contained in the above 
sample of soda ush, without calculating it from the carbonate found 
as above, 3'1 gm. is treated as before, and the number of c.c. 
required is the percentage of sodic oxide. In the same sample 
52'6 c.c. would be required = 52*6 per cent, of sodic oxide, or 90 
per cent, of sodic carbonate. 

Method for percentag-e of Purity in Commercial Substances. The 
rules, therefore, for obtaining the percentage of pure substance in 
any commercial article, such as alkalies, acids, and various salts, 
by means of systematic normal solutions such as have been 
described are these 

1. With normal solutions ^ or (according to its atomicity) 
of the molecular weight in grams of the substance to be analyzed 
is to be weighed for titration, and the number of c.c. required to 
produce the desired reaction is the percentage of the substance 
whose atomic weight has been used. 

With decinormal solutions -$ or TT of the molecular weight 
in grams is taken, and the number of c.c. required will, in like 
manner, give the percentage. 

Where the grain system is used it will be necessary, in the case 
of titrating with a normal solution, to weigh the whole or half the 
molecular weight of the substance in grains, and the number of 
decems required will be the percentage. 

With decinormal solutions, y 1 ^- or J^- of the molecular weight in 
grains is taken, and the number of decems will be the percentage. 

It now only remains to say, with respect to the system of weights- 



12. VOLUMETRIC PROCESSES. 31 

and measures to be used, that the analyst is at liberty to choose his 
own plan. Both systems are susceptible of equal accuracy, and he 
must study his own convenience as to which he Avill adopt. The 
normal solutions prepared on the gram system are equally applicable 
for that of the grain, and vice versa, so that there is no necessity 
for having distinct solutions for each system. 

Factors, or Coefficients, for the Calculation of Analyses. It 
frequently occurs that from the nature of the substance, or from 
its being in solution, this percentage method cannot be conveniently 
followed. For instance, suppose the operator has a solution con- 
taining an unknown quantity of caustic potash, the strength of 
which he desires to know ; a weighed or measured quantity of it 
is brought under the acid burette and exactly saturated, 32 c.c. 
being required. The calculation is as follows : 

The molecular Aveight of potassic hydrate being 56 : 100 c.c. of 
normal acid will saturate 5*6 gm. ; therefore, as 100 c.c. are to 5*6 srni.* 



so are 32 c.c. to #,' JQQ~~= 1*792 gm. KH.O. 

The simplest way, therefore, to proceed, is to multiply the 
number of c.c. of test solution required in any analysis, by the 
TTTO (T ( or T<TO o- ^ bivalent) of tli3 molecular weight of the substance 
sought, which gives at once the amount of substance present. 

An example may be given 1 gm. of marble or limestone 'is 
taken for the estimation of pure calcic carbonate, and exactly 
saturated with standard nitric or hydrochloric acid (sulphuric or 
oxalic acid are, of course, not admissible) 17 '5 c.c. are required, 
therefore 17 -5 x 0*050 (the o-oVir f the molecular weight of 
CaCO 3 ) gives 0'875 gm., and as 1 gm. of substance only was 



taken = 87 "5% of calcic carbonate. 



ON THE DIRECT AND INDIRECT PROCESSES OF 
ANALYSIS AND THEIR TERMINATION. 

12. THE direct method includes all those analyses where the 
substance under examination is decomposed by simple contact with 
a known quantity or equivalent proportion of some other body 
capable of combining with it, and where the end of the decomposition 
is manifested in the solution itself. 

It also properly includes those analyses in which the substance 
reacts upon another body to the expulsion of a representative 
equivalent of the latter, which is then estimated as a substitute 
for .the thing required. 

Examples of this method are readily found in the process for 
the determination of iron by permanganate, where the beautiful 
rose colour of the permanganate asserts itself as the end of the 
reaction. 

The testing of acids and alkalies comes, also, under this class, the 



32 VOLUMETRIC ANALYSIS. 12. 

great sensitiveness of litmus, or other indicators, allowing the 
most trifling excess of acid or alkali to alter their colour. 

The indirect method is exemplified in the analysis of manganese 
ores, and also other peroxides and oxygen acids, by boiling with 
hydrochloric acid. The chlorine evolved is estimated as the 
equivalent of the quantity of oxygen which has displaced it. We 
are indebted to Bun sen for a most accurate and valuable series of 
processes based on this principle. 

The residual method is such that the substance to be analyzed is 
not estimated itself, but the excess of some other body added for 
the purpose of combining with it or of decomposing it ; and the 
quantity or strength of the body added being known, and the con- 
ditions under which it enters into combination being also known, 
by deducting the remainder or excess (which exists free) from the 
original quantity, it gives at once the proportional quantity of the 
substance sought. 

An example will make the principle obvious : Suppose that a 
sample of native calcic or baric carbonate is to be tested. It is not 
possible to estimate it with standard nitric or hydrochloric acid in 
the exact quantity it requires for decomposition. There must be 
an excess of acid and heat applied also to get it into solution ; if, 
therefore, a known excessive quantity of standard acid be first 
added and solution obtained, and the liquid then titrated backward 
with an indicator and standard alkali, the quantity of free acid can 
be exactly determined, and consequently that which is combined 
also. 

In some analyses it is necessary to add a substance which shall 
be an indicator of the end of the process j such, for instance, is 
litmus or the azo colours in alkalimetry, potassic chromate in silver 
and chlorine, and starch in iodine estimations. 

There are other processes, the end of which can only be 
determined by an indicator separate from the solution ; such is 
the case in the estimation of iron by potassic bichromate, where 
a drop of the liquid is brought into contact with another drop of 
solution of red potassic prussiate on a white slab or plate ; when 
.a blue colour ceases to form by contact of the two liquids, -the end 
of the process is reached. 



14 



INDICATORS. 



33 



PAET II. 



ANALYSIS BY SATURATION. 



ALKALIMETRY. 

13. GAY LUSSAC based his system of alkalimetry upon a 
standard solution of sodic carbonate, with a corresponding solution 
of sulphuric acid. It possesses the recommendation, that a pure 
standard solution of sodic carbonate can be more readily obtained 
than any other form of alkali. Mohr introduced the use of 
caustic alkali instead of a carbonate, the strength of which is 
established by a standard solution of oxalic or sulphuric acid. 
The advantage in the latter system is, that in titrating acids with 
a caustic alkali, the well-known interference produced in litmus 
by carbonic acid is avoided ; this difficulty is now overcome 
wherever it is desired by the new indicators to be described. 

INDICATORS USED IN ALKALIMETRY. 

14. 1. Litmus Solution. It has 
been the custom since the introduction 
of the azo indicator, to regard litmus as 
old fashioned and of very doubtful 
sensitiveness. This is a mistake, for if 
properly prepared, it is, in the absence 
of carbonic acid, one of the most 
sensitive of the indicators used for 
alkalies. The difficulty which occurs 
when carbonates are titrated may be 
overcome by boiling off the gas, but 
this is tedious, and like most of the 
indicators in use, it is less sensitive in hot 
than in cold liquids, nevertheless it has 
excellent qualities, arid will hold its 
position against many more modern 
indicators. The litmus of commerce 
differs considerably in purity and colour, 
but a careful examination will at once 
detect a good specimen by the absence 
of a greyish muddy colour, due to 
inert matters, both of vegetable and mineral nature. 

A simple solution may be made by treating the cubes with 
repeated small quantities of hot water; mixing all the extracts, 
and allowing the liquid to stand in a covered beaker for a day or 
night. The clear blue liquid is then poured off and placed in the 
stock bottle, together with two or three drops of chloroform, this 

D 




34 VOLUMETRIC ANALYSIS. 14. 

latter agent prevents the development of bacteria, and if the 
bottle is simply covered with a piece of paper, through which the 
pipette is passed, the solution will keep for a long period. If the 
colour is a deep blue it must be modified by a few drops of diluted 
hydrochloric acid, until it is a faint purple. In course of time it 
may lose its colour, but this may be restored by simple exposure 
in a basin. Another method of preparing an extract of litmus in 
a concentrated form for dilution whenever required is as follows : 
extract all soluble matters from the solid litmus by repeated 
quantities of hot w r ater ; evaporate the mixed extracts to a moderate 
bulk, and add acetic acid in slight excess to decompose carbonates; 
evaporate to a thick extract, transfer this to a beaker, and add a large 
proportion of hot 85 per-cent. alcohol or methylated spirit ; by this 
treatment the blue colour is precipitated, and the alkaline acetates, 
together with some red colouring matter, remain dissolved; the 
fluid with precipitate is thrown on a filter, washed with hot spirit, 
and the pure colouring matter finally evaporated to a paste, which 
is placed in a wide-mouthed bottle, together with a few drops of 
chloroform ; this extract will keep for years unchanged. 

Another recent method gives the best results of any. The 
crushed litmus is extracted with warm distilled water, as before 
described, and the several extracts mixed, then allowed to stand 
in a beaker till quite clear this clear extract is poured off, 
freely acidified with hydrochloric acid, and put into a dialyser, 
which is surrounded by running water and kept so for about 
a week. The colouring matter of litmus being a colloid, all the 
calcium and other salts are removed, and a pure soluble colour in 
hot distilled water remains, which may be preserved in the 
manner previously described, or evaporated to a pasty condition 
and kept for use at any time when required. 

Free carbonic acid interferes considerably with the production of 
the blue colour, and its interference in titrating acid solutions with 
alkaline carbonates can only be got rid of by boiling the liquid 
during the operation, in order to displace the gas from the solution. 
If this is not done, it is easy to overstep the exact point of neutrality 
in endeavouring to produce the blue colour. The same difficulty is 
also found in obtaining the pink-red when acids are used for 
titrating alkaline carbonates, hence the great value of the caustic 
alkaline solutions free from carbonic acid when this indicator is used. 

It sometimes occurs that titration by litmus is required at night. 
Ordinary gas or lamp light is not adapted for showing the reaction 
in a satisfactory manner ; but a very sharp line of demarcation 
between red and blue may be found by using a monochromatic 
light. With the yellow sodium flame the red colour appears 
perfectly colourless, while the blue or violet appears like a mixture 
of black ink and water. The transition is very sudden, and even 
sharper than the change by daylight. 

The operation should be conducted in a perfectly dark room ; 



14 INDICATORS. 35 

and the flame may be best obtained by heating a piece of platinum 
coil sprinkled with salt, or a piece of pumice saturated with a 
concentrated solution of salt, in the Bunsen flame. 

2. Litmus Paper. Is simply made by dipping strips of 
calendered unsized paper in the solution and drying them ; the 
solution used being rendered blue, red, or violet as may be 
required. 

3. Cochineal Solution. This indicator possesses the advantage 
over litmus, that it is not so much modified in colour by the presence 
of carbonic acid, and may be used by gas-light. It may also be used 
with the best effect with solutions of the alkaline earths, such as 
lime and baryta water ; the colour with pure alkalies and earths is 
especially sharp and brilliant. The solution is made by digesting 
1 part of crushed cochineal with 10 parts of 25 per-cent. alcohol. 
Its natural colour is yellowish-red, which is turned to violet by 
alkalies ; mineral acids restore the original colour ; it is not so 
easily affected by weak organic acids as litmus, and therefore for 
these acids the latter is preferable. It cannot be used in the 
presence of even traces of iron or alumina compounds or acetates, 
which fact limits its use. 

4. Turmeric Paper. Pettenkof er, in his estimation of car- 
bonic acid by baryta water, prefers turmeric paper as an indicator. 
For this purpose it is best prepared by digesting pieces of the root, 
first in repeated small quantities of water to remove a portion of 
objectionable colouring matter, then in alcohol, and dipping strips 
of calendered unsized paper into the alcoholic solution, drying and 
preserving them in the dark. 

Thomson in continuance of his valuable studies on various 
indicators, found that turmeric paper is of very little use for 
ammonia, or the alkaline carbonates, or sulphides and sulphites, 
but he prepared a special paper of a light red-brown colour, by 
dipping it into the alcoholic tincture of turmeric rendered slightly 
alkaline by caustic soda. If this paper is wetted with water the 
colour is intensified to a dark red-brown ; when partly immersed in 
a very dilute solution of an acid, the wetted portion becomes bright 
yellow, while immediately above this a moistened dark red-brown 
band is formed, and the upper dry portion retains its original 
colour. This appearance only occurs in the titration of a com- 
paratively large proportion of an acid, when the latter is nearly all 
neutralized, and thus serves to indicate the near approach to the 
end-reaction. When neutral or alkaline, the colour of the immersed 
portion of paper is simply intensified as already described. This 
intensification is quite as decided as a change of tint. This red- 
brown paper is equally as sensitive as phenolphthalein for the 
titration of citric, acetic, tartaric, oxalic and other organic acids by 
standard soda or potash, and may be used for highly coloured 

D 2 



36 VOLUMETRIC ANALYSIS. 14. 

solutions. It is also available, like phenolphthalein, for the 
estimation of small quantities of acid in strong alcohol. 

Indicators derived from the Azo Colours, etc. 

A great stride has been taken in the application of these 
modern indicators, and the best thanks of all chemists are due to 
R. T. Thomson for his valuable researches on them, read before 
the Chemical Section of the Philosophical Society of Glasgow, and 
published in their Transactions ; also reprinted (C. A 7 ", xlvii. 123, 
185; xlix. 32, 119; J. &. C. I. vi. 195). The experiments 
recorded in these papers are most carefully carried out, and the 
truthfulness of their results has been verified by Lunge and other 
practical men as well as by myself. 

Space will only permit here of a record of the results, fuller 
details being given in the publications to which reference has been 
made. 

Much discussion has arisen as to the comparative sensitiveness 
of litmus and methyl orange, but there can be no doubt that in 
the absence of CO 2 litmus bears the palm, especially with very 
dilute solutions. In the titration of alkaline carbonates litmus 
may safely be used, if a considerable excess of standard acid is 
first added, the CO 2 completely boiled off, the liquid rapidly 
cooled, then titrated back with standard alkali free from CO' 2 . 
Where very great delicacy is required, not only must the standard 
solutions be free from CO 2 but the distilled water used for dilution 
should have been recently boiled. 

5. Methyl Orang-e, or para-dimethylaniline-azo-benzone-sulphonic 
acid is prepared by the action of diazotized sulphanilic acid upon 
dimethylaniline, the commercial product being either an ammonium 
or sodium salt of the sulphonic acid thus produced. If carefully 
prepared from the purest materials it possesses a bright orange 
colour, perfectly soluble in water ; but the commercial product is 
often of a dull colour, due to slight impurities in the substances 
from which it is produced, and often not completely soluble in 
water. These impurities may generally IDC removed by 
recrystallizatiori from hot alcoholic solution. Complaints have 
been made by some operators that the commercial article is some- 
times unreliable as an indicator ; it may be so, but although I have 
examined many specimens, I have not yet found any in which the- 
impurities sensibly affected its delicate action when used in the 
proper manner. The common error is the use of too much of it ; 
again, there is the personal error of observation, some eyes being- 
much more sensitive to the change of tint than others. The 
great value of this indicator is that, unlike litmus and some other 
agents, it is comparatively unaffected by carbonic acid, sulphuretted 
hydrogen, hydrocyanic, silicic, boric, arsenious, oleic, stearic,. 
palmitic, and carbolic acids, etc. It must not be used for the- 



1-1. INDICATORS. 37 

organic acids, such as oxalic, acetic, citric, tartaric, etc., since 
the end-reaction is indefinite ; nor can it be used in the presence 
of nitrous acid or nitrites, which decompose it. It may safely be 
used for the estimation of free mineral acids in alum, ferrous 
sulphate or chloride, zinc sulphate, cupric sulphate or chloride. The 
acid radical (and consequently -its equivalent metal) in cupric 
sulphate and similar salts may be estimated with accuracy by pre- 
cipitating the solution with sulphuretted hydrogen, filtering, and 
titrating the filtrate at once with normal alkali and methyl orange. 

Methyl orange is especially useful for the accurate standardizing 
of any of the mineral acids by means of pure sodic carbonate in the 
cold, the liberated carbonic acid having practically no effect, as is the 
case with many indicators. Its effect is also excellent with ammonia 
or its salts. A convenient strength for the indicator is 1 gram of 
the powder in a liter of distilled water ; a single small drop of the 
liquid is sufficient for 100 c.c. or more of any colourless solution 
the colour being faint yellow if alkaline, and pink if acid ; if too 
much is used the end-reaction is slower and much less definite. 
All titrations with methyl orange should be carried on at ordinary 
temperatures if the utmost accuracy is desired. 

There are two other azo-compounds in use, more especially by 
continental chemists, which possess the same properties and give 
much the same effect as methyl orange, namely Fischer's 
dimethylamido-azobenzene and tropoeolin 00. My experience is 
that these preparations are less sensitive than methyl orange, and 
wherever they are recommended for any process of titration the 
latter may be substituted with advantage. 

6. Phenacetolin. This substance is prepared by boiling together 
for several hours equal molecular proportions of phenol, acetic 
anhydride, and sulphuric acid. The product is well washed with 
water to remove excess of acid and dried for use ; it is soluble 
only in alcohol, and a convenient strength is 2 gm. per liter. The 
solution is dark brown, which gives a scarcely perceptible yellow 
with caustic soda or potash, when a few drops are used with the 
ordinary volumes of liquid. With ammonia and the normal 
alkaline carbonates it gives a dark pink, with bicarbonate a much 
more intense pink, and with mineral acids a golden yellow. This 
indicator may be used to estimate the amount of caustic potash 
or soda in the presence of their normal carbonates if the proportion 
of the former is not very small, or of caustic lime in the presence 
of carbonate. 

Practice however is required with solutions of known composition, 
so as to acquire knowledge of the exact shades of colour. 

7. Phenolphthalein. This substance is obtained by heating 
together at 120 C., for ten or twelve hours, five parts of phthalic 
anhydride, ten of phenol, and four of sulphuric acid j the product 



38 VOLUMETRIC ANALYSIS. 14 

is boiled with water, and the residue dissolved in dilute soda and 
filtered. The nitrate contains the phenolphthalein, which may 
be precipitated by neutralizing with acetic and hydrochloric acids, 
and purified by solution in alcohol, boiling with animal charcoal 
and re-precipitating with boiling water ; it is of a resinous nature, 
but quite soluble in 50 per-cent. alcohol. A convenient strength 
is 10 gm. per liter. 

A few drops of the indicator show no colour in the ordinary 
volumes of neutral or acid liquids ; the faintest excess of caustic 
alkalies, on the other hand, gives a sudden change to purple-red. 

This indicator is useless for the titration of free ammonia, or its 
compounds, or for the fixed alkalies when salts of ammonia are- 
present, except with alcoholic solutions, in which case caustic soda 
or potash displace the ammonia in equivalent quantities at ordinary 
temperatures, and the indicator forms no compound with the 
ammonia. 

It may, however, be used like phenacetoKn for estimating the 
proportions of hydrate and carbonate of soda or potash in the 
same sample where the proportion of hydrate is not too small. 
Unlike methyl orange, this indicator is especially useful in titrating 
all" varieties of organic acids ; viz., oxalic, acetic, citric, tartaric, 
etc. 

One great advantage possessed by phenolphthalein is, that it 
may be used in alcoholic solutions, or mixtures of alcohol and 
ether,* and therefore many organic acids insoluble in water may 
be accurately titrated by its help ; in addition to this it may be 
used to estimate the acid combined with many organic bases, 
such as morphia, quinia, brucia, etc., the base having no effect 
on the indicator. 

8. Kosolic Acid is soluble in 50 per-cent. alcohol, and a con- 
venient strength is 2 gm. per liter. Its colour is pale yellow, 
unaffected by acids, but turning to violet-red with alkalies. It 
possesses the advantage over litmus and the other indicators, that 
it can be relied upon for the neutralization of sulphurous acid 
with ammonia to normal sulphite (Thomson). Its delicacy- 
is sensibly affected by salts of ammonia and by carbonic acid. 
It is excellent for all the mineral, but useless for the organic acids^ 
excepting oxalic, 

9. Lacmoid. This indicator is a product of resorcin, and is- 
therefore somewhat allied to litmus ; nevertheless, it differs from it 
in many respects, and has a pronounced and valuable character of 
its own, especially when used in the form of paper. It may be 

* H. 1ST. and. C. Draper (C. N. Iv. 143) have shown that this indicator is rapidly 
decomposed by atmospheric carbonic acid, which is more readily absorbed, by alcohol 
than by water. Fortunately this is less the case with hot solutions than with cold ; 
titrations of this kind should therefore be quickly done, and with not too small 
a quantity of the indicator. 




14. INDICATORS. 

prepared by heating gradually to 110 C. a mixture of 100 parts 
of resorcin, five parts of sodic nitrite, and five parts of water; 
after the violent reaction moderates, it is heated to 120 C. until 
evolution of ammonia ceases. The residue is dissolved in warm 
water, and the lacmoid precipitated therefrom by hydrochloric 
acid ; it is well washed from free acid and dried for use. Lacmoid 
is soluble in dilute alcohol, and the indicator is best made by 
dissolving 2 gm. to the liter. 

10. Lacmoid Paper. This is prepared by dipping slips of 
calendered unsized paper into the blue or red solution, and drying 
them. 

Thomson states that, in nearly every particular, lacmoid paper, 
either blue or red, is an excellent substitute for methyl orange, 
and may be employed in titrating coloured solutions where the 
latter would be useless. Solution of lacmoid, on the other hand, 
is not so valuable as the paper, inasmuch as it is more easily affected 
by weak acids such as carbonic, boric, etc. 

There are a host of other indicators belonging to the same category 
as those mentioned above, such as Congo red, Porrier's blue, 
fluorescin, etc. ; but as they have no special advantages over them, 
and indeed are practically inferior in delicacy, no description of 
them will be given here. 

Two or more indicators are sometimes useful in one and the 
same solution, and will be described as occasion requires. 

Special indicators for certain purposes, such as potassic chromate 
for silver, ferric sulphate for sulphocyanides, etc., will be described 
in their proper place. 

Extra Sensitive Indicators. Mylins and Fb'rster (Bericlite, 
xxiv. 1482 ; also C. N. Ixiv. 228, et seq.) describe a series of 
experiments on the estimation of minute traces of alkali and the 
recognition of the neutrality of water by means of an etheieal 
solution of iodeosin or erythrosin. This body is a derivative of 
fluorescin, and occurs plentifully in commerce as a dye for fabrics 
and paper. The commercial material is purified by solution in 
aqueous ether, and the filtered solution is shaken with dilute 
caustic soda which removes the colour; the latter is then precipitated 
with stronger alkali. The salt is then filtered off, washed with 
spirit . and finally recrystallized from hot alcohol. The indicator 
used by the operators was made by dissolving 1 decigram 
of the dry powder in a liter of aqueous pure ether. The 
ether of commerce is purified and rendered neutral by washing 
with weak alkali, afterwards with pure water, and keeping 
the ether over pure water. The indicator so prepared is quite 
useless for the ordinary titration of acids and alkalies ; its 
chief use is for the detection and measurement of very minute 
proportions of alkali such as would occur in water kept in glass 



40 VOLUMETRIC ANALYSIS. 14. 

vessels, or the solubility of calcium or other earthy carbonates in 
water free from carbonic acid, or in the use of millinormal solutions 
of alkalies and acids, also the neutrality of so-called pure salts 
or water. The method of using the indicator is that of shaking 
up say 20 c.c. of the indicator with 100 c.c. of the liquid to 
be examined, when, if alkali is present, a rose colour will be 
communicated to the layer of ether which rises to the top. The 
indicator may be used in conjunction with millinormal standard 
solutions, or colorimetrically, like the well-known Xessler test. 
Further details of its use are described in the contributions 
mentioned. Another similar indicator is mentioned by Ru hem ami 
(J. C. S. Trans. Ixi. 285), the imide of dicinnamylphenylazimide. 
This material gives a violet rose colour with the most minute traces 
of alkali, such, for instance, as would occur from merely heating 
alcohol in a test tube, the faint trace of alkali thus derived from 
the glass being sufficient to cause a rapid development of colour. 



SHORT SUMMARY OF THOMSON'S RESULTS WITH 
INDICATORS AND PURE SALTS OF THE ALKALIES 
AND ALKALINE EARTHS, 

The whole of the base or acid in the following list of substances 
may be estimated with delicacy and precision unless otherwise 
mentioned. 

Litmus Cold. Hydrates of soda, potash, ammonia, lime, baryta, 
etc. ; arsenites of soda and potash, and silicates of the same bases ; 
nitric, sulphuric, hydrochloric, and oxalic acids. 

Litmus Boiling. The neutral and acid carbonates of potash, 
soda, lime, baryta, and magnesia, the sulphides of sodium and 
potassium, and silicates of the same bases. 

Methyl Orange Cold. The hydrates, carbonates, bicarbonates, 
sulphides, arsenites, silicates, and borates of soda, potash, ammonia, 
lime, magnesia, baryta, etc., all the mineral acids, sulphites, half the 
base in the alkaline and earthy alkaline phosphates and arseniates. 

Rosolic Acid Cold. The whole of the base or acid may be 
estimated in the hydrates of potash, soda, ammonia, and arsenites 
of the same ; the mineral and oxalic acids. 

Rosolic Acid Boiling. The alkaline and earthy hydrates and 
carbonates, bicarbonates, sulphides, arsenites, and silicates. 

Phenacetolin Cold. The hydrates, arsenites, and silicates of the 
alkalies ; the mineral acids. 

Phenacetolin Boiling. The alkaline and earthy hydrates, car- 
bonates, bicarbonates, sulphides, arsenites, and silicates. 



14. INDICATORS. 41 

Phenolphthalein Cold. -The alkaline hydrates, except ammonia; 
the mineral acids, oxalic, citric, tartaric, acetic, and other organic 
acids. 

Phenolphthalein Boiling. The alkaline and earthy hydrates, 
carbonates, bicarbonates, and sulphides, always excepting ammonia 
and its salts. 

Lacmoid Cold. The alkaline and earthy hydrates, arsenites and 
borates, and the mineral acids. Many salts of the metals which are 
more or less acid to litmus are neutral to lacmoid, such as the 
sulphates and chlorides of iron, copper, and zinc ; therefore this 
indicator serves for estimating free acids in such solutions. 

Lacmoid Boiling. The hydrates, carbonates, and bicarbonates of 
potash, soda, and alkaline earths. 

Lacmoid Paper The alkaline and earthy hydrates, carbonates, 
bicarbonates, sulphides, arsenites, silicates, and borates ; the mineral 
acids ; half of the base in sulphites, phosphates, arseniates. 

This indicator reacts alkaline with the chromates of potash or 
soda, but neutral with the bichromates, so that a mixture of the 
two, or of bichromates with free chromic acid, may be titrated by 
its aid, which could also be done with methyl orange were it not 
for the colour of the solutions. 

The following substances can be determined by standard 
alcoholic potash, with phenolphthalein as indicator. One c.c. 
normal caustic potash (1 c.c. = '056 gm. KHO) is equal to 
(Hehner and Allen) 

088 gm. butyric acid. '1007 gm. tributyrin 

282 ,, oleic acid. '2947 ,, triolein. 

256 ,, palmitic acid '2687 tripalmitin. 

284 ,, stearic acid '2967 ,, tristearin. 

410 ,, cerotic acid. '6760 ,, myricin. 

329 ,, resin acids (ordinary colophony, chiefly sylvic acid). 

General Characteristics of the Foregoing Indicators. 

It is interesting to notice the different degrees of sensitiveness 
shown by indicators used in testing acids and alkalies. This is well 
illustrated by Thomson's experiments, where he used solutions 
of the indicator containing a known weight of the solid material, 
and so adjusted as to give, as near as could be judged, the same 
intensity of colour in the reaction. 

It was found that lacmoid, rosolic acid, phenacetolin, and 
phenolphthalein were capable of showing the change of colour 
with one-fifth of the quantity of acid or alkali which was required 
in the case of methyl orange or litmus ; that is to say, in 100 c.c. 
of liquid, where the latter took 0*5 c.c., the same effect with the 
former was gained by O'l c.c. 



42 VOLUMETRIC ANALYSIS. ' 14 

Another important distinction is shown in their respective 
behaviour with mineral and organic acids. 

It is true the whole of them are alike serviceable for the 
mineral acids and fixed alkalies ; but they differ considerably in 
the case of the organic acids and ammonia. Methyl orange 
and lacmoid appear to be most sensitive to alkalies, while 
phenolphthalein is most sensitive to acids ; the others appear to^ 
occupy a position between these extremes, each showing, however, 
special peculiarities. The distinction, however, is so marked, that, 
as Thomson says, it is possible to have a liquid which may be 
acid to phenolphthalein and alkaline to lacmoid. 

The presence of certain neutral salts has, too, a definite effect 
on the sensitiveness of certain indicators. Sulphates, nitrates, 
chlorides, etc., retard the action of methyl orange slightly, while in 
the case of phenacetolin and phenolphthalein they have no effect. 
On the other hand, neutral salts of ammonia have such a disturbing 
influence on the latter as to render it useless, unless with special 
precautions. 

Nitrous acid alters the composition of methyl orange ; so also 
do nitrites when existing in any quantity. Forbes Carpenter 
has noted this effect in testing the exit gases of vitriol chambers 
(J. S. C. I. v. 287). 

Sulphites of the fixed alkalies and alkaline earths are practically 
neutral to phenolphthalein, but alkaline to litmus, methyl orange, 
and phenacetolin. 

Sulphides, again, can be accurately titrated with methyl orange 
in the cold, and on boiling off the H 2 S a tolerably accurate result can 
be obtained with litmus and phenacetolin, but with phenolphthalein 
the neutral point occurs when half the alkali is saturated. The 
phosphates of the alkalies, arseniates, and arsenites, also vary in 
their effects on the various indicators. 

Again, boric acid and the borates can be very accurately titrated 
by help of methyl orange or lacmoid paper, but the other indicators 
are practically useless, except with the modification on page 44. 

Thomson classifies the usual neutrality indicators into three 
groups. The methyl orange group, comprising that substance, 
together with lacmoid, dimethylamidobenzene, cochineal and Congo 
red ; the phenolphthalein group, consisting of itself and turmeric ; the 
litmus group, including litmus, rosolic acid, and phenacetolin. The 
methyl orange group are most susceptible to alkalies, the phen- 
olphthalein to acids, and the litmus somewhat between the two. 
This classification has nothing to do with delicacy of reaction, but 
with the special behaviour of the indicator under the same circum- 
stances ; for instance, saliva, which is generally neutral to litmus 
paper, is always strongly alkaline to lacmoid or Congo red, and acid 
to turmeric paper. Fresh milk reacts very much in the same way. 
No absolutely hard and fast line can however be drawn. 

Thomson gives the following table as an epitome of the results 



14 



INDICATORS. 



obtained with indicators, and 011 which several processes have beert 
based. The figures refer to the number of atoms of hydrogen 
displaced by the monatomic metals, sodium or potassium, in the 
form of hydrates. Where a blank is left it is meant that the end- 
reaction is obscure. The figures apply also to ammonia, except 
where phenolphthalein is concerned, and when boiling solutions are 
used. Calcic and baric hydrates also give similar results, -.except 
where insoluble compounds are produced. Lacmoid paper acts in 
every respect like methyl orange, except that it is not affected by 
nitrous acid or its compounds. Turmeric paper behaves exactly 
like phenolphthalein with the mineral acids and also with thio- 
sulphuric and organic acids. 



Acids. 


Methyl Orange. 'Phenolphthalein. 


Litmus. 


Name. 


Formula. 


Cold. 


Cold. 


Boiling. 


Cold. 


Boiling. 


Sulphuric . . 


H 2 SO 4 


2 


2 


2 


2 


2 


Hydrochloric . 


HC1 


I 


1 


1 


1 


1 


Nitric . . . 


HNO 3 


1 


1 


1 


1 


1 


Thiosulpliuric . 


H-'S 2 3 


2 


2 


2 


2 


2 


Carbonic . . 


H 2 C0 3 





1 dilute! 


- 





Sulphurous 


H 2 SO :? 


1 


2 








Hydrosulphuric 


H 2 S 





1 dilute 


o 





Phosphoric 


HAPO 4 


1 


2 


; 








Arsenic . . . 


H 3 AsO 4 


1 


2 











Arsenious . . 


IFAsO 3 

















Nitrous . . . 


HNO 2 


indicator destroyed * 





1 





Silicic . . . 


H 4 Si0 4 

















Borio . . . 


H 3 B0 3 

















Chromic . . 


H 2 Cr0 4 


1 


2 


2 








Oxalic . . . 


H 2 C 2 4 





2 


2 


2 


2 


Acetic . . . 


HC 2 H 3 2 


1 





1 nearly 





Butyric . . . 


HC 4 H7O 2 


1 





1 nearly 





Succiuic . . . 


H 2 C 4 H 4 O 4 


2 





2 nearly 





Lactic . . . 


HC 3 H 5 3 





1 





1 





Tartaric . . . 


H 2 C 4 H 4 ( 


. 


2 


. 


2 





Citric . . . 


H 3 C 6 H 5 0' 





3 












Allen (Pliarm. Jour., May llth, 1889) clearly points out that 
the acid which enters into the composition of an indicator must be 
weaker than the acid which it is required to estimate by its means. 
The acid of which methyl orange is a salt is a tolerably strong one, 
since.it is only completely displaced by the mineral acids; the 
organic acids are not strong enough to overpower it completely, 
hence the uncertainty of the end-reaction. The still weaker acids, 
such as carbonic, hydrocyanic, boric, oleic, etc., do not decompose 
the indicator at all, hence their salts may be titrated by it, just as 
if the bases only were present. On the other hand the acid of 
phenolphthalein is extremely weak, hence its salts are easily 
decomposed by the organic and carbonic acids. A combination of 
the two indicators is frequently of service ; say, for instance, in a 



44 VOLUMETRIC ANALYSIS. 15. 

mixture of normal and acid sodic carbonate, if first titrated with 
plienolphthalein and standard mineral acid, the rose colour dis- 
appears exactly at the point when the normal carbonate is saturated, 
the bicarbonate can then be found by continuing the operation 
with methyl orange. The study of these new indicators is still 
imperfect, and requires further elucidation ; more especially if we 
take into consideration some new aspects of the question mentioned 
in a paper by K. T. Thomson (/. S. C. I. xii. 432). The 
experiments there recorded and which are too voluminous to 
produce here, are of a very interesting character and point to the 
supposition that molecular condition, viscosity of the liquid or 
some such influence was at work, so as to modify very considerably 
the action of the indicator. The irregularities occurring in the cases 
mentioned are no doubt exceptional, and need not disturb the faith 
hitherto reposed in well-known and much-used methods of titration. 

The particular indicator whose erratic action was under 
discussion was phenolphthalein and it was demonstrated, that in 
using this indicator in the titration of boric acid with soda, no 
satisfactory end-reaction could be got in a merely aqueous solution, 
but that by the addition of not less than 30 per cent, of glycerine 
to the mixture, a perfectly correct determination could be made. 
Other substances such as starch, glucose, and cane sugar had 
a similar effect, but not to the same extent as glycerine. 

The result of these investigations, is to give a satisfactory 
method of estimating volumetrically the boric acid existing in its 
natural compounds, which has hitherto been a much desired thing. 

PREPARATION OF THE NORMAL ACID AND ALKALINE 

SOLUTIONS. 

15. IT is quite possible to carry out the titration of acids 
and alkalies with only one standard liquid of each kind; but it 
frequently happens that standard acids or alkalies are required 
in other processes of titration beside mere saturation, and it is 
therefore advisable to have a variety. 

Above all things it is absolutely necessary to have, at least, one 
standard acid and alkali prepared with the most scrupulous 
accuracy to use as foundations for all others. 

I prefer sulphuric acid for the normal acid solution, inasmuch as 
there is no difficulty in getting the purest acid in commerce. The 
normal acid made with it is totally unaffected by boiling, even 
when of full strength, which cannot be said of either nitric or 
hydrochloric acid. Hydrochloric acid is however generally pre- 
ferred by alkali makers, owing to its giving soluble compounds 
with lime and similar bases. Nitric and oxalic acids are also 
sometimes convenient. 

Sodic carbonate, on the other hand, is to be preferred for the 
standard alkali, because it may readily be prepared in a pure 



15. NORMAL SOLUTIONS. 45 

state, or may be easily made from pure- bicarbonate as 
described further on. Differences of opinion exist among 
chemists as to the best material to be used as a standard, in 
preparing the various solutions used in alkalimetry and acidimetry. 
Some give the preference to borax with methyl orange as indicator 
for alkalies. Others to potassic quadroxalate for acids with 
phenolphthalein as indicator. My experience satisfies me, that 
although many of these modifications may serve very well as 
controls, there is no more reliable standard than pure sodie 
carbonate. 

The chief difficulty with sodic carbonate is, that with litmus as 
indicator, the titration must be carried on at a boiling heat in 
order to get rid of carbonic acid, which hinders the pure blue 
colour of the indicator, notwithstanding the alkali may be in 
great excess. This difficulty is now set aside by the use of methyl 
orange. In case the operator has not this indicator at hand, litmus- 
gives perfectly accurate results, if the saturation is first conducted 
by rapidly boiling the liquid in a thin flask for a minute after each 
addition of acid until the point is reached when one drop of acid 
in excess gives a pink-red colour, which is not altered by further 
boiling. This is used as a preliminary test, but as titrations are 
usually conducted at ordinary temperatures, the final adjustment 
should be made by adding in the second trial a moderate excess of 
the acid, then boiling to get thoroughly rid of the CO 2 , rapidly 
cooling the liquid in a closed flask, and titrating back with an 
accurate standard alkali. A slight calculation will then give the 
figures for adjustment. 

As has been previously said, these two standards must be pre- 
pared with the utmost care, since upon their correct preparation and 
preservation depends the verification of other standard solutions. 

It may, however, be remarked, that in place of a standard solution 
of sodic carbonate, which is of limited use for general purposes, 
the pure anhydrous salt may be used for the rigid adjustment of 
normal acid. In this case about 3 grams of pure ^N"a 2 C0 3 or 4 gm. 
of pure XaHCO 3 are heated to dull redness for ten minutes in a 
weighed platinum crucible, cooled under an exsiccator, the exact 
weight quickly taken, then transferred to a flask by means of a 
funnel, through which it is washed and dissolved - with distilled 
water, methyl orange added, and the operation completed by 
running the acid of unknown strength from a burette divided into- 
-j c.c. into the soda solution in small quantities until exact 
saturation occurs. 

A second trial should now be made, but preferably with 
a different weight of the salt. The saturation is carried out 
precisely as at first. The data for ascertaining the exact strength 
of the acid solution by calculation are now in hand. 

A strictly normal acid should at 16 C. exactly saturate sodic-. 
carbonate in the proportion of 100 c.c. to 5 -3 gm. 



46 VOLUMETRIC ANALYSIS. 15. 

Suppose that 2'46 gm. sodic carbonate required 41 '5 c.c. of tlie 
acid in the first experiment, then 

2-46 : 5-3 : : 41'5 : x = 89'4 c.c. 

Again: 2*153 gm. sodic carbonate required 36*32 c.c. of acid, 
then 

2-153 : 5-3 : : 36'32 : x = S9'4 c.c. 

The acid may now be adjusted by measuring 890 c.c. into the 
graduated liter cylinder, adding 4 c.c. from the burette, or 
with a small pipette, and filling to the liter mark with distilled 
water. 

Finally, the strength of the acid so prepared must be proved by 
taking a fresh quantity of sodic carbonate, or by titratioii with a 
strictly normal sodic carbonate solution previously made, and using 
not less than 50 c.c. for the tit-ration, so as to avoid as much 
as possible the personal errors of measurement in small quantities. 
If the measuring instruments all agree, and the operations are 
.all conducted with due care, a drop or two in excess of either 
acid or alkali in 50 c.c. should suffice to reverse the colour of 
the indicator. 

In all alkalimetric titrations it must not be forgotten that some 
glass vessels yield a notable quantity of alkali to boiling water,. 
and even more to hot alkaline solutions. The use of vessels made 
of Jena glass is therefore preferable. 

1. Normal Sodic Carbonate. 
53 gm. !S T a 2 C0 3 per liter.. 

This solution is made by quickly weighing and dissolving 
53 gm. of pure sodic monocarboiiate, previously gently ignited and 
cooled under the exsiccator in hot distilled water, and when cooled 
diluting to 1 liter at 16 C. Absolutely pure sodic carbonate is 
-difficult to find in commerce, and even if otherwise pure, is 
generally contaminated with insoluble dust contracted in the 
process of drying ; very pure bicarbonate is not difficult to find, 
but its purity must be proved, the usual impurities are traces of 
chlorides, sulphates, and occasionally thiosulphate or sulphite. 

To obtain a salt which shall be suitable for a standard, the best 
white bicarbonate should be selected, and 20 or 30 grams 
dissolved in about half a liter of hot water. If the solution is 
free from any sediment or floating particles, a portion is acidified 
with pure nitric acid in a small beaker and tested with silver 
nitrate for chlorine, another portion for sulphate with baric 
chloride ; if either of these are found the salt is freed from them 
by packing, say half a pound, into a clean funnel, the neck of 
which is stopped with a plug of cotton wool. Cold distilled water 
free from any trace of chloride or sulphate is then poured on the 
.salt in repeated small quantities, and allowed to filter through 
until the testing shows the absence of these impurities. Of 



15. NORMAL SOLUTIONS. 47 

course this means a waste of some bicarbonate, but as the salt is 
not very soluble in cold water it is of no consequence. When 
the impurities are found to be removed, the funnel is allowed to 
drain completely, the contents spread out on a clean flat dish or 
plate, tied over loosely with porous paper, and placed on the water 
bath or in some other warm position to dry, finally put into 
a stoppered bottle for conversion into monocarbonate as required. 

If on the other hand the sample has not dissolved quite clear, 
another method must be adopted by making a saturated solution 
of the salt in boiling distilled water, filtering at once through 
paper in a heated funnel into a clean porcelain dish and keeping 
the solution stirred until quite cold ; by this means a pure salt 
deposits in a granular state which, after pouring off the superfluous 
liquid, may be dried and kept for "use as before described. In 
using this salt for the standard the procedure is as follows : 

About 85 gm. is heated to dull redness (not to fusion) in a 
platinum crucible, for fully ten minutes, stirring it occasionally 
with a platinum wire, then placed under an exsiccator to cool ; 
when placed upon the balance it will be found that very little 
more than 53 gm. remains. The excess is removed as quickly as 
possible, and the contents of the crucible washed into a beaker 
with hot distilled water; when the salt is dissolved the solution is 
cooled to a proper temperature, decanted into a liter flask and 
filled up to the mark with distilled water at 16 C. If cold 
water is used a hard cake is produced which dissolves very slowly. 

2. Normal Potassic Carbonate. 

69 gm. K 2 C0 3 per liter. 

This solution is sometimes, though rarely, preferable to the soda 
salt, and is of service for the estimation of combined acids in certain 
cases, where, by boiling the compound with this agent, an inter- 
change of acid and base occurs. 

It cannot be prepared by direct weighing of the potassic carbonate, 
and is therefore best established by titrating a solution of unknown 
strength with strictly normal acid. 

3. Normal Sulphuric Acid. 
49 grn. H 2 S0 4 per liter. 

About 30 c.c. of pure sulphuric acid of sp. gr. 1'840, or there- 
abouts, are mixed with three or four times the volume of distilled 
water and allowed to cool, then put into the graduated cylinder and 
diluted up to the liter at the proper temperature. The solution 
may now be titrated by strictly normal alkali, or with sodic 
carbonate. 

25 c.c. of the solution, diluted to 250 c.c., may be con- 
trolled by precipitation with baric chloride at a boiling heat, 
in which case 100 c.c. should produce as much baric sulphate as is 
equal to 49 gm. per liter. 



48 VOLUMETRIC ANALYSIS. 15. 

In using this control it is best to make two determinations, and 
preferably with different quantities of the acid, the mean is then 
taken for basis of calculation. 

4. Normal Oxalic Acid. 
63 gm. C 2 4 H 2 ,2H 2 0, or 45 gm. C 2 4 H 2 per liter. 

This solution cannot very well be established by direct weighing, 
owing to uncertain hydration ; hence it must be titrated by normal 
alkali of known accuracy. 

The solution is apt to deposit some of the acid at low tempera- 
tures, but keeps well if preserved from direct sunlight, and will 
bear heating without volatilizing the acid. Very dilute solutions 
of oxalic acid are very unstable ; therefore, if a decinormal or 
centinorrnal solution is at any time required, it should be made 
when wanted. 

5. Normal Hydrochloric Acid. 

36-37 gm. HC1 per liter. 

It has been shown by Roscoe and Dittmar (J. C. S. xii. 128, 
1860) that a solution of hydrochloric acid containing 20'2 per cent, 
of the gas when boiled at about 760 m.m. pressure, loses acid and 
water in the same proportion, and the residue will therefore 
have the constant composition of 20 "2 per cent., or a specific 
gravity of 1-10. About 181 gm. of acid of this gravity, diluted 
to one liter, serves very well to form an approximate normal 
acid. 

The actual strength may be determined by precipitation with 
"silver nitrate, or by titration with an exactly weighed quantity of 
pure sodic monocarbonate. Hydrochloric acid is useful on account 
of its forming soluble compounds with the alkaline earths, but it has 
the disadvantage of volatilizing at a boiling heat. Dittmar says 
that this may be prevented by adding a few grains of sodic sulphate. 
In many cases this would be inadmissible, for the same reason that 
sulphuric acid cannot be used. The hydrochloric acid from which 
standard solutions are made must be free from chlorine gas or 
metallic chlorides, and should leave no residue when evaporated in 
a platinum vessel. 

6. Normal Nitric Acid. 
63 gm. H2x T 3 per liter. 

A rigidly exact normal acid should be established by sodic 
carbonate, as in the case of normal sulphuric and hydrochloric acids. 

The nitric acid used should be colourless, free from chlorine 
and nitrous acid, sp. gr. about T3. If coloured from the 
presence of nitrous or hyponitrous acids, it should be mixed with 
two volumes of water, and boiled until white. When cold it may 
be diluted and titrated as previously described for sulphuric acid. 



15. NORMAL SOLUTIONS. 49 

7. Normal Caustic Soda or Potash. 
40 gin. NallO or 56 gm. KHO per liter. 

Pure caustic soda made from metallic sodium may now be readily 
obtained in commerce, and hence it is easy to prepare a standard 
solution of exceeding purity, by simply dissolving the substance in 
distilled water till of about 1*05 sp. gr., or about 50 gm. to the 
liter, roughly estimating its strength by normal acid and methyl 
orange or litmus, then finally adjusting the exact strength by 
titrating 50 c.c. with normal acid. 

However pure caustic soda or potash may otherwise be, they are 
both in danger of absorbing carbonic acid, and hence in using 
litmus the titration must be conducted with boiling. Methyl 
orange permits the use of these solutions at ordinary temperature 
notwithstanding the presence of CO 2 . 

Soda and potash may both be obtained in commerce sufficiently 
pure for all ordinary titration purposes, but in case they are not at 
hand the requisite solutions may be prepared as follows : 

Two parts of pure sodic or potassic carbonate are to be dissolved 
in twenty parts of distilled water, and boiled in a clean iron pot ; 
during the boiling, one part of fresh quick-lime, made into a cream 
with water, is to be added little by little, and the whole boiled until 
all the carbonic acid is removed, which may be known by the clear 
solution producing no effervescence on the addition of dilute acid ; 
the vessel is covered closely and set aside to cool and settle ; when 
cold, the clear supernatant liquid should be poured or drawn off 
and titrated by normal acid, and made of the proper strength 
as directed for sulphuric acid. 

Soda or potash solutions may be freed from traces of chlorine, 
.sulphuric, silicic, and carbonic acids, by shaking with Mi lion's 
base, trimercur-ammonium (C. N. xlii. 8). Carbonic acid may be 
removed by the cautious addition of baric hydrate in solution, 
shaking well, and then after settling clear ascertaining the exact 
strength with correct standard acid. 

In preparing these alkaline solutions, they should be exposed as 
little as possible to the air, and when the strength is finally settled, 
should be preserved in the bottle shown in fig. 24, or in full bottles 
having their glass stoppers slightly greased with vaseline. 

8. Semi-normal Ammonia. 
8-5 gm. NH :3 per liter. 

For some years past I have used this strength of standard 
ammonia for saturation analyses, and have been fully satisfied with 
its behaviour ; it is cleanly, does not readily absorb carbonic acid, 
holds its strength well for two or three months when kept in a cool 
place and well stoppered ; and can at any time be prepared in a few 



50 VOLUMETKIC ANALYSIS. 15. 

minutes, by simply diluting strong solution of ammonia with fresh 
distilled water. 

A normal solution cannot be used with safety, owing to evapora- 
tion of the gas at ordinary temperatures. 

It is necessary to add that, even in the case of ~ strength, 
the solution should be titrated from time to time against correct 
normal acid. ^ ammonia keeps its strength for a long time 
in well-closed bottles. 

9. Decinormal Caustic Baryta. 

The solution of caustic baryta is best made from the crystallized 
hydrate, approximately of ~ strength. This is best done by 
shaking up in a stoppered bottle powdered crystals of baric 
hydrate with distilled water, and allowing it to stand a day or 
two until quite clear ; there should be an excess of the hydrate, 
in which case the clear solution, when poured off into a stock 
bottle fitted with CO 2 tube, will be nearly twice the required 
strength. It is better to dilute still further (after taking its 
approximate titre with HC1 and phenolphthalein) with freshly 
boiled and cooled distilled water ; the actual working strength 
may be checked by evaporating 20 or 25 c.c. to dryness with 
a slight excess of sulphuric acid, then igniting over a Bun sen 
flame and weighing the BaSO 4 . The corresponding acid may be 
either ^ oxalic, nitric, or hydrochloric, and the proper indicator 
is phenolphthalein. Oxalic acid is recommended byPettenkofer 
for carbonic acid estimation, because it has no effect upon the 
baric carbonate suspended in weak solutions; but there is the 
serious drawback in oxalic acid, that in dilute solution it is liable 
to rapid decomposition ; and as in my experience ~ hydrochloric 
acid in very dilute mixtures has no effect upon the suspended 
baric carbonate, it is preferable to use this acid. 

The baryta solution is subject to constant change by absorption 
of carbonic acid, but this may be prevented to a great extent by 
preserving it in the bottle shown in fig. 24. A thin layer of light 
petroleum oil on the surface of the liquid preserves the baryta at 
one strength for a long period in the bottle shown in fig. 25. 

The reaction between baryta and yellow turmeric paper is very 
delicate, so that the merest trace of baryta in excess gives a decided 
brown tinge to the edge of the spot made by a glass rod on the 
turmeric paper. If the substance to be titrated is not too highly 
coloured, phenolphthalein should invariably be used. 

10. Normal Ammonio-Cupric Solution for Acetic Acid and free 
Acids and Bases in Earthy and Metallic Solutions. 

This acidimetric solution is prepared by dissolving pure cupric 
sulphate in warm water, and adding to the clear solution liquid 
ammonia, until the bluish-green precipitate which first appears is- 



16. NORMAL SOLUTIONS. 51 

nearly dissolved ; the solution is then filtered into the graduated 
cylinder, and titrated by allowing it to flow from a pipette graduated 
in i or y 1 ^- c.c. into 10 or 20 c.c. of normal sulphuric or nitric acid 
(not oxalic). While the acid remains in excess, the bluish-green 
precipitate which occurs as the drop falls into the acid rapidly 
disappears ; but so soon as the exact point of saturation occurs, the 
previously clear solution is rendered turbid by the precipitate 
remaining insoluble in the neutral liquid. 

The process is especially serviceable for the estimation of the free 
acid existing in certain metallic solutions, i.e. mother-liquors, etc., 
where the neutral compounds of such metals have an acid reaction 
on litmus such as the oxides of zinc, copper, and magnesia, and 
the protoxides of iron, manganese, cobalt, and nickel; it is also 
applicable to acetic and the mineral acids. 

If cupric nitrate be used for preparing the solution instead of 
sulphate, the presence of barium, or strontium, or metals precipitable 
by sulphuric acid is of no consequence. The solution is stand- 
ardized by normal nitric or sulphuric acid ; and as it slightly alters 
by keeping, a coefficient must be found from time to time by 
titrating with normal acid, by which to calculate the results 
systematically. Oxides or carbonates of magnesia, zinc, or other 
admissible metals, are dissolved in excess of normal nitric acid, 
and titrated residually with the copper solution. 

Example : 1 gra. of pure zinc oxide was dissolved in 27 c.c. of normal 
acid, and 2'3 c.c. of normal copper solution required to produce the 
precipitate ==24*7 c.c. of acid; this multiplied by 0'0405, the coefficient 
i'or zinc oxide, - I'OOO gm. ' 

ESTIMATION OF THE CORRECT STRENGTH OF STANDARD 
SOLUTIONS NOT STRICTLY NORMAL OR SYSTEMATIC. 

16. IN discussing the preparation of the foregoing standard 
solutions, it has been assumed that they shall be strictly and 
absolutely correct; that is to say, if the same measure be filled 
first with any alkaline solution, then with an acid solution, and the 
two mixed together, a perfectly neutral solution shall result, so that 
a drop or two either way will upset the equilibrium. 

Where it is possible to weigh directly a pure dry substance, this 
approximation may be very closely reached. Sodic monocarbonate, 
for instance, admits of being thus accurately weighed. On the 
other hand, the caustic alkalies cannot be so weighed, nor can 
the liquid acids.- An approximate quantity, therefore, of these 
substances must be taken, and the exact power of the solution 
found by experiment. 

In titrating such solutions it is exceedingly difficult to make them 
so exact in strength, that the precise quantity, to a drop or two, 
shall neutralize each other. In technical matters a near approxima- 
tion may be sufficient, but in scientific investigations it is of the 
greatest importance that the utmost accuracy should be obtained ; 

E 2 



52 VOLUMETRIC ANALYSIS. 16. 

it is therefore advisable to ascertain the actual difference, and to 
mark it upon the vessels in which the solutions are kept, so that 
a slight calculation will give the exact result. 

Suppose, for instance, that a standard sulphuric acid is prepared, 
which does not rigidly agree with the normal sodic carbonate (not 
at all an uncommon occurrence, as it is exceedingly difficult to hit 
the precise point) ; in order to find out the exact difference it must 
be carefully titrated as in 15. Suppose the weight of sodic 
carbonate to be 1*9 gm., it is then dissolved and titrated with the 
standard acid, of which 36*1 c.c, are required to reach the exact 
neutral point. 

If the acid were rigidly exact it should require 35 '85 c.c. ; in 
order, therefore, to find the factor necessary to bring the quantity 
of acid used in the analysis to an equivalent quantity of normal 
strength, the number of c.c. actually used must be taken as the 
denominator, and the number which should have been used, had 
the acid been strictly normal, as the numerator, thus 

35 ' 8 - 5 -0-99V 
36-1 ' 

Q'993 is therefore the factor by which it is necessary to multiply 
the number of c.c. of that particular acid used in any analysis 
in order to reduce it to normal strength, and should be marked 
upon the bottle in which it is kept. 

On the other hand, suppose that the acid is too strong, and that 
35 '2 c.c. were required instead of 35*85, 



1-0184 is therefore the factor by which it is necessary to multiply 
the number of c.c. of that particular acid in order to bring it to 
the normal strength., This plan is much better than dodging about 
with additions of water or acid. 

Under all circumstances it is safer to prove the strength of any 
standard solution by experiment, even though its constituent has 
been accurately weighed in the dry and pure state. 

Further, let us suppose that a solution of caustic soda is to be. 
made by means of lime as described previously. After pouring off 
the clear liquid, water is added to the sediment to extract more 
alkaline solution ; by this means we may obtain two solutions, one 
of which is stronger than necessary, and the other weaker. Instead, 
of mixing them in various proportions and repeatedly trying the 
strength, we may find, by two experiments and a calculation, 
the proportions of each necessary to give a normal solution, thus : 

The exact actual strength of each solution is first found, by 
separately running into 10 c.c. of normal acid as much of each 
alkaline solution as will exactly neutralize it. We have, then, in 
the case of the stronger solution, a number of c.c. required less 
than 10. Let us call this number V. 



16. NORMAL SOLUTIONS. 53 

In the weaker solution the number of c.c. is greater than 10, 
represented by v. A volume of the stronger solution x will 
saturate 10 c.c. of normal acid as often as V is contained in x. 

A volume of the weaker solution = y will, in like manner, saturate 

10 y 10 x 10 ?/, 

- c.c. ot normal acid; both together saturate y -\ -- 

and the volume of the saturated acid is precisely that of the two 
liquids, thus- 1() x 1Q 

-I 

mence 



= x 



1Q>* + 10 Vy = *.* + Vp y 

v x (10 - Y) = V y (v - 10). 

And lastly, , r / , m 

x_ _ V (v + 10) 

~y ~ v (10 - V)" 

An example will render this clear. A solution of caustic soda 
was taken, of which 5*8 c.c. were required to saturate 10 c.c. normal 
acid; of another solution, 12*7 c.c. were required. The volumes of 
each necessary to form a normal solution were found as follows : 

5-8 (12-7 -10) = 15-66 
12-7 (10 -5-8) = 53-34 

Therefore, if the solutions are mixed in the proportion of 15'66 
c.c. of the stronger with 53 '34 c.c. of the weaker, a correct solution 
ought to result. The same principle of adjustment is, of course, 
applicable to standard solutions of every class. 

Again: suppose that a standard solution of sulphuric acid has 
been made, approximating as nearly as possible to the normal 
strength, and its exact value found by precipitation with baric 
chloride, or a standard hydrochloric acid with silver nitrate, and 
such a solution has been calculated to require the coefficient 0*995 
to convert it to normal strength, by the help of this solution, 
though not strictly normal, we may titrate an approximately normal 
alkaline solution thus : Two trials of the acid and alkaline solu- 
tions show that 50 c.c. alkali =48'5 c.c. acid, having a coefficient 
of 0'995 = 48'25 c.c. normal ; then, according to the equation, 
x 50 := 48'25 is the required coefficient for the alkali. 

= 0-965. 



And here, in the case of the alkaline solution being sodic carbonate, 
we can bring it to exact normal strength by a calculation based on 
the equivalent weight of the salt, thus 

1 : 0-965 : : 53 : 5M45. 

The difference between the two latter numbers is 1'855 gm., and 
this weight of pure sodic carbonate, added to one liter of the 
solution, will bring it to normal strength. 



54 



VOLUMETRIC ANALYSIS. 



TABLE FOR THE SYSTEMATIC ANALYSIS OF ALKALIES, 
ALKALINE EARTHS AND ACIDS. 



Substance. 


Formula. 


Atomic 
Weight. - 


Quantity to be 
weighed so that 1 
c.c. Normal Solu- 
tion=l per cent, 
of substance. 


Normal 
Factor.* 


Soda 


Na 2 O 


62 


3'1 gm. 


0-031 


Sodic H} T drate . . 


NaHO 


40 


4-0 gm. 


0-040 


Sodic Carbonate . . 


Na 2 C0 3 


106 


5'3 gm. 


0-053 


Sodic Bicarbonate 


NaHCO 3 


84 


8'4 gm. 


0-084 


Potash 


K 2 O 


94 


4' 7 gm. 


0-047 


Potassic Hydrate . . 


KHO 


56 


5'6 gm. 


0-056 


Potassic Carbonate . 


K-CO 3 


138 


6'9 gm. 


0-069 


Potassic Bicarbonate 


KHCO 3 


100 


lO'O gm. 


O'lOO 


Ammonia .... 


NH 3 


17 


1:7 gm. 


0-017 


Ainmonic Carbonate 


(NH 4 ) 2 CO 3 


96 


4'8 gm. 


0-048 


Lime (Calcic Oxide) . 


CaO 


56 


2-8 gm. 


0-028 


Calcic Hydrate . . 


CaH 2 2 


74 


3'7 gm. 


0-037 


Calcic Carbonate . . 


CaCO 3 


100 


5'0 gm. 


O'OoO 


Baric Hvdrate . . 


BaH 2 O 2 


171 


8'55 gm. 


0'0855 


Do. (Crystals) . . 


BaO 2 H 2 (H 2 0) s 


315 


1575 gm. 


0-1575 


Baric Carbonate . . 


BaCO 3 


197 


9-85 gm. 


0-0985 


Strontia 


SrO 


103'5 


5*175 gm. 


0-05175 


Strontic Carbonate . 


SrCO 3 


147'5 


7-375 gm. 


0-07375 ' 


Magnesia .... 


MgO 


40 


2'00 gm. 


0-020 


Magnesic Carbonate. 


MgCO 3 


84 


4'20 gm. 


0-042 


Nitric Acid. . . . 


HNO 3 


63 


6'3 gm. 


0-063 


Hydrochloric Acid . 


HC1 


36-37 


3'637 gm. 


0-03637 


Sulphuric Acid . . 


H 2 SO 4 


98 


4'9 gm. 


0-049 


Oxalic Acid . . . 


C 2 O 4 H 2 (H 2 O) 2 


126 


6'3 gm. 


0-063 


Acetic Acid . . - . 


C 2 O 2 H 4 


60 


6'0 gm. 


0-060 


Tartaric Acid . . . 


C 4 G H r, 


150 


7-5 gin. 


0075 


Citric Acid .... 


C0'H S + H 2 


210 


7-0 gm. 


0-070 


Carbonic Acid . . . 


CO 2 


44 




0'022 



* This is the coefficient by which the number of c.c. of normal solution used in 
any analysis is to be multiplied, in order to obtain the amount of pure substance 
present in the material examined. 

If grain weights are used instead of grams, the decimal point must be moved 
one place to the right to give the necessary weight for examination; thus sodic 
carbonate, instead of 5'3 gm., would be 53 grains, the normal factor in this case would 
also be altered to 0'53. 



17. ALKALINE SALTS. 55 

THE TITRATION OF ALKALINE SALTS. 
1. Total Alkali in Caustic Soda or Potash, or their Carbonates. 

17. THE necessary quantity of substance being weighed or 
measured, as the case may be, and mixed with distilled water to a 
proper state of dilution (say about one per cent, of solid material), 
an appropriate indicator is added, and the solution is ready for the 
burette. Xormal acid is then cautiously added from a burette 
till the change of colour occur. In the case of caustic alkalies 
free from CO 2 , the end-reaction is very sharp with any of the 
indicators ; but if CO' 2 is present, the only available indicators in 
the cold are methyl orange or lacmoid paper. If the other indica- 
tors are used, the CO 2 must be boiled off after each addition of acid. 

In examining carbonates of potash or soda, or mixtures of caustic 
and carbonate, where it is only necessary to ascertain the total 
alkalinity, the same method applies. 

In the examinations of samples of commercial refined soda or 
potash salts, it is advisable to proceed as follows : 

Powder and mix the sample thoroughly, weigh 10 gm. in a platinum or 
porcelain crucible, and ignite gently over a spirit or gas lamp, and allow the 
crucible -to cool under the exsiccator. Weigh again, the loss of weight gives 
the moisture ; wash the contents of the crucible into a beaker, dissolve and 
filter if necessary, and dilute to the exact measure of 500 c.c. with distilled 
water in a half-liter flask ; after mixing it thoroughly take out 50 c.c. 1 gm. 
of alkali with a pipette, and empty it into a small flask, bring the flask under 
a burette containing normal acid and graduated to i or T V c.c., allow the acid 
to flow cautiously as before directed, until the neutral point is reached : the 
process may then be repeated several times if necessary, in order to be certain 
of the correctness of the analysis. 

Residual Titration : As the presence of carbonic acid with litmus and the 
other indicators, except methyl orange, always tends to confuse the exact end 
of the process, the difficulty is best overcome, in the case of not using methyl 
orange, by allowing an excess of acid to flow into the alkali, boiling to expel 
the CO 2 , and then cautiously adding normal caustic alkali, drop by drop, 
until the liquid suddenly changes colour; by deducting the quantity of 
caustic alkali from the quantity of acid originally used, the exact volume of 
acid necessary to saturate the 1 gm. of alkali is ascertained. 

This method of re-titration gives a very sharp end-reaction, as 
there is 110 carbonic acid present to interfere with the delicacy of 
the indicator. It is a procedure sometimes necessary in other cases, 
owing to the interference of impurities dissipated by boiling, e.g. 
sulphuretted hydrogen, which would otherwise bleach the indicator, 
except in the case of methyl orange and lacmoid paper, either of 
which are indifferent to H 2 S in the cold. An example will make 
the plan clear : - 

Example : 50 c.c. of the solution of alkali prepared as directed, equal to 
1 gm. of the sample, is put into a flask, and 20 c.c. of normal acid allowed to 
flow into it ; it is then boiled and shaken till all CO 2 is expelled, and normal 
caustic alkali added till the neutral point is reached ; the quantity required 
is 3'4 c.c v which deducted from 20 c.c, of acid leaves 16'6 c.c. The following 



56 VOLUMETKIC ANALYSIS. IT. 

calculation, therefore, gives the percentage of real alkali, supposing it to 
be soda : 31 is the half molecular weight of anhydrous soda (Na-O) and 1 c.c. 
of the acid is equal to 0'031 gm., therefore 16'6 c.c. is multiplied by 0'031, 
which gives 5146 ; and as 1 gm. was taken, the decimal point is moved 
two places to the right, which gives 5T46 per cent, of real alkali ; if calculated 
as carbonate, the 16'6 would be multiplied by 0'053, which gives 0'8798 gm. 
= 87'98 per cent. 



2. Mixed Caustic and Carbonated Alkaline Salts. 

The alkaline salts of commerce, and also alkaline lyes used in 
soap, paper, starch, and other manufactories, consist often of a 
mixture of caustic and carbonated alkali. If it be desired to 
ascertain the proportion in which these mixtures occur, the total 
alkaline power of a weighed or measured quantity of substance (not 
exceeding 1 or 2 gm.) is ascertained by normal acid and noted; a 
like quantity is then dissolved in about 150 c.c. of water in a 
200 c.c. flask, and exactly enough solution of baric chloride added 
to remove all carbonic acid from the soda or potash. 

Watson Smith has shown (J. S. C. I. i. 85) that whenever an 
excess of baric chloride is used in this precipitation so as to form 
baric hydrate, there is an invariable loss of soda : exact precipita- 
tion is the only way to secure accuracy. 

The flask is now filled up to the 200 c.c. mark with distilled 
water, securely stoppered, and put aside to settle. When the 
supernatant liquid is clear, take out 50 c.c. with a pipette, and 
titrate with normal hydrochloric acid to the neutral point. The 
number of c.c. multiplied by 4 will be the quantity of acid required 
for the caustic alkali in the original weight of substance, because 
only one-fourth was taken for analysis. The difference is calculated 
as carbonate, or the precipitated baric carbonate may be thrown 
upon a dry filter, washed well and quickly with boiling water, and 
titrated with normal acid, instead of the original analysis for the 
total alkalinity ; or both plans may be adopted as a check upon 
each other. 

^--iThe principle of this method is, that when baric chloride is added 
to a mixture of caustic and carbonated alkali, the carbonic acid of 
the latter is precipitated as an equivalent of baric carbonate, while 
the equivalent proportion of caustic alkali remains in solution as 
baric hydrate. By multiplying the number of c.c. of acid required 
to saturate this free alkali with the y^Vo atomic weight of caustic 
potash or soda, according to the alkali present, the quantity of 
substance originally present in this state will be ascertained. 

As caustic baryta absorbs CO 2 very readily when exposed to the 
atmosphere, it is preferable to allow the precipitate of baric 
carbonate to settle in the flask as here described, rather than to 
filter the solution as recommended by some operators, especially 
also as the filter obstinately retains some baric hydrate. 

A very slight error, however, occurs in all such cases, in 



17. ALKALINE SALTS. 57 

consequence of the volume of the precipitate being included in 
the measured liquid. 

K. Williams (/. S. C. I. vi. 346) estimates the caustic soda in. 
soda ash by digesting a weighed quantity in strong alcohol in a 
tightly stoppered flask with frequent shaking and finally allowing 
to stand overnight ; the undissolved carbonate is filtered off, 
washed with alcohol until a drop gives no alkaline reaction 
the nitrate and washings are then titrated with normal acid and 
phenolphthalein. 

Peter Hart recommends the following technical method of 
ascertaining the relative proportions of caustic and carbonated 
soda in sola ash : 50 grains of the sample are dissolved in 
10 ounces of water, phenolphthalein added, and the standard 
acid (1 dm. - 0'5 grn. Na 2 0) slowly run in until the colour- 
disappears. At this point all the caustic soda and one-naif the 
carbonate has been neutralized, say 30 dm. has been used. To 
the same solution (in which the soda now exists as bicarbonate) 
methyl orange is added, and the titration continued until pink ; 
the burette now reads 50 dm. Then 50 - 30 = 20 as NaHCO", 
but as this originally existed in the sample as Na 2 CO :) , this figure 
must be doubled = 40, which deducted from 50 leaves 10 dm. as 
representing the caustic soda in the sample. 



3. Estimation of Hydrates of Soda or PotasB. with small 
proportions of Carbonate. 

This may be accomplished by means of phenacetolin (Lunge, 
/. & C. I. i. 56). The alkaline solution is coloured a scarcely 
perceptible yellow with a few drops of the indicator. The standard 
acid is then run in until the yellow gives place to a pale rose 
tint; at this point all the caustic alkali is saturated, and the 
volume of acid used is noted. Further addition of acid now 
intensifies this red colour until the carbonate is decomposed, when 
a clear golden yellow results. The neutralization of the NaHO or 
the KHO is indicated by a rose tint permanent on standing ; that 
of Xa 2 C0 3 or K 2 C0 3 by the sudden passage from red to yellow. 

Practice is required with solutions of known composition to 
accustom the eye to the changes of colour. Phenolphthalein may 
also be employed for the same purpose as follows : 

Add normal acid to the cold alkaline solution till the red colour 
is discharged, taking care to use a very dilute solution, and keeping 
the spit of the burette in the liquid so that no CO 2 escapes. The 
point at which the colour is discharged occurs when all the hydrate 
is neutralized and the carbonate converted into bicarbonate ; the 
volume of acid is noted, and the solution heated to boiling, with 
small additions of acid, till the red colour produced by the decom- 
position of the bicarbonate is finally destroyed. 

In both these methods it is preferable, after the first stage, to 

E8E LIB, 

OF THE 

UNIVERSITY, 

. m OF 



58 VOLUMETRIC ANALYSIS. 17. 

add excess of acid, boil off the CO 2 , and titrate back with normal 
alkali. The results are quite as accurate as the method of precipi- 
tation with barium. 

4. Estimation of Alkaline Bicarbonates in presence of 
Normal Carbonates (Lring-e, J. S. C. I. i. 57). 

To a weighed quantity of the solid bicarbonate, or a measured 
quantity of a solution, there is added an excess of J ammonia, 
followed by an excess of solution of baric chloride. The mixture 
is made in a measuring flask, and the whole diluted with hot 
distilled water to the mark. 

A portion of the clear settled liquid, or filtered through a dry 
filter, is then titrated with normal acid : the alkaline strength due 
to the excess of ammonia, above that required to convert the bicar- 
bonate into normal carbonate, deducted from the total ammonia 
added, gives the equivalent of the bicarbonate present. 

Example (Lunge) : 20 gm. sodic bicarbonate in the course of manufac- 
ture were dissolved to a liter. 50 c.c. of this solution required 12' 1 c.c. 
normal acid=0'3751 gm. Na 2 O ; 50 c.c. were then mixed with 50 c.c. of 
standard ammonia (50 ' c.c. =24'3 normal acid) and the whole treated with 
excess of baric chloride. One half of the clear liquid required 6'25 c.c. 
of normal acid, 24'3 (6'25 x 2) = 11'8 c.c. : this is, therefore, the equivalent 
of the CO 2 as bicarbonate. 

' XaHCO 3 : 11-8 x -084= '9912 gm. 
Xa-CO 3 : (12-1 11-8) x "053= "0159. 

A simpler plan than the above has been devised by Thomson, 
which gives good results when carefully carried out. 

To the cold solution of the sample, an excess of normal caustic 
soda, free from CO 2 , is added, the CO 2 is then precipitated with 
neutral solution of baric chloride, and the excess of sodic hydrate 
found by standard acid, using phenolphthalein as indicator. The 
precipitate of baric carbonate has no effect on the indicator in the 
cold. The calculation is the same as before. 



5. Estimation of small quantities of Sodic or Potassic 
Hydrates in presence of Carbonates. 

This method, by Thomson, has just been alluded to, and 
consists in precipitating the carbonates by neutral solution of baric 
chloride in the cold : the baric carbonate being neutral to phenol- 
phthalein, this indicator can be used for the process. When the 
barium solution is added, a double decomposition occurs, resulting 
in an equivalent quantity of sodic or potassic chloride, while the 
baric carbonate is precipitated, and the alkaline hydrate remains in 
solution. 

Example (Thomson): 2 gm. of pure sodic carbonate were mixed in 
solution with '02 gm. of sodic hydrate; excess of baric chloride was then 



17. ALKALINE SALTS. 



which in three trials an average of 5 c.c. was required ; therefore, 
5 x '004 = '02 gm. exactly the quantity used. 

In this process the presence of chlorides, sulphates, and sulphites 
does not interfere ; neither do phosphates, as baric phosphate is 
neutral to the indicator. With sulphides, half of the base will be 
estimated ; but if hydrogen peroxide be added, and the mixture 
allowed to rest for a time, the sulphides are oxidized to sulphates, 
Avhich have no effect. If silicates or aluminates of alkali are 
present, the base will of course be recorded as hydrate. 

Thomson further says : 

"The foregoing method can also be applied to the determination 
of hydrate of sodium or potassium in various other compounds, 
which give precipitates with baric chloride neutral to phenolph- 
thalein, such as the normal sulphites and phosphates of the alkali 
metals. An illustration of the use to which the facts I have 
stated in this and former papers may be put will be found in the 
analysis of sulphite of sodium. Of course sulphate, thiosulphate, 
and chloride are determined as usual, but to estimate sulphite, 
carbonate and hydrate, or bicarbonate of sodium by methods in 
ordinary use is rather a tedious operation. To find the proportion 
of hydrate, all that is necessary is to precipitate with baric chloride 
and titrate with standard acid, as above described. Then, by 
simple titration of another portion of the sample in the cold, using 
phenolphthalein as indicator, the hydrate and half of the carbonate 
can be found, and finally, by employment of methyl orange as 
indicator, and further addition of acid, the other half of the 
carbonate and half of the sulphite can- be estimated. By simple 
calculations, the respective proportions of these three compounds 
can be obtained, a result which can be accomplished in a few 
minutes. It must be borne in mind that if a large quantity of 
sodic carbonate is in the sample the proportion of that compound 
found will only be an approximation to the truth, as the 
end-reaction is only delicate with small proportions of sodic 
carbonate. If there is no hydrate found, bicarbonate of sodium 
can be tested for, and determined by Lunge's method described 
above" ( 17.4). 

6. Estimation of Alkalies in the presence of Sulphites. 

It is not possible to estimate the alkaline compounds in the 
presence of sulphites by titration with acids, as a certain' quantity 
of acid is taken up by the sulphite, SO 2 being evolved. This 
difficulty may be completely overcome by the aid of hydrogen 
peroxide, which speedily converts the sulphites into sulphates 
(Grant and Cohen, J. & C. I. ix. 19). These operators proved 
that neither caustic or carbonate alkali were affected by H 2 2 , nor 
had the latter any prejudicial effect on methyl orange in the cold. 



60 VOLUMETRIC ANALYSIS. 17. 

The quantity of H 2 2 required in any given analysis must depend 
on the amount of sulphite present ; for instance, the caustic 
salts of commerce contain about 50 % of sulphite, and it suffices 
to take 10 o.c. of ordinary 10 vol. H 2 2 for every O'l gm. of the 
salts in solution. In the case of mixtures containing less or more 
sulphite the quantity may be varied. 

The Analysis : A measured volume of the peroxide is run into a beaker, 
and three or four drops of methyl orange added. As the H-O- is invariably 
faintly acid, the acidity is carefully corrected by adding- drop by drop from a 
pipette T T caustic soda. The required quantity of salt to be analyzed is 
then added in solution, and the mixture gently boiled, during the boiling the 
methyl orange will be bleached. The liquid is then cooled, a drop or two 
more of methyl orange added, and the titration for the proportion of alkali 
carried out with normal acid in the usual way. The results are very 
satisfactory. 

7. Estimation of Caustic Soda, or Potach by standard 
Bichromate of Potash. 

This process was devised by Richter, or rather the inverse of 
it, for estimating bichromate with caustic alkali by the aid of 
phenolphthalein. Exact results may be obtained by it in titrating 
soda or potash as hydrates, but not ammonia as recommended 
by Richter. 

For the process there are required a decinormal solution of bichromate con- 
taining 1474 gm. per liter, and ^ soda or potash solution titrated against 
sulphuric acid. A comparison liquid containing about 1 gm. of monochro- 
mate of potash in 150 200 c.c. water is advisable for ascertaining the exact 
end of the reaction ; 50 c.c. of the alkali being diluted with the same volume 
of water, is coloured with phenolphthalein, and the bichromate run in from 
a burette ; the fine red tint changes to reddish yellow, which remains till 
the neutral point is nearly reached, when the yellow colour of the mono- 
chromate is produced; the change is not instantaneous as with mineral acids, 
so that a little time must be allowed for the true colour to declare itself. 

8. Estimation of Potash in Neutral Salts free from Soda. 

Stolba precipitates the potash from a tolerably concentrated solution of 
the substances with hydrofluosilicic acid and strong alcohol. The method is 
also applicable to the estimation of potash in potassic platinum chloride. To 
ensure complete decomposition, it is well to warm the mixture for a little 
time before adding the alcohol, which must be of about the same volume as 
the liquid itself. After some hours, when the precipitate has settled, the 
solution is filtered off, the beaker and precipitate well washed with equal 
mixtures of alcohol and water, the whole transferred to a white porcelain 
basin, water rather freely added, and heated to boiling, a few drops of 
litmus added, and normal or semi-normal alkali run in until exact 
saturation occurs ; or a known excess of alkali may be added, and the amount 
found by residual titration with normal acid. The results are generally 
about 1' too low, owing to the difficulty of fully decomposing the pre- 
cipitate. 

2 eq. alkali = 1 eq. potash. 

The process is very limited in its use, and is not applicable when 



17. ALKALINE SALTS. 61 

sulphates are present, nor in the presence of any great amount of 
free acid. Sulphuric acid may be previously removed by calcic 
acetate and alcohol ; other acids by moderate ignition previous to 
precipitation. Large proportions of ammonia salts must also be 
removed ; and, of course, all other matters precipitable by hydro- 
fluosilicic acid, especially soda. 



9. Direct estimation of Potash in tb.e presence of Soda. 

Fleischer recommends the following method; and my own 
experiments confirm his statements, so far at least as the pure salts 
are concerned. 

The solution must contain no other bases except the alkalies, nor any acids 
except nitric, hydrochloric, or acetic. This can almost invariably be easily 
accomplished. Earthy alkalies are removed by ammonic carbonate or 
phosphate ; sulphuric, chromic, phosphoric, and arsenic acids by baric 
chloride, followed by ammonic carbonate. 

The solution should be tolerably concentrated, and the volume about 25 or 
30 c.c. ; 1015 c.c. of neutral solution of ammonic acetate of sp. gr. 
T035 are added ; followed by finely powdered pure tartaric acid in sufficient 
quantity to convert the potash into acid tartrate, with an excess to form some 
ammonic tartrate, but not enough to decompose the whole. This is the weak 
part of the method ; however, as a guide, it is not advisable to add more 
than 5 gm. tartaric acid for 10 c.c. of ammonic acetate. If the quantity of 
potash is approximately known, it is best to add about one-third more than 
is sufficient to convert the whole into acid tartrate. 

After adding the tartaric acid the mixture must be well stirred for five or 
ten minutes, without rubbing the sides of the beaker; a like volume of 95 
per-cent. alcohol is added, and again well stirred. The precipitate contains 
the whole of the potash as tartrate, and a portion of ammonium tartrate. 
After standing half an hour with occasional stirring, the precipitate is 
collected on a porous filter, and repeatedly washed with alcohol and water in 
equal parts until clean. 

When the washing is finished the precipitate will be entirely free from 
soda ; filter and precipitate are transferred to a porcelain basin, treated with 
sufficient hot water to dissolve the tartrates, then exactly neutralized with 
normal alkali and litmus, and the volume so used noted. A like volume, or 
preferably, a larger known volume of normal alkali is now added, and the 
mixture boiled to expel all ammonia ; the end may be known by holding 
litmus paper in the steam. The excess of normal alkali is now found by 
titration with normal acid ; the amount so found must be deducted from that 
which was added in excess after the exact titration of the tartrate : the 
difference equals the ammonia volatilized. By deducting this difference 
from the volume of normal alkali originally required, the volume corre- 
sponding to potash is found. 

Example : 29'4 c.c. of normal alkali were required in the first instance to 
neutralize a given precipitate ; 40 c.c. of the same alkali were then added, 
the boiling accomplished, and 22'5 c.c. normal acid used for the excess ; then 
40 22-5 = 17-5 c.c., and again 29'4 I7'5 = ir9, which multiplied by the 
factor for KHO = 0'056 gives 0'6664 gm. 

The soda in the nitrate may be obtained by evaporation with 
hydrochloric acid as sodic chloride, and estimated as in 42. 



62 VOLUMETRIC ANALYSIS. 17. 

10. Mixed Caustic Soda and Potash. 

This process depends upon the fact, that potassic bitartrate is 
almost insoluble in a solution of sodic bitartrate. 

Add to the solution containing 1 the mixed salts a standard solution of 
tartaric acid till neutral or faintl} 7 " acid this produces neutral tartrates 
of the alkalies now add the same volume of standard tartaric acid as before 
they are now acid tartrates, and the potassio bitartrate separates almost 
completely, filter off the sodic bitartrate and titrate the filtrate with normal 
caustic soda ; the quantity required equals the soda originally in the 
mixture the quantity of tartaric acid required to form bitartrate with the 
soda subtracted from the total quantity added to the mixture of the two 
alkalies, gives the quantity required to form potassic bitartrate, and thus 
the quantity of potash is found. 

This process is only applicable for technical purposes. 

Mixtures of potash and soda in the form of neutral chlorides are 
estimated by J. T. White as follows (C. N. Ivii. 214) : 20 c.c. of the 
solution containing about 0'2 gm. of the mixed salts are placed into 
a 100 c.c. flask, and 5 c.c. of a hot saturated solution of ammoiiic bicarbonate 
added ; the mixture is cooled, and alcohol added in small quantities, with 
shaking, until the measure is made up to 100 c.c. After three or four hours, 
10 c.c. of the clear liquid are removed with a pipette, evaporated and ignited, 
the residue is moistened with a few drops of ammonic chloride solution 
and again ignited; the sodic chloride so obtained is then titrated 
with standard silver solution, 1 c.c. of which represents '001 gm. Cl ; this is 
calculated to iSaCl and the KC1 found by difference. 

11. Potash as Platino-chloride: 

111 cases where potash exists in combination as a neutral salt, 
such as kainit or kieserit, etc., or as a constituent of minerals, 
it has to be first separated as double chloride of potassium and 
platinum. The method usually adopted is that of collecting the 
double salt upon a tared filter, when the weight of the dry double 
salt is obtained, the w r eight of potash is ascertained by calculation. 

It may, however, be arrived at by volumetric means as follows : 

The potash having been converted into double chloride in the usual way 
is dried, collected, and mixed with about double its weight of pure sodic 
oxalate, and gently smelted in a platinum crucible ; this operation results in 
the production of metallic platinum, chlorides of sodium and potassium,, 
with some carbonate of soda. The quantity of potash present is, however,, 
solely measured by the chlorine ; in order to arrive at this, the fused mass is 
lixiviated with water, filtered, nearly neutralized with acetic acid, and the 
chlorine estimated with -$ silver and chromate, the number of c.c. of silver 
required is multiplied by the factor 0'00157, which gives at once the weight 
of potash. This factor is used because 1 molecule of double chloride contains 
3 atoms chlorine, hence the quantity of ^V silver used is three times as much 
as in the case of sodic or potassic chloride. 

L. de Koninck (Chem. Zeit. xix. 301) has improved this process 
materially by the use of formic; acid as a reducing agent. The 
chloroplatinate is filtered and washed in the usual way, dissolved in boiling 
water and decomposed by calcic formate free from Cl. The liquid is heated 
until the platinum is fully separated and the solution colourless; it is 
neutralized with a small quantity of pure calcic carbonate, filtered, washed, 
and the chlorine determined by titratiou with & silver solution and 
chromate. 



17. ALKALINE COMPOUNDS. 63 

12. Separation of the Potash as Bitartrate. 

The mixed salts being rendered as nearly neutral as possible, a saturated 
solution of sodic bitartrate is added in excess, and the whole evaporated to 
dryness in the water bath. The dry mass is then deprived of the excess of 
sodic bitartrate by washing it on a filter with a saturated solution of potassic 
bitartrate ; when all the soda salt has been removed, the potash salt is 
dissolved in hot water, and titrated with normal alkali, of which 1 c.c. 
represents 0'039 gm. K. In cases where potash is to be separated as 
bitartrate, the operator should consult 26, 2 and 3. 

TECHNICAL EXAMINATION OF SOME ALKALINE 

COMPOUNDS FOUND IN COMMERCE OH OCCURRING IN 

COURSE OF MANUFACTURE. 

There is now considerable unanimity among English and foreign 
manufacturers of alkaline compounds, as to methods of analysis to 
be adopted either for guidance in manufacture or commercial 
valuation. Lunge has contributed important papers on the 
subject (/. S. C. I. i. 12, 16, 55, 92), also in conjunction with 
Hurter in the Alkali Makers' Pocket Book* which contains 
valuable tables and processes of analysis. So far as volumetric- 
methods are concerned, the same processes will be given here with 
others. 

13. Soda Ash, Black Ash, Mother-liquors, etc. 

Soda Ash or Refined Alkali. 5 or 10 gm. are dissolved in about 150 c.c. 
of warm distilled water, and any insoluble matter filtered off (German, 
chemists do not filter), and the volume diluted to \ or 1 liter. 

The total quantity of alkali is determined in 50 c.c. by normal sulphuric, 
nitric, or hydrochloric acid, as in 17. l.f 

The quantity of caustic alkali present in any sample is determined as 
in 17. 2 or 5. 

The presence of sulphides is ascertained by the smell of sulphuretted 
hydrogen when the alkali is saturated with an acid, or by dipping paper, 
steeped in sodic nitro-prusside into the solution : if the paper turns blue or 
violet, sulphide is present. 

The quantity of sulphide and sulphite may be determined by saturating 
a dilute solution of the alkali with a slight excess of acetic acid, adding starch 
and titrating with /^ iodine solution till the blue colour appears. The 
quantity of iodine required is the measure of the sulphuretted hydrogen 
and sulphurous acid present. 

The proportion of sulphide is estimated as follows : 13'820 gm. of pure 
silver are dissolved in dilute nitric acid, and the solution, together with an 
excess of liquid ammonia, made up to a liter. Each c.c. 0'005 gm. Na 2 S. 

The Analysis : 100 c.c. of the alkali liquor is heated to boiling, some 
ammonia added, and the silver solution dropped in from a burette until no 
further precipitate of Ag-S is produced. Towards the end filtration will be 
necessary, in order to ascertain the exact point, to which end the Be ales 
filter is serviceable (fig. 23). The amount of Na 2 S so found is deducted 
from the total sulphide and sulphite found by iodine. 

Sodic chloride (common salt) may be determined by carefully neutralizing 
1 gin. of the alkali with nitric acid, and titrating 'with deciuormal silver 

* Bell & Sons, York Street, Covent Garden. 

f This gives a alight error, owing- to traces of aluminat3 of o;li ani lime, which 
consume acid. 



<64= VOLUMETRIC ANALYSIS. 17. 

solution and potassic chromate. Each c.o. represents 0'005837 gm. of 
common salt. Since the quantity of acid necessary to neutralize the alkali 
has already been found, the proper measure of T ^ nitric acid may at once 
,be added. 

Sodic sulphate is determined, either directly or indirectl}% as in 76. 
Each cc. of normal baric chloride is equal to 0*071 gm. of dry sodic 
sulphate. 

Examination of Crude Soda Lyes and Red Liquors. K aim aim 
andSpiiller (Dingl. polyf. */., 264, 456 459) recommend a process based 
on the insolubility of baric sulphite and the solubility of baric thiosulphate 
in alkaline solutions. The estimation is performed in the following 
manner: 1. The total alkalinity is determined in a measured volume of 
'the liquor under examination by titration with normal acid, methyl orange 
being used as indicator. The acid consumed equals sodic carbonate 4- sodio 
sulphide, + sodic hydroxide, + one-half sodic sulphite (Na 2 SO 3 is alkaline 
and NaHSO 3 neutral to methyl orange). 2. An equal volume of the 
liquor is titrated with T \ solution of iodine, the volume consumed corres- 
ponding with the sodic sulphide + the sodic sulphite, + the sodic 
thiosulphate. 3. Twice the volume of liquor as that used in (1) and (2) 
is precipitated with an alkaline zinc solution, and the mixture made up to 
a certain measure, one-half of which is filtered, acidified, and titrated with 
yV iodine. The iodine consumed equals sodic sulphite + sodic thiosulphate. 
4. Three or four times the volume of liquor used in (1) and (2) is treated 
with an excess of a solution of baric chloride, the mixture made up to 
a known volume with water, and filtered, (a) One-third or one-fourth 
(as the case may be) of the filtrate is titrated with normal acid, the amount 
used corresponding with the sodic hydroxide + the sodic sulphite. 
(b) A new third or fourth of the filtrate is acidified and titrated with -^ 
iodine, the iodine consumed being equal to sodic sulphite + sodic thio- 
.sulphate. The calculation is made as follows : 

2 4i A c.c. ^ iodine corresponding to ... Na 2 SO 3 

2 3 = B c.c. yV iodine corresponding to Na 2 S 

46 (2 3) ... = C c.c. $ iodine corresponding to Na 2 S 2 O 3 

4a r VB = D c.c. normal acid corresponding to ... NaOH 

1 (4a + T V A) =E c.c. normal acid corresponding to ... Na' 2 CO 3 

Black Ash. Digest 50 gm. with warm water in a half-liter flask, fill up to 
mark, and allow to settle clear. 

(1) Total Alkali existing as carbonate, hydrate, and sulphide, is found 
by titrating 10 c.c. = l gm. of ash with standard acid and metlryl orange in 
Ihe cold. 

(2) Caustic Soda. 20 c.c. of the liquid are put into a 100 c.c. flask with 
10 c.c. of solution of baric chloride of 10 per cent, strength, filled up with 
hot water, well shaken, and corked after settling a few minutes. The clarified 
liquid is filtered, and 50 c.c. = 1 gm. ash, titrated with standard acid and 
metlryl orange ; or it may be titrated without filtration if standard oxalic 
acid and phenolphthalein are used, this acid having no effect on the baric 
carbonate. Each c.c. normal acid = 0'031 Na 2 0. This includes sulphides. 

(3) Sodic Sulphide. Put 10 c.c. of liquor into a flask, acidulate with 
acetic acid, dilute to about 200 c.c. and titrate with T ^ iodine and starch. 
Each c.c. = 0-0039 Na 2 S, or 0'0031 Na 2 O. 

(4) Sodic Chloride. 10 c.c. are neutralized exactly with normal nitric 
acid, and boiled till all H-S is evaporated. Any sulphur which may have 
been precipitated is filtered off, and the filtrate titrated with T N silver and 
chromate. Each c.c. =0'00 5837 gm. NaCl. 

(5) Sodic Sulphate. This is best estimated by precipitation as baric 
.sulphate, and weighing, the quantity being small. If, however, volumetric 
estimation is desired, it may be done as in 76, taking 50 c.c. of liquor. 



17. ALKALINE COMPOUNDS. 65 

For other methods of examining the various solid and liquid 
alkali wastes used for soda and sulphur recovery, etc., the reader is 
referred to the Alkali Makers' Pocket Book already mentioned. 

14. Salt Cake. 

Is the impure sodic sulphate used in alkali manufacture or left in 
the retorts in preparing hydrochloric acid from sulphuric acid and 
salt, or nitric acid from sodic nitrate. It generally contains free 
sulphuric acid existing as sodic bisulphate, the quantity of which 
may be ascertained by direct titration with normal alkali. 

The common salt present is estimated by decinormal silver solution and 
chromate ; having first saturated the free acid with pure sodic carbonate, 
1 c.c. silver solution is equal to 0'005837 gm. of salt. 

Sulphuric acid, combined with soda, is estimated either directly or 
indirectly as in 76 ; 1 c.c. of normal barium solution is equal to 0'07l gm. 
or 0'71 grn. of dry sodic sulphate. 

Iron is precipitated from a filtered solution of the salt cake with amtaonia 
in excess, the precipitate of ferric, oxide re-dissolved in sulphuric acid, 
reduced to the ferrous state with zinc and titrated with permanganate. 

Grossman adopts a method suggested by Bohlig (see 32), 
and has worked out the process in the case of salt cake in careful 
detail (C. N. xli. 114) as follows : 

The neutral solution of salt cake (3'55 gm.) is put into a 500 c.c. 
flask, 250 c.c. of a cold saturated solution of baric hydrate added, 
the flask filled with water, and shaken up. Of the filtered clear liquid 
250 c.c. are put in an ordinary flask, carbonic acid passed through 
for about ten minutes, and then the contents of the flask boiled so 
as to decompose any baric bicarbonate which may be in solution. After 
cooling, the contents of the flask are again transferred to the 500 c.c. 
flask, the latter filled up with water to the mark, shaken up, and filtered. 
250 c.c. of the filtrate i.e., one-fourth of the original quantity used are 
then titrated with one-fourth normal sulphuric acid. The number of c.c. of 
one-fourth normal acid used multiplied by two will give the percentage of 
sodic sulphate. 

There are, however, sources of error in the experimental working of this 
method which make certain corrections necessary. They arise 

(1) From the impurities of the caustic baryta. 

(2) Prom the precipitate formed in the measured liquid. 

(3) Prom certain constant losses. 

The commercial caustic baryta always contains baric nitrate, and sometimes 
baric chloride. It is evident that on adding a solution of baric hydrata 
which contains baric nitrate to a solution of sodic sulphate, a quantity of the 
latter, equivalent to the quantity of the baric nitrate present, will be 
converted into sodic nitrate, and thus escape the alkalimetric test, as will be 
seen by the following equations : 

Ba(N O 3 ) 2 + Na 2 SO 4 =BaSO 4 + 2NaNO 3 . 
Ba(NO 3 ) 2 +2NaOH + CO 2 =BaCO 3 +2NaN0 3 +H 2 O. 

It is therefore necessary to measure approximately the quantity of baryta 
solution used, so as to know the amount of baric nitrate introduced into the 
process. The latter can be easily ascertained by passing carbonic acid in 
excess through the cold saturated solution of baric hydrate, boiling, filtering, 
and precipitating tke baryta left in solution by sulphuric acid as usual. 

P 



66 VOLUMETRIC ANALYSIS. 17. 

250 c.c. of a baryta solution used for experiment yielded 0*0280 gin. of BaSO 4 , 
which corresponds to Q'0171 gni. of Na 2 S0 4 , or 0*96 c.c. of one-fourth normal 
acid ; and it follows that for every 250 c.c. of this baryta solution was found 
0-0171 gm. of Na 2 SO 4 too little ; or, that there must be added 0'24 c.c. of 
one-fourth normal acid to the result of the final titration (of one-fourth of 
the original quantity). If the baryta contain caustic alkali, a corresponding 
quantity of baric nitrate will be found less by the test ; but it is easily 
understood, that the calculations will not be influenced as long as the baric 
nitrate is in excess of the caustic alkali, which is always the case in good 
commercial baryta. 

The second error arises from the precipitates of baric sulphate and carbonate 
taking up some space in the 500 c.c. flask, the final results thus being found 
too high. If it is assumed that a cold saturated solution of baryta contains 
about 23 gm. of BaO per liter, it will be near enough for all practical purposes 
if in the experiment, working with 3'55 gm. of Na-SO 4 and 250 c.c, of baryta 
solution, 0'4 per cent, is subtracted from the final results for this error. 

Three experiments made with 3' 55 gm. of pure ignited sodic sulphate gave 
the following results : 

Used one-fourth normal acid ... 49'37 c.c. 
Add for Ba (NO 3 ) 2 0'24 c.c. 



49*61 c.c. 
=99-22 per cent. Na 2 SO 4 . 

II. 

Used one-fourth normal acid ... 49'21 c.c. 
Add for Ba(NO 3 ) 2 0'24 c.c. 



49-45 c.c. 
=98*90 per cent. Na 2 SO 4 . 

III. 

Used one-fourth normal acid ... 49'37 c.c. 
Add for Ba(NO 3 ) 2 0'24 c.c. 



49-61 c.c. 
=99'22 per cent. Na 2 SO 4 . 

The average of these three experiments gives 99'1 per cent. ; and if 0"4 
per cent, be subtracted for the precipitate, the result is 98'7 per cent, instead 
of 100. 

Grossman states that this loss of 1'3 per cent, in working with 3'55 gm. 
of sulphate in the given dilution is a constant, and by dividing all results by 
0'987 correct results are obtained. 

It now remains to show the applicability of this method to the assay of 
salt cake and like substances. The following is a complete analysis of a 
sample of salt cake made in the usual way : 

Moisture 0'49 

Insoluble ... 0'29 

Free sulphuric hydrate 0*38 

Aluminium sulphate 0*23 

Ferric sulphate 0'42 

Calcic sulphate 1'17 

Sodic chloride 2'00 

Sodic sulphate (by difference) 95'02 



lOO'OO 



17. ALKALINE COMPOUNDS. 67 

In order to make a good analysis of salt cake by weight it is necessary to 
estimate seven constituents, to find by difference the quantity of actual sodic 
sulphate, which is the only constituent wanted. 

When baric hydrate is added to a solution of salt cake the free acid is 
precipitated, so are alumina and iron, and the sulphuric acid combined with 
them and with lime. The lime is partly thrown down as such, and what is 
left as lime in solution is precipitated as carbonate in the second operation. 
Thus, whatever other sulphates be present, only the sodic sulphate is given; 
and by one simple test we are thus able to get a result which formerly could 
onh r be attained by a tedious complete analysis. 

The salt cake, of which a complete analysis is given above, was tested by 
the alkalimetric method. 3"55 gm. required 

One-fourth normal acid 46'95 c.c. 

Add for Ba(NO 3 ) 2 0'24 c.c. 



47'19 c.c. 
=94-38 per cent. Na 2 SO 4 . 

(94-38 0-40)=93'98. 

93-98 : 0-987=95-2 per cent. 1SVSO 4 . 

Thus, by the alkalimetric test, 95*2 per cent, of JS"a 2 S0 4 occurs, 
whereas the analysis gives 95 '02 per cent. If it be considered how 
difficult it is to wash soda salts completely from precipitates, it is 
not surprising to find the result too low in the complete analysis, 
as in five precipitates a very minute quantity will make up 0*2 
per cent. 

It is hardly necessary to point out that none of the figures for 
the correction of the errors enumerated above can be used by any 
one else working by this method, but that they must be ascertained 
in every individual case. It is absolutely necessary to ascertain 
after the first operation that there is no sulphate, and after the 
second (before titrating) that there is no baryta in solution. 

15. Raw Salt, Brine, etc. 

Lime may be estimated by precipitation with ammonic oxalate, and the 
precipitate titrated with permanganate, as in 52. 

Sulphuric acid as in 76. 

Magnesia is precipitated as ammoniacal phosphate, by a solution of sodic 
phosphate containing ammonia, first removing the lime by ammonic oxalate, 
the precipitate of double phosphate of magnesia and ammonia is brought on 
a filter, washed with cold water containing ammonia, then dissolved in acetic 
acid, and titrated with standard uranium solution, or by the process for 
P-0 5 ( 24). 

The quantity of real salt in the sample may be ascertained by treating a 
weighed quantity in solution with caustic baryta, boiling, setting aside that 
the excess of baryta may precipitate itself as carbonate, or more quickly by 
udding ammonic carbonate, filtering, evaporating the solution to dryness, and 
gently igniting the residue is pure salt. The loss of weight between this 
and the original specimen taken for analysis, will show the percentage of 
impurities. 

16. Silicates of Soda and Potash. 

A weighed quantity of the substance is gently ignited, until no aqueous 
vapours are given off, and the residue weighed thus the respective per- 
centages of water and anhydrous material are obtained. 

F 2 



68 VOLUMETRIC ANALYSIS. 17. 

Another portion of the substance is dissolved in hot water, and titrated 
with litmus and normal acid boiling, or with methyl orange after cooling. The 
volume of acid is calculated to soda or potash. Solid alkaline silicates require 
to be finely powdered previous to solution in hot water. 

17. Soap. 

The methods here given are a combination of those published 
by A. R. Leeds (C. N. xlviii. 166) and C. R. A. Wright (Journ. 
Soc. Arts, 1885, 1117, also J. S. C. I. iv. 631), and others. 

(1) Moisture and Volatile Matters. 15 gin. are dried to a constant 
weight, first at 100, then at 110 C. 

(2) Free Fats. Residue of (1) is exhausted in a S o x h 1 e t tube, with light 
petroleum ether, and the extract, after evaporation of the ether, weighed. 

(3) Fatty Acids, Chlorides, Sulphates, G-lycerine, etc. The residue 
from (2), w r hich has been treated with ether, represents 15 gm. soap ; it is 
weighed, and two-thirds of it are dissolved in water, and normal nitric acid 
added in excess to separate the fatty acids. These are collected on a tared 
filter, dried, and weighed. The acid filtrate is now titrated with normal soda 
or potash (free from chlorides or sulphates), with phenolphthalein as indicator ; 
the difference between the volumes of acid and alkali used gives roughly the 
total alkali. The residual neutral liquid is divided into two equal parts, in 
one of which chlorine is estimated with T N ^ silver and chromate, and in the 
other sulphuric acid by normal baric chloride. If glycerine is present, 
it may be estimated by Muter's copper test in the absence of sugar. 
Sugar is, however, often largely used in transparent soaps in place of 
glycerine; when both are present, the separate estimation is difficult, but 
"Wright suggests the method of Fehliug for the sugar, first inverting 
with acid ; the copper retained in solution by the glycerine being estimated 
colorimetrically, using for comparison a liquid containing both sugar and 
glycerine to known extents, treated side by side with the sample tested. 

(4) Free and Total Alkali. These are obtained by Wright's alcohol test. 
Two or three grams of the soap are boiled with 95 per cent, alcohol, the extract 
filtered off and residue washed with alcohol. The solution so obtained may 
be either positively alkaline with caustic alkali, or negatively alkaline from 
the presence of fatty acids or a diacid soap, according to the kind of soap 
used. Phenolphthalein is added, which shows at once whether free alkali is 
present, and in accordance with this either standard alcoholic acid or alkali is 
used for titration. The residue on the filter is then dissolved in water, and 
titrated Avith normal or decinormal acid ; the alkali so found may include 
carbonate, silicate, borate, or aluminate of soda or potash, and also any 
soluble lime. The sum of the two titrations will be the total alkalinity in 
case both showed an alkaline reaction ; if otherwise, the alkali used to 
produce a colour in the alcoholic extract is deducted from the volume of acid 
used in the water extract. This method of taking the alkalinity of a soap 
is very fairly exact; the error ought never to exceed _+ 0'5 per cent. 
J. A. Wilson (C. N. lix. 280) states that the estimation of free caustic 
alkali in high class soaps, containing no free glycerides, by the alcoholic 
method is correct, but in the case of common commercial soaps it is 
entirely misleading. 

(5) Combined A IJcali Wilson (C. N. Ixiv. 205) proceeds as follows : 
1. The alkali, in all forms, is determined by titration with standard acid in the 
usual manner. 2. Another weighed quantity of the soap is decomposed in an 
Erlenmeyer flask with a slight excess of dilute H 2 SO 4 , and the flask kept 
on the water-bath till the fatty acids separate quite clear. The flask is then 
placed in ice-water to cool, and then filtered. The fatty acids are washed three 
times successively Avith 250 c.c. of boiling water and allowed to cool each time 



18. ALKALINE EARTHS. 69 

uiid filtered. The united filtrates are diluted to 1 liter, and 500 c.c. placed 
in a clear white beaker and tinted with methyl orange ; T ^ alkali is then 
dropped in till the liquid acquires the usual colour, after which a little 
phenolphthalein is added, and the addition of standard alkali continued till 
a permanent pink is established. The number of c.c. used in the latter 
titration are due to the soluble acids, and are calculated to caprylic acid. 
The fatty acids in the flask, and any little on the filter are dried and 
weighed, and then dissolved in alcohol, and titrated with | alcoholic alkali. 
The alkali so used, together with that required for neutralization of the 
soluble acids, and deducted from the total alkali, gives the alkali existing 
In other forms than as soap. Of course, if desired, the soap may be 
decomposed with standard H 2 SO 4 , and the alkali required to neutralize the 
methyl orange noted, which, deducted from the total acid used, would give 
the acid equivalent to the alkali existing in all forms. 

The method of C. Hope is undoubtedly the quickest and best for the 
examination of the alcoholic solution of soap. Two grams of soap are dissolved 
in hot absolute alcohol, a drop of phenolphthalein indicator added, and some 
bubbles of CO 2 passed through till the colour disappears. The liquid is 
filtered ; the residue, consisting of total impurities, is washed with hot alcohol, 
weighed and titrated with r \ acid and methyl orange, which gives the alkali 
not existing as soap. The alcoholic solution is evaporated to dryness at 
100 C. and the dry soap weighed. It is then gently ignited, dissolved in 
water, and titrated with T ^- acid and methyl orange to find the alkali existing 
as soap. The difference between this and the soap residue, before ignition, 
gives the fatty anhydrides, which multiplied by 1*03 gives the fatty acids. 
The water is found by deducting the weights of the impurities arid dry soap 
from 100. Fuller information on this subject may be found in Allen's 
Organic Analysis and in Lant Carpenter's treatise on Soap and Candles. 

TITRATION OF AL.KALINE EARTHS. 

18. STANDARD hydrochloric or nitric acid must in all cases be 
used for the titration of the caustic or carbonated alkaline earths, as 
these are the only acids yielding soluble compounds, except in the 
case of magnesia. The hydrates, such as caustic lime, baryta, 
strontia, or magnesia, may all be estimated by any of the indicators, 
using the residual method, i.e., adding a known excess of standard 
acid, boiling to expel any trace of CO 2 , and re-titrating with 
standard alkali. 

The carbonates of the same bases may of course also be 
estimated in the same way, bearing in mind, that when methyl 
orange is used, the liquid is best cooled before re-titration. All 
heating may be avoided when using methyl orange in titrating 
mixtures of hydrates and carbonates, or the latter only, unless it is 
impossible to dissolve the substance in the cold. A good excess 
of acid is generally sufficient. 

The total amount of base in mixtures of caustic and carbonated 
alkaline earths is also estimated in the same way. 

(1) Estimation of Mixed Hydrates and Carbonates. This may 
be done either by phenacetolin or phenolphthalein as indicator. 
The former has been recommended byDegener and Lunge : the 
method, however, requires practice in order to mark the 
change of colour. 



70 VOLUMETRIC ANALYSIS. 18. 

The liquid containing the compound in a fine state of division is tinted 
with the indicator so as to be of a faint 3 T ellow; standard acid is then 
cautiously added until a permanent pink occurs (at this stage all the hydrate 
is saturated), more acid is now cautiously added until the colour becomes deep 
yellow, the volume of acid so used represents the carbonate. 

The method is especially adapted to mixtures of calcic hydrate 
and carbonate. It is also applicable to barium, but not to 
magnesium, owing to the great insolubility of magnesic hydrate in 
dilute acid. If phenolphtlialein is used as indicator, the method 
is as follows : 

Heat the liquid to boiling, and cautiously add standard acid until the red 
colour is just discharged. The carbonates of lime and baryta, rendered 
dense by boiling, are both quite neutral to the indicator. To obtain the 
whole of the base, excess of standard acid is used, and the mixture re-titrated 
with standard alkali. 

Magnesia in solution as bicarbonate may be accurately estimated 
in the cold with methyl orange as indicator. 

(2) Estimation of Calcium, Barium, Magnesium, and Strontium 
in Neutral Soluble Salts. The amount of base in the chlorides and 
nitrates of the alkaline earths may be readily estimated as 
follows : 

The weighed salt is dissolved in water, cautiously neutralized if acid or 
alkaline, phenolphtlialein added, heated to boiling, and standard sodic 
carbonate delivered in from time to time with boiling until the red colour 
is permanent. 

Magnesium salts cannot however be estimated in this way, 
or even mixtures of lime and magnesia, as magnesic carbonate 
affects the indicator in a different manner to the other carbonates. 

(3) Precipitation of the Alkaline Earths from their Central Salts as 
Carbonates. Soluble salts of lime, bar} T ta, and stroutia, such as chlorides, 
nitrates, etc., are dissolved in water, and the base precipitated as carbonate, 
with excess of ammonic carbonate and some free ammonia. The mixture is 
heated to about 60 C. for a few 7 minutes. The precipitated carbonate is then 
to be filtered, well washed with hot water till all soluble matters, especially 
ammonia, are removed, and the precipitate with filter titrated with normal 
acid, as already described. 

Magnesia salts require caustic soda or potash instead of ammonic carbonate ; 
but the process gives results slightly too low, owing to the slight solubility of 
magnesic hydrate in the alkaline liquid. 

(4) Lime and Magnesia Carbonates in Waters. The amount of calcic 
or calcic and magnesic carbonates, dissolved in ordinary non- alkaline waters 
may be very readily, and with accuracy, found by taking 200 or 300 c.c. 
of the water, heating to near boiling, adding phenacetolin or lacmoid, and 
titrating cautiously with T ^- nitric or sulphuric acid. An equally accurate 
result may be obtained by methyl orange in the cold liquid. 

(5) Magnesia. The magnesia existing in the commercial Stassfurt salts 
used for manures, etc., and other soluble magnesia salts, may very readily 
be determined with accuracy by Stolba's method, as given for P 2 O 5 in 
24.2, or in all cases where separation can be made as ammonio-magnesic 



ALKALINE EAKTHS. 71 

phosphate. The precipitation ma} 7 be hastened considerably by precipitating 
with microcosmic salt, in the presence of a tolerably large proportion of 
ammonic chloride, accompanied with vigorous stirring. Half an hour quite 
suffices to bring down the whole of the double phosphate, and its adherence 
to the sides of the beaker is of no consequence, if the titration is made in 
the same beaker, and with the same glass rod, using an excess of standard 
acid, and titrating back with weak standard ammonia and methyl orange. 

The precipitate may also be titrated with standard uranium ( 72). 
Precht (Z. a, C. 1879, 438) adopts the following method for soluble 
magnesia salts in kaiuit, kieserit, etc., depending upon the insolubility of 
magnesic hydrate in weak caustic potash : 

10 gm. of the substance are dissolved, filtered, and mixed with 25 c.c. of 
normal caustic potash, if it contains less than 50 per cent, of magnesic 
sulphate ; or 50 c.c. if it contains more than 50 per cent. The mixture is 
warmed somewhat, transferred to a 500 c.c. flask, and the volume made up 
with water. After standing at rest for half an hour, 50 c.c. of the clear 
liquid are withdrawn, and the excess of normal alkali estimated in the usual 
way with normal acid. Ammonium and metallic salts must be absent. 

1 c.c. normal potash 0'02 gm. MgO. 

(6) Hardness of Water estimated without Soap Solution. As is 
generally known, the soap-destroying power of a water is ascertained 
in Clark's process by a standard solution of soap in weak alcohol, 
titrated against a standard solution of calcic chloride. The 
valuation is in so-called degrees, each degree being equal to I grain 
of calcic carbonate, or its equivalent, in the imperial gallon. The 
process is an old and familiar one, but open to many objections 
from a scientific point of view. The scale of degrees is arbitrary, 
and is seriously interfered with by the presence of varying 
proportions of magnesia.. 

We are indebted, primarily to Mohr, and subsequently to 
Hehner, for an ingenious method of determining both the 
temporary and permanent hardness of a water without the use 
of soap solution. 

The standard solutions required are -^ sodic carbonate and 
-$ sulphuric acid. Each c.c. of standard acid exactly neutralizes 
1 m.gm. of CaCO 3 , and each c.c. of the alkali precipitates the like 
amount of CaCO 3 , or its equivalent in magnesia, in any given 
water. 

Process : 100 c.c. of the water are tinted with an indicator of suitable 
character, heated to near boiling, and standard acid cautiously added until 
the proper change of colour occurs. Hehner recommends phenacetolin ; 
but my own experiments give the preference to lacmoid, which is also 
commended by Thomson. Draper (C. N. li. 206) points out the value 
of lacmoid and carminic acid for such a process, and I fully endorse his 
opinion with respect to both indicators. Practice is desirable in order 
to recognize the precise end-reaction. The number of c.c. of acid used 
represents the number of Clark's degrees of temporary hardness per 100,000. 
To obtain degrees per gallon, multiply the number of c.c. by 0'7. The 
permanent hardness is ascertained by taking 100 c.c. of the water and adding 
to it a rather large known excess of the standard sodic carbonate. The 
quantity must of course be regulated by the amount of sulphates, chlorides, 
or nitrates of lime and magnesia present in the water ; as a rule, a volume 



72 VOLUMETRIC ANALYSIS. 19. 

equal to the water will more than suffice. Evaporate in a platinum dish to 
dryness (glass or porcelain will not do, as they affect the hardness), then 
extract the soluble portion with small quantities of distilled water, through 
a very small filter, and titrate the filtrate with the standard acid for the 
excess of sodic carbonate : the difference represents the permanent hardness. 

Some waters contain alkaline carbonates, in which case there is 
of course no permanent hardness, because the salts to which this is 
clue are decomposed by the alkaline carbonate. In examining a 
water of this kind, the temporary hardness will be shown to be 
greater than it really is, owing to the alkaline carbonate ; and the 
estimation for permanent hardness will show more sodic carbonate 
than was actually added. If the difference so found is deducted 
from the temporary hardness, as first noted, the remainder will be 
the true temporary hardness. 



AMMONIA. 



19. IN estimating the strength of solutions of free ammonia 
by the alkalimetric method, it is better to avoid the tedious process 
of weighing any exact quantity, and to substitute for it the following 
plan, which is applicable to most liquids for the purpose of 
ascertaining both their absolute and specific weights. 

Let a small and accurately tared flask, beaker, or other convenient vessel 
be placed upon the balance, and into it 10 c.c. of the ammoniacal solution 
delivered from a very accurately graduated 10 c.c. pipette. The weight 
found is, of course, the absolute weight of the liquid in grams ; suppose it to 
be 9'65 gm., move the decimal point one place to the left, and the specific 
weight or gravity is at once given (water being 1), which in this case is 0'965. 

It must be borne in mind that this system can only be used properly with 
tolerably delicate balances and ver} r accurate pipettes. The latter should 
invariably be tested by weighing distilled water at 16 C. 

The 10 c.c. weighing 9'65 gm., are now mixed with water and titrated with 
nesmal acid of which 49 c.c. are required, therefore 49 x 0'017=0'833 gm. NH 3 
=8'64 per cent, of real ammonia; according to Otto's table 0'965 sp. gr. is 
equal to 8'50 per cent. Ammonic carbonate, and a mixture of the same with 
bicarbonate, as it most commonly occurs in commerce, may be titrated direct 
with normal acid for the percentage of real ammonia, using methyl orange 
as indicator. The carbonic acid can be determined by precipitating the 
solution while hot with baric chloride, and when the precipitate is well 
washed, dissolving it with an excess of normal acid and titrating backward 
with normal alkali; the number of c.c. of acid used multiplied by 0'022 
(the i mol. wt. of CO 2 ) will give the weight of carbonic acid present in 
the sample. 

1. Estimation of Combined Ammonia "by distillation with Alkalies 
or Alkaline Earths. 

This method allows of the expulsion of ammonia from all its 
salts. Caustic soda, potash, or lime, may any of them be used 
where no organic nitrogenous compound exists in the substance ; 



19. AMMONIA. 73 

but should such be the case, it is preferable to use freshly ignited 
magnesia. 

The distilling apparatus may conveniently be arranged by con- 
necting an ordinary well-stoppered small retort to a small Liebig 
condenser, and leading the distilled gas into a vessel containing an 
excess of normal acid. After the operation is ended, the excess 
of acid is ascertained by residual titration with normal alkali or 
ammonia, and thus the amount of displaced ammonia is found. 

The retort must be so supported that its neck inclines well 
upwards, in order that any alkali mechanically carried into it by 
the spray which occurs during ebullition shall not reach the 
condenser. An angle of about 30 suffices ; and in order that a 
convenient connection may be made with the condenser, the end of 
the retort is bent downward, and the connection securely made with 
india-rubber tubing. In like manner, the end of the condenser is 
elongated by a glass tube and india-rubber joint, so that the tube 
dips into a two-necked bottle or bulb, containing the measured 
normal acid ; the end of this tube should be cut obliquely, and 
reach nearly, but not quite, to the surface of the acid. The outlet 
of the receiver is fitted with a tube containing glass wool, broken 
glass, or fibrous asbestos, wetted with a portion of the normal 
acid, so that any traces of ammonia which may possibly escape 
condensation in the bulk of the acid may be retained. 

The retort containing the ammoniacal compound in solution 
being securely fixed, and all the apparatus tightly connected, the 
stopper of the retort is removed, and a strong solution of caustic 
alkali, or, in case of compounds in which ammonia is quickly 
released, pieces of solid alkali are rapidly introduced, the stopper 
inserted, and the distillation forthwith commenced. Lime or 
magnesia, suspended in water, must be added through a small 
funnel ; the distillation is continued until the steam has washed all 
traces of ammonia out of the condenser tube into the normal acid. 
Cold water is of course run continuously through the condenser as 
usual. Finally, the tubes connected with the receiver are well 
washed out into the bulk of normal acid, methyl orange added, and 
the titration completed with normal alkali or J ammonia. 

Each c.c. of normal acid neutralized by the displaced ammonia 
represents O'OIT gm. NH 3 . 

The apparatus shown in fig. 28 is of great value in determining 
accurately all the forms of ammonia which can be displaced by 
soda, potash, or lime, and the gas so evolved collected in a known 
volume in excess of normal acid, the excess of acid being after- 
wards found by residual titration with normal alkali or ammonia. 

Many modifications of this apparatus have been suggested, such 
as the introduction of a condenser between the two flasks to cool 
the distillate; another is the use of a (J tube containing some 
standard acid in place of c. I do not find that any of these 
modifications are required to secure accuracy, if the apparatus 



74 



VOLUMETRIC ANALYSIS. 



is tightly fitted. It is necessary that a bulb should exist in 
the distilling tube, just above the cork of the distilling flask, 
otherwise the spray from the boiling liquid is occasionally projected 
into the tube, and is blown over with the condensed steam. 




Fig. 28. 

Another precaution is advisable where dilute liquids are boiled, 
and much steam generated, that is, to immerse the condenser flask 
in cold water. 

The little flask, holding about 200 c.c. and placed upon the wire 
gauze, contains the ammoniacal substance. The tube d is filled with strong 
solution of caustic potash or soda. The large flask holds about half a liter, 
and contains a measured quantity of normal acid, part being contained in 
the tube c, which is filled with glass wool or broken glass, and through which 
the normal acid has been poured. The stoppers of the flasks should be 
caoutchouc, failing which, good corks soaked in > melted paraffin may be 
used. 

The substance to be examined is weighed or measured, and put into the 
distilling flask with a little water, the apparatus then being made tight at 
every part; some of the caustic alkali is allowed to flow in by opening the 
clip, and the gas or spirit lamp is lighted under it. 



19. AMMONIA. 75 

The contents are brought to gentle boiling, taking care that the froth, if 
any, does not enter the distilling tube. It is well to use a movable gas 
burner or common spirit lamp held under the flask in the hand ; in case 
there is any tendency to boil over, the heat can be removed immediately, and 
the flask blown upon by the breath, which reduces the pressure in a moment. 
In examining guano and other substances containing ammouiacal salts and 
organic matter by this means, the tendency to frothing is considerable; and 
unless the above precautions are taken, the accuracy of the results will be 
interfered with. A small piece of bee's wax or solid paraffin is very 
serviceable in allaying the froth. 

The distilling tube has both ends cut obliquely ; and the lower end nearlv, 
but not quite, reaches to the surface of the acid, to which a little methyl 
orange may be added. The quantity of normal acid used must, of course, be 
more than sufficient to combine with the ammonia produced ; the excess is 
afterwards ascertained by titration with normal alkali or ^ ammonia. 

It is advisable to continue the boiling for say ten or fifteen minutes, 
waiting a minute or two to allow all the ammonia to be absorbed ; then opening 
the clip, blow through the pipette so as to force all the remaining gas into 
the acid flask. The tube c must be thoroughly washed out into the flask 
with distilled water, so as to carry down the acid with any combined gas which 
may have reached it. The titration then proceeds as usual. This process 
is particularly serviceable for testing commercial ammouiacal salts, gas 
liquor, etc. (see below). The results are extremely accurate. 

2. Indirect Method. 

In the case of tolerably pure ammoniacal salts or liquids, free 
from acid, or in which the free acid is previously estimated, 
a simple indirect method can be used, as follows : 

If the ammoniacal salt be boiled in an open vessel with normal caustic 
alkali, the ammonia is entirely set free, leaving its acid combined with the 
fixed alkali. If, therefore, the quantity of alkaline solution is known, the 
excess beyond that, necessary to supplant the ammonia, may be found by 
titration with standard acid. The boiling of the mixture must be continued 
till a piece of red litmus paper, held in the steam from the flask, is no longer 
turned blue. 

Example : 1*5 gm. of purest sublimed ammonic chloride was placed in a 
wide-mouthed flask with 40 c.c. of normal soda, and boiled till all ammonia 
was expelled, then titrated back with normal sulphuric acid, of which 
11'9 c.c. were required; 28'1 c.c. of normal alkali had therefore been 
neutralized, which multiplied by 0'05337, the factor for ammonic chloride, 
gave T499 gm., instead of 1'5 gm. originally taken. 

3. Technical Analysis of Gras Liquor, Sulphate of Ammonia, Sal 
Ammoniac, etc., arranged for the use of Manufacturers. 

This process depends upon the fact, that when ammoniacal salts 
are heated with caustic soda, potash, or lime, the whole of the 
ammonia is expelled in a free state, and may by a suitable apparatus 
(fig. 29) be estimated with extreme accuracy (see 19. 1). 

The set of apparatus here described consists of a distilling flask 
B, and condensing flask F, fitted together in such a manner, that no 
loss of free ammonia can occur ; the whole of the ammonia being 
liberated from the distilling flask into a measured quantity of free 



76 VOLUMETRIC ANALYSIS. 19. 

acid contained in the condensing flask, where its amount is after- 
wards found by the method hereinafter described. 

Analysis of Gas Liquor. This liquid consists of a solution of 
carbonates, sulphates, hyposulphites, sulphides, cyanides, and other 
salts of ammonia. The object of the ammonia manufacturer is to 
get all these out of his liquor into the form of sulphate or chloride 
as economically as possible. The whole of the ammonia existing 
as free or carbonate in the liquor, can be distilled off at a steam 
heat ; the fixed salts, however, require to be heated with soda, 
potash, or lime (the latter is generally used on a large scale as most 
economical), in order to liberate the ammonia contained in them. 

The valuation of gas liquor is almost universally made in Great 
Britain by Twaddle's hydrometer, every degree of which is taken 
to represent what is technically called "two-ounce strength;" that is 
to say, a gallon of such liquor should neutralize exactly two ounces by 
weight of concentrated oil of vitriol thus 5 degrees, Twaddle, is 
called " ten-ounce " liquor but experiment has clearly proved, that 
although the hydrometer may be generally a very convenient 
indicator of the commercial value of gas liquor, it is not accurate 
enough for the manufacturer who desires to work with the utmost 
economy. Sometimes the liquor contains a good deal of free 
ammonia, and in such case the hydrometer would show it to be 
weaker than it really is ; on the other hand, sometimes, from 
accidental causes, other solid matters than ammonia salts occur in 
the liquor, and the hydrometer shows it to be stronger than it really 
is. The method of saturation, by mixing standard acid with the 
liquor, is perhaps more correct than the hydrometer; but this 
system is entirely at fault in the presence of much fixed ammonia, 
and is, moreover, a very offensive and poisonous operation. 

The apparatus here described is exactly the same on a small 
scale as is necessary in the actual manufacture of sulphate of 
ammonia in quantities ; and its use enables any manufacturer to 
tell to a fraction how much sulphate of ammonia he ought to 
obtain from any given quantity of gas liquor. It also enables him 
to tell exactly how much ammonia can be distilled off with heat 
alone, and how much exists in a fixed condition requiring lime. 

The measures used in this process are on the metrical system ; 
the use of these may, perhaps, at first sight appear strange to 
English manufacturers ; but as the only object of the process is to 
obtain the percentage of ammonia in any given substance, it is 
a matter of no importance which system of measures or weights is 
used, as when once the percentage is obtained, the tables will at 
once show the result in English terms of weight or measure. 

a is a small pipette, holding 10 cubic centimeters to the mark in neck : 
this is the invariable quantity of liquor used for the analysis, whatever the 
strength may be. This measure is filled to the mark by suction and 
transferred, without spilling a drop, to flask B the fittings being previously 



UJ 




78 VOLUMETRIC ANALYSIS. 19. 

removed the tube C is then filled in the same manner, with strong 
caustic soda solution from a clean cup or other vessel, in order to do 
which, the clip at the top must be opened : the cork is then replaced, and 
the flask B is then securely imbedded in perfectly dry sand, in the sand- 
bath D. The graduated pipette E is then filled in the same manner to 
the O mark, with standard acid, and 20, 30, 40, or 50 c.c. (according 
to the estimated strength of the liquor) allowed to flow into the flask P, 
through the cup G, which is filled with broken glass placed on a layer of 
glass wool or fibrous asbestos. The broken glass should be completely 
wetted with the acid, so that any vapours of ammonia which may escape 
the acid in the flask shall become absorbed by the acid. The quantity 
of standard acid to be used is regulated by the approximately known 
strength of the liquor, which of course can be told by Twaddle's 
hydrometer: thus, for a liquor of 3 C Twaddle=6-oz. liquor, 20 c.c. 
8-oz., 25 c.c. 10-oz., 30 c.c. of acid will be sufficient but there must 
always be an excess. The required quantity can always be approx- 
imately known, since every 10 c.c. of acid represents 1 per cent, of 
ammonia. The standard acid having been carefully passed through G, 
the apparatus is fitted together at H by the elastic tube, and the india-rubber 
stoppers securely inserted in both flasks; this being done, the lamp is 
lighted under the sand-bath, and at the same time the spring-clip on C is 
pressed, so as to allow about two-thirds of the caustic soda to flow into B ; 
the rest will gradually empty itself during the boiling. The heat is continued 
to boiling, and allowed to go on till the greater bulk of the liquid in B 
is boiled away into P. A quarter of an hour is generally sufficient for this 
purpose, but if the boiling is continued till the liquid in B just covers the 
bottom of the flask, all the ammonia will have gone over to P; during 
the whole operation the distilling tube must never dip into the acid in P. 
In order to get rid of the last traces of ammonia vapour out of B, the lamp 
is removed, and the mouth being applied to the tube over the spring-clip, 
the latter is opened, and a good blast of air immediately blown through. 
The apparatus may then be detached at H ; distilled or good boiled drinking 
water is then poured repeatedly through G in small quantities, till all traces of 
acid are removed into flask P. This latter now contains all the ammonia out of 
the sample of liquor, with an excess of acid, and it is necessar} r now to find out 
the quantity of acid in excess. This is done by means of the burette I, and 
the standard solution of soda, w : hich soda is of exactly the same strength as 
the standard acid. In order to find out how much of the standard acid has been 
neutralized by the ammonia in the liquor distilled, the burette I is filled to 
with standard soda, and one drop of methyl orange, or a sufficiency of any 
other indicator, other than phenolphthalein, being added to the cooled contents 
of flask P, the soda is slowly dropped into it from the burette, with constant 
shaking, until the indicator changes colour. The number of c.c. of soda so 
used, deducted from the number of c.c. of standard acid used, will show 
the number neutralized by the ammonia in the liquor distilled ; therefore, if 
the number of c.c. of soda used to destroy the pink colour be deducted 
from the number of c.c. of standard acid originally used, it will show the 
number of c.c. of standard acid neutralized by the ammonia, which has been 
distilled out of the liquor, and the strength of the solutions is so arranged 
that this is shown without any calculation. The following examples will 
suffice to show this : Suppose that a liquor is to be examined which marks 
5 Twiddle, equal to 10-ounce liquor ; 10 c.c. of it are distilled into 30 c.c. 
of the standard acid, and it has afterwards required 6 c.c. of standard 
soda to neutralize it ; this leaves 24 c.c. as the volume of acid saturated 
by the distilled ammonia, and this represents 2'4 per cent. ; and on referring 
to the table it is found that this number corresponds to a trifle more than 
11 ounces, the actual figures being 2*384 per cent, for 11 ounce strength. 

The strength of the standard soda and acid solutions is so 



19. AMMONIA. 79 

arranged, that when 10 c.c. of liquor are distilled, every 10 c.c. of 
acid solution represents 1 per cent, of ammonia in the liquor. In. 
like manner 13 c.c. of acid will represent 1'3 per cent, of ammonia 
corresponding to 6-ounce liquor. 

The burette is divided into tenths of a cubic centimeter, and 
those who are familiar with decimal calculations can work out the 
results to the utmost point of accuracy ; the calculation being, that 
every 1 per cent, of ammonia requires 4 '61 ounces of concentrated 
oil of vitriol (sp. gr. 1 *S45) per gallon, to convert it into sulphate : 
thus, suppose that 10 c.c. of any given liquor have been distilled, 
and the quantity of acid required amounts to 18 '6 c.c., this is 
1*86 per cent., and the ounce strength is shown in ounces and 
decimal parts as follows : 

4-61 
1-86 



2766 
3688 
461 

8 '5746 ounces of oil of vitriol. 



The liquor is therefore a trifle over 8|-ounce strength. 

Spent Liquors. It is frequently necessary to ascertain the 
percentage of ammonia in spent liquors, to see if the workmen 
have extracted all the available ammonia. In this case the same 
measure, 10 c.c. of the spent liquor, is taken, and the operation 
conducted precisely as in the case of a gas liquor. 

Example : 10 c.c. of a spent liquor were distilled, and found to neutralize 
3 c.c. of acid : this represents three-tenths of a per-cent. equal to 1-oz. and 
four-tenths of an ounce, or nearly 1 oz. Such a liquor is too valuable to 
throw away, and should be worked longer to extract more ammonia. 

Process for Sulphate of Ammonia or Sal Ammoniac : An average 
sample of the salt being drawn, ten grams are weighed, transferred without 
loss to a beaker or a flask having a 100 c.c. mark upon it, distilled or boiled 
drinking water poured on it, and well stirred till dissolved, and finally 
water added exactly to the mark. The 10 c.c. measure is then filled with 
the solution, and emptied into the distilling flask B ; 30 c.c. of standard 
acid are put into flask E and the distillation carried on precisely as in 
the case of a gas liquor. The number of c.c. of standard acid required 
shows directly the percentage of ammonia ; thus, if 24*6 c.c. are used in 
the case of sulphate, it contains 24'6 per cent, of ammonia. 

The liquors when tested must be measured at ordinary tempera- 
tures, say as near to 60 F. as possible. The standard solutions 
must be kept closely stoppered and in a cool place. 

The following table is given to avoid calculations ; of course, it 
will be understood that the figures given are on the assumption 
that the whole of the ammonia contained in the liquor is extracted 



80 



VQLUMETKIC ANALYSIS. 



19. 



in the manufacture as closely as it is in the experiment. With the 
most perfect arrangement of plant, however, this does not as a rule 
take place ; but it ought to be very near the mark with proper 
apparatus, and care on the part of workmen. 



Approxi- 
mate 
measure of 
Standard 
Acid in c.c. 


Percentage 
of Ammonia 
NH3 


Ounce 
strength 
pei- 
gallon. 


Weight of Sulphuric Acid in pounds 
and decimal parts required for each 
gallon of liquor. 


Yield of 
Sulphate 
per gallon in 
Ibs. and 
decimal 


C. O. V. 


T> n \r Chamber 
U< V- AnirJ 


and tenths. 






169 Tw. 


144, Tw ACIO. 
Iw - 120= Tw. 


parts. 


2-2 


2168 


1 


0625 '0781 


0893 


0841 


4'3 


4336 


2 


1250 


1562 


1786 


1682 


6-5 


6504 


3 


1875 


2343 


2679 


2523 


87 '8672 


4 


2500 


3124 


3572 


3364 


lO'l 1-0840 


5 


3125 


3905 


4465 


4205 


13'0 1-3000 


6 


3750 


4686 


'5358 


5046 


15-2 1-5176 


7 


4375 


5467 


6251 


5887 


17-3 


17344 


8 


5000 


6248 


7144 


6728 


19-5 


1-9512 ; 9 


5625 


7029 


8037 


7569 


21-7 


2-1680 


10 


6260 


7810 


8930 


8410 


23-8 


2-3840 


11 


6875 


8591 


9823 


9251 


26-0 


2-6016 


12 


7600 


9372 


1-0716 


1-0092 


28-2 


2-8184 


13 


8125 


1-0153 


1-1609 


1-0933 


30-4 


3-0350 


14 


8750 


1-0934 


1-2502 


1-1774 


32-5 


3-2520 


15 


9375 


1-1715 i 1-3395 


1-2615 


34-7 


3-4688 


16 I'OOOO 


1-2496 1-4288 


1-3456 


36-9 


3-6856 


17 1-0625 


1-3277 


1-5181 


1-4297 


39-0 


3-9024 


18 1-1250 


1-4058 


1-6074 


T5138 


41-2 


4-1192 


19 


T1875 


1-4839 


1-6967 


1-5979 


43'3 


4-3360 


20 


1-2500 


1-5620 ! 1-7860 


1-6820 



The weight of sulphuric acid being given in decimals renders it 
very easy to arrive at the weight necessary for every thousand 
gallons of liquor, by simply moving the decimal point ; thus 8-oz. 
liquor would require 500 Ibs. of concentrated oil of vitriol, 625 Ibs. 
of brown oil of vitriol, or 714J Ibs. chamber acid for every 1000 
gallons, and should yield in all cases 672*8 (say 673) Ibs. of 
sulphate. 

4. Combined Nitrog-en in Organic Substances. 

The old-fashioned process consists in heating the dried substance 
in a combustion tube with soda lime, by which the nitrogen is con- 
verted into ammonia; and this latter being led into a measured 
volume of normal acid contained in a suitable bulb apparatus, 
combines with its equivalent quantity ; the solution is then 
titrated resiclually with standard alkali for the excess of acid, 
and thus the quantity of ammonia found. 

As the combustion tube with its arrangements for organic 
analysis is well known, and described in any of the standard books 
on general analysis, it is not necessary to give a description here. 




19. AMMONIA. 

5. Kjeldahl's Method. 

This has met with considerable acceptance in lieu of the 
combustion method, on account of its easy management and 
accurate results. Unlike the combustion method, the ammonia is 
obtained free from organic matters or colour, and moreover salts of 
ammonia and nitrates may be estimated with extreme accuracy. 
It was first described by Kjeldahl (Z. a. C. xxii. 366), and has 
since been commented upon by many operators, among whom are 
Warington (G. N. lii. 162), Pfeiffer and Lehmann (Z. a. C. 
xxiv. 388), Marcker and others (Z. a. C. xxiii. 553; xxiv. 
199,393; xxv. 149, 155; xxvi. 92; xxvii. 222, 398); Gunning 
(idem xxviii. 188); Arnold and Wedermeyer (idem xxxi. 525); 
and recently by Bernard Dyer (J. C. S. Ixvii.-viii. 811). 

The original process consisted in heating the nitrogenous substance 
in a flask, with concentrated sulphuric acid, to its boiling point, 
and when the oxidation through the agency of the acid is nearly 
completed, adding finely powdered permanganate of potash in small 
quantities till a green or pink colour remains constant ; the whole 
of the nitrogen is thus converted into ammonic sulphate. The 
flask is then cooled, diluted with water somewhat, excess of 
caustic soda added, the ammonia distilled off' into standard acid, 
and the amount found by titration in the usual way. 

Some practical difficulties occurred in the process as originally 
devised; and, moreover, with some organic substances a very 
long time was required to destroy the carbon set free by the 
strong acid. 

Another difficulty was, that if nitrates were present in the 
compound analyzed their reduction to ammonia was not certain nor 
regular, and unless this difficulty could be overcome the value of 
the process was limited. 

The experience of many hundreds of operators since this 
method was first introduced has resulted in rendering it as perfect 
as need be, and the results of this experience in all essential 
particulars will -now be described, omitting the details as to some 
of the special forms of apparatus, and which are not absolutely 
essential. The method requires the following re-agents and 
apparatus : 

1 . Standard acid, which may be either sulphuric or hydrochloric ; 
a convenient strength is semi-normal. 

2. Standard alkali, either ammonia, soda, or potash, of corres- 
ponding strength to the acid. 

3. Concentrated sulphuric acid free from nitrates and ammonic 
sulphate.* 

* Commercial oil of vitriol frequently contains ammonia, owing to the fact that 
makers sometimes add ammonic sulphate during concentration in order to get rid of 
nitrous compounds. Meldola and Moritz state that any traces of ammonia may be 
destroyed by digesting the acid for two or three hours at a temperature below boiling 

G 



82 VOLUMETRIC ANALYSIS. J 9. 

4. Mercuric oxide prepared in the wet way or metallic 
mercury. 

5. Powdered potassic sulphate. 

6. Granulated zinc. 

7. Solution of potassic sulphide in water, 40 gm. in the 
liter. 

8. A saturated solution of caustic soda free from nitrates or 
nitrites. 

9. An indicator litmus, methyl orange, or cochineal are 
suitable, but any other except phenolphthalein may be used. 

10. Digestion flasks with long neck and round bottom, holding 
about 200 250 c.c. These flasks should be well annealed and 
not too thick, preferably made of Jena glass the neck about 
-f inch wide, and 3| 4 inches long. 

1 1 . Distillation flasks of hard Bohemian glass and Erlenmeyer 
pattern, 550 600 c.c. capacity, fitted with a rubber stopper and 
a bulb above with curved delivery tube, to prevent the spray of 
the boiling alkaline liquid from being carried over into the 
condenser tubes. Copper distilling bottles or flasks are used by 
some operators for technical purposes with good results, but in this 
case it is advisable to put the soda into the vessel first then add 
the acid liquid. 

12. The condenser. Owing to the undoubted solubility of 
glass in fresh distilled water containing ammonia, it is advisable to 
have the condenser tube made of block tin. This should be about 
three-eighths of an inch wide externally, and is connected with the 
bulb tube of the distilling flask with stout pure rubber tube. It is 
surrounded by either a metal or glass casing, through which 
cold water is passing in the usual manner. It is very easy to 
fit up such an arrangement with the condenser tubes made 
entirely of glass sold by the dealers in chemical apparatus. The 
end of the condenser tube may be simply inserted into the neck 
of a flask in an oblique position, containing the standard acid, 
or it may have a delivery tube connected by rubber leading 
into a beaker. There is no necessity for dipping the delivery 
tube into the acid unless the temperature of the place is 
very high. 

In places where it is difficult to arrange for a flow of water to 
keep the distilling tube cool the simple apparatus shown in fig. 30 
may be serviceable, and unless the temperature of the place is 
exceedingly high there is no loss of ammonia, This arrangement 
is used by Stutzer, whose results with it compare well 

with sodic or potassic nitrite in the proportion of 0'5 gm. of the salt to 100 c.c. of 
acid. Lunge objected to this treatment, because of the probable formation of nitro- 
sulphuric acid. Experiments have since been made by Mori tz which prove that the 
objection is groundless, provided the digestion is carried on for a period sufficient to 
expel the nitrous acid (J. S. C. I. ix. 443). The purification of the acid may of course 
be obviated by ascertaining once for all the amount of ammonia in any given stock of 
acid, by making a blank experiment with pure sugar and allowing in all cases for the 
amount of NH-i so found. 




19. AMMONIA.. 83 

with others made in condensers surrounded by flowing 

water ; and equally 
accurate figures 
have been got in 
comparison with 
the ordinary con- 
denser, using the 
same quantity of 
substance for 
digestion. The 
explanation of this 
is, no doubt, the 
very strong affinity 
of ammonia for 
Tig. 30. water, and when 

in very minute 

quantity it is held very tenaciously, even at a tolerably high 
temperature. The tube should be not less than 3 feet long. 
Where a large number of estimations are being carried on it is 
convenient to have a special condenser, which will allow of six 
or more distillations being worked at the same time. Several 
forms of such arrangements have been devised, and are obtainable 
of the apparatus dealers. 

For use in my own laboratory where a large number of agricul- 
tural samples are examined, the form shown in fig. 31 has been 
adopted, and has been found to answer well. The body of the 
condenser consists of an ordinary 10-gallon iron drum filled AA 7 ith 
water; the block tin distilling tubes run through this at equal 
distances from each other, and emerge at the bottom of sufficient 
length to dip into the vessels containing the standard acid. With 
this arrangement there is no necessity for running water, and six 
distillations may be carried on simultaneously without fear of 
losing ammonia ; the body of water is so great that the lower 
portion is quite cool after the distillations are finished. In case 
of extremely hot weather or in a very hot laboratory, the cover 
may be removed and a lump of ice placed in the water, if a large 
number of distillations are required. 

The distilling flasks are closed with rubber stoppers, and fitted 
with ball top arrangement shown more plainly in fig. 30.* These 
are connected with the tin tubes by rubber joints, and supported on 
an iron frame over which is laid a strip of wire gauze. The Bunsen 
burners are of Fletcher's make, with nickel gauze tops which 
give a smokeless flame of any desired size. So well does this 
arrangement work, that during many hundreds of distillations not 
one breakage has occurred, due to the heating or the distillation. 
The tin condensing tubes do not in this case dip into the standard 
acid, as various experiments have proved it unnecessary. 

*These may be had of Gerhard t, Bonn, and probably of other apparatus dealers. 

G 2 



84 



VOLUMETRIC ANALYSIS. 



19. 



Dyer uses a block tin condensing tube rising 15 18 inches 
vertically from the distilling flask with no condenser, but bent 




Pig. 31. 

downwards and fitting into a pear-shaped adapter (with large 
expansion to allow of varied pressure), whose narrowed end dips 
actually into the acid. 

13. " A convenient stand for 
holding the digestion flasks is shown 
in fig. 32, and they rest in an 
oblique position. Heat is supplied 
by small Buns en burners. With 
a little care the naked flame can be 
applied directly to the flask with- 
out danger. Some operators prefer 
to close the digestion flasks with 
a loosely fitting glass stopper 
elongated to a point, and having 
a balloon-shaped top. This aids in the condensation of any acid 




rig. 32. 



19. AMMONIA. 85 

which may distil, but if the flasks are tolerably long in the neck, 
there is practically no loss of acid except as SO 2 which occurs in 
any case. It is almost needless to say that the digestion should be 
done in a fume closet with good draught. 

The Kjeldahl-Grunning Process : From 05 to 5 gm. of the substance 
according to its nature is brought into a digestion flask with approximately 
O'o gm. of mercuric oxide or a small globule of metal and 20 c.c. of 
sulphuric acid (in case of bulky vegetable substances 30 c.c. or more may 
be necessary). The flask is placed on wire gauze over a small Bun sen 
burner in an upright position, or in the frame above described in an inclined 
position, and heated below the boiling-point of the acid for from five to 
fifteen minutes, or until frothing has ceased. The heat is then raised till 
the acid boils briskly, this is continued for about fifteen minutes, when 
about 10 grams of potassic sulphate are added, and the boiling continued. 
No further attention is required till the contents of the flask have become 
a clear liquid, which is colourless, or at least has only a very pale straw 
colour. The flask is then removed from the frame, and after cooling, the 
contents are transferred to the distilling flask with repeated quantities of 
water amounting in all to about 250 c.c., and to this 25 c.c. of potassic 
sulphide solution are added, 50 c.c. of the soda solution*, or sufficient to 
make the reaction strongly alkaline, and a few pieces of granulated zinc. 
The flask is at once connected with the condenser, and the contents are 
distilled till all ammonia has passed over into the standard acid, and the con- 
centrated solution can no longer be safely boiled. This operation usually 
requires from twenty to thirty minutes. The distillate is then titrated with 
standard alkali. 

The use of mercury or its oxide in this operation greatly shortens the 
time necessary for digestion, which is rarely over an hour, and in the case 
of substances most difficult to oxidize, is more commonly less than an 
hour. Potassic sulphide removes all mercury from solution, and so prevents 
the formation of mercuro-ammonium compounds which are not completely 
decomposed by soda solution. The addition of zinc gives rise to an evolution 
of hydrogen, and prevents violent bumping. Previous to use the stock of 
reagents should always be tested by a blank experiment; in many cases 
if potassic sulphate is used there is no necessity for mercury, and therefore 
no sulphide is required. 

The following modifications must be used for the determination 
of nitrogen in substances which contain nitrates. 

Estimation of Nitrog-en, including- Nitrates, by the Kjeldahl- 
Grunning 1 - Jodlbauer Process. 

The requisite quantity of substance to be analyzed is put into the digesting 
flask together with 1 or 2 gm. of zinc dust. From 20 to 30 c.c. of 
sulphuric acid containing 2 gm. of salicylic acid are then quickly poured over 
the mixture so as to cover it at once. The whole is then gently heated till 
frothing is over, and the process finished with or without the potassic sulphate 
as before described. 

The following observations by Bernard Dyer are of consider- 
able importance in connexion with the modified process : 
" When nitrates are present in addition to organic or ammoniacal 

*Some operators prefer to close the distilling flask with, a caoutchouc stopper, 
through which in addition to the distilling tube, a funnel with tap is fixed for running 
in the alkali, this is to guard against possible loss of ammonia. 



86 VOLUMETRIC ANALYSIS. 19. 

nitrogen, Jodlbauer's modification (Cliem. Centr. iii., xvii., 433) 
suffices to determine accurately the total nitrogen. This process 
consists in previously adding to the sulphuric acid used for 
oxidation, either phenol or, preferably, salicylic acid generally 
about 2 grams for a determination. While the contents of the 
flask are still cold, from 1 to 2 grams of zinc dust are added (as 
well as a drop of mercury or some oxide) and allowed to dissolve 
before the flask is heated. The process is then continued exactly 
as previously described, when the whole of the nitrogen is 
obtained as ammonia. There is no difficulty whatever in 
determining the nitrogen in potassium or sodium nitrate in 
this manner ; but I find that when ammonia salts are present as 
well as potassium or sodium nitrate, the results are invariably too 
low, unless the sulphuric acid containing the salicylic acid is 
poured quickly into the flask out of a beaker, so that the nitrate 
shall be completely covered by the acid before the lapse of an 
appreciable interval of time ; this prevents the formation of the 
lower oxides of nitrogen, and consequent loss. When even 
ammonium nitrate is treated in this way, the whole of the nitrogen 
is retained in solution. I allude to this detail, because I have 
nowhere seen attention drawn to it, and because I think there is 
a probability of large errors occurring in the analysis of compound 
fertilisers, including mixtures of ammonia salts and alkali nitrates, 
if the acid is allowed to flow on to the sample from a pipette in 
the usual way." The experiments carried on by this chemist, and 
recorded in the paper already mentioned are extremely valuable. 
They show that the Kjeldahl process either with the modifications 
of Gunning and Arnold, or with that of the same and 
Jodlbauer is capable of accurately estimating the nitrogen in 
a very large variety of complex substances, and with the 
expenditure of very little time as compared with older 
methods. 

As respects the substances available for the accurate estimation 
of their nitrogen by the Kjeldahl method, Dyer finds that if 
zinc alone (without the use of phenol or salicylic acid) be used 
with aromatic nitro-compounds there is loss of nitrogen, as though 
it were necessary that more carbon should be present. 

The Kjeldahl-Gunning method fails to furnish the calculated 
quantity of nitrogen in azobenzene or amido-azobenzene. Mere 
reduction by zinc suffices with amido-azobenzene, but in the case of 
azobenzene the complete Jodlbauer modification is necessary. 
With amido-azotoluene the correct amount was obtained by the 
Kjeldahl-Gunning process supplemented by reduction with 
zinc and with carbazol by the Kjeldahl-Gunning method 
alone. 

Hydroxylamine hydrochloride, which contains 20-21 per cent, 
of nitrogen, yielded only 3 per cent, by the Kjeldahl-Gunning 
method; by reduction with zinc about 10 per cent, was obtained ; 



19. AMMONIA. 87 

by the Kjeldahl-Gunning-Jodlbauer method about 19 per 
cent, j by reduction with sugar and zinc less than 19 per cent. 
The Kjeldahl-Gunning-Jodlbauer method with the addition 
of sugar as well as zinc, however, gave the calculated quantity in 
each of three separate determinations. Acetaldoxime, by the 
K j eld a hl-Gu lining method, gave somewhat low results, but with 
the addition of sugar and zinc furnished correct results. 
jXaphthoquinone-oxinie yields its full percentage by the Kjeldahl- 
Guniiing method. 

Potassium cyanide and ethyl cyanide both give nearly correct 
results by the K j e 1 d a h 1 - G u n n i n g method ; no trace of 
hydrocyanic acid is evolved if the sulphuric acid used be strong. 
Potassium ferrocyanide also yields accurate results. Potassium 
ferricyanide, however, only gives sufficiently accurate results when 
reduced by the addition of sodium thiosulphate. Sodium 
nitroprusside failed with any modification of the method to yield 
all its nitrogen. 

Phenylhydrazine derivatives cannot by any modification of the 
method tried be made to give correct results ; there is invariably 
loss of nitrogen, presumably liberated in the free state. 

H. C. Sherman (Jour. Amer. Chem. Soc. xvii. 567) states 
that no known modification will give accurate results, where large 
proportions of both chlorides and nitrates exist in the substance 
digested. 

The Stock Method. A method based on the same principle as 
that of Kjeldahl has been devised by W. F. K. Stock (Analyst 
xvii. 109, idem xviii. 58), but the oxidation in this case depends 
not on the sulphuric acid but on manganic oxide. From 0'5 to 
1 '0 gm. of the substance is digested at a temperature below boiling, 
with 10 c.c. of strong sulphuric acid and 5 gm. of finely ground 
MnO 2 until the black carbonaceous matters are destroyed and 
a greenish liquid results ; this is distilled in a special apparatus, 
arranged by the author of the method much in the same way as 
in Kje Id a hl's process, with excess of soda and titrated in the 
same way. 

The results obtained by me with organic substances have almost 
invariably been low in comparison with the Kjeldahl method 
described above, and this is probably due to the same cause as that 
existing in the original Kjeldahl method where a lower 
temperature was used, and the oxidizing influence of permanganate 
was relied on for completing the decomposition. 

All modern authorities appear to agree in discarding the use of 
permanganate in the Kjeldahl method as not only useless but 
even harmful. 

It is only fair to say that very good results have been obtained 
in the case of certain nitrogen compounds by the Stock method, 
and further research may result in its being improved. 



88 VOLUMETRIC ANALYSIS. 20. 

ACIDIMETRY OB- THE TITRATION OF ACIDS. 

20. THIS operation is simply the reverse of all that has been 
said of alkalies, and depends upon the same principles as have 
been explained in alkalimetry. 

With free liquid acids, such as hydrochloric, sulphuric, or nitric, 
the strength is generally taken by means of the hydrometer or 
specific-gravity bottle, and the amount of real acid in the sample 
ascertained by reference to the tables constructed by Otto, 
Bine an, or lire. The specific gravity may very easily be taken 
with the pipette, as recommended with ammonia, and of course the 
real acid may be quickly estimated by normal caustic alkali and an 
appropriate indicator. 

In the case of titrating concentrated acids of any kind it is 
preferable in all cases to weigh accurately a small quantity, dilute 
to a definite volume, and take an aliquot portion for titration. 

Delicate End-reaction in Acidimetry. 

If an alkaline iodate or bromate be added to a solution of an 
alkaline iodide in the presence of a mineral acid, iodine is set free 
and remains dissolved in the excess of alkaline iodide, giving the 
solution the well-known colour of iodine. This reaction has been 
long observed, and is capable of being used with excellent effect as 
an indicator for the delicate titration of acids, and therefore of 
alkalies, by the residual method. Kjeldahl, for instance, uses it 
in his ammonia process, where the distillate contains necessarily an 
excess of standard acid. The reaction is definite in character, and 
may be used in various ways in volumetric processes. For instance, 
potassic bromate liberates iodine in exact proportion to its contained 
oxygen in the presence of excess of dilute mineral acid, and the 
iodine so liberated may be accurately titrated w r ith sodic thiosulphate. 
In acidimetry, however, the method is simply used for its exceeding 
delicacy as an end-reaction, one drop of T ^ sulphuric, nitric, or 
hydrochloric acid being quite sufficient to cause a deep blue colour 
in the presence of starch. 

The adjustment of the standard liquids is made as follows : 
2 or 3 c.c. of - acid are run into a flask, diluted somewhat with 
water, and a crystal or two of potassic iodide thrown in ; 1 or 2 c.c. 
of a 5 per cent, solution of potassic iodate are then added, which 
at once produces a brown colour, due to free iodine. A solution 
of sodic thiosulphate is added from a burette, with constant 
shaking, until the colour is nearly discharged ; a few drops of clear 
freshly prepared starch solution are now poured in, and the blue 
colour removed by the very cautious addition of thiosulphate.. 
The quantity of thiosulphate used represents the comparative 
strengths of it and the standard acid, and is used as the basis 
of calculation in other titrations. The first discharge of the blue- 
colour must be taken in all cases as the correct ending, because OH 



21. ACIDIMETRY. 89 

standing a few minutes the blue colour returns, due to some 
obscure reaction from the thiosulphate. This has been probably 
regarded as one of the drawbacks of the process, and another is the 
instability of the thiosulphate solution ; but these by no means 
invalidate its accuracy, and it moreover possesses the advantage of 
being applicable to excessively dilute solutions, and may be used 
by artificial light. The organic acids cannot be estimated by this 
method, the action not being regular. Neutral alkaline and 
alkaline earthy salts have no interference, but salts of the organic 
acids and borates must be absent. 



ACETIC ACID. 

C 2 H 4 2 = 60. 

21. IN consequence of the anomaly existing between the sp. gr. 
of strong acetic acid and its actual strength, the hydrometer is not 
reliable, but the volumetric estimation is now rendered extremely 
accurate by using phenolphthaleiii as indicator, acetates of the 
alkalies and alkaline earths having a perfectly neutral behaviour to 
this indicator. Even coloured vinegars may be titrated when 
highly diluted. Where, however, the colour is too much for this 
method to succeed Pettenkofer's plan is the best, and this opinion 
is endorsed by A. K. Leeds (Jour. Am. Chem. Soc. xvii. 741). 
The latter takes 50 c.c. of the vinegar and 50 c.c. of water with 
a drop of phenolphthalein, adds - baryta to slight excess which 
causes the organic colouring matters to separate either in the cold 
or on warming, the excess of baryta is then found by titration 
with -^ acid and turmeric paper. 

Several processes have at various times been suggested for the 
accurate and ready estimation of acetic acid, among which is that 
of Greville Williams, by means of a standard solution of lime 
syrup. The results obtained were very satisfactory. 

C. Mohr's process consists in adding to the acid a known 
excessive quantity of precipitated neutral and somewhat moist 
calcic carbonate. When the decomposition is as nearly as possible 
complete in the cold, the mixture must be heated to expel the CO 2 , 
and to complete the saturation ; the residual carbonate is then 
brought upon a filter, washed with boiling water, and titrated with 
excess of normal acid and back with alkali. 

In testing the impure brown pyroligneous acid of commerce, 
this method has given fairly accurate results.* 

The titration of acetic acid or vinegar may also be performed by 
the ammonio-cupric solution described in 15.10. 

*A. E. Leeds (loo. cit.) has not found this method to answer, which I think must 
be due to using dried calcic carbonate. I have only used it for commercial wood acid, 
and the figures obtained by me were the highest among several other methods ; but an 
error has been committed in not mentioning that the CaCQ3 should not be thoroughly 
dried, and the alkalinity of which is known. 



90 VOLUMETRIC ANALYSIS. 21. 

1. Free Mineral Acids in Vineg-ar. Hehlier has devised ail 
excellent method for this purpose (Analyst i. 105). 

Acetates of the alkalies are always present in commercial vinegar ; 
and when such vinegar is evaporated to dryness, and the ash ignited, 
the alkalies are converted into carbonates having a distinct alkaline 
reaction on litmus; if, however, the ash has a neutral or acid 
reaction, some free mineral acid must have been present. The 
alkalinity of the ash is diminished in exact proportion to the 
amount of mineral acid. added to the vinegar as an adulteration. 
Hence the following process : 

50 c.c. of the vinegar are mixed with 25 c.c. of -$ soda or potash, 
evaporated to dryness, and ignited at a low red heat to convert the acetate? 
into carbonates ; when cooled, 25 c.c. of ^r acid are added ; the mixture 
heated to expel CO', and filtered ; after washing the residue/the filtrate and 
washings are exactly titrated with ^ alkali ; the volume so used equals the 
amount of mineral acid present in the 50 c.c. of vinegar. 

1 c.c. / alkali=0'0049 gra. H 2 SO 4 or 0'003037 gm.liCl. 

If the vinegar contains more than 0'2 per cent, of mineral acid, 
more than 25 c.c. of ~ alkali must be used to the 50 c.c, vinegar 
before evaporating and igniting. 

2. Acetates of the Alkalies and Earths. These salts are 
converted by ignition into carbonates, and can be then residually 
titrated with normal acid ; no other organic acids must be present, 
nor must nitrates, or similar compounds decomposable by heat. 
1 c.c. normal acid^O'06 gm. acetic acid. 

3. Metallic Acetates. Neutral solutions of lead and iron acetates 
may be precipitated by an excess of normal sodic or potassic carbonate, the 
precipitate well boiled, filtered, and washed with hot water, the filtrate and 
washings made up to a definite volume, and an aliquot portion titrated with 
N or ^ acid ; the difference between the quantity so used and calculated for 
the original volume of alkali will represent the acetic acid. 

If such solutions contain free acetic or mineral acids, they must 
be exactly neutralized previous to treatment. 

If other salts than acetates are present, the process must be 
modified as follows : 

Precipitate with alkaline carbonate in excess, exactly neutralize with 
hydrochloric acid, evaporate the whole or part to dryuess, ignite to convert 
the acetates into carbonates, then titrate residually with normal acid. Any 
other organic acid than acetic will, of course, record itself in terms of acetic 
acid. 

4. Commercial Acetate of Lime. The methods just described 
are often valueless in the case of this substance, owing to tarry 
matters, which readily produce an excess of carbonates. 

Presenius (Z. a. c. xiii. 153) adopts the following process for tolerabl} r 
pure samples : 5 gm. are weighed and transferred to a 250 c.c. flask, 
dissolved in about 150 c.c. of water, and 70 c.c. of normal oxalic acid added ; 
the flask is then well shaken, and filled to the mark, 2 c.c. of water are added 



21. ACETIC ACID. 91 

to allow for the volume occupied by the precipitate, the whole is again well 
shaken, and left to settle. The solution is then filtered through a dry filter 
into a dry flask : the volume so filtered must exceed 200 c.c. 

100 c.c. are first titrated with normal alkali and litmus ; or, if highly 
coloured, by help of litmus or turmeric paper ; the volume used multiplied 
by 2' 5 will give the volume for 5 gm. 

Another 100 c.c. are precipitated with solution of pure calcic acetate in 
slight excess, warmed gently, the precipitate allowed to settle somewhat, 
then filtered, well washed, dried, and strongly ignited, in order to convert 
the oxalate into calcic carbonate or oxide, or a mixture of both. The 
residue so obtained is then decomposed with excess of normal acid, and 
titrated residually with normal alkali. By deducting the volume of acid 
used to neutralize the precipitate from that of the alkali used in the first 
100 c.c., and multiplying by 2'5, is obtained the volume of alkali expressing 
the weight of acetic acid in the 5 gm. of acetate. 

In the case of very impure and highly coloured samples of 
acetate, it is only possible to estimate the acetic acid by repeated 
distillations with phosphoric acid and water to incipient dryness, 
and then titrating the acid direct with ~ alkali, each c.c. of which 
represents 0'006 gm. acetic acid. 

The distillation is best arranged as suggested by Still well and 
Gladding, or later by Harcourt Phillips (C. N. liii. 181). 

A 100 to 120 c.c. retort, the tubulure of which carries a small funnel 
fitted in with a caoutchouc stopper, and the neck of the funnel stopped 
tightly with a glass rod shod with elastic tube, is supported upon a stand in 
such a way that its neck inclines upwards at about forty-five degrees : the 
end of the neck is drawn out, and bent so as to fit into the condenser by 
help of an elastic tube. The greater part of the retort neck is coated with 
flannel, so as to prevent too much condensation. 

1 gm. of the sample being placed in the retort, 10 c.c. of a 40 per cent, 
solution of P 2 O 5 are added, together with as much water as will make about 
50 c.c. A small naked flame is used, and if carefully manipulated, the 
distillation may be carried on to near dryness without endangering the 
retort. After the first operation the retort is allowed to cool somewhat, then 
50 c.c. of hot water added through the funnel, another distillation made as 
before, and the same repeated a third time, Avhich will suffice to carry 
over all the acetic acid. The distillate is then titrated with alkali and 
phenolphthalein. 

By this arrangement the frothing and spirting is of no con- 
sequence, and the whole process can be completed in less than 
an hour. The results are excellent for technical purposes. 

Weber (Z, a. C. xxiv. 614) has devised a ready and fairly 
accurate method of estimating the real acetic acid in samples of 
acetate of lime, based on the fact that acetate of silver is insoluble 
in alcohol. 

Process : 10 gm. of the sample in powder are placed in a 250 c.c. flask, 
a little water added, and heated till all soluble matters are extracted, cooled, 
and made up to the measure : 25 c.c. are then filtered through a dry filter, 
put into a beaker, 50 c.c. of absolute alcohol added, and the acetic acid at 
once precipitated with an alcoholic solution of silver nitrate. The silver 
acetate, together with any chloride, sulphate, etc., separates free from 
colour. The precipitate is brought on a filter, well washed with 60 per cent, 
alcohol till the free silver is removed ; precipitate is then dissolved in weak 



92 VOLUMETRIC ANALYSIS. 22. 

nitric acid, and titrated with ^ salt solution. Each c.c. represents O'OOG gm. 
acetic acid. 

Several trials made in comparison with the distillation method 
with phosphoric acid gave practically the same results. 

A good technical process has been devised by Grim aha w 
(Allen's Organic Analysis i. 397). 10 gm. of the sample are treated 
with water and an excess of sodic bisulphate (ISfaHSO 4 ), the 
mixture diluted to a definite volume, filtered, and a measured 
portion of the filtrate titrated with standard alkali ; a similar 
portipn meanwhile is evaporated to dryness with repeated 
moistening with water, to drive off all free acetic acid. The 
residue is dissolved and titrated with standard alkali, when the 
difference between the volume now required and that used in the 
original solution will correspond to the acetic acid in the sample. 
Litmus paper is the proper indicator. 

BORIC ACID AND EQUATES. 

Boric anhydride B 2 3 =70. 

22. THE soda in borax may, according to Thomson, be 
very accurately estimated by titrating the salt with standard H 2 S0 4 
and methyl orange or lacmoid paper. Litmus and phenacetolin 
give very doubtful end-reactions : phenolphthaleiii is utterly useless. 

Example : T683 gm. sodic pyroborate in 50 c.c. of water required in one 
case 16'7 c.c. normal acid, and in a second 16'65 c.c. The mean of the two 
represents 0'517 gm. Na 2 O. Theory requires 0'516 gm. 

The estimation of boric acid as such has up to the present time 
presented great difficulties, and no volumetric method of any 
value has been available. 

R. T. Thomson has now removed this difficulty by finding 
a method easy of execution, and of considerable accuracy 
(J. S.C.I, xii. 432), see also page 44 in this book. 

Process : To determine boric acid in articles of commerce it is 
necessary to use methyl orange, to which indicator boric acid is perfectly 
neutral. In the case of boric acid in borax 1 gm. is dissolved in water, 
metlryl orange added, and then dilute sulphuric acid till the pink colour just 
appears. Boil for a short time to expel carbonic acid, cool, and add normal 
or fifth-normal soda till the pink colour of the methyl orange (a little more 
of which should be added if necessary) just assumes a pure yellow tinge. 
At this stage all the boric acid will exist in the free state. Add glycerine in 
such proportion that the total solution after titration will contain 30 per 
cent, at least, then add a little phenolphthaleiii, and lastly normal or fifth- 
normal soda from a burette until a permanent pink colour is produced. 
More glycerine may be added during the estimation if it is found necessary. 
The proportion of boric acid present is calculated from the number of c.c. of 
soda consumed. 

1 c.c. normal NaOH=0'0620 gm. H 3 BO 3 

1 c.c. =0-0505 gm. Na 2 B 4 7 

1 c.c. =0-0955 gm. Na 2 B 4 O 7 +10H-O 



23. CARBONIC ACID. 93 

In the case of boric acid of commerce, which generally contains salts of 
ammonium, 1 gin. may be dissolved in hot water, a slight excess of sodic 
carbonate added, and the solution boiled down to about half its bulk to expel 
ammonia. Any precipitate which appears may then be filtered off, and the 
filtrate titrated as already described. 

The method may also be applied to boracite or borate of lime by dissolving 
1 gm. of either of these minerals in dilute hydrochloric acid with the aid of 
heat, nearly neutralizing with caustic soda, boiling to expel carbonic 
acid, cooling, exactly neutralizing to rnetlryl orange, and continuing the 
determination as in borax. If much iron is present, however, it may be 
removed by a preliminary treatment with sodic carbonate, and removal of 
oxide of iron as well as the carbonates of calcium and magnesium ~by 
nitration. 

Thomson has also attempted to apply the process to the 
estimation of boric . acid in milk and other foodstuffs. This of 
course necessitates the removal of phosphoric acid from the ash of 
the milk, for which purpose a barium salt was found to be 
a successful precipitant, and if the solution be sufficiently dilute 
will leave the boric acid in solution. The experiments have not 
as yet been completely successful. 



CARBONIC ACID AND CARBONATES. 

23. ALL carbonates are decomposed by strong acids ; the 
carbonic acid which is liberated splits up into water and carbonic 
anhydride (CO 2 ), which latter escapes in the gaseous form. 

It will be readily seen from what has been said previously as 
to the estimation of the alkaline earths, that carbonic acid in 
combination can be estimated volumetrically with a very high 
degree of accuracy (see 18). 

The carbonic acid to be estimated may be brought into 
combination with either calcium or barium, these bases admitting 
of the firmest combination as neutral carbonates. 

If the carbonic acid exist in a soluble form as an alkaline mono- 
carbonate, the decomposition is effected by the addition of baric or 
calcic chloride as before directed ; if as bicarbonate, or a compound 
between the two, ammonia must be added with either of the 
chlorides. 

As solution of ammonia frequently contains carbonic acid, this 
must be removed by the aid of baric or calcic chloride, previous 
to use. 

1. Carbonates Soluble in Water. 

It is necessary to remember, that when calcic chloride is used as 
the precipitant in the cold, amorphous calcic carbonate is first 
formed ; and as this compound is sensibly soluble in water, it is 
necessary to convert it into the crystalline form. In the absence of 
free ammonia this can be accomplished by boiling. When ammonia 
is present, the same end is obtained by allowing the mixture to 



94 VOLUMETRIC ANALYSIS. 23. 

stand for eight or ten hours in the cold, or by heating for an hour 
or two to 70 80 C. "With barium the precipitation is regular. 

Another fact is, that when ammonia is present, and the precipi- 
tation occurs at ordinary temperatures, ammonic carbamate is 
formed, and the baric or calcic carbonate is only partially precipi- 
tated. This is overcome by heating the mixture to near boiling for 
a couple of hours, and is best done by passing the neck of the 
flask through a retort ring, and immersing the flask in boiling 
water. 

When caustic alkali is present in the substance to be examined, 
it is advisable to use barium as the precipitant ; otherwise, for all 
volumetric estimations of CO 2 calcium is to be preferred, because 
the precipitate is much more quickly and perfectly washed than 
the barium compound. 

Example : 1 gra. of pure anhydrous sodic carbonate was dissolved in 
water, precipitated while hot with baric chloride, the precipitate allowed to 
settle well, the clear liquid decanted through a moist filter, more hot water 
containing a few drops of ammonia poured over the precipitate, which was 
repeatedly done so that the bulk of the precipitate remained in the flask, 
being washed by decantation through the filter ; when the washings showed 
no trace of chlorine, the filter was transferred to the flask containing the 
bulk of the precipitate, and 20 c.c. of normal nitric acid added, then titrated 
with normal alkali, of which 1/2 c.c. was required=18'8 c.c. of acid ; this 
multiplied by 0'022, the coefficient for carbonic acid, gave O4136 gin. CO-= 
41'36 per cent., or multiplied by 053, the coefficient for sodic carbonate, 
gave 0*9964 gm. instead of 1 gm. 

2. Carbonates Soluble in Acids. 

It sometimes occurs that substances have to be examined for 
carbonic acid, which do not admit of being treated as above 
described ; such, for instance, as the carbonates of the metallic 
oxides (white lead, calamine, etc.), carbonates of magnesia, iron, 
and copper, the estimation of carbonic acid in cements, mortar, and 
many other substances. In these cases the carbonic acid must be 
evolved from the combination by means of a stronger acid, and 
conducted into an absorption apparatus containing ammonia, then 
precipitated with calcic chloride, and titrated as before described. 

The following form of apparatus (fig. 33) affords satisfactory 
results. 

The weighed substance from which the carbonic acid is to be evolved is 
placed in b with a little water; the tube d contains strong hydrochloric 
acid, and c broken glass wetted with ammonia free from carbonic acid. 
The flask is about one-eighth filled with the same ammonia : the bent tube 
must not enter the liquid. When all is ready and the corks tight, warm the 
flask a gently so as to fill it with vapour of ammonia, then open the clip and 
allow the acid to flow circumspectly upon the material, which may be heated 
until all carbonic acid is apparently driven off ; then by boiling and shaking 
the last traces can be evolved, and the operation ended. When cool, the 
apparatus may be opened, the end of the bent tube washed into a, and also 
a good quantity of boiled distilled water passed through c, so as to carry 



23. 



CARBONIC ACID. 



95> 



down any ammonic carbonate that may have formed. Then add solution of 
calcic chloride, boil, filter, and titrate the precipitate as before described. 

-During- the filtration, and while ammonia is present, there is a great 
avidity for carbonic acid, therefore boiling water should be used for washin- 
and the funnel kept covered with a small glass plate. 

In many instances CO 2 may be estimated by its equivalent in 
chlorine with -- silver and potassic chromate, as in 39. 




Fig. 33. 
3. Carbonic Acid. G-as in 



etc. 



Waters, 

The carbonic acid existing in waters as neutral carbonates of the 
alkalies or alkaline earths may very elegantly and readily be titrated 
directly by ^ acid (see- 18). 

"Well or spring water, and also mineral waters, containing free 
carbonic acid gas, can be examined by collecting measured quantities 
of them at their source, in bottles containing a mixture of calcic 
and ammonic chloride, afterwards heating the mixture in boiling 
water for one or two hours, and titrating the precipitate as before 
described. 

Pettenkofer's method with caustic baryta or lime is decidedly 
preferable to any other. Lime water may be used instead of' 



96 VOLUMETRIC ANALYSIS. 23. 

baryta with equally good results, but care must be taken that the 
precipitate is crystalline. 

The principle of the method is that of removing all the carbonic 
acid from a solution, or from a water, by excess of baryta or lime 
water of a known strength ; and, after absorption, finding the 
excess of baryta or lime by titration with -^ acid and turmeric 
paper. 

The following is the best method to be pursued for ordinary 
drinking waters not containing large quantities of carbonic acid : 

100 c.c. of the water are put into a flask with 3 c.c. of strong solution of 
calcic or baric chloride, and 2 c.c. of saturated solution of ammonic chloride ; 
45 c.c. of baryta or lime water, the strength of which is previously ascertained 
by means of decinormal acid, are then added, the flask well corked and put 
aside to settle ; when the precipitate is f ully subsided, take out 50 c.c. of the 
clear liquid with a pipette, and let this be titrated with decinormal acid. 
The quantity required must be multiplied by 3 for the total baryta or lime 
solution, there being 50 c.c. only taken ; the number of c.c. so found must be 
deducted from the original quantity required for the baryta solution added ; 
the remainder multiplied by 0'0022 will give the weight of carbonic acid 
existing free and as bicarbonate in the 100 c.c. 

The addition of the baric or calcic chloride and ammonic chloride is made 
to prevent any irregularity which might arise from alkaline carbonates or 
sulphates, or from magnesia. 

If it be desirable to ascertain the volume of carbonic acid from 
the weight, 1000 c.c. of gas, at and 0*76 m.m., weigh 
1 '96663 gm. 100 cubic inches weigh 47'26 grains. 

4. Carbonic Acid in Aerated Beverages, etc. 

For ascertaining the quantity of carbonic acid in bottled aerated 
waters, such as soda, seltzer, potass, and others, the following 
.apparatus is useful. 

Fig. 34 is a brass tube made like a cork -borer, about five inches long, having 
four small holes, two on each side, and about two inches from its cutting end ; 
the upper end is securely connected with the bent tube from the absorption 
flask (tig. 35) by means of a vulcanized tube ; the flask contains a tolerable 
quantity of pure ammonia, into which the delivery tube dips ; the tube 
a contains broken glass moistened with ammonia. 

Everything being ready the brass tube is greased, and the bottle being 
-held in the right hand, the tube is screwed a little aslant through the cork 
by turning the bottle round, until the holes appear below the cork and the 
gas escapes into the flask. When all visible action has ceased, after the 
bottle has been well shaken two or three times to evolve all the gas that can 
i be possibly eliminated, the vessels are quietly disconnected, the tube a washed 
out into the flask, and the contents of the bottle added also ; the whole is 
then precipitated with calcic chloride and boiled, and the precipitate titrated 
as usual. This gives the total carbonic acid free and combined. 

To find the quantity of the latter, another bottle of the same manufacture 
must be evaporated to dryness, and the residue gently ignited, then titrated 
with normal acid and alkali ; the amount of carbonic acid in the mono- 
-carbonate deducted from the total, will give the weight of free gas originally 
present. 







CARBONIC ACID. 



97 



The volume may be found as follows : 1000 c.c. of carbonic acid at 0, 
and 76 m.m., weigh T96663 gm. Suppose, therefore, that the total weight 
of carbonic acid found in a bottle of ordinary soda water was 2'8 gm., and 
the weight combined with alkali 0'42 gm., this leaves 2'38 gm. CO 2 in 
a free state 



1-96663 : 2'38 



1000 : x = 1210 



If the number of c.c. of carbonic acid found is divided by the 
number of c.c. of soda water contained in the bottle examined, the 
quotient will be the volume of gas compared with that of the soda 
water. The volume of the contents of the bottle is ascertained by 
marking the height of the fluid previous to making the experiment ; 
the bottle is afterwards filled to the same mark with water, emptied 
into a graduated cylinder and measured ; say the volume was 
292 c.c., therefore 

4-14 vols. CO 2 . 



rig. si. 




5. Carbonic Acid in Air. 

A dry glass globe or bottle capable of being securely closed by 
a rubber stopper, and holding 4 to 6 liters, is filled with the air 
to be tested by means of a bellows aspirator ; baryta water is then 
introduced in. convenient quantity and of known strength as 
compared with T ~ acid.'" The vessel is securely closed, and the 
liquid allowed to flow round the sides at intervals during half an 
hour or more. When absorption is judged to be complete, the 

* Clowes and C o 1 e m a n prefer to use saturated lime water in place of baryta, and 
have obtained good results : see their Quantitative Analysis, 2nd. edit. p. 416. 

II 



98 VOLUMETRIC ANALYSIS. 23. 

baryta is emptied out quickly into a stoppered bottle, and the 
excess of baryta at once ascertained in a measured portion by T ^y- 
hydrochloric acid and turmeric paper as described in 15.9. The 
final calculation is of course made on the total baryta originally 
used, and upon the exact measurement of the air-collecting vessel. 
It is above all things necessary to prevent the absorption of CO 2 
from extraneous sources during the experiment. The error may be 
reduced to a minimum by carrying on the titration in the vessel 
itself, which is done by fixing an accurately graduated pipette 
through the cork or caoutchouc stopper of the air vessel, to the 
upper end of which is attached a stout piece of elastic tube, closed 
with a pinch-cock ; and this being filled to the mark with dilute 
standard acid acts as a burette. The baryta solution tinted with 
phenolphthaleiii is placed in the air bottle which must be of 
colourless glass, and after absorption of all CO 2 , the excess of 
baryta is found by running in the acid until the colour disappears. 
The cork or stopper must have a second opening to act as 
ventilator ; a small piece of glass tube does very well. 

If a freshly made solution of oxalic acid is used containing 
0*2863 gm. per liter, each c.c. represents 1 mgm. CO 2 . The liquid 
holds its strength correctly for a day, and can be made as required 
from a strong solution, say 28*636 gm. per liter. 

Another method of calculation is, to convert the volume of 
baryta solution decomposed into its equivalent volume in ~ acid, 
1 c.c. of which = 0*0022 gm. CO 2 or by measurement at C. and 
760 m.m. pressure represents 1*119 c.c. The method above 
described is a combination of those of Pettenkofer and Dal ton, 
and though much used, is liable to considerable error from various 
causes. 

A. H. Gill in a report from the Sanitary and Gas Analysis 
Laboratory of the Technical Institute at Boston, U.S.A. 
(Analyst xvii. 184), gives a somewhat modified arrangement of the 
Pettenkofer method. Ordinary green glass bottles of one or 
two gallon capacity are measured with water, and the contents in 
c.c. ascertained preferably by weighing on a good balance. 

The bottles are dried before being used, this may easily be done 
by rinsing first with alcohol or methylated spirit, draining, then 
rinsing with ether and after again draining, the bottle is quickly 
dried by blowing air through it witli the ordinary laboratory bellows. 
If this plan is not used they must be dried after draining well, in 
a warm closet. A special form of bellows for filling the bottle 
with air is used by Gill, but the usual aspirator made on the 
accordion pattern suffices, or a small fan blower, the driving parts 
of which are connected by rubber bands to render it noiseless, may 
be used. 

The bottle is fitted with a rubber stopper carrying a glass tube, 
closed by a plug of solid rubber. 

The air to be tested is drawn into the bottle by repeated use of 



23. CAKBONIC ACID. 99 

the aspirator so as to collect a representative sample, and if the 
test is made in a room everything should be quiet, and care must 
be taken to avoid draughts or the proximity of a number of 
persons. 

Process : 50 c.c. of the standard barium hydrate are run into the 
bottle rapidly from a burette (the tip passing entirely through the tube in 
the stopper), the nipple replaced, and the solution spread completely over the 
sides of the bottle while waiting three minutes for the draining of the 
burette, before reading, unless it be graduated to deliver 50 c.c. The bottle 
is now placed upon its side, and shaken at intervals for forty to sixty 
minutes, taking care that the whole surface of the bottle is moistened with 
the solution each time. The absorption of the last traces of carbon dioxide 
is very slow indeed, half an hour in many cases being insufficient. 

At the time at which the barium solution is added, the temperature and 
pressure should be noted. At the end of the above period, shake well to 
insure homogeneity of the solution, remove the cap from the tube, and 
invert the large bottle quickly over a 60 or 70 c.c. glass stoppered bottle, so 
that the solution shall come in contact with the air as little as possible. With- 
out waiting for the bottle to drain, withdraw a portion of 15 or 25 c.c. with 
a narrow-stemmed spherical-bulbed pipette and titrate with sulphuric acid* 
(1 c.c.=l mgm. CO 2 ), using rosolic acid as an indicator. The difference 
between the number of c.c. of standard acid required to neutralize the 
amount of barium hydrate (e.g., 50 c.c.) before and after absorption, gives 
the number of milligrams of carbon dioxide present in the bottle. 

This is expressed in cubic centimeters under standard conditions, and 
divided by the capacity of the bottle under standard conditions, and the 
results reported in parts per 10,000. To reduce the air in the bottle to standard 
conditions, a hygrometric measurement of the air in the room from which 
the sample was taken, is necessary. This in ordinary cases is usually 
omitted, as the object of the investigation is comparative results, as regards 
the efficiency of ventilation, and the rooms in the same building Avould not 
vary appreciably in the amount of moisture in the atmosphere. This 
correction may make a difference of about 0'15 parts per 10,000. 

Another method on the same principle is to attacli a bulb 
apparatus, containing a measured quantity of baryta or lime 
water, to an aspirator bottle filled with water; the tap of the 
"bottle is opened to such an extent as to allow the air to bubble 
through the test solution at a moderate rate. The process of 
titration is the same as above. This method takes longer time, 
and the volume of air, which should not be less than five or six 
liters, is ascertained by measuring the volume of water allowed to 
run out of the aspirator, and the rate of flow being regulated so 
that from two to three hours is required to pass the above volume 
of air. If a flask, fitted with tubes, is used in place of, the bulb 
apparatus, the titration may be done without transferring the test 
solution. 

* Sulphuric acid, in distinction to oxalic acid, enables one to estimate the excess of 
"barium hydrate in presence of the suspended barium carbonate, and also of caustic 
alkali, which is a frequent impurity of commercial barium hydrate. Professor 
Johnson, in the American edition of Fresenius' Quantitative Analysis, calls 
attention to the fact that the normal alkaline oxalates decompose the alkaline earthy 
carbonates, so that the reaction continues alkaline if the least trace of soda or potash 
be present. The sulphuric acid may be prepared by diluting 46*51 c.c. normal sulphuric 
acid to a liter. 

H 2 



100 VOLUMETRIC ANALYSIS. 23, 

6. Sckeibler's Apparatus for the estimation of Carbonic Acid 

by "Volume. 

This apparatus is adapted for the estimation of the CO 2 contained 
in native carbonates, as well as in artificial products, and has been 
specially contrived for the purpose of readily estimating the CO 2 
in the bone-black used in sugar refining. The principle upon, 
which the apparatus is founded is simply this : That the quantity 
of CO 2 contained in calcic carbonate can be used as a measure 
of the quantity of that salt itself ; and instead of determining, as 
has usually been the case, the quantity of gas by weight, this 
apparatus admits of its. estimation by volume ; and it is by this 
means possible to perform, in a few minutes, operations which 
would otherwise take hours to accomplish, while, moreover, the 
operator need scarcely possess any knowledge of chemistry. The 
results obtained by this apparatus are correct enough for technical 
purposes. 

The apparatus is shown in fig. 36, and consists of the following 
parts : The glass vessel, A, serves for the decomposition of the 
material to be tested for CO 2 , which for that purpose is treated 
with dilute HC1 ; this acid is contained, previous to the experiment,. 
in the gutta-percha vessel s. The glass stopper of A is perforated, 
and through it firmly passes a glass tube, to which is fastened the 
india-rubber tube r, by means of which communication is opened 
with B, a bottle having three openings in its neck. The central 
opening of this bottle contains a glass tube (r) firmly fixed, which 
is in communication, on the one hand, with A, by means of the 
flexible india-rubber tube already alluded to, and, on the other 
hand, inside of B, with a very thin india-rubber bladder, K. 
The neck (</) of the vessel B is shut off during the experiment by 
means of a piece of india-rubber tubing, kept firmly closed with 
a spring clamp. The only use of this opening of the bottle B,, 
arranged as described, is to give access of atmospheric air to the 
interior of the bottle, if required. The other opening is in 
communication with the measuring apparatus C, a very accurate 
cylindrical glass tube of 150 c.c. capacity, divided into 0*5 c.c. ; 
the lower portion of this tube C is in communication with the 
tube D, serving the purpose of controlling the pressure of the gas. 
The lower part of this tube D ends in a glass tube of smaller 
diameter, to which is fastened the india-rubber tube p, leading 
to E, but the communication between these parts of the apparatus 
is closed, as seen at p, by means of a spring clamp. E is a water 
reservoir, and on removal of the clamp at p, the water contained 
in C and D runs off towards E ; when it is desired to force the 
water contained in E into C and I), this can be readily done 
by blowing with the mouth into V, and opening the clamp 
at p. 

The main portion of the apparatus above described, with the 
exception, however, of the vessel A, is fixed by means of brass 



23. 



CARBONIC ACID. 



101 



fittings to a wooden board ; a thermometer is also attached. The 
filling of the apparatus with water is very readily effected by 
pouring it through a suitable funnel placed in the open end of the 
tube D, care being taken to remove, or at least to unfasten, the 
.spring clamp at p ; in this manner the water runs into E, which 




should be almost entirely filled. Distilled water is preferable for 
this purpose, especially as the filling only requires to be done once, 
because the water always remains in E as long as the apparatus is 
intended to be kept ready for use. When it is required to fill the 
tubes C and I) with water, so as to reach the zero of the scale 




102 VOLUMETRIC ANALYSIS. 2S. 

of the instrument, it is best to remove the glass stopper from A. 
The spring clamp at p is next unfastened, and air is then blown by 
means of the mouth into the tube V, which communicates with E ; 
by this operation the water rises up into the tubes C and D, 
which thus become filled with that liquid to the same height. 
Care should be taken not to force the water up above the zero* 
of the scale at C, and especial care should be taken against forcing 
so much of the fluid up that it would run over into the tube ?/,. 
and thence find its way to B, whereby a total disconnection of 
all the parts of the apparatus would become necessary. If by any 
accident the water should have been forced up above the zero at C,, 
before the operator had closed the spring clamp at p, this is easily 
remedied by gently opening that clamp, whereby room is given for 
the water to run off to E in such quantity as may be required to- 
adjust the level of that fluid in C precisely with the zero of the scale. 
The filling of the tube C with water has the effect of forcing the 
air previously contained in that tube into E, where it causes the 
compression of the very thin india-rubber ball placed within B. 
If it should happen that this india-rubber ball has not become 
sufficiently compressed and flattened, it is necessary to unfasten the 
spring clamp at q, and to cautiously blow air into B, through 
the tube q, by which operation the complete exhaustion of the- 
india-rubber bladder placed within B is readily performed. This- 
operation is also required only once, because during the subsequent 
experiments the india-rubber bladder K is emptied spontaneously. 
It may happen, however, that while the filling of the tubes I) and C 
with water is being proceeded with, the india-rubber bladder K 
has become fully exhausted of air before the water in C reaches 
the zero of the scale. In that case the level of the water in the 
tubes D and C will not be the same, but will be higher in D : 
it is evident, however, that this slight defect can be at once 
remedied by momentarily unfastening the spring clamp at q. 

The apparatus should be placed so as to be out of reach of direct 
sunlight, and should also be protected against the heat of the 
operator's body by intervention of a glass screen, and is best placed 
near a north window, so as to afford sufficient light for reading off 
the height of the water in the tubes. 

In testing carbonates the method is as follows : 

Put the very finely powdered portion of carbonate into the perfectly dry 
decomposing glass A, fill the gutta-percha tube with 10 c.c. hydrochloric 
acid of 1'12 sp. gr., place the tube cautiously in the decomposing glass, and 
then close the bottle with the well-tallowed stopper. Here the water will 
sink a little in C and rise in D ; open q for a moment, and the equilibrium 
Avill be restored. Now note the thermometer and barometer, grasp the bottle 
with the right hand round the neck to avoid warming, raise it, incline it 
slightly so that the hydrochloric acid may mix with the substance gradually, 
and at the same time with the left hand regulate p, so that the water in the 
two tubes may be kept at exactly the same height ; continue these operations 
without intermission, till the level of the water in C does not change for 
a few seconds. Now bring the columns in C and D to exactly the same height, 



24. CITRIC ACID. 103 

read off the height of the water, and note whether the temperature has 
remained constant. If it has, the number of c.c. read off indicates the 
liberated CO' 2 , but as a small quantity has been dissolved by the hydrochloric 
acid, it is necessary to make a correction. Scheibler has determined the 
small amount of carbonic acid which remains dissolved in the 10 c.c. 
hydrochloric acid at the mean temperature, and he directs to add 0'8 c.c. to 
the volume of the carbonic acid read off. Warington (C. N. xxxi. 253) 
states that this is not a constant quantity, but is dependent upon the volume 
of gas evolved, and this ratio he fixes at 7 per cent, of the gas measured. 
Lastly, the volume being reduced to 0, 760 m.m., and the dry condition, the 
weight is found. 

Under no circumstances can the method be considered actually accurate, 
but for technical purposes it is convenient, as the operation is performed in 
a very short time, and is specially suitable for comparative examinations 
of various specimens of the same material. 

If it is desired to dispense with all corrections, each set of 
experiments may be begun by establishing the relation between 
the CO 2 obtained in the process (i.e. the CO 2 actually yielded 
+ 0*8 c.c.) and pure calcic carbonate. This relation is, of course, 
dependent on the temperature and pressure prevailing on the 
particular day. For example, from 0'2737 gm. calcic carbonate 
containing 0*1204 gm. CO 2 , 63*8 c.c. were obtained, including 
the 0*8 c.c. ; and in an analysis of dolomite under the same 
circumstances from 0'2371 gm. substance, 57*3 c.c. were obtained, 
including the 0*8 c.c. 

Therefore 63*8 : 57*3 : : 0*1204 : x, or = 0*1082, consequently 
the dolomite contains 45*62 per cent, of CO 2 . 

For the special procedure in testing bone-black, used in sugar 
refining, the reader is referred to the printed instructions supplied 
with the apparatus.* 

"Wigner (Analyst i. 158) has obtained exceedingly good results 
in the analysis of lead carbonates, etc., with Me Leod's gas 
apparatus. The nitrometer has also been turned to good account 
for the same purpose. 

CITRIC ACID. 

C 6 7 H 8 xH 2 = 210. 

24. THIS acid in the free state may readily be titrated 
with pure normal soda and phenolphthalein. 1 c.c. normal alkali 
= 0*07.gm. crystallized citric acid. 

1. Citrates of the Alkalies and Earths. These citrates may be 
treated with neutral solution of lead nitrate or acetate, in the absence of 
other acids precipitable by lead. The lead citrate is washed with a mixture 
of equal parts alcohol and water, the precipitate suspended in water, and 
H 2 S passed into it till all the lead is converted into sulphide; the clear 
liquid is then boiled to remove IPS, and titrated with normal alkali. 

* It is perhaps almost needless to say that the modern apparatus designed by 
Hemp el, Lunge, and others, for technical gas analysis, practically supersedes that 
of Scheibler. The methods are all, however, open to the objection that an uncertain 
portion of CO- is lost by aqueous absorption. 



104 VOLUMETRIC ANALYSIS. 25. 

2. Fruit Juices, etc. If tartaric is present, together with free 
citric acid, the former is first separated as potassic bitartrate, 
which can very well be done in the presence of citric acid, as 
follows : 

A cold saturated proof spirit solution of potassic acetate is added to a 
somewhat strong solution of the mixed acids in proof spirit, in sufficient 
quantity to separate all the tartaric acid as bitartrate, which after stirring 
well is allowed to stand some hours ; the precipitate is then transferred to a 
filter, and first washed with proof spirit, then rinsed off the filter with a cold 
saturated solution of potassic bitartrate, and allowed to stand some hours, 
with occasional stirring; this treatment removes any adhering citrate. The 
bitartrate is again brought on to a filter, washed once with proof spirit, then 
dissolved in hot water, and titrated with normal alkali, 1 c.c. of which 
0'15 gni. tartaric acid. 

The first filtrate may be titrated for the free citric acid present after 
evaporating the bulk of the alcohol. 

3. Lime and Lemon Juices. The citric acid contained in lemon, 
lime, and similar juices, may be very fairly estimated by 
Warington's method (J. C. S. 1875, 934). 

15 or 20 c.c. of ordinary juice, or 34 c.c. of concentrated juice, are first 
exactly neutralized with pure normal soda, made up, if necessary, to about 
50 c.c., heated to boiling in a salt bath, and so much solution of calcic 
chloride added as to be slightly in excess of the organic acids present. The 
mixture is kept at the boiling point for about half-an-hour, the precipitate 
collected on a filter and washed with hot water, filtrate and washings concen- 
trated to about 15 c.c., and a drop of ammonia added ; this will produce a 
further precipitate, which is collected separately on a very small filter by 
help of the previous filtrate, then washed with a small quantity of hot water. 
Both filters, with their precipitates, are then dried, ignited at a low red heat, 
and the ash titrated with normal_or ^ acid, each c.c. of which represents 
respectively O'OY or O'OOT gm H 3 Ci + H 2 O. 

FORMIC ACID. 

HCOOH = 4G. 

25. H. C. JONES (Amer. Cliem, Jour. xvii. 539 541) has 
worked out a method which though not acidimetric may be 
quoted here. It is based on a' process originally devised by 
Peau de Saint-Gilles, by titration with potassic permanganate 
in the presence of an alkaline carbonate. Lieben confirmed 
this, using a more elaborate process. The method is on the same 
principle, but the procedure differs from that of Lieben. 

Process : The solution containing the formic acid is made alkaline with 
Na 2 CO 3 , warmed and an excess of standard permanganate added. All the 
formic acid is thus oxidized, and a precipitate of manganese hydroxide 
thrown down. The solution is acidified with H-'SO 4 , and a measured volume 
of oxalic acid run in until all the precipitate has dissolved and the 
permanganate disappeared. The excess of oxalic acid is then titrated with 
standard permanganate. A volume of oxalic acid equal to that taken is also 
titrated with the permanganate solution, and the difference between the result 
<md the total permanganate used gives the quantity of permanganate required 
fco oxidize the formic acid. The experimental results agree well among 
themselves and with those obtained by other methods. 



26. HYDROFLUOKIC ACID. 105 

The author further shows that Saint- Gilles' statement that 
oxalic acid can be titrated in acid solution in the presence of 
formic acid is unreliable, since formic acid is also oxidized to some 
extent by the permanganate under these conditions. 

F. Freyer (Chem, Zeit. xix. 1184), having occasion to determine 
the formate in a mixture of calcium acetate and formate, has 
devised the following method. 

Process : The mixed calcium salts are distilled with dilute sulphuric acid 
in a current of steam until the distillate is no longer acid ; an aliquot portion 
of the distillate is titrated with alkali to determine the total acid, wiiilst 
another portion is evaporated, if necessary, with excess of caustic soda to 
concentrate it, and is treated as follows : 10 to 20 c.c , containing about 
0'5 gm. of formic acid, are heated for half an hour to an hour with 50 c.c. 
of a 6 per cent, solution of potassic bichromate and 10 c.c. of concentrated 
sulphuric acid in a flask provided with an inverted condenser. The liquid is 
now made up to 200 c.c., and the unaltered chromic acid determined in 
10 c.c. of it. For this purpose, 1 to 2 gm. of pure potassic iodide, 10 c.c. 
of a 25 per cent, solution of phosphoric acid, and some water are added ; and 
after five minutes the solution is diluted to about 100 c.c. with boiled water, 
and titrated with T ^- thiosulphate solution in the usual manner: The 
phosphoric acid is added according to Meineke's recommendation, and is 
for the purpose of rendering the change from the blue colour of the iodide 
of starch to the green of the chromium salt more visible ; the commercial 
glacial acid may be dissolved in water, oxidized by potassium permanganate 
until it has a faint rose colour, and filtered before being used. 

The bichromate solution used for the oxidation is titrated in the same 
way. One mol. potassic bichromate is equivalent to three mols. formic acid. 

The results quoted by the author show that the method is fairly 
accurate, both in the absence and in presence of acetic acid. 



HYDROFLUORIC ACID, SILICOFLUORIC ACID, 
AND FLUORIDES. 

1 c.c. of ^ alkali = 0-02 gm. of HF = 0'024 gm. of H 2 SiF. 

26. COMMERCIAL hydrofluoric acid, which is now a not 
inconsiderable article of commerce, is as a rule far from pure. It 
generally contains in addition to hydrofluoric acid, silicofluoric acid, 
sulphuric acid, sulphurous acid, and frequently traces of iron and 
lead. Two analyses of commercial acid gave the following 
figures : 

1. 2. 

Hydrofluoric acid 4S'00 45'80 

Silicofluoric acid 13'05 9'49 

Sulphuric acid 4'07 3'23 

Sulphurous acid 0*49 ...... 1'06 

Left on evaporation 0'16 

Water by difference 34-23 4-0*42 



100-00 100-00 



106 VOLUMETRIC ANALYSIS. 26. 

If it is desired to prepare pure acid, the best way is to add to the 
commercial acid peroxide of hydrogen till it ceases to bleach 
iodine, and then potassic hydric fluoride sufficient to fix all the 
silicofluoric and sulphuric acids. Re-distillation in a lead retort 
with a platinum condenser will then give perfectly pure acid. 

The total amount of free acid may be estimated with normal 
alkali (preferably potash), using phenolphthalein or litmus, the 
former is best. Methyl orange and lacmoid do not give 
good results. In the case of pure acid, each c.c. of ~ alkali 
indicates 0*02 gm. of HF, and the reaction when phenolphthalein is 
employed is very sharp, When, however, commercial acid is thus 
titrated a difference is observed ; the pink colour obtained on 
adding the alkali only endures for a second or so and then fades 
away, and this may be repeated for some time till at last 
a permanent pink is produced. The cause of this is the presence 
of silicofluoric acid. The first appearance of pink ensues when 
the reaction H 2 SiF 6 + K 2 = K-'SiF 6 + H 2 occurs. Then another 
reaction sets in 

K 2 SiF 6 + 2 K 2 = (KF) 6 + SiO 2 , 

but from the slight solubility of the potassium silicofluoricle some 
time elapses before it is complete. 

The sulphuric and sulphurous acid must also be estimated if the 
real amount of HF is required. 

Estimation of Sulphuric Acid in Presence of Hydrofluoric Acid 
(W. B. Giles). Long experience has convinced the author of this new 
process, that all methods depending upon the supposed solubility of barium 
fluoride, and the corresponding insolubility of the sulphate in either hot or 
cold diluted hydrochloric acid give most erroneous results. For instance, 
a sample of hydrofluoric acid known to contain 4/ of H 2 SO 4 was treated in 
the way described by Fresenius, using a large volume of hot dilute 
hydrochloric acid, and' the precipitate was copiously washed with the same 
weak acid. The barium precipitate obtained was equal to 6'08 / of SO 3 or 
over 50 / more than was present, and it was found that on repeatedly 
moistening the precipitate with dilute H-SO 4 , and re-igniting, that the 
weight increased materially, showing co-precipitation of barium fluoride. 
The author therefore devised the following process for the estimation of the 
SO 3 which gives accurate results. Its basis is - 

1. The conversion of HF into H-'SiF, which is easily accomplished. 

2. The precipitation of the SO 3 from this solution by means of lead silico- 
fluoride. 

3. The total insolubility of PbSO 4 in a solution containing an excess of 
the said lead salt. 

Process : A convenient weight of the hydrofluoric acid is placed in 
a platinum dish, about half its volume of water is added, and then 
precipitated silica in evident excess, and the whole is allowed to stand with 
occasional stirring for a few hours. It is then filtered, using an ebonite 
funnel, into another suitable platinum basin, and the excess of silica 
thoroughly washed, the filtrate and washings are then evaporated to 
a convenient bulk, and solution of lead silicofluoride is added in excess. If 
the least trace of sulphuric acid was contained in the acid originally, an 
almost immediate precipitate of PbSO 4 Avill form, as it is exceedingly 



26. HYDROFLUORIC ACID. 107 

insoluble in the presence of the lead silicofluoride. The solution is allowed! 
to stand an hour or two, and the PbSO 4 separated by nitration, when it can 
of course be treated in any convenient volumetric way for the estimation of 
the lead, or it may be weighed direct. 

Lead silicofluoride is easily prepared by saturating commercial HF with 
coarsely powdered flint in a lead basin, and then agitating with powdered 
litharge. Its solubility is very great, and the specific gravity of the 
solution may reach 2'000 or more. 

Example : To 37'89 gm. of chemically pure HF of 1250 sp. gr., there 
was added 25 c.c. of normal acid=ro gm. SO 3 . The mixture was then 
treated as described above, and gave PbSO 4 3'782 gm.=l'0002 gm. of SO 3 . 

Estimation of the Silicofluoric Acid. To a convenient quantity of the 
acid contained in a platinum dish, a solution of potassic acetate in strong 
methylated spirit is added in excess, and then more spirit is added, so that 
there may be about equal volumes of liquid and spirit. Allow to stand for 
several hours, and then filter and wash with a mixture of half spirit and 
half water. The resulting potassium silicofluoride may then be titrated 
with normal alkali according to the equation : 



or if the filter was a weighed one, it may be dried at 100 C. and weighed 
direct. 

Example : 2 gm. of chemically pure precipitated silica were dissolved in 
a large excess of pure diluted HF. Treated as above described, it yielded 
7'35 gm. of K 2 SiF which equals 2'004 gm. of silica; 2 gm. of some 
powdered flint treated in the same way with 50 gm. of pure HF (of 40 / ) 
gave 7-168 gm. of K 2 SiF 6 =l'958 gm. of silica. 

Sulphurous Acid. This is easily estimated by taking the solution which 
results from the total acidity determination and titrating with decinormal 
iodine. Commercial hydrofluoric acid generally contains from 0'5 to 2'0 % 

The amount of each of the impurities being thus known, the 
percentage of real HF is easily calculated; e.g., 10 gm. of an 
acid was found to neutralize 276'0 c.c. of normal alkali. It was 
found to give the following results : 

c c. normal alkali 8'0 = 3 -2 3 SO 3 

39-0= 9-36H 2 SiF 
276 -4:7 = 229 c.c. x 0'02 = 45-80 % HF. 

41-61% H 2 by difference 



100-00 
In this instance the amount of SO 2 was not allowed for. 

Bifluorides. -These salts have lately been used to some extent 
on the Continent by distillers. They may be titrated in the same 
way as the acid with phenolphthalein. They generally contain 
some silicofluoride.* 

The estimation of fluorine in soluble fluorides has been done 

* The whole of this section, to this point, is kindly contributed by W. B. Giles, 
F.I.C., who has had large practical experience on the subjects treated. 



108 VOLUMETRIC ANALYSIS. 26. 

volumetrically by Knob loch (Pliarm. Zeitslirift xxxix. 558). 
The process is based on the following facts : 

"When a solution of ferric chloride is mixed with its equivalent 
quantity of potassic fluoride the decomposition is complete, and 
the resulting ferric fluoride solution is colourless. In this state 
the iron is not detectable by such tests as thiocyanate, salicylic 
acid, etc. Still more interesting is the fact that ferric fluoride 
does not liberate iodine from iodides. 

The following standard solutions, &c., are required : 

-A potassic fluoride ; 5 '809 gm. of the pure ignited salt in 
a liter of water. 

^j- solution of ferric chloride, which the author prepared by 
diluting 19 gm. of the officinal ferric chloride of the Prussian 
pharmacopoeia to a liter. 

~Q sodic thiosulphate solution. 

Zinc iodide solution, made by mixing 10 gm. of iodine, 5 gm. 
of zinc powder, and 25 c.c. of water in a flask, and warming till 
the violent action is over and the solution colourless, then diluting 
to 40 c.c. and filtering. 

Process: The liquid containing the fluorides in solution is mixed with 
a known excess of ferric chloride solution, then with excess of zinc iodide, 
and allowed to remain in a closed vessel at 35 40 C. for half an hour ; the 
liberated iodine is then titrated with thiosulphate. The volume of the 
latter used is deducted from that of the ferric chloride the difference is the 
measure of the fluorine, 1 c.c. thiosulphate = 0'0019 gm. P. 

The author states that calcium and strontium in their soluble 
salts may also be estimated by the same method by acidifying 
their solutions with hydrochloric acid, adding equal volumes, first 
of potassic fluoride and then ferric chloride solutions in excess, 
excess of zinc iodide is then added, and digested at 35 40 C. 
and the liberated iodine ascertained as before, 1 c.c. of thiosulphate 
-0-002 Ca. 

None of these reactions have been verified by me, but the 
method as given here is novel, and probably capable of being 
developed by experience. 

A very interesting paper on the acidimetry of hydrofluoric acid 
is contributed by Hag a and Osaka (J. C. S. xvii. xviii. 251), being 
the results of independent experiments made by them in the 
laboratory of the Imperial University, Japan. 

The authors examined the behaviour of the usual indicators in 
the neutralization of hydrofluoric acid. That its alkali salts blue 
litmus, and that its avidity number places it among the vegetable 
acids rather than with the strong mineral acids, appear to be the 
only two facts yet recorded bearing upon its acidimetry. ' 

To get uniform indications it was found necessary to have not 
only the acid pure, but the titrating solutions also ; a little silica, 
alumina, or carbon dioxide affecting the titration more than it 
would in the case of the ordinary mineral acids. 



28. PHOSPHORIC ACID. 

Phenolphthalein is the best indicator, and leaves nothing to be 
desired when potash or soda is used for the titration. Rosolic acid 
is almost equal to it, and can be used besides with ammonia. 
"With both indicators the change of colour has the advantage of 
being very evident in platinum vessels. Methyl orange is useless. 
Litmus, lacinoid and phenacetolin are all capable of being made to 
yield accurate results in the hands of an experienced operator. 

The fact that accurate results can only be obtained with very- 
pure acid and reagents, militates against the value of any 
acidimetric process, and therefore the indirect method by Giles, 
described above, is of greater technical value. 

OXALIC ACID. 

C 2 H 2 4 2H 2 0-126. 

27. THE free acid can be accurately titrated with normal 
alkali and phenolphthalein. 

In combination with alkalies, the acid can be precipitated with calcic 
chloride as calcic oxalate, where no other matters occur precipitable by 
calcium; if acetic acid is present in slight excess it is of no consequence, as 
it prevents the precipitation of small quantities of sulphates. The precipi- 
tate is well washed^dried, ignited, and titrated with normal acid, 1 c.c. of 
which = 0-063 gm. O~. 

Acid oxalates are titrated direct for the amount of free acicL 
The reaction continues to be acid until alkali is added in such 
proportion that 1 molecule acid = 2 atoms alkali metal. 

The combined acid may be found by igniting the salt, and 
titrating the residual alkaline carbonate as above. 

PHOSPHORIC ACID. 



28. FREE tribasic phosphoric acid cannot be titrated directly 
with normal alkali in the same manner as most free acids, owing ta 
the fact, that when an alkaline base (soda, for instance) is added to 
the acid, a combination occurs in which at one and the same time 
red litmus paper is turned blue and blue red. This fact has been 
repeatedly noticed in the case of some specimens of urine, also in 
milk. In order, therefore, to estimate phosphoric acid, or alkaline 
phosphates, alkalimetrically, it is necessary to prevent the formation 
of soluble phosphate of alkali, and to bring the acid into a definite 
compound with an alkaline earth. Such a method gives tolerably 
good results when carried out as follows : 

The solution of free acid, or its acid or neutral combination with alkali in 
a somewhat dilute state, is placed in a flask, and a known volume of normal 
alkali in excess added, in order to convert the whole of the acid into a basic 
salt; a drop or two of rosolic acid is added, then sufficient neutral baric 



110 VOLUMET1MC ANALYSIS. 28. 

chloride poured in to combine with all the phosphoric acid, the mixture is 
heated nearly to boiling; and, while hot, the excess of alkali is titrated with 
normal acid. The suspended baric phosphate, together Avith the liquid, 
possesses a rose-red colour until the last drop or two of acid, after continuous 
heating, and agitation, gives a permanent Avhite or slightly yellowish, milky 
appearance, when the process is ended. 

The volume of normal alkali, less the volume of acid, represents the 
amount of alkali required to convert the phosphoric acid into a chemically 
neutral salt, e.g. trisodic phosphate. 1 c.c. alkali = 0'02366 gm. P 2 O 5 . In 
dealing with small quantities of material, it is better to use f or ^ standard 
solutions. 

Thomson has shown in his researches on the indicators, that 
phosphoric acid, either in the free state, or in combination with 
soda or potash, may with very fair accuracy be estimated by the 
help of methyl orange and phenolphthalein. If, for instance, 
normal potash be added to a solution of phosphoric acid" until the 
pink colour of methyl orange is discharged, KH 2 P0 4 is formed 
(112 KHO-142 P 2 5 ). If now phenolphthalein is added, and 
the addition of potash continued until a red colour occurs, K 2 HP0 4 
is formed. (Again 112 KHO = 142 P 2 5 .) On adding standard 
hydrochloric or sulphuric acid, until the pink colour of methyl 
orange reappears, the titration with standard potash may be 
repeated. 

Many attempts have been made to utilize these reactions for the 
accurate estimation of P 2 5 in manures, etc., but, so far as my 
own experience goes, without adequate success. 

Titration as Ammonio-magnesian Phosphate- Stolba (Cliem. 
Gent, 1866, 727, 728) adopts an alkalimetric method, which 
depends upon the fact, that one molecule of the double salt 
requires two molecules of a mineral acid for saturation. 

The precipitation is made with magnesia mixture, the precipitate well 
washed with ammonia, and the latter completely removed by Avashing with 
alcohol of 50 or 60 per cent. The precipitate is then dissolved in a measured 
excess of ^ acid, methyl orange added, and the amount of acid required found 
by titration with ^ alkali. Care must be taken that all free ammonia is 
removed from the filter and precipitate, and that the whole of the double 
salt is decomposed by the acid before titration, which may always be insured 
by using a rather large excess and warming. The titration is carried on cold. 

This method has given me very good results in comparison with 
the gravimetric method. The same process is applicable to the 
estimation of arsenic acid, and also of magnesia, 



1 c.c. of T ^ acid -0-00355 gm. P 2 O 
= 0-00575 gm. As 2 3 
= 0-002 gm. MgO 



= - 



The reaction in the case of phosphoric acid may be expressed as 
follows : 

Mg (NEP) PO 4 + 2HC1 - (NH 4 ) H 2 P0 4 + MgCl 2 . 



29. SULPHURIC ANHYDRIDE. Ill 

Method for the Determination of Phosphoric Acid in its Pure 
Solutions E. Segalle (Z.A.C. xxxiv. 3339) has investigated various 
methods for the above purpose with the following result: 

By far the most accurate results are obtained by Glucksinann's method. 
In this, the phosphoric acid is precipitated by an excess of " magnesia 
mixture " of known strength in free ammonia, the precipitate filtered off, 
and the free ammonia left in solution is titrated by standard acid. From 
the equation 

H 3 P0 4 + MgSO 4 + 3 NIP - MgNH 4 P0 4 + (NH 4 ) 2 SO 4 

it will be seen that H 3 PO 4 =3NH 3 . 

The following modification is recommended as being more convenient and 
simple. To the phosphoric acid solution, contained in a graduated flask, an 
excess of standard ammonia (preferably normal) is added, followed by an 
excess of a saturated neutral solution of magnesium sulphate. The liquid 
is then diluted to the mark, well shaken, and filtered, and the residual 
ammonia titrated in an aliquot part of the filtrate. 

On account of its simplicity, the modified method is well adapted for 
ascertaining the strength of the solutions of phosphoric acid employed in 
pharmacy. 

SULPHURIC ANHYDRIDE. 

SO 3 = 80. 

29. NORDHAUSEX or fuming sulphuric acid consists of a 
mixture of SO 3 and H 2 SO. When it is rich in SO 3 it occurs in 
a solid form, and being very hygroscopic cannot be weighed in the 
ordinary manner. Its strength is therefore best taken in the way 
recommended by Mess el as follows : A very thin, bulb tube with 
capillary ends is inserted into a bottle of the melted acid. The 
ends are bent like the letter /, the bulb being in the middle. The 
bottle should be of such size, that one end of the tube projects out 
of its mouth. As soon as the bulb is filled, the upper capillary 
end is sealed, the tube lifted out, wiped, inverted, and the other 
end sealed ; the tube is then carefully wiped with blotting paper 
till dry and clean, then weighed. A stoppered bottle, just large 
enough to allow the tube being placed loosely inside it, is then 
about one-third filled with water, the tube gently inserted, the 
stopper replaced, held firmly in by the hand, and a vigorous shake 
.given so as to break the tube. A sudden vibration occurs from 
contact of the acid with the water, but no danger is incurred. 
A white cloud is seen on the sides of the bottle, which disappears 
on shaking for a few minutes. After the bottle is cooled the 
contents are emptied into a measuring flask. An aliquot portion 
is then taken out and titrated with iodine for SO 2 , which is 
always present in small quantity : another portion is titrated with 
standard alkali and methyl orange for sulphuric acid. No other 
indicator is available, and as Lunge has pointed out (Zeii, Angew. 
Cliem. 1895, 221), neutrality is reached when the acid sulphite is 
formed, and not when the whole of the SO 2 is neutralized. 



112 VOLUMETRIC ANALYSIS. 30. 

TARTARIC ACID. 

C 4 H 6 G =150. 

30. THE free acid may be readily titrated with normal alkali 
and phenolphthalein. 

1 c.c. alkali = 0*075 gm. tartaric acid. 

The amount of tartaric acid existing in tartaric acid liquors is 
best estimated by precipitation as potassic bitartrate ; the same is 
also the case with crude argols, lees, etc. Manufacturers are highly 
indebted to War in gt on and Grosjean for most exhaustive 
papers on this subject, to which reference should be made by all 
who desire to study the nature and analysis of all commercial 
compounds of citric and tartaric acids (Warington, J. C. S. 1875 r 
925994; Grosjean, J. C. S. 1879, 341356). 

Without entering into the copious details and explanations given 
by these authorities, the methods may be summarized as follows : 

1. Commercial Tartrates. 

In the case of good clean tartars, even though they may contain sulphates 
and carbonates, very accurate results may he obtained by indirect methods. 

(a] The very finely powdered sample is first titrated with normal alkali, 
and thus the amount of tartaric acid existing as bitartrate is found; another 
portion of the sample is then calcined at a moderate heat, and the ash 
titrated. By deducting from the volume of acid so used the volume used 
for bitartrate, the amount of base corresponding to neutral tart rates is 
obtained. 

(b) The whole of the tartaric acid is exactly neutralized with caustic 
soda, evaporated to dryness, calcined, and the ash titrated with normal acid ; 
the total tartaric acid is then calculated from the volume of standard acid 
used ; any other organic acid present Avill naturally be included in this 
amount. In the case of fairly pure tartars, etc., this probable error may he- 
disregarded. 

Warington's description of the first process is as follows : 

5 gni. of the finely powdered tartar are heated with a little water to 
dissolve any carbonates that may be present. If it is wished to guard against 
crystalline carbonates, 5 c.c. of standard HC1 are added in the first instance 1 , 
and the heating is conducted in a covered beaker. Standard alkali is next 
added to the extent of about three-fourths of the amount required by a good 
tartar of the kind examined, plus that equivalent to the acid used, and the 
whole is brought to boiling; when nearly cold, the titration is finished. 
From the amount of alkali consumed, minus that required by the HC1, the 
tartaric acid present as acid tartrate is calculated. 

2 gm. of the powdered tartar are next weighed into a platinum crucible 
with a well-fitting lid ; the crucible is placed over an argand burner ; heat is 
first applied very gently to dry the tartar, and then more strongly till 
inflammable gas ceases to be evolved. The heat should not rise above very 
low redness. The black ash is next removed with water to a beaker. If the 
tartar is known to be a good one, 20 c.c. of standard H 2 SO 4 are now run 
from a pipette into the beaker, a portion of the acid being used to rinse the 
crucible. The contents of the beaker are now brought to boiling, filtered, 
and the free acid determined with standard alkali. As the charcoal on the 
filter under some circumstances retains a little acid, even when well washed,. 



.30. TARTARIC ACID. 113 

it is advisable when the titration is completed to transfer the iilter and its 
contents to the neutralized fluid, and add a further amount of alkali if 
necessar} r . From the neutralizing power of a gram of burnt tartar is 
subtracted the acidity of a gram of unburnt tartar, both expressed in c.c. of 
standard alkali, the difference in the neutralizing power of the bases existing 
as neutral tartrates, and is then calculated into tartaric acid on this 
assumption.* 

If the tartar is of low quality, 5 c.c. of solution of hydrogen peroxide 
(1 volume 10 volumes O) are added to the black ash and water, and 
immediately afterwards, the standard acid ; the rest of the analysis proceeds 
.as already described ; the small acidity usually belonging to the peroxide 
.-solution must, however, be known and allowed for in the calculation. By 
the use of hydrogen peroxide the sulphides formed during ignition are 
reconverted into sulphates, and the error of excess which their presence 
would occasion is avoided. 

The above method does not give the separate amounts of acid 
and neutral tartrates in the presence of carbonates, but it gives the 
correct amount of tartaric acid ; it is also correct in cases where 
free tartaric acid exists, so long as the final results show that some 
acid existed as neutral salt. Whenever this method shows that 
the acidity of the original substance is greater than the neutralizing 
power of the ash, it will be necessary to use the method b, which 
is the only one capable of giving good results when the sample 
contains much free tartaric acid. 

Instead of the alkalimetric estimation in both the above methods, 
equally good results may be got by a carbonic acid determination 
in the asli with S c he i bier's apparatus ( 23.6), or any of the 
usual methods. 

2. Tartaric Acid Liquors. 

Old factory liquors contain a great variety of substances gradually 
accumulated, from which the actual tartaric acid can only be 
separated as bitartrate by the following process : 

(c) A quantity of liquor containing 24 gm. of tartaric acid, and of 
30 40 c.c. volume, is treated Avith a saturated solution of neutral potassic 
citrate, added drop by drop with constant stirring. If free sulphuric acid is 
present no precipitate is at first produced ; but as soon as the acid is satisfied, 
the bitartrate begins to appear in streaks on the sides of the vessel. When 
this is seen, the remainder of the citrate is measured in to avoid an undue 
excess : 4 c.c. of a saturated solution of potassic citrate will be found 
sufficient to precipitate the maximum of 4 grams of tartaric acid supposed to 
be present. If the liquor contain a great deal of sulphuric acid, a fine 
precipitate of potassic sulphate will precede the formation of bitartrate, but 
is easily distinguished from it. With liquors rich in sulphuric acid, it is 
advisable to stir the mixture vigorously at intervals for half an hour, then 
proceed as in 3 d. 

Grosjean modifies this process by precipitating the liquor with an excess 
of calcic carbonate, then boiling the mixture with excess of potassic oxalate. 

* It is obvious that the neutralizing power of the ash of an acid tartrate is exactly 
the same as the acidity of the same tartrate before burning-. In making the calcula- 
tions, it nmst be remembered that the value of the alkali in tartaric acid is twice as 
great in the calculation made from the acidity of the unburnt tartar, as in the 
. calculation of the acid existing as neutral tartrates. 

I 



114 VOLUMETRIC ANALYSIS. 31. 

By this means the alumina, iron, phosphoric and sulphuric acids are thrown 
down with the calcic oxalate, and the precipitate allows of ready nitration. 
The separation as bitartrate then follows, as in d. 

3. Very impure Lees and Argrols. 

Grosjean (/. C. S. 1879, 341) gives a succinct method for 
the treatment of these substances, based on War ing ton's original 
oxalate process, the principle of which is as follows : 

The finely ground sample ( about 2 gm. tartaric acid) is first moistened 
with a little water, heated to 100 C., then digested for 15 minutes or so with 
an excess of neutral potassic oxalate (the excess must not be less than 
1'5 gm.), and nearly neutralized with potash. After repeated stirring, the 
mixture is transferred to a vacuum filter, and the residue washed; the 
liquid so obtained contains all the tartaric acid as neutral potassic tartrate ; 
excess of citric acid is added, which precipitates the whole of the tartaric 
acid as bitartrate, and the amount is found by titration with standard alkali 
in the usual way. 

One of the chief difficulties in treating low qualities of material is the 
filtration of the nearly neutral mixture above mentioned. Grosjean adopts- 
the principle of Casamajor's filter (C. N. xxxii. 45), using an ordinary 
funnel with either platinum, lead, or pumice disc ; but whether this, or 
Bun sen's, or other form of filter is used, the resulting filtrate and washings- 
(which for 2 gm. tartaric acid should not much exceed 50 c.c.) are ready for 
the separation of the bitartrate in the following improved way : 

(d) To the 50 c.c. or so of cold solution 5 gm. of powdered potassic- 
chloride are added, and stirred till dissolved : this renders the subsequent 
precipitation of bitartrate very complete. A 50 per-cent. solution of citric 
acid is then mixed with the liquid in such proportion, that for every 2 gm. 
of tartaric acid an equal, or slightly greater amount of citric acid is present. 
By continuously stirring, the whole of the bitartrate comes down in ten 
minutes (Grosjean) ; if the temperature is much above 16, it is preferable 
to wait half an hour or so before filtering. This operation is best done on 
the vacuum filter, and the washing is made with a 5 per-cent. solution of 
potassic chloride, saturated at ordinary temperature with potassic bitartrate ; 
if great accurac} 7 is required, the exact* acidity of the solution should be 
found by ^5- alkali, and the washing continued until the washings show no- 
greater acidity, thus proving the absence of citric acid. Finally, the washed 
precipitate is gently pressed into a cake to free it from excess of liquid,, 
transferred to a beaker Avith the filter, hot \vater added, and titrated with 
standard alkali. 

The troublesome filtration can be avoided in many cases by taking* 
30 40 gm. of substance, and after decomposition by oxalate, and neutralizing- 
with potash, making up the volume to 150 or 200 c.c., adding water in 
corresponding proportion to the bulk of the residue, then taking an aliquot 
portion for precipitation. A blank experiment made by Grosjean in this 
way, gave a volume of 3'75 c.c. for the residue in 10 gm. lees. Other things, 
being equal, therefore, 30 or 40 gm. may respectively be made up to 161 and 
215 c.c., then 50 c.c. taken for precipitation. > 

ESTIMATION OF COMBINED ACIDS AND BASES IN 
NEUTRAL SALTS. 

31. THIS comprehensive method of determining the quantity 
of acid in neutral compounds (but not the nature of the acid), is 
applicable only in those cases where the base is perfectly precipitated 



31. COMBINED ACIDS AND BASES. 115 

by an excess of caustic alkali or its carbonate. The number of 
bodies capable of being so precipitated is very large, as has been 
proved by the researches of Langer and "VVawnikiewicz (Ann. 
Chem. u. Phar. 1861, 239), who seem to have worked out the 
method very carefully. These gentlemen attribute its origin to 
Bunsen ; but it does not seem certain who devised it. The best 
method of procedure is as follows : 

The substance is weighed, dissolved in water in a 300-c.c. flask, heated to 
boiling or not, as may be desirable ; normal alkali or its carbonate, according 
to the nature of the base, is then added from a burette, until the whole is 
decidedly alkaline. It is then diluted to 300 c.c. and put aside to settle, and 
100 c.c. are taken out and titrated for the excess of alkali ; the remainder 
multiplied by 3, gives the measure of the acid combined with the original 
salt, i.e. supposing the precipitation is complete. 

Example : 2 gm. crystals of baric chloride were dissolved in water, heated 
to boiling, and 20 c.c. normal sodic carbonate added, diluted to 300 c.c. and 
100 c.c. of the clear liquid titrated with normal nitric acid, of which 1*2 c.c. 
was required ; altogether, therefore, the 2 gm. required 16'4 c.c. normal 
alkali ; this multiplied by 0'122 gave 2'0008 gm. Bad 2 2H 2 O instead of 
2 gm. ; multiplied by the factor for chlorine 0'03537, it yielded 0'58007 gm. 
Theory requires 0'5809 gm. chlorine. 

The following substances have been submitted to this mode of 
examination with satisfactory results : 

Salts of the alkaline earths precipitated with an alkaline 
carbonate while boiling hot. 

Salts of magnesia, with pure or carbonated alkali. 

Alum, with carbonate of alkali. 

Zinc salts, boiling hot, with the same. 

Copper salts, boiling hot, with pure potash. 

Silver salts, with same. 

Bismuth salts, half an hour's boiling, with sodic carbonate. 

Nickel and cobalt salts, with the same. 

Lead salts, with the same. 

Iron salts, boiling hot, with pure or carbonated alkali. 

Mercury salts, with pure alkali. 

Protosalts of manganese, boiling hot, with sodic carbonate. 

Chromium persalts, boiling hot, with pure potash. 

Where the compound under examination contains but one base 
precipitable by alkali, the determination of the acid gives, of 
course, the quantity of base also. 

Wolcott Gibbs (C. N. 1868, i. 151) has enunciated a new 
acidimetric principle applicable in cases where a base is precipitable 
at a boiling temperature by hydric sulphide, and the acid set free 
so as to be estimated with standard alkali. Of course the method 
can only be used where complete separation can be obtained, and 
where the salt to be analyzed contains a fixed acid which has no 
effect upon hydric sulphide. A weighed portion is dissolved in 

i 2 



116 VOLUMETRIC ANALYSIS. 31. 

water, brought to boiling, and the gas passed in until the metal is 
completely precipitated ; which is known by testing a drop of the 
clear liquid upon a porcelain tile with sulphuretted hydrogen 
water, or any other appropriate agent adapted to the metallic salt 
under examination. 

The liquid is filtered from the precipitate, and the latter well 
washed, and the solution made up to a definite measure. An 
aliquot portion is then titrated with normal alkali as usual, Avith 
one of the phenol indicators. 

In the case of nitrates or chlorides, where nitric or hydrochloric 
acid would interfere with the hydric sulphide, it was found that the 
addition in tolerable quantity of a neutral salt containing an organic 
acid (e.g. sodic or potassic tartrate, or the double salt) obviated all 
difficulty. 

The results obtained by Gibbs in the case of copper, lead,, 
bismuth, and mercury, as sulphate, nitrate, and chloride, agreed 
very closely with theory. 

Though not strictly belonging to the domain of acidimetry, 
a method worked out by Neumann (Z. A. C. xxxiv. 454) may here 
be mentioned for the technical estimation of some of the heavy 
metals precipitable by sodic sulphide. The strength of the sulphide 
solution is ascertained by boiling it with a measured excess of 
standard acid till all the H 2 S is dissipated ; the excess of acid is 
then found by titration with standard alkali, using phenolphthalein 
as indicator. Having established the working strength of the 
sulphide solution, the neutral solution of the metal to be estimated 
is first precipitated with a known excess of standard sulphide, 
and the solution containing the suspended sulphide or hydroxide 
is rendered clear, if necessary, by the addition of strong sodium 
chloride solution, and diluted to a definite volume at 16 C. 
An aliquot part of the solution is then filtered off, or removed 
by means of a pipette, and the excess of sulphide indirectly 
determined in it. This indirect process is necessary, because the 
alkaline sulphide destroys the colour of litmus or of phenolphthalein. 
The estimation of the amounts of metal in the following salts by 
this method gave excellent results : alum, chrome alum, silver 
sulphate, copper sulphate, cobalt sulphate, cadmium sulphate, lead 
nitrate, manganese sulphate, nickel sulphate, ferrous sulphate, 
ferrous ammonium sulphate, ferric chloride. This method, of course, 
is not applicable if the solutions contain any free acid. Solutions 
of chlorides containing free hydrochloric are first evaporated on 
the water-bath, the residue moistened with alcohol, and again 
evaporated to dryness. Sulphates are first converted into chlorides 
by treatment with barium chloride and hydrochloric acid, and 
the solutions so obtained are treated as before described for the 
removal of the free HC1. .Nitrates are twice evaporated to dryness 
with concentrated HC1, excess of the latter being finally removed 
in the above-mentioned manner. 



32, ALKALIMETBIC METHODS. 117 

EXTENSION OF ALKALIMETBIC METHODS. 

32. BOHLIG (Z. a. C. 1870, 310) has described a method 
for the estimation of sulphuric acid, baryta, chlorine, iodine, and 
bromine, which appears worthy of some consideration, since the 
only standard solutions required are an acid and an alkali v 

Alkaline sulphates are known to be partially decomposed, in 
contact with baric carbonate, into alkaline, carbonates and baric 
sulphate. The decomposition is complete in the presence of free 
carbonic anhydride ; acid carbonates of the alkali-metals are left in 
solution, together with some acid baric carbonate, which can be 
removed by boiling. The solution is filtered, and the alkaline 
carbonate determined by means of a standard acid solution, and 
the amount of sulphuric acid or alkaline sulphate calculated 
from the amount of normal acid required. This process has been 
satisfactorily used by Hanbst for sulphates in waters (C. N. 
xxxvi. 227), and by Grossmann for salt cake (C. N. xli. 114). 
See also 17.14. 

Neutral chlorides, bromides, and iodides, more especially of the 
alkali-metals, are most readily decomposed by pure silver oxide 
into insoluble silver salts, leaving the alkali-metal in solution as 
hydrate (ammonia salts always excepted), which can then be 
determined as usual by standard acid. 

The author treats solutions containing sulphates of the heavy 
metals, of the earths or alkaline earths, and free from acids whose 
presence would influence the method, viz., phosphoric, arsenic, 
oxalic, etc., with a solution of potassic carbonate so as to precipitate 
the bases and leave about double or treble the amount of alkaline 
carbonate in solution. From 1 to 1J gm. of substance is operated 
upon in a flask. The solution is made up to 500 c.c., well shaken, 
and the precipitate allowed to subside. 50 c.c, are then filtered, 
and titrated with standard acid and methyl orange. Another 100 c.c. 
are filtered in like manner into a strong quarter-liter flask, and 
diluted with about 100 c.c. of hot water; the requisite quantity of 
normal acid is then run in at once from a burette ; the solution 
diluted to 250 c.c. ; and about a gram of dry baric carbonate 
(free from alkali) added. The flask is next closed, and the liquid 
well agitated. The decomposition of the alkaline sulphate is 
complete in a few minutes. The flask should be opene.d now and 
then to allow the carbonic anhydride to escape. Finally, about 
J gm. of pulverized baric hydrate is added, the whole well shaken, 
and a portion of the rapidly clearing liquid tested qualitatively for 
barium and sulphuric acid. The result should be a negative one. 
50 c.c., corresponding to 20 c.c. of the original solution, are then 
filtered and titrated with normal acid, and the quantity of sulphuric 
acid (sulphate) calculated as usual. 

The source of carbonic anhydride is thus placed in the liquid 
itself, provided the quantity of potassic carbonate be not -too small. 



118 VOLUMETIIIC ANALYSIS. 32. 

Equivalent quantities of K 2 S0 4 + 2K 2 C0 3 + 2HC1 + BaCO 3 when 
mixed with sufficient water change into BaSO 4 + 2KHC0 3 + 2KC1, 
and it is therefore more than sufficient to add twice the quantity 
of potassic carbonate compared with the alkaline sulphate operated 
upon. 

Baric hydrate is added with a view of removing any carbonic 
anhydride left in the liquid after boiling, which would otherwise 
dissolve some of the excess of baric carbonate contained in the 
precipitate. 

Any baric hydrate not required to remove CO 2 is acted upon by 
the acid potassic carbonate, but does not influence the final result. 

Phosphoric and oxalic acids the author proposes to remove by 
means of calcic chloride ; chromic acid by deoxidizing agents, 
such as alcohol and hydrochloric acid. Bohlig recommends this 
method for estimating sulphuric acid in ashes, crude soda, Stassfurth 
salts, etc, 

Solutions containing baryta are estimated in like manner by 
precipitation as carbonate, and decomposition with potassic sulphate 
in a solution containing free carbonic acid. Chlorine is determined 
in solutions by first precipitating any metallic chloride with potassic 
carbonate added in moderate excess. The filtrate is made up to 
250 c.c., and the excess of potassic carbonate determined in 50 c.c. 
by means of a normal solution of HC1. 125 c.c. of the solution 
are next treated with excess of silver oxide and made up to 250 c.c., 
well shaken (out of contact with the light) and filtered. 100 c.c. 
of the nitrate are titrated with normal hydrochloric acid. The 
difference between the quantity of acid required in the last and 
that of the first experiment, multiplied by 5, gives the amount 
of chlorine contained in the original solution. A portion of the 
filtrate should be tested for chlorine by means of mercurous nitrate. 

The filtrate is obtained perfectly clear only in the presence of 
some potassic or sodic carbonate, and by employing argentic oxide 
free from argentous oxide. A few drops of pure potassic per- 
manganate added to the argentic oxide preserved in water prevent 
formation of the latter. The oxide to be employed for each 
experiment is filtered when required, and thoroughly washed. 

Bromine and iodine are determined in like manner, The author 
has not been able, however, to estimate the mixtures of the halogen 
salts ; but Jie has made the interesting observation that potassic 
iodide, when boiled with potassic permanganate, is completely 
oxidized into iodate. This facilitates the detection of small 
quantities of chlorine and bromine, in the presence of much 
iodide. The greater part of iodate may be separated also by 
precipitation with baric nitrate before determining chlorine. The 
standard acid solutions which Bohlig employed contained not more 
than one-third of the equivalent of HC1 or SO 3 per liter. 

For further particulars the reader is referred to the original 
paper (Arch. Pharm. 3 cxlv. 113). 



32. ALKALIMETRIC METHODS. 119 

Siebold (Year Bool- of Pharmacy, 1878, 518) describes a very- 
ingenious process, devised by himself, for the titration of caustic 
and carbonated alkalies by means of prussic acid, the principle of 
which is explained in 59. The process is useful in the case of 
carbonates, since CO 2 is no hindrance. 

0'5 to 1 gm. of the alkali or alkaline carbonate is dissolved in about 100 c.c. 
of water, and an excess of hydrocyanic acid (say 10 or 20 c.c.) of 5 per cent, 
solution added; then ~ silver solution cautiously added with constant 
stirring until a faint permanent turbidity occurs. Each c.c. of T ^j- silver 
= 0-0138 gm. K 2 CO 3 , or 0'0106 gm. Na 2 CO 3 . 

In the case of chlorides being present, their quantity may be 
determined by boiling down the mixture to about half its volume 
to expel all free prussic acid, adding a drop or two of potassic 
chromate as indicator, then titrating with ~$ silver. Any excess 
.above that required in the first titration will be due to chlorine, 
and may be calculated accordingly. 



120 VjOLUMETKIC ANALYSIS. 33. 

PART III. 
ANALYSIS BY OXIDATION OR REDUCTION. 

33. THE series of analyses which occur under this system are 
very extensive in number, and not a few of them possess extreme 
accuracy, such in fact, as is not possible in any analysis by weight. 
The completion of the various processes is generally shown by 
a distinct change of colour ; such, for instance, as the occurrence 
of the beautiful rose-red permanganate, or the blue iodide of 
starch ; and as the smallest quantity of these substances will 
colour distinctly large masses of liquid, the slightest excess of the 
oxidizing agent is sufficient to produce a distinct effect. 

The principle involved in the process is extremely simple. 
Substances which will take up oxygen are brought into solution, 
and titrated with a substance of known oxidizing power ; as, for 
instance, in the determination of ferrous salts by permanganic- 
acid. The iron is ready and willing to receive the oxygen, 
the permanganate is equally willing to part with it ; while the iron 
is absorbing the oxygen, the permanganate loses its colour almost 
as soon as it is added, and the whole mixture is colourless ; but 
immediately the iron is satisfied, the rose colour no longer disappears, 
there being no more oxidizable iron present. In the case of potassic 
permanganate the reaction is: lOFeO + 2MnK0 4 = 5Fe 2 :! + 
2MnO + K-'O. Oxalic acid occupies the same position as the 
ferrous salts ; its composition is C 2 4 H 2 + 2H-O = 126. If perman- 
ganate is added to it in acid solution, the oxalic acid is oxidized to 
carbonic acid, and the manganic reduced to manganous oxide, thus 
Mn 2 7 + 5C 2 0*H 2 + 2H 2 S0 4 = 10C0 2 + 2MnS0 4 + 7H 2 O. When 
the oxalic acid is all decomposed, the colour of the permanganate 
no longer disappears. On the other hand, substances which will 
give up oxygen are deoxidized by a known excessive quantity of 
reducing agent, the amount of which excess is afterwards ascertained 
by residual titration with a standard oxidizing solution; the strength 
of the reducing solution being known, the quantity required is 
a measure of the substance which has been reduced by it. 

The oxidizing agents best available are potassic permanganate, 
iodine, potassic bichromate, and red potassic prussiate. 

The reducing agents are sulphurous acid, sodic hyposulphite,'"" 
sodic thiosulphate, oxalic acid, ferrous oxide, arsenious anhydride, 
stannous chloride, yellow potassic prussiate, and zinc or magnesium. 

With this variety of materials a great many combinations may be 
arranged so as to make this system of analysis very comprehensive; 
but the following are given as sufficient for almost all purposes, 

:;: S c li il t z e 11 b e r g e r ' s preparation is here meant. 



L; 

*r | 

UNIVERSITY 




34. STANDARD PERMANGAN 

and as being susceptible of the greatest amount of purity and 
stability of material, with exceedingly accurate results: 

1. Permanganate and ferrous salts (with the rose colour as 
indicator) ; permanganate and oxalic acid (with the rose colour 
as indicator). 

2. Potassic bichromate and ferrous salts (with cessation of blue 
colour when brought in contact with red potassic prussiate, as 
indicator). 

3. Iodine and sodic thiosulphate (with starch as indicator) ; 
iodine and sodic arsenite (with starch as indicator). 



PREPARATION OF STANDARD SOLUTIONS. 

PERMANGANIC ACID AND FERROUS OXIDE. 

1. Potassic Permang-anate. 
Mn 2 K 2 8 = 315'6. Decinormal Solution = 3 '156 gm. per liter. 

34. THE solution of this salt is best prepared for analysis by 
dissolving the pure crystals in fresh distilled water, and should be 
of such a strength that 17*85 c.c. will oxidize 1 decigram of iron. 
The solution is then decinormal. If the salt can be had perfectly 
pure and dry, 3'156 gm. dissolved in a liter of water at 16 C., will 
give an exactly decinormal solution ; but, nevertheless, it is always 
well to verify it as described below.* If kept in the light in 
ordinary bottles it will retain its strength for several months, if in 
bottles covered with black paper much longer, nevertheless, it 
should from time to time be verified by titratioii in one of the 
following ways : 

2. Titration of Permanganate. 

(a) With. Metallic Iron. The purest iron to be obtained is 
thin annealed binding-wire free from rust, generally known as 
flower wire.f Its actual percentage of pure iron may be taken 
as 99-6 

* Very fairly pure permanganate, in large crystals, may now be obtained in commerce, 
and if this salt is recrystallized twice from hot distilled water and dried thoroughly at 
1')J J C., it will be found practically pure. 

t Miss C. F. Roberts (Amer. Jour. Sci. 1894, 286, 290) advocates the use of pure 
iron, prepared by electrolysis, as follows : About 10 gin. -of ferrous-ammonium sulphate 
are dissolved in 150 c.c. of water. 5 c.c. of a saturated solution of potassic oxalate 
added, and then heated with a sufficiency of solution of animonic oxalate until clear. 
A weighed piece of platinum foil, shaped so as to be easily placed into a rather large 
weighing bottle, is then pvit into a beaker containing the iron solution, and the latter 
decomposed with a current of about two amperes between two platinum electrodes. In 
about two hours enough iron will be deposited for a tit-ration. The deposited metal is 
of course well washed, dried, and weighed in the weighing bottle, then dissolved in 
dilute acid, precisely as in the case of iron wire. 



122 VOLUMETRIC ANALYSIS. 34. 

Process : Fit a tight cork or rubber stopper, with bent delivery tube, into 
a flask holding about 300 c.c., and clamp it in a retort stand in an inclined 
position, the tube so bent as to dip into a small beaker containing pure 
water. Pill the flask one-third with dilute pure sulphuric acid, and add 
a few grains of sodic carbonate in crystals ; the CO 2 so produced will drive 
out the air. While this is being done weigh about O'l gram of the wire; 
put it quickly into the flask when the soda is dissolved, and apply a gentle 
heat till the iron is completely in solution, a few black specks of carbon are 
of no 'consequence. The flask is then rapidly cooled under a stream of cold 
water, diluted if necessary with some recently boiled and cooled water, and 
the permanganate run in cautiously from a T V c.c. tap burette, Avith constant 
shaking, until a faint rose-colour is permanent. Instead of this arrange- 
ment for dissolving the iron the apparatus shown in the section on iron 
analysis ma}^ be used, 63. 

The decomposition which ensues from titrating ferrous oxide by 
permanganic acid may be represented as follows : 

lOFeO and Mn 2 7 - 2MnO and 5Fe 2 :) . 

The weight of wire taken, multiplied by 0*996, will give the 
actual weight of pure iron upon which to calculate the strength of 
the permanganate. 

Example: Exactly O'l gm. of wire was dissolved and titrated with 
a permanganate solution, of which the quantity required was 17*6 c.c. 
The equation O'l : 0'0996 : : 17'85=^ gives 17'45, the permanganate is 
therefore a trifle too strong, but correct enough for all practical purposes. 

(b) "With Ferrous-ammonium Sulphate. In order to ascertain 
the strength of the permanganate, it may be titrated with 
a weighed quantity of this substance instead of metallic iron. 

This salt is a convenient one for titrating the permanganate, as it saves the 
time and trouble of dissolving the iron, and Avhen perfectly pure, it can be 
depended on without risk. To prepare it, 139 parts of the purest crystals of 
ferrous sulphate, and 66 parts of pure crystallized ammonic sulphate are 
separately dissolved in the least possible quantity of distilled water of about 
40 C. (if the solutions are not perfectly clear they must be filtered) ; mix 
them at the same temperature in a porcelain dish, adding a few drops of 
pure sulphuric acid, and stir till cold. During the stirring the double salt 
will fall in a finely granulated form. Set aside for a few hours, then pour 
off the supernatant liquid, and empty the salt into a clean funnel with a little 
cotton wool stuffed into the neck, so that the mother-liquor may drain away ; 
the salt may then be quickly and repeatedly pressed between fresh sheets of 
clean filtering paper. Lastly, place in a current of air to dry thoroughly, 
so that the small grains adhere no longer to each other, or to the paper in 
which they are contained, then preserve in a stoppered bottle for use. 



The formula of the salt is Fe (XH 4 )' 2 (SO 4 ) 2 , 6H 2 = 392. 
Consequently it contains exactly one-seventh of its weight of iron ; 
therefore 0*7 gm. represents O'l gm. Fe, and this is a convenient 
quantity to weigh for the purpose of titrating the permanganate. 

Process : 0'7 gm. being brought into "dilute cold solution in a flask or 
beaker, and 20 c.c. of dilute sulphuric acid (1 to 5) added (the titration of 
permanganate, or any other substance by it, should always take place in the 
presence of free acid, and preferably sulphuric), the permanganate is delivered 



34 STANDARD PERMANGANATE. 123 

from a burette with glass tap divided in T V c.c., as before described, until 
a point occurs when the rose colour no longer disappears on shaking. 

(<?) With Oxalic Acid. This is a very quick method of titrating 
permanganate, if the exact acidimetric value of the solution of pure oxalic 
acid is known. 10 c.c. of normal solution are brought into a flask with dilute 
sulphuric acid, as in the case of the iron salt, and considerably diluted with 
water, then warmed to about 60 C., and the permanganate added from the 
burette. The colour disappears slowly at first, but afterwards more rarpidly, 
becoming first brown, then yellow, and so on to colourless. More care must 
be exercised in this case than in the titration with iron, as the action is not 
momentary. 100 c.c. should be required to be strictly decinormal. The 
chemical change which occurs is explained on page 120. 

(d) With Lead Oxalate. Stolba prefers this salt to oxalic acid, for 
the reasons that it contains no water, is not liable to absorb an}' from 
exposure, and has a high molecular weight, 1 part of the salt representing 
0'42799 of oxalic acid, or 63 oxalic acid=147'2 lead oxalate. 

The method of titration is similar to that with oxalic acid, using dilute 
sulphuric acid, and warming the mixture to ensure the complete decomposi- 
tion of the salt into lead sulphate and free oxalic acid. Sodic oxalate is 
also anhydrous and equally serviceable. 

The lead oxalate is prepared by precipitating pure lead acetate with oxalic 
acid in excess, and washing the precipitate by decantation with warm water 
till all free acid is removed; the precipitate is then dried at 120 C., and 
preserved for use. Some operators prefer to use ammonic oxalate in place of 
oxalic acid or lead oxalate, as being a substance of definite hydration, and 
easily obtained in a pure state. Its formula is (NH 4 ) 2 C 2 O 4 , H 2 6 - 142=2Fe. 
The titration is carried on precisely as in the case of oxalic acid. 

0) With Hydrogen Peroxide in the Nitrometer. In a paper on 
this subject by Lunge ('/. S. C. I. ix. 21) it is shown by very carefully 
conducted experiments with purest materials and verified apparatus that 
-exceedingly accurate results may be obtained by the modified nitrometer 
with patent tap (illustrated at the end of Part VII.). Lunge's experiments 
were made on a semi-normal solution of permanganate (1 c.a=0'004 gin. O), 
but whether equally exact results would be obtained with T \ permanganate 
I cannot say, not having tried it ; but of course an approximately semi-normal 
solution may be made and reduced to either -| or T ^ strength, if desired, 
by dilution with fresh distilled water. The exact method of using this 
instrument will be described under the head of Nitrometer in Part VII. ; 
but so far as permanganate is concerned it was found that convenient 
quantities of substances to use were 10 c.c. of permanganate, 15 c.c. 
of ordinary 10 volume H 2 2 , and 30 c.c. of sulphuric acid 1 : 5. The 
nitrometer having been charged with water, the mixture was shaken up 
and allowed to stand ten minutes, shaken again and read off after five 
minutes. The volume of oxygen so obtained was corrected for temperature 
and pressure, then calculated into weight. The results of three experiments 
using the quantities mentioned above were as follows : 

1. Corrected volume of O 55'92 c.c.=0'0040C7 gm. 

2. 55-82 c.c.=0-004000 

3. 55'82 c.c.=0'004000 
Average 0'004002 gm. of oxygen per c.c. of solution. 

Three experiments with the same permanganate solution gave, when iron 
wire was used, an average of 0*00399 gm., and with oxalic acid 0'OU3997 gm. 
of oxygen respectively per c.c. 

Lunge says : *' Wo cannot but infer that standardizing a solution 



124 VOLUMETRIC ANALYSIS. 35. 

of permanganate with hydrogen peroxide in the nitrometer when 
observing the prescribed precautions is one of the most accurate 
known methods for this purpose, and withal possesses the great 
advantage that it is carried out within an extremely short time, 
without requiring a fundamental substance of accurately known 
composition." 

Many other substances have been proposed for standardizing 
permanganate, such as potassic ferrocyanate, thiocyanate, vanadic 
oxide, etc., but they are all inferior in value to those above named. 

3. Precautions in Titrating with Permanganate. 

Tt must be borne in mind that free acid is always necessary in 
titrating a substance with permanganate, in order to keep the 
resulting mangaiious oxide in solution. Sulphuric acid, in a dilute 
form, has no prejudicial effect on the pure permanganate, even at 
a high temperature. With hydrochloric acid the solution to be 
titrated must be very dilute and of low temperature, otherwise 
chlorine will be liberated and the analysis spoiled. This acid acts 
as a reducing agent on permanganate in concentrated solution, 
thus 

Mn 2 7 + 14HC1 = 7H 2 + 10C1 + 2MnCK 

The irregularities due to this reaction may be entirely obviated 
by the addition of a few grams of manganous or magnesic sulphate 
before titration. 

Organic matter of any kind decomposes the permanganate, and 
the solution therefore cannot be filtered through paper, nor can it 
be used in Mohr's burette, because it is decomposed by the 
india-rubber tube. It may, however, be filtered through gun 
cotton or glass wool. 

TITRATION OF FERRIC SALTS BY PERMANGANATE. 

35. ALL ferric compounds requiring to be estimated by 
permanganate must, of course, be reduced to the ferrous state. 
This is best accomplished by metallic zinc or magnesium in 
sulphuric acid solution. Hydrochloric may also be used with the 
precautions mentioned. 

The reduction occurs on simply adding to the warm diluted 
solution small pieces of zinc (free from iron, or at least with 
a known quantity present) or coarsely powdered magnesium until 
colourless ; or until a drop of the solution brought in contact with 
a drop of potassic thiocyanate produces no red colour. All the 
zinc or magnesium must be dissolved previous to the titration. 

The reduction may be hastened considerably as shown in 61.3. 

When the reduction is complete, no time should be lost in 
titrating the solution. 



36. STANDARD PERMANGANATE. 125 

CALCULATION OF ANALYSES MADE WITH 
PERMANGANATE SOLUTION. 

36. THE calculation of analyses with permanganate, if the 
solution is not strictly decinormal, can be made by ascertaining its 
coefficient, reducing the number of c.c. used for it to decinormal 
strength, and multiplying the number of c.c. thus found by -^3^-$ 
of the equivalent weight of the substance sought ; for instance 

Suppose that 15 c.c. of permanganate solution have been found 
to equal Ol gm. iron; it is required to reduce the 15 c.c. 
to decinormal strength, which would require 1000 c.c. of per- 
manganate to every 5'6 gm. iron, therefore 5*6 : 1000 : : O'l : x = 
17-85 c.c. ; 17-85 x 0-0056 = 0-09996 gm. iron, which is as near to 
O'l gm. as can be required. Or the coefficient necessary to reduce 
the number of c.c. used maybe found as follows: O-'l : 15 : : 

5'6 : .= 84 c.c,, therefore -o-p=-- 1 '19. Consequently 1-19 is the 

coefficient by which to reduce the number of c.c, of that special 
permanganate used in any analysis to the decinormal strength, from 
whence the weight of substance sought may be found in the 
usual way. 

Another plan is to find the quantity of iron or oxalic acid repre- 
sented by the permanganate used in any given analysis, and this being 
done the following simple equation gives the required result : 

Fe (56) eq. weight of the weight the weight of 

or : the substance : : of Fe or : substance 

O (63) sought O found sought 

In other words, if the equivalent weight of the substance analyzed 
be divided by 56 or 63 (the respective equivalent weights of iron or 
oxalic acid), a coefficient is obtained by which to multiply the 
weight of iron or oxalic acid, equal to the permanganate used, 
and the product i the weight of the substance titrated. 

For example : sulphuretted hydrogen is the substance sought, 
the eq. weight of H' 2 S corresponding to 2 eq. Fe is 17; let this 

17 
number therefore be divided by 56, -^ = 0-3036, therefore, if the 

quantity of iron represented by the permanganate used in an 
estimation of H 2 S be multiplied by 0'3036, the product will be 
the weight of the sulphuretted hydrogen sought. 

Again : in the case of manganic peroxide whose equivalent 
weight is 43*4. 

5=0.775 

The weight of iron therefore found by permanganate in any analysis 
multiplied by the coefficient 0*775 will give the amount of manganic 
peroxide, MnO 2 . Again: if m gm. iron = k c.c. permanganate, 

then 1 c.c permanganate = -r gm. metallic iron. 



126 VOLUMETRIC ANALYSIS. 37. 

The equivalents here given are on the hydrogen scale, in 
accordance with the normal system of solutions adopted ; and thus 
it is seen that two equivalents of iron are converted from the 
ferrous to the ferric state by the same quantity of oxygen as 
suffices to oxidize one equivalent of oxalic acid, sulphuretted 
hydrogen, or manganic peroxide. 

1 c.c. decinormal permanganate is equivalent to 
0'0056 gm. Fe estimated in the ferrous state 



0-0072 


FeO 





0-008 


Fe 2 O 3 




0-003733 


Fe 


from' PeS 


0-0059 


Bn 


SnCl 2 


0-00295 


Sn 


SnS 2 


0-00315 


Cu 


CuS 


0-00274 


Mn 


MnS 


0-00315 


Cu 


Cu+Fe 2 CF 


0-0063 


Cu 


CuO+Fe 


0-0017 


H 2 S 


55 35 


0-0008 







0-0063 


CT 




0-002 


Ca from 


CaC 2 O 4 


0-0120 


Ur 


UrO, etc., etc. 



When possible the necessary coefficients will be given in the 
tables preceding any leading substance. 



CHROMIC ACID AND FERROUS OXIDE. 

37. POTASSIC bichromate, which appears to have been first 
proposed by Penny, possesses the advantage over permanganate, 
that it is absolutely permanent in solution, may easily be obtained 
in a pure state, and its solution may be used in Mohr's burette 
without undergoing the change peculiar to permanganate : on the 
other hand, the end of the reaction in the estimation of iron can 
only be known by an external indicator ; that is to say, a drop of 
the mixture is brought in contact with a drop of solution of red 
potassic prussiate (freshly prepared) upon a white slab or plate. 
While the ferrous oxide is in tolerable excess, a rich blue colour 
occurs at the point of contact between the drops but as this 
excess continues to lessen by the addition of the bichromate, the 
blue becomes somewhat turbid, having first a green, then a grey, 
and lastly a brown shade. When the greenish-blue tint has all 
disappeared, the process is finished. This series of changes in the 
colour admits of tolerably sure reading of the burette, after some 
little practice is obtained. 

The .Reaction between chromic acid and ferrous oxide may be 
represented by the formula : 

2Cr0 8 + 6FeO - CrW + 3Fe 2 3 . 
The decomposition takes place immediately, and at ordinary 



STANDARD BICHROMATE. 

temperatures, in the presence of free sulphuric or hydrochloric acid. 
Xitric acid is of course inadmissible. 

The reduction of ferric compounds to the ferrous state may be 
accomplished by zinc,""" magnesium, sodic sulphite, ammonic 
bisulphite, or sulphurous acid; or, instead of these, stannous 
chloride may be used, which acts very rapidly as a reducing agent 
upon ferric oxide, the yellow colour of the solution disappearing 
almost immediately. 

In the analysis of iron ores, reduction by the latter is very rapid 
and serviceable ; the greatest care, however, is necessary that the 
stannous chloride is not present in excess, as this would consume 
the bichromate solution equally with the ferrous oxide, and so 
lead to false results. 

The discharge of the yellow colour of the iron solution may with 
care be made a very sure indicator of the exact point of reduction. 
But in order to obviate the inaccuracy which would be produced 
by an excess of tin in the state of protosalt, an aqueous solution of 
mercuric chloride should be added to the mixture in slight excess ; 
the stannous chloride is then all converted into stannic chloride, 
and the titration with bichromate may proceed fas usual ; 
a precipitate of Hg 2 Cl 2 does not interfere. The concentrated 
hydrochloric solution of iron is heated to gentle boiling, and the 
moderately dilute tin solution added with a pipette, waiting 
a moment for each addition till the last traces -of colour have 
disappeared ; the solution is then poured into a beaker, diluted 
with boiled and cooled water, mercuric solution added, and titrated 
with the bichromate as above described. See also 64. 

It is absolutely necessary that the solution of potassic ferri- 
cyanide used as the indicator with bichromate should be free from 
f errocyanide ; and as a solution when exposed to air for a short 
time becomes in some measure converted into the latter, it is 
necessary to use a freshly prepared liquid. 

1. Preparation of the Decinormal Solution of Bichromate. 
4 '9 13 gm. per liter. 

The reaction which takes place between potassic bichromate and 
ferrous oxide is, 

6FeO + Cr 2 K 2 0" - 3Fe 2 3 + Cr 2 3 + K 2 

It is therefore necessary that J eq. in grams should be used for the 
liter as a normal solution and ^ for the decinormal ; and as it is 
preferable on many accounts to use a dilute solution, the latter is 
the more convenient for general purposes. 

Taking the equivalent number of chromium as 52 '4, that of 
potassic bichromate is 294'S ; if, therefore, /^ of this latter number 

* When zinc is used, the zinc ferricyanide somewhat obscures the critical point in. 
testing with the indicator. 



128 VOLUMETRIC ANALYSIS. 38. 

= 4'913 gm. of the pure \vell dried salt be dissolved in a liter of 
water, the decinormal solution is obtained. 

1 c.c. of this solution is capable of yielding up y^^ eq. in 
grams of oxygen, and is therefore equivalent to the yo^jo^ eq. of 
any substance which takes up 1 equivalent of oxygen. 

2. Solution of Stannous Chloride. 

About 10 gm. of pure tin in thin pieces are put into a large 
platinum capsule, about 200 c.c. strong pure hydrochloric acid 
poured over it, and heated till it is dissolved ; or it may be 
dissolved in a porcelain capsule or glass flask, adding pieces of 
platinum foil to excite a galvanic current. The solution so 
obtained is diluted to about a liter with distilled water, and pre- 
served in the bottle (fig. 24) to which the air can only gain access 
through a strongly alkaline solution of pyrogallic acid. When 
kept in this manner, the strength will not alter materially in 
a month. If not so preserved, the solution varies considerably 
from day to day, and therefore should always be titrated before 
use as described in 64 if required for quantitative analysis. 

Examples of Iron Tit ration : 0'7 gm. of pure and dry ammonio-ferrous 
sulphate=0'l gm. iron, was dissolved in water, and titrated with decinormal 
bichromate, of which 17'85 c.c. were required; this multiplied by 0'03913 
gave 0'699 gm. instead of 0'7 gm. 

0'56 gm. of iron wire required 99'8 c.c. =0*5588 gm. ; as it is impossible 
to obtain iron wire perfectly pure, the. loss is undoubtedly owing to the 
impurities. 

IODINE AND SODIC THIOSULPHATE. 

38. THE principle of this now beautiful and exact method of 
analysis was first discovered by Dupasquier, who used a solution 
of sulphurous acid instead of sodic thiosulphate. Bun sen im- 
proved his method considerably by ascertaining the sources of 
failure to which it was liable, which consisted in the use of a too 
concentrated solution of sulphurous acid. The reaction between 
iodine and very dilute sulphurous acid may be represented by the 
formula 

SO 2 + 1 2 + 2H 2 = 2HI + H 2 SO. 

If the Sulphurous acid is more concentrated, i.e. above 4 04 per 
cent., in a short time the action is reversed, the irregularity of 
decomposition varying with the quantity of water present, and the 
rapidity with which the iodine is added.'"" 

Sulphurous acid, however, very rapidly changes by keeping even 
in the most careful manner, and cannot therefore be used for 
a standard solution. The substitution of sodic thiosulphate is 
a great advantage, inasmuch as the salt is easily obtained in 

* Tliis irregularity is now obviated by tie' method of Giles and Shearer ( 75.5), 
in which solutions of SO 2 or sulphites of any strength may be accurately titrated 
with iodine, by adding the latter to the former in excess, and when the reaction is 
complete titrating the excess of iodine with tMosulphate. 



38. IODOMETRY. 129 

a pure state, and may be directly weighed for the standard solution. 
The reaction is as follows : 



2Na 2 S 2 3 + 2I = 2]S 

the result being that thiosulphuric acid takes oxygen from the 
water, with the production of tetrathionic and hydriodic acids in 
combination with soda. 

In order to ascertain the end of the reaction in analysis by this 
method an indicator is necessary, and the most delicate and sensitive 
for the purpose is starch, which produces with the slightest trace of 
free iodine in cold solution the well-known blue iodide of starch. 
Hydriodic or mineral acids and iodides have no influence upon the 
colour. Caustic alkalies destroy it. 

The principle of this method, namely, the use of iodine as an 
indirect oxidizing body by its action upon the elements of water, 
forming hydriodic acid with the hydrogen, and liberating the oxygen 
in an active state, can be applied to the determination of a great 
variety of substances with extreme accuracy. 

Bodies which take up oxygen, and decolorize the iodine solution, 
such as sulphurous acid, sulphites, sulphuretted hydrogen, alkaline 
thiosulphites and arsenites, stannous chloride, etc., are brought 
into dilute solution, starch added, and the iodine delivered in 
with constant shaking or stirring until a point occurs at which 
a final drop of iodine colours the whole blue a sign that the 
substance can take up no more iodine, and that the drop in excess 
has shown its characteristic effect upon the starch. 

Free chlorine, or its active compounds, cannot, however, be 
titrated with thiosulphate directly, owing to the fact that, instead 
of tetrathionic acid being produced as with iodine, sulphuric acid 
occurs, as may be readily seen by testing with baric chloride. 
In such cases, therefore, the chlorine must be evolved from its 
compound and passed into an excess of solution of pure potassic 
iodide, where it at once liberates its equivalent- of iodine, which 
can then, of course, be estimated with thiosulphate. 

All bodies which contain available oxygen, and which evolve 
chlorine when boiled with strong hydrochloric acid, such as the 
chromates, manganates, and all metallic peroxides, can be readily 
and most accurately estimated by this method. 

1. Preparation of the Decinormal Solution of Iodine. 

I = 127 ; 12 '7 gm. per liter. 

Chemically pure iodine may be obtained by intimately mixing 
dry commercial iodine with about one-fourth of its weight of 
potassic iodide, and gently heating the mixture between two large 
watch-glasses or porcelain capsules ; the lower one being placed 
upon a heated iron plate, the iodine sublimes in brilliant plates, 
which, with the exception of a trace of moisture, are pure. 

K 



130 VOLUMETRIC ANALYSIS. 38. 

The watch-glass or capsule containing the iodine is placed under 
the exsiccator to cool, and also to deprive it of any traces of watery 
vapour; then 12*7 gm. are accurately weighed, and together with 
about 18 gm. of pure potassic iodide (free from iodate)""' dissolved, 
in ahout 250 c.c. of water, and diluted to a liter. The flask must 
not be heated in order to promote solution, and care must be taken 
that iodine vapours are not lost in the operation. 

The solution is best preserved in stoppered bottles, kept in the 
dark, and which should be completely filled ; but under any 
circumstances it does not hold its strength well for any length 
of time, and consequently should be titrated before use in 
analysis. 

The verification of the iodine solution may be done in many 
ways. Pure sodic thiosulphate prepared as described below, or 
a strictly -f$ solution of it, or again pure arsenious acid or its ^ 
solution, with the addition of a little sodic bicarbonate, or baric- 
thiosulphate as proposed by Plimpton and Chorley, may be 
used; this latter salt possesses a high molecular weight, 267 parts 
being equivalent to 127 of iodine, but being sparingly soluble in 
water the titration must be carefully done, inasmuch as the 
crystalline powder has to be gradually decomposed by the iodine,, 
and the end-point may easily be overstepped. A weighed quantity 
of the salt is put into a stoppered bottle with water, and the 
iodine run in from a burette with continuous shaking, until the 
salt is nearly dissolved; starch indicator is then added, and the 
iodine continued with shaking until the blue colour is faintly 
permanent. 

Pure baric thiosulphate is easily prepared by mixing together 
a warm solution of 50 gm. of sodic thiosulphate in 300 c.c. of 
water, and 40 gm. of baric chloride in a like volume of warm 
w r ater ; after stirring well, the salt soon separates in fine powdery 
crystals. These are collected in a funnel stopped with glass or 
cotton wool, repeatedly washed with cold water till all chlorine 
is removed, then dried at below 30 C. on a glass or porcelain 
plate until all extraneous moisture is removed ; or the crystals may 
be treated, after thorough washing with alcohol and ether, as. 
described below for sodic thiosulphate. 

2. Decinormal Sodic Thiosulphate. 
]S T a 2 S 2 3 , 5H 2 = 248-27 = 24*827 gm. per liter. 

It is not difficult either to manufacture or procure pure sodic 
thiosulphate, but there may be uncertainty as to extraneous water 

* Morse and Burton (Amer. Chem. Jour,, 1888) state that potassic iodide may be 
completely freed from iodate by boiling a solution of it with zinc amalgam, prepared 
by shaking zinc dust in good proportion with mercury in presence of tartaric acid, and 
washing with water. The iodate is completely reduced with formation of zinc hydroxide. 
The pure solution of iodide is filtered for use through a paper filter saturated with hot 
water. 



38. IODOMETEY. 131 

held within the crystals. In order to avoid this, Me in eke 
(Gli em. Zeit. xviii. 33) recommends that the otherwise pure crystals 
be broken to coarse powder, washed first with pure alcohol, then 
with ether, and lastly dried in a current of dry air at ordinary 
temperature. The salt so prepared may be weighed directly, and 
dissolved in a liter of distilled water, and then titrated with the 
iodine solution and starch indicator ; or it may be checked with 
j bichromate as recommended by Mohr, by digesting a measured 
volume of the bichromate with an excess of potassic iodide, and 
hydrochloric acid, in a well-stoppered flask at moderate heat. 
When the mixture has cooled, the liberated iodine is measured by 
the thiosulphate, and the working power of the latter ascertained. 
It is advisable to preserve the solution in the dark. After a time 
all solutions of thiosulphate undergo a slight amount of oxidation, 
and sulphur deposits upon the bottle ; it is therefore always 
advisable to titrate it previous to use. 

Beside the clecinornial iodine and thiosulphate, it is convenient 
in some cases to use centinormal solutions, which can readily be 
prepared by diluting the decinormal solution when required. 

In using the iodine solution Mohr's burette may be employed, 
but care must be taken that the solution is not left in it for 
any length of time, as decomposition slowly takes place, and the 
tube becomes hard ; the tap burette is on this account preferable. 

3. Starch. Indicator; 

One part of clean potato starch, or arrowroot, is first mixed 
smoothly with cold water into a thin paste, then gradually poured 
into about 150 or 200 times its weight of boiling water, the boiling 
continued for a few minutes, then allowed to stand and settle 
thoroughly ; the clear solution only is to be used as the indicator, 
of which a few drops only are necessary/' 5 ' The solution may 
be preserved for some long time by adding to it a few drops of 
chloroform, and shaking well in a stoppered bottle. 

Lintnar's soluble starch acts well as an indicator, as it gives at 
once a clear solution in boiling water. It is prepared by steeping 
potato starch, at ordinary temperature, for a week in dilute hydro- 
chloric acid, washing out the acid with repeated quantities of cold 
water,, and drying the starch at a moderate temperature. The 
colour which occurs with this form of starch is not quite so pure 
a blue as fresh ordinary starch, owing to the presence of some 
dextrine produced unavoidably in the preparation, but it is no 
hindrance to the end-point in practice. 

Concentrated Solution of Starch. This will keep any length of 
time. Made by rubbing about 5 gm. starch to a smooth emulsion, 

* In iodometric analyses it is always advisable in titrating- the free iodine with thio- 
sulphate or arsenious solution to delay adding the starch until the iodine colour is nearly- 
removed j a much more delicate ending may be obtained and with very little starch. 

K 2 



132 VOLUMETRIC ANALYSIS. 39. 

with about 50 c.c. water. Then add 25 c.c. of strong solution of 
caustic potash and shake well, dilute with half a liter of water, 
boil, and allow to settle. This indicator answers very well in 
cases where the alkali is of no consequence, but is not available 
for the delicate acidimetric method by iodic acid unless the alkali 
is exactly corrected. It answers well, however, with the addition 
of 2 gm. of potassic iodide as a reagent for nitrites, and keeps 
perfectly though exposed to light. 

ANALYSIS OF SUBSTANCES BY DISTILLATION WITH 
HYDROCHLORIC ACID. 

39. THERE are a great variety of substances containing oxygen, 
which when boiled with hydrochloric acid yield chlorine, equivalent 




to the whole or a part only of the oxygen they contain according to 
circumstances. Upon this fact are based the variety of analyses 
which may be accomplished by means of iodine and sodic thio- 
sulphate, or arsenite ; the chlorine so evolved, however, is not itself 
estimated, but is conveyed by means of a suitable apparatus into 
a solution of potassic iodide, thereby liberating an equivalent 
quantity of iodine. This latter body is then estimated by thio- 
sulphate ; the quantity so found is, therefore, a measure of the 
oxygen existing in the original substance, and consequently 
a measure of the substance itself. Analyses of this class may be 
made the most exact in the whole range of volumetric analysis, 
far outstripping any process by weight. 



39. 



IODOMETRY. 



The apparatus used for distilling the substances, and conveying 
the liberated chlorine into the alkaline iodide, may possess a variety 
of forms, the most serviceable, however, being the three kinds 
devised respectively by Bun sen, Fresenius, and Mohr. 

Bunsen's arrangement consists of an inverted retort, into the 
neck of which the tube from the small distilling flask is passed. 

Owing to the great solubility of HC1 in the form of gas, the 
apparatus must be so constructed that when all Cl is liberated and 
HC1 begins to distil, the liquid may not rush back to the flask 
owing to condensation. 




Fig. 38. 

The best preventive of this regurgitation is, however, suggested 
by Fresenius, and applicable to each kind of apparatus; namely, 
the addition of a few pieces of pure magnesite. This substance 
dissolves but slowly in the hydrochloric acid, and so keeps up 
a constant flow of CO 2 , the pressure of which is sufficient to 
prevent the return of the liquid. 

The apparatus contrived by Fresenius is shown in fig. 37, and 
is exceedingly useful as an absorption apparatus for general 
purposes. 



VOLUMETRIC ANALYSIS. 39. 

Mo hr's apparatus is shown in fig. 38 and is, on account of its 
simplicity of construction, very easy to use. 

The distilling flask is of about 2 oz. capacity, and is fitted with 
a cork soaked to saturation in melted paraffin ; through the cork 
the delivery tube containing one bulb passes, and is again passed 
through a common cork, fitted loosely in a stout tube about 12 or 
13 inches long and 1 inch wide, closed at one end like a test tube. 
This tube, containing the alkaline iodide, is placed in an hydrometer 
glass, about 12 inches high, and surrounded by cold water ; the 
delivery tube is drawn out to a fine point, and reaches nearly to 
the bottom of the condenser. No support or clamp is necessary, 
as the hydrometer glass keeps everything in position. The 
substance to be distilled is put into the flask and covered with 
strong hydrochloric acid, the magnesite added, the condenser 
supplied with a sufficient quantity of iodide solution, and the 
apparatus put together tightly. Either an argand or common 
spirit lamp, or gas, may be used for heating the flask, but the 
flame must be manageable, so that the boiling can be regulated at 
will. In the case of the common spirit lamp it may be held in the 
hand, and applied or withdrawn according to the necessities of the 
case : the argand spirit or gas lamp can, of course, be regulated by 
the usual arrangements for the purpose. If the iodine liberated 
by the chlorine evolved should be more than will remain in 
solution, the cork of the condensing tube must be lifted, and more 
solution added. "When the operation is judged to be at an end, 
the apparatus is disconnected, and the delivery tube washed out 
into the iodide solution, which is then emptied into a beaker or 
flask and preserved for titration, a little fresh iodide solution is 
put into the condenser, the apparatus again put together, and 
a second distillation commenced, and continued for a minute or so, 
to collect every trace of free chlorine present. This second 
operation is only necessary as a safeguard in case the first should 
not have been complete. 

The solutions are then mixed together and titrated in the 
manner previously described. In all cases the solution must be 
cooled before adding the thiosulphate, otherwise sulphuric acid 
might be formed. 

Instead of the large test tube, some operators use a (J tube to 
contain the potassic iodide, having a bulb in each limb, but the 
latter is not necessary if magnesite is used. 

The solution of potassic iodide may conveniently be made of 
such a strength that T 2 ^- eq. or 33 '2 gin. are contained in the liter. 
1 c.c. will then be sufficient to absorb the quantity of free iodine, 
representing 1 per cent, of oxygen in the substance analyzed, 
supposing it to be weighed in the metric system. In examining 
peroxide of manganese, for instance, 0'436 gm. or 4*36 grn. would 
be used, and supposing the percentage of peroxide to be about 
sixty, 60 c.c. or dm. of iodide solution would be sufficient to absorb 



IODOMETRY. 



135 



the chlorine and keep in solution the iodine liberated by the 
process ; it is advisable, however, to have an excess of iodide, and, 
therefore, in this case, about 70 c.c. or dm. should be used. 
A solution of indefinite strength will answer as well, so long as 
enough is used to absorb all the iodine. It may sometimes happen 
that not enough iodide is present to keep all the liberated iodine in 
solution, in which case it will separate out in the solid form ; more 
iodide, however, may be added to dissolve the iodine, and the 
titration can then be made as usual. 

The process of distillation above described may be avoided in 
many cases. There are a great number of substances which, by 
mere digestion with hydrochloric acid and potassic iodide at an 
elevated temperature, undergo decomposi- | 
tion quite as completely as by distillation, sjjf 
For this purpose a strong bottle with 
a very accurately ground stopper is neces- 
sary ; and as the ordinary stoppered bottles 
of commerce are not sufficiently tight, it is 
better to re-grind the stopper with a little 
very fine emery and water. It must then 
be tested by tying the stopper tightly 
down and immersing in hot water ; if any 
bubbles of air find their way through the 
stopper the bottle is useless. The capa- 
city may vary from 30 to 150 c.c., accord- 
ing to the necessities of the case. 

The stopper may be secured by fine copper binding-wire, or 
a kind of clamp contrived by Mohr may be used, as shown in 
fig. 39 ; by means of the thumb-screws the pressure upon the 
stopper may be increased to almost any extent. 

The substance to be examined, if in powder, is put into the 
bottle with pure flint pebbles or small garnets, so as to divide it 
better, and a sufficient quantity of saturated solution of potassic 
iodide and pure hydrochloric acid added ; the stopper is then 
inserted, fastened down, and the bottle suspended in a water 
bath, and the water is gradually heated to boiling by a gas 
name or hot plate as may be most convenient. When the 
decomposition is complete the bottle is removed, allowed to cool 
someAvhat, then placed in cold water, and, after being shaken, 
emptied into a beaker, and the liquid diluted by the washings 
for titration. 

The salts of chloric, iodic, bromic, and chromic acids, together 
with many other compounds, may be as effectually decomposed by 
digestion as by distillation ; many of them even at ordinary tem- 
peratures. Recently precipitated oxides, or the natural oxides, 
when reduced to fine powder are readily dissolved and de- 
composed by very weak acid in the presence of potassic iodide 
(Pickering). 




136 VOLUMETRIC ANALYSIS. 40. 

The potassic iodide used in the various analyses must be abso- 
lutely free from potassic iodate and free iodine, or if otherwise, the 
effect of the impurity must be known by blank experiment. 

ARSENIOTJS ACID AND IODINE. 

40. THE principle upon which this method of analysis is 
based is the fact, that when arsenious acid is brought in contact 
with iodine in the presence of water and free alkali, it is converted 
into arsenic acid, the reaction being 

AS 2 3 + 41 + 2K' 2 = As 2 5 + 4KI. 

The alkali must be in sufficient quantity to combine with the 
hydriodic acid set free, and it is necessary that it should exist in 
the state of bicarbonate, as caustic or monocarbonated alkalies 
interfere with the colour of the blue iodide of starch used as indicator. 

If, therefore, a solution of arsenious acid containing starch is 
titrated with a solution of iodine in the presence of an alkaline 
bicarbonate, the blue colour does not occur until all the arsenious 
acid is oxidized into arsenic acid. In like manner, a standard 
solution of arsenious acid may be used for the estimation of iodine 
or other bodies which possess the power of oxidizing it. 

The chief value, however, of this method is found in the 
estimation of free chlorine existing in the so-called chloride of 
lime, chlorine water, hypochlorites of lime, soda, etc., in solution; 
generally included under the term of chlorimetry. 

Preparation of the ^ Solution of Alkaline Arsenite. 
As 2 3 = 198 ; 4-95 gm. per liter. 

The iodine solution is the same as described in 38. 

The corresponding solution of alkaline arsenite is prepared by 
dissolving 4'95 gm. of the purest sublimed arsenious oxide in 
about 250 c.c. of distilled water in a flask, with about 20 gm. of 
pure sodic carbonate.'"" It is necessary that the acid should be in 
powder, and the mixture needs warming and -shaking for some 
time in order to complete the solution ; when this is accomplished 
the mixture is diluted somewhat, cooled, then made up to the 
liter. 

In order to test this solution, 20 c.c. are put into a beaker with 
a little starch indicator, and the iodine solution allowed to flow in 
from a burette, graduated in -^ c.c. until the blue colour appears. 
If exactly 20 c.c. are required, the solution is strictly decinormal ; 
if otherwise, the necessary factor must be found for converting it 
to that strength. 

* In the previous edition of this book, the arsenkms solution was recommended to 
be made with alkaline bicarbonate, but this has, after keeping, been found to give 
defective results with bleach analyses from some cause not yet understood. 



40. . IODOMETRY. 137 

Iodized Starch-paper. Starch solution cannot be used for the 
direct estimation of free chlorine, consequently resort must be had 
to an external indicator ; and this is very conveniently found in 
starch-iodide paper, which is best prepared by mixing a portion of 
starch solution with a few drops of solution of potassic iodide on 
a plate, and soaking strips of pure filtering paper therein. The 
paper so prepared is used in the damp state, and is far more 
sensitive than when dried. 

Example of Titration : 50 c.c. of chlorine water were mixed with solution 
of sodic carbonate, and brought under the arsenic burette, and 20 c.c. of 
solution added ; on touching the prepared paper with the mixture no colour 
was produced, consequently the quantity used was too great; starch was 
therefore added, and decinorinal iodine, of which 3"2 c.c. were required to 
produce the blue colour. This gave 16'8 c.c. of arsenious solution, which 
multiplied by 0'003537, gave 0*05942 gm. of Cl in the 50 c.c. A second 
operation with the same water required 16'8 c.c. of arseuious solution direct, 
before the end of the reaction with iodized starch-paper was reached. 




138 VOLUMETRIC ANALYSIS. 41. 



PART IV. 
ANALYSIS BY PRECIPITATION. 

41. THE general principle of this method of determining the 
quantity of any given substance is alluded to in 1, and in all 
instances is such that the body to be estimated forms an insoluble 
precipitate with a titrated reagent. The end of the reaction is, 
however, determined in three ways. 

1. By adding the reagent until no further precipitate occurs, 
as in the determination of chlorine by silver. 

2. By adding the reagent in the presence of an indicator con- 
tained either in the liquid itself, or brought externally in contact 
with it, so that the slightest excess of the reagent shall produce 
a characteristic reaction with the indicator ; as in the estimation 
of silver with sodic chloride by the aid of potassic chromate, or 
with thiocyanate and ferric sulphate, or that of phosphoric acid 
with uranium by yellow potassic prussiate. 

3. By adding the reagent to a clear solution, until a precipitate 
occurs, as in the estimation of cyanogen by silver. 

The first of these endings can only be applied with great accuracy 
to silver and chlorine estimations. Very few precipitates have the 
peculiar quality of chloride of silver ; namely, almost perfect 
insolubility, and the tendency to curdle closely by shaking, so as to 
leave the menstruum clear. Some of the most insoluble precipitates, 
such as baric sulphate and calcic oxalate, are unfortunately excluded 
from this class, because their finely divided or powdery nature 
prevents their ready and perfect subsidence. 

In all these cases, therefore, it is necessary to find an indicator, 
which brings them into class 2. 

The third class comprises only two processes; viz., the deter- 
mination of cyanogen by silver, and that of chlorine by mercuric 
nitrate. 

Since the estimation of chlorine by precipitation with silver, 
and that of silver by thiocyanic acid, can be used in many cases 
for the indirect estimation of many other substances with great 
exactness, the preparation of the necessary standard solutions will 
now be described. 

SILVER AND CHLORINE. 
I. Decinormal Solution of Silver. 

10-766 gm. Ag or 16-^66 gm. AgXO :! per liter. 

10 '7 6 6 gm. of pure silver are dissolved in pure dilute nitric acid 
with gentle heat in a flask, into the neck of which a small funnel 
is dropped to prevent loss of liquid by spirting. T\ r hen solution 
is complete, the funnel must be washed inside and out with 



41. PKECIPITATIOX ANALYSES. 139 

distilled water into the flask, and the liquid diluted to a liter ; but 
if it be desired to use potassic chromate as indicator in any analysis, 
the solution must be neutral ; in which case the solution of silver 
in nitric acid is evaporated to dryness, and the residue dissolved in 
a liter; or, what is preferable, 16-966 gm. of pure silver nitrate, 
previously heated to 120 C. for ten minutes, are dissolved in 
a liter of distilled water. 

2. Decinormal Solution of Salt. s 

5-837 gm. 2s r aCl per liter. 

5-837 gm. of pure sodic chloride are dissolved in distilled water, 
and the solution made up to a liter. 

There are two methods by which the analysis may be ended : 

(a) By adding silver cautiously, and well shaking after each 
addition till no further precipitate is produced. For details 
see 73. 

(/;) By using a few drops of solution of pure potassic rnono- 
chromate as indicator, as devised by Mohr. If the pure salt is 
not at hand, some silver nitrate should be added to the solution of 
the ordinary salt, to remove chlorine, and the clear liquid used. 

The method I is exceedingly serviceable, on the score of saving 
both time and trouble. The solutions must be neutral, and cold. 
When, therefore, acid is present in any solution to be examined, it 
should be neutralized with pure sodic or calcic carbonate in very 
slight excess.* 

Process : To the neutral or faintly alkaline solution, two or three 
drops of a cold saturated solution of chromate are added, and the silver 
solution delivered from the burette until the last drop or two produce 
a faint blood-red tinge, an evidence that all the chlorine has combined with 
the silver, and the slight excess has formed a precipitate of silver chromate ; 
the reaction is very delicate and easily distinguished. The colour reaction 
is even more easily seen by gas-light than by daylight. It may be rendered 
more delicate by adopting the plan suggested by I) up re (Analyst v. 123). 
A glass cell, about 1 centimeter in depth, is filled with water tinted with 
chromate to the same colour as the solution to be titrated. The operation is 
performed in a white porcelain basin. The faintest appearance of the red 
change is at once detected on looking through the coloured cell. Tor the 
analysis of waters weak in chlorine this method is very serviceable, 
but contrary to what has been generally accepted, the accuracy of the 
results are seriously interfered with by great dilution or high temperature 
(W. G. Young, Analyst xviii. 125). As is the case with most volumetric 
processes, it is therefore necessary in order to secure a high degree of 
accuracy to titrate under the same conditions under which the standard was 
fixed. 

Example: 1 gm. of pure sodic chloride was dissolved in 100 c.c. of water, 
a few drops of chromate added, and titrated with ^V silver, of which 17'1 c.c. 
were required to produce the red colour ; multiplied by the T *V factor for 
sodic chloride = 0'005837 the result was 0'998 gm. NaCl, instead of 1 gm. 

* Silver chromate is sensibly soluble in the presence of alkaline or earthy nitrates, 
especially at a high temperature ; sodic and calcic hydrates have the least effect ; 
arninonic, potassic, and inagnesic nitrates the greatest. See also Forbes Carpenter 
(J. S. C. I. v. 286). 



140 VOLUMETRIC ANALYSIS. 42. 

INDIRECT ESTIMATION OF AMMONIA, SODA., POTASH, 
LIME, AND OTHER ALKALIES AND ALKALIN * EAxtTH ?, 
WITH THEIR CARBONATES, NITRATES, AND CHLO- 
RATES, ALSO NITROGEN, BY MEANS OF DECINORMAL 
SILVER SOLUTION, AND POTASSIC CHROMATE, AS 
INDICATOR. 

1 CiC . _*_ silver solution = T olyo^ ^. eq. of each substance. 



42. MOHR, with, his characteristic ingenuity has made use 
of the delicate reaction between chlorine and silver, with potassic 
chromate as indicator, for the determination of the bodies men- 
tioned above. All compounds capable of being converted into. 
neutral chlorides by evaporation to dryness with hydrochloric acid 
may be determined with great accuracy. The chlorine in a com- 
bined state is, of course, the only substance actually determined ; 
but as the laws of chemical combination are exact and well known, 
the measure of chlorine is also the measure of the base with which 
it is combined. 

In most cases it is only necessary to slightly supersaturate the 
alkali, or its carbonate, with pure hydrochloric acid ; evaporate on 
the water bath to dryness, and heat for a time to 120 C. in the air 
bath, then dissolve to a given measure, and take a portion for 
titration ; too great dilution must be avoided. 

Alkalies and Alkaline Earths with organic acids are ignited to 
convert them into carbonates, then treated with hydrochloric acid, 
and evaporated as before described. 

Carbonic Acid in combination may be determined by precipita- 
tion with baric chloride, as in 23. The washed precipitate is 
dissolved on the filter with hydrochloric acid (covering it with 
a watch-glass to prevent loss), and then evaporated to dryness 
repeatedly till all HC1 is driven off. In order to titrate with 
accuracy by the help of potassic chromate, the baryta must 
be precipitated by means of a solution of pure sodic or potassic 
sulphate, in slight excess ; the precipitated baric sulphate does 
not interfere with the delicacy of the reaction. If this precaution 
Avere not taken, the yellow baric chromate would mislead. 

Free Carbonic Acid is collected by means of ammonia and baric 
chloride (as in 23), and the estimation completed as in the case of 
combined CO 2 . 

Chlorates are converted into chlorides by ignition before titration. 

Nitrates are evaporated with concentrated hydrochloric acid, and 
the resulting chlorides titrated, as in the previous case. 

Nitrogen. The ammonia evolved from guano, manures, oilcakes, 



42. PRECIPITATION ANALYSES. 141 

and sundry other substances, when burned with soda lime or 
obtained by the Kj el da hi method, is conducted through dilute 
hydrochloric acid; the liquid is carefully evaporated to dryness 
before titration. 

In all cases the operator will, of course, take care that no chlorine 
from extraneous sources other than the hydrochloric acid is present; 
or if it exists in the bodies themselves as an impurity, its quantity 
must be first determined. 

Example : 0'25 gm. pure sodic carbonate was dissolved in water, and 
hydrochloric acid added till in excess ; it was then dried on the water bath 
till no further vapours of acid were evolved; the resulting white mass was 
heated for a few minutes to about 150 C., dissolved and made up to 30 c.c. 
100 c.c. required 15'7 c.c. T ^ silver, this multiplied by 3 gave 47'1 c.c., which 
multiplied by the T ^ factor for sodic carbonate 0'0053, gave 0'24963 gm. 
instead of 0*25 gm. 

Indirect Estimation of Potash and Soda existing as Mixed 
Chlorides. It is a problem of frequent occurrence to determine the 
relative quantities of potash and soda existing in mixtures of the 
two alkalies, such as occur, for instance, in urine, manures, soils, 
waters, etc. The actual separation of potash from soda by means 
of platinum is tedious, and not always satisfactory. 

The following method of calculation is frequently convenient, 
since a careful estimation of the chlorine present in the mixture is 
the only labour required ; and this can most readily be accom- 
plished by ~Q silver and chromate, as previously described. 

(1) The weight of the mixed pure chlorides is accurately found and noted. 

(2) The chlorides are then dissolved in water, and very carefully titrated 
with / silver and chromate for the amount of chlorine present, which is 
also recorded ; the calculation is then as follows : 

The weight of chlorine is multiplied by the factor 2'103 ; from the product 
so obtained is deducted the weight of the mixed salts found in 1. The 
remainder multiplied by 3'6288 will give the weight of sodic chloride 
present in the mixture. 

The weight of sodic chloride deducted from the total as found in 1 will 
give the weight of potassic chloride. 

Sodic chloride x 0'5302=Soda (Na e O). 
Potassic chloride x 0'63l7=Potash (K 2 O). 

The principle of the calculation, which is based on the atomic constitution 
of the individual chlorides, is explained in most of the standard works on 
general analysis. Indirect methods like this can only give useful results 
when the atomic weights of the two substances differ considerably, and when 
the proportions are approximately equal. 

Another method of calculation in the case of mixed potassic and 
sodic chlorides is as follows : 

The weight of the mixture is first ascertained and noted ; the chlorine is 
then found by titration with T ^ silver, and calculated to NaCl : the weight 
so obtained is deducted from the original weight of the mixture, and the 
remainder multiplied by 2'42857 will give the potassium. 



142 VOLUMETRIC ANALYSIS. 43. 

SILVER AND THIOCYANIC ACID. 

43. THIS excellent and most accurate method has been devised 
"by Volhard and is fully described by the author (Lielig's Ann. cL 
CJiem. cxc. 1), and has been favourably noticed by Falck (Z. a. C. 
xiv. 227), Briigelman (Z. a. C. xvi. 7), and Drechsel (Z. a. C. 
xvi. 351), and many other well known chemists. It differs from 
Mohr's chromate method in that the silver solutions may contain 
free nitric acid, which renders it of great service in indirect 
analyses. 

This method is based on the fact that when solutions of silver 
and an alkaline thiocyanate are mixed in the presence of a ferric 
salt, so long as silver is in excess, the thiocyanate of that metal 
is precipitated, and any brown ferric thiocyanate which may 
form is at once decomposed. When, however, the thiocyanate is 
added in the slightest excess, brown ferric thiocyanate is formed, 
and asserts its colour even in the presence of much free acid. 
The method may of course be used for the estimation of silver, 
and by the residual process, for the estimation of substances which 
are completely precipitated by silver.'"" 

It may be used for the estimation of silver in the presence of 
copper up to 70 per cent. ; also in presence of antimony, arsenic, 
iron, zinc, manganese, lead, cadmium, bismuth, and also cobalt and 
nickel, unless the proportion of these latter metals is such as to 
interfere by intensity of colour. 

It may further be used for the indirect estimation of chlorine, 
bromine, and iodine, in presence of each other, existing either in 
minerals or inorganic compounds, and for copper, manganese, and 
zinc ; these will be noticed under their respective heads. 

1, Decinormal Ammonic or Potassic Thiocyanate. 

This solution cannot be prepared by weighing the thiocyanate 
direct, owing to the deliquescent nature of the salts; therefore 
about 8 gm. of the ammonium, or 10 gm. of the potassium salt 
may be dissolved in about a liter of water as a basis for getting an 
exact solution, which must be finally adjusted by a correct 
decinormal silver solution. 

The standard solution so prepared remains of the- same strength 
for a very long period if preserved from evaporation. 

2. Decinormal Silver Solution. 

This is the same as described in a preceding section ( 41), and 
may contain free nitric acid if made direct from metallic silver. 

* In cases where chlorine is precipitated by excess of silver, and the excess has to be 
found by thiocyanate, experience has proved that it is absolutely necessary to filter off 
the chloi'ide and titrate the filtrate and washings. If this be not done the solvent 
effect of the thiocyanate upon the AgCl will give inaccurate results. This fact seems 
to have been overlooked at the time the method was first introduced. 



44. COLOUR REACTIONS. 143 

3. Ferric Indicator. 

This may consist simply of a saturated solution of iron alum ; 
or may be made by oxidizing ferrous sulphate with nitric acid, 
evaporating with excess of sulphuric acid to dissipate nitrous 
fumes, and dissolving the residue in water so that the strength 
is about 10 per cent. 

5 c.c. of either of these solutions are used for each titration, 
which must always take place at ordinary temperatures. 

4. Pure Nitric Acid. 

This must be free from the lower oxides of nitrogen, secured by 
diluting the usual pure acid with about a fourth part of water, 
and boiling till perfectly colourless. It should then be preserved 
in the 'dark. 

The quantity of nitric acid used in the titration may vary 
within wide limits, and seems to. have no effect upon the precision 
of the method. 

The Process for Silver : 50 c.c. of & silver solution are placed into 
a flask, diluted somewhat with water, and 5 c.c. of ferric indicator added, 
together with about 10 c.c. of nitric acid. If the iron solution should 
cause a yellow colour, the nitric acid will remove it. The thiocyauate is 
then delivered in from a burette; at first a white precipitate is produced 
rendering the fluid of a milky appearance, and as each drop of thiooyanate 
falls in, it produces a reddish-brown cloud which quickly disappears on 
shaking. As the point of saturation approaches, the precipitate becomes 
flocculent and settles easily ; finally, a drop or two of thiocyanate produces 
a faint brown colour which no longer disappears on shaking. If the 
solutions are correctly balanced, exactly 50 c.c. of thiocyanate should be 
required to produce this effect. 

The colour is best seen ~by holding the flask so as to catch the reflected 
light of a white Avail or a suspended sheet of white paper. 

PRECISION IN COLOUR REACTIONS: 

44. DUPRE adopts the following ingenious method for colour 
titrations (Analyst v. 123) : As is well known, the change from 
pale yellow to red, in the titration of chlorides by means of silver 
nitrate with neutral chromate as indicator, is more distinctly 
perceived by gas-light than by daylight ; and in the case of potable 
waters, containing from one to two grains of chlorine per gallon, 
it is absolutely necessary to concentrate by evaporation previous 
to titration, or else to perform the titration by gas-light. The 
adoption of the following simple plan enables the operator to 
perceive the change of colour as sharply, and with as great 
a certainty, by daylight as by gas-light. Nevertheless, as has 
been before mentioned, it is impossible to get accurate results 
with very weak solutions of chlorine unless the silver solution 
is standardized upon similar solutions. 

The water is placed into a white porcelain dish (100 c.c. are 
a useful quantity), a moderate amount of neutral chromate is added 



144 VOLUMETRIC ANALYSIS. 4-i. 

(sufficient to impart a marked yellow colour to the water), but 
instead of looking at the water directly, a flat glass cell containing 
some of the neutral chromate solution is interposed between the 
eye and the dish. The effect of this is to neutralize the yellow 
tint of the water ; or, in other words, if the concentration of the 
solution in the cell is even moderately fairly adjusted to the depth 
of tint imparted to the water, the appearance of the latter, looked 
at through the cell, is the same as if the dish were filled with pure 
water. If now the standard silver solution is run in, still looking 
through the cell, the first faint appearance of a reel coloration 
becomes strikingly manifest; and what is more, when once the 
correct point has been reached the eye is never left in doubt, how- 
ever long we may be looking at the water. A check experiment 
in which the water, with just a slight deficiency of silver, or excess 
of chloride, is used for comparison is therefore unnecessary. 

A similar plan will be found useful in other titrations. Thus, 
in the case of turmeric, the change from yellow to brown is per- 
ceived more sharply and with greater certainty when looking 
through a flat cell containing tincture of turmeric of suitable 
concentration than with the naked eye. The liquid to be titrated 
should, as in the former case, be placed into a white porcelain dish. 
Again, in estimating the amount of carbonate of lime in a water 
by means of decinormal acid and cochineal, the exact point of 
neutrality can be more sharply fixed by looking through the 
cell filled with a cochineal solution. In this case the following 
plan is found to answer best. The water to be tested about 
250 c.c. is placed into a flat porcelain evaporating dish, part of 
which -is covered over with a white porcelain plate. The water is 
now tinted with cochineal as usual, and the sulphuric acid run in, 
the operator looking at the dish through the cell containing the 
neutral cochineal solution. At first the tint of the water and the 
tint in which the porcelain plate is seen are widely different ; as, 
however, the carbonate becomes gradually neutralized, the two 
tints approach each other more and more, and when neutrality is 
reached they appear identical ; assuming that the strength of the 
cochineal solution in the cell, and the amount of this solution 
added to the water, have been fairly well matched. Working in 
this manner it is not difficult (taking |- liter of water) to come 
within 0*1 c.c. of decinormal acid in two successive experiments, 
and the difference need never exceed 0'2 c.c. In the cell employed 
the two glass plates are a little less than half an inch apart. 

A somewhat similar plan may be found useful in other titrations, 
or, in fact, in many operations depending on the perceptions of 
.colour change. 



45. ALUMINIUM. 145 

PAET V. 

APPLICATION OF THE FOREGOING PRINCIPLES OF 
ANALYSIS TO SPECIAL SUBSTANCES. 

AL.TJMINITJM. 

Al = 27-3. 

45. ALUMINIUM salts (the alums and aluminium sulphates 
used in dyeing and paper-making) may be titrated for alumina 
in the absence of iron (except in mere traces) by mixing the acid 
solutions with a tolerable quantity of sodic acetate, then a known 
volume in excess of y^ phosphate solution (2O9 gm. of ammonio- 
sodic phosphate per liter), heating to boiling, without filtration; the 
excess of phosphate is found at once by titration with standard 
uranium. If iron in any quantity is present, it may be estimated 
in a separate portion of the substance, and its amount deducted 
before calculating the alumina. The latter is precipitated as 
A1P0 4 , and any iron in like manner as FePO 4 . Each c.c. of ~ 
phosphate = 0*005 13 gm. A1 2 O 3 . Only available for rough purposes. 

Baeyer's Method. As originally proposed, this process for 
estimating alumina in alums and aluminic sulphates was carried 
out by two titrations, a measured portion of the solution being first 
treated with an excess of normal soda in sufficient quantity to 
dissolve the precipitate of hydrate of alumina first formed. It was 
then diluted to a definite volume, half being titrated with normal 
acid and litmus, other half with tropoeolin 00, the difference being 
calculated to alumina. 

A considerable improvement however has been made by using 
phenolphthalein as the indicator, one titration only being necessary. 
The method is based on the fact that, if to a solution of alumina, 
containing the indicator, normal soda is added in excess, or until 
the red colour is produced, normal acid be then added until the 
colour disappears, the volume of acid so required is less than the 
soda originally added in proportion to the quantity of alumina 
present. 

The volume of acid which so disappears is in reality the quantity 
necessary to combine with the alumina set free by the alkali ; and 
if this deficient measure of acid be multiplied by the factor 
0-01716 (J mol. wt. of A1 2 3 ), the weight of alumina will be 
obtained. This factor is given on the assumption that the normal 
sulphate APS SO 4 is formed. 

The titration must take place in the cold and in dilute solutions. 
Very fair technical results have been obtained by me with 

L 



146 VOLUMETRIC ANALYSIS. 45. 

potash and ammonia alums and the commercial sulphates of 
alumina. 

Alumina existing as aluminate of alkali in caustic soda, for 
instance, may be very well estimated by taking advantage of the 
fact, that such alumina is quite indifferent to methyl orange, but 
reacts acid with phenolphthalein. This fact has been recorded by 
Thomson and others, but the priority of discovery appears to be 
due to Baeyer (Z. a. C. xxiv. 542), who, however, used litmus in 
the place of phenolphthalein and tropceolin 00 instead of methyl 
orange. 

Cross and Be van (J. S. C. I. viii. 252) in their examination of 
caustic soda for alumina, found by experiment, that the mean of 
the differences between the titration with methyl orange and 
phenolphthalein required the factor O0205 per c.c. of normal acid 
for the alumina, pointing to the salt as 2AP0 3 : 5S0 3 . 

The estimation of the alumina in caustic soda has given rise to 
much discussion between even very experienced operators, notably 
MM. Cross and Bevan and Lunge, but the former chemists 
have proved, as far as possible, by various methods, the accuracy 
of their views that the above-named equation is correct. The 
method adopted by them consists in boiling the weighed sample 
with a slight excess of standard acid, allowing to cool and titrating 
back with standard soda and phenolphthalein. The acid so con- 
sumed represents the total alkali present. To a similar portion 
a slight excess of acid is added and titrated back with soda and 
methyl orange. 

Estimation of free Acid. Alum cakes or aluminic sulphates of 
various kinds often contain free H 2 S0 4 , and many methods have 
been proposed for its estimation. Baeyer titrates a 10 per cent, 
solution of the substance in water with normal soda, and tropceolin 
00 or methyl orange. 

R. Williams (C. N. Ivi. 194) adopts the -alcohol method by 
digesting the substance for at least twelve hours with strong 
alcohol, filtering off and washing with the same agent, and titrating 
the solution without dilution or evaporation with -$ acid and 
phenolphthalein. 

Beilsteiii and Grosset (Bull, de I'Academie Imp. des Sciences 
de St. Petersburg, xiii. 41) have examined with great care all the 
proposed methods for this purpose, and have devised one which 
gives very good technical results. 

Process : 1 to 2 gin. of substance is dissolved in 5 c.c. of water, 5 c.c. 
of a cold saturated neutral solution of ammonic sulphate added, and stirred 
for a quarter of an hour. 50 c.c. of 95 per cent, alcohol are then added, the 
mixture thrown on a small filter, and washed with 50 c.c. of the same 
alcohol. The nitrate is evaporated on the water bath, the residue dissolved 
in water, and titrated with ^ alkali and litmus. The whole of the neutral 
aluminic sulphate is precipitated as ammonia alum, the alcohol contains all 
the free acid. 



46. ANTIMONY. 147 

ANTIMONY. 

Sb = 120. 

1. Conversion of Antimonious Acid in Alkaline Solution into 
Antimonic Acid by Iodine (Mohr). 

46. ANTIMONIOUS oxide, or any of its compounds, is brought 
into solution as tartrate by tartaric acid and water ; the excess of 
acid neutralized by sodic carbonate ; then a cold saturated solution 
of sodic bicarbonate added, in the proportion of 10 c.c. to about 
O'l gm. Sb 2 3 ; to the clear solution starch is added and 
iodine until the blue colour occurs. No delay must occur in the 
titration when the bicarbonate is added, otherwise a portion of the 
metal is precipitated as antimonious hydrate, upon which the iodine 
fas little effect.* 

For the estimation of antimonic acid and its salts, G.vonKnorre 
(Zeit. Angeic. Chem., 1888, 155) gives the following method as 
accurate : 

The solution of the salt, strongly acidified with hydrochloric acid, is treated 
in a roomy flask with strong solution of sodic sulphide, added gradually in 
small portions. It is then vigorously boiled until all SO 2 is expelled, a drop 
of phenolphthalein is added, then caustic potash until red ; this is in turn 
removed by a small excess of tartaric acid. Sodic bicarbonate is then added, 
and the titration with iodine carried out in the usual way. 

The colour disappears after a little time, therefore the first 
appearance of a permanent blue is accepted as the true measure 
of iodine required. 

1 c.c. T ^ iodine = 0-0060 gm. Sb. 

Estimation of Antimony in presence of Tin (Type and Britannia 
metal, etc.). The finely divided alloy is dissolved in strong hydro- 
chloric acid by heat, adding frequently small quantities of potassic 
chlorate. The liquid is boiled to remove free chlorine, cooled, 
a slight excess of strong solution of potassic iodide added, and 
the liberated iodine estimated by standard thiosulphate. Some 
operators prefer to collect the liberated iodine in carbon bisulphide 
previous to titration. 

120 Sb liberate 253 I, and the weight of I found multiplied 
by 0-47 5 = Sb. 

If iron or other metal capable of liberating iodine be present, 
treat the alloy with nitric acid, and evaporate to obtain the oxides 
of antimony and tin wash, boil in hydrochloric acid, and proceed 
as before described. The rationale is, that antimonic chloride is 
reduced to antimonious chloride, while stannic chloride is not affected. 

H. Oxidation by Potassic Bichromate or Permanganate (Kessler), 
Bichromate or permanganate added to a solution of antimonious 

*Dunstaii and Boole (Pharm. Jour., Nov., 1888) have proved that the accurate 
estimation of the antimony in tartar emetic may be secured by this method, using the 
precautions above mentioned. 

L 2 



148 VOLUMETRIC ANALYSIS. 46. 

chloride, containing not less than J of its volume of hydrochloric 
acid (sp. gr. 1*12), converts it into antimonic chloride. 

The reaction is uniform only when the minimum quantity of acid 
indicated above is present, but it ought not to exceed i the volume, 
and the precautions before given as to the action of hydrochloric 
acid on permanganate must be taken into account, hence it is 
preferable to use bichromate. 

Kessler (Poggend. Annal. cxviii. 17) has carefully experimented 
upon this method and adopts the following processes. 

A standard solution of arsenious acid is prepared containing 
5 gm. of the pure acid, dissolved by the aid of sodic hydrate, 
neutralized with hydrochloric acid, 100 c.c. concentrated hydro- 
chloric acid added, then diluted with water to 1 liter ; each c.c. of 
this solution contains 0'005 gm. As 2 3 , and represents exactly 
0-007253 gm. Sb' 2 3 . 

Solutions of potassic bichromate and ferrous sulphate of known 
strength in relation to each other, are prepared in the usual way ; 
and a freshly prepared solution of potassic ferricyanide used as 
indicator. 

The relation between the bichromate and arsenious solution is 
found by measuring 10 c.c. of the latter into a beaker, 20 c.c. 
hydrochloric acid of sp. gr. 1*12, and from 80 to 100 c.c. of water 
(to insure uniformity of action the volume of HC1 must never be 
less than J or more than J) ; the bichromate solution is then added 
in excess, the mixture allowed to react for a few minutes, and the 
ferrous solution added until the indicator shows the blue colour. 
To find the exact point more closely, J or 1 c.c. bichromate solution 
may be added, and again iron, until the precise ending is obtained. 

Process : The material, free from organic matter, organic acids, or heavy 
metals, is dissolved in the proper proportion of HC1, and titrated precisely 
as just described for the arsenious solution ; the strength of the bichromate 
solution having been found in relation to As 2 O 3 the calculation as respects 
Sb 2 O 3 presents no difficulty. Where direct titration is not possible the same 
course may be adopted as with arsenic ( 47) ; namely precipitation with 
H 2 S and digestion with mercuric chloride. 

In the case of using permanganate it is equally necessary to have 
the same proportion of HC1 present in the mixture, and the 
standard solution must be added till the rose colour is permanent. 
The permanganate may be safely used with J the volume of HC1, 
with the addition of some magnesic sulphate, and as the double 
tartrate of antimony and potassium can readily be obtained pure, 
and the organic acid exercises no disturbing effect in the titration, 
it is a convenient material upon which to standardize the solution. 

3. Distillation of Antimonious or Antimonic Sulphide -with. 
Hydrochloric Acid, and Titration of the evolved Sulphuretted 
Hydrogen (Schneider). 

When either of the sulphides of antimony is heated with 
hydrochloric acid in Bunsen's, Fresenius', or Mohr's distilling 



47. AKSENIC. 149 

apparatus ( 39), for every 1 eq. of antimony present as sulphide, 
3 eq. of HAS are liberated. If, therefore, the latter be estimated, 
the quantity of antimony is ascertained. 

Process : The antimony to be determined is brought into the form of ter- 
or penta -sulphide (if precipitated from a hydrochloric solution, tartaric acid 
must be previously added to prevent the precipitate being contaminated with 
chloride), which, together with the filter containing it, is put into the distilling 
flask with a tolerable quantity of hydrochloric acid not too concentrated. 
The absorption tube contains a mixture of caustic soda or potash, with a 
definite quantity of ^V arsenious acid solution in sufficient excess to retain all 
the sulphuretted hydrogen evolved. The flask is then heated to boiling, and 
the operation continued till all evolution of sulphuretted hydrogen has ceased ; 
the mixture is then poured into a beaker, and acidified with hydrochloric 
acid, to precipitate all the arseuious sulphide. The whole is then diluted to, 
say 300 c.c., and 100 c.c. taken with a pipette, neutralized with sodic 
carbonate, some bicarbonate added, and the titration for excess of arsenious 
acid performed with ^ iodine and starch, as directed in 40. The separa- 
tion of antimony may generally be insured by precipitation as sulphide. If 
arsenic is precipitated at the same time, it may be removed by treatment 
with ammonic carbonate.- 

ARSENIC. 

As = 75. As 2 3 = 198. As 2 5 ^230. 
1. Oxidation by Iodine (Mo hr). 

47. THE principle upon which the determination of arsenious 
acid by iodine is based is explained in 40. 

Experience has shown, that in the 'estimation of arsenious 
compounds by the method there described, 'it is necessary to use 
sodic bicarbonate for rendering the solution alkaline as in the case 
of antimony. 

Process : To a neutral aqueous solution, add about 20 c.c. of saturated 
solution of sodic bicarbonate to every O'l gm. or so of As 2 O 3 , and then titrate 
with 3^ iodine and starch. When the solution is acid, the excess may be 
removed by neutral sodic carbonate, then the necessary quantity of bicar- 
bonate added, and the titration completed as before. 

Process for Arsenic Acid: This is best done by dissolving the acid in water, 
and boiling with potassic iodide in the presence of hydrochloric acid in large 
excess until all iodine vapours are dissipated. The'AsHO 4 is completely 
reduced to AsHO 3 . The liquid is then cooled, sodic carbonate added to 
neutrality, then some bicarbonate, and the arsenious acid titrated with 
iodine in the usual way. Younger (J. 8. C. I. ix. 158) has verified this 
method and proved that the reduction is complete : he also states that when 
the boiled solution cools, the liberation of a slight amount of iodine occurs, 
which may be prevented by using a few c.c. of glycerine. Of course the 
arsenic acid must contain no nitric acid, nitrates, or similar interfering 
bodies. 

1 c.c. T \ iodine - 0-00495 gm. As 2 3 , or 0*00575 gm. As 2 5 . 

Arsenic in Copper, Iron, Pyrites, etc. The method generally 
adopted is the distillation of the arsenic obtained as sulphide, 
with ferric chloride, and titration of the distillate with iodine as 
above described, but F. Flatten (J. S. C. I. xvii. 324) has made 



150 VOLUMETRIC ANALYSIS. 47. 

use of the discovery, that if As 2 S 3 is simply boiled with pure 
water for a period of from 1 to 3 hours or until the liquid is 
quite colourless and all H 2 S dissipated, the whole of the arsenic 
will exist as As 2 3 , and may be titrated with T f y- iodine direct. 
The results obtained by this method are as exact as any other, and 
saves an immense amount of work. 

Titration of Alkaline Arseniates. In a previous edition of this 
book it was recommended, on the authority of Barnes, to estimate 
the arsenic acid in commercial arseniates of soda, etc., by reduction 
with sulphurous acid (passing the gas through the liquid), boiling 
off the excess of SO' 2 , neutralizing with sodic bicarbonate, and 
titrating with iodine as described above. This method has not 
given me satisfactory results. The mere passing the gaseous SO 2 
through the liquid does not, in all cases, insure the complete 
reduction to arsenious acid. 

Holthof (Z. a. C. xxii. 378) and McKay (0. N. liii. 221243) 
have experimented largely on this method of estimating arsenic, 
which was really originally suggested by Mohr, but never widely 
adopted, owing to the defect already mentioned. Holthof proved 
that various forms of arsenic, on being converted into arsenic acid, 
would bear evaporation to dryness with HC1 without loss, and that 
arsenic sulphide could be oxidized by strong nitric acid, or with 
HC1 and KC10 3 to arsenic acid, and reduced to the lower state of 
oxidation by copious treatment with SO 2 , the method being to add 
300 or 400 c.c. of strong solution of SO 2 , digest on the water bath 
for two hours, then boil down to one-half, and when cool add 
sodic bicarbonate, and titrate with iodine and starch. 

McKay shortens the method considerably by placing the 
mixture in a well-stoppered bottle, tying down the stopper, and 
digesting in boiling water for one hour. At the end of that time 
the bottle is removed and allowed to cool somewhat, then emptied 
into a boiling flask, diluted with about double its volume of water, 
and boiled down by help of a platinum spiral to one-half. The 
liquid is cooled, diluted, and either the whole or an aliquot portion 
titrated in the usual way. 

For quantities of material representing about 0*1 gm. As, 30 c.c. 
of saturated solution of SO 2 will suffice, and the reduction may 
therefore be made in a bottle holding 50 or 60 c.c. (fig. 39). The 
results are very satisfactory. In the case of titrating commercial 
alkaline arseniates, which often contain small quantities of 
arsenious acid, this must be estimated first, and the amount 
deducted from the total obtained after reduction. 

2. Oxidation by Potassic Bichromate (Kessler). 

This method is exact!}' the same as is fully described in 46 for antimony. 

The arsenious compound is mixed with T \ bichromate in excess in presence 
of hydrochloric acid and water, in such proportion that at least ^ of the total 
volume consists of hydrochloric acid (sp. gr. 1'12). 



47. ARSENIC. 151 

The excess of bichromate is found by a standard solution of pure iron, or 
of double iron salt, with potassic ferricyanide as indicator ; the quantity of 
bichromate reduced is, of course, the measure of the quantity of arsenious 
converted into arsenic acid. 

1 c.c. T \ bichromate -0-00495 gm. As 2 3 . 

In cases where the direct titration of the hydrochloric acid solution cannot 
be accomplished, the arsenious acid is precipitated with H' 2 S (with arsenates 
at 70C.) 5 the precipitate well washed, the filter and the precipitate placed in 
a stoppered flask, together with a saturated solution of mercuric chloride in 
hydrochloric acid of 1/12 sp. gr., and digested at a gentle heat until the 
precipitate is white, then water "added in such proportion that not less than ^ 
of the volume of liquid consists of concentrated HC1 ; ^V bichromate is then 
added, and the titration with standard ferrous solution completed as usual. 

3. Indirect Estimation by Distilling- with Chromic and 
Hydrochloric Acids (B u n s e n) . 

The principle of this very exact method depends upon the fact, 
that when potassic bichromate is boiled with concentrated hydro- 
chloric acid, chlorine is liberated in the proportion of 3 eq. to 1 eq. 
chromic acid. 

If, however, arsenious acid is present, but not in excess, the 
chlorine evolved is not in the proportion mentioned above, but so 
much less as is necessary to convert the arsenious into arsenic acid. 

A S 2Q 3 + 4C1 + 2H 2 - As 2 5 + 4HC1. 

Therefore every 4 eq. of chlorine, short of the quantity yielded 
when bichromate and hydrochloric acid are distilled alone, represent 
1 eq. arsenious acid. The operation is conducted in the apparatus 
fig. 37 or 38. 

4. By Precipitation as TJranic Arsenate (Bodeker). 

The arsenic must exist in the state of arsenic acid (As-O 5 ), and the process 
is in all respects the same as for the estimation of phosphoric acid, devised 
by N e u b a u e r , P i n c u s , and myself ( 72) . The strength of the uranium 
solution may be ascertained and fixed by pure sodic or potassic arseuate, or 
by means of a weighed quantity of pure arsenious acid converted into arsenic 
acid by evaporation with strong nitric acid, and neutralizing with alkali, then 
dissolved in acetic acid. The method of testing is precisely the same as with 
phosphoric acid ; the solution of uranium should be titrated upon a weighed 
amount of arsenical compound, bearing in mind here, as in the case of P'O , 
that the titration must take place under precisely similar conditions as to 
quantity of liquid, the amount of sodic acetate and acetic acid added, and 
the depth of colour obtained by contact of the fluid under titration with the 
yellow prussiate solution. 

Bo am ((7. N. Ixi. 219), who has had large experience in the 
examination of arsenical ores, recommends this method as being 
rapid and accurate, and carries it out as follows : 

1 to 1-5 gm. of dried and powdered ore is boiled to dryness with 2025 
c.c. of strong nitric acid ; when cool about 30 c.c. of 30 % caustic soda 
solution is added and boiled for a few minutes ; then diluted, filtered and 




152 VOLUMETRIC ANALYSIS. 47. 

made up to 250 c.c. 25 c.c. of the liquid are acidified with a solution 
containing 10 per cent, of sodic acetate in 50 per cent, acetic acid, and 
heated to near boiling, then titrated with the standard uranium as usual. For 
this latter, the same authority recommends what he terms a fourth normal 
solution of uranium, containing 17'1 gm. uranic acetate, and 15 c.c. glacial 
acetic acid made up to 2 liters with water, 1 c.c. being equal to T25 m.gm. 
As. But if the method has to be considered accurate, this suggestion can 
scarcely be adopted, since the uranic acetate of commerce is of indefinite 
hydration; and moreover, to insure exactitude, it is necessary that the 
titration should be carried out with the same proportions of saline matters, 
acetic acid, etc., as existed in originally standardizing the uranium. I there- 
fore unhesitatingly recommend that the uranium should be standardized 
with a known weight of pure arsenic or arsenate in the presence of the same 
proportions of sodic hydrate and acetate, acetic acid, etc., as will actually 
be used in the analysis of an ore. The method may be used for all ores 
which can be attacked by nitric acid. It is also available for iron pyrites 
containing tolerable quantities of arsenic ; the ferric arsenate being readily 
decomposed by excess of NaHO, thus allowing the ferric hydrate to be 
filtered off free from As. 

The solution of arsenic acid must of course be free from 
metals liable to give a colour with the indicator and from 
phosphates. Alkalies, alkaline earths, and zinc are of no con- 
sequence, but it is advisable to add nearly the required volume of 
uranium to the liquid before heating. The arsenic acid must be 
separated from all bases which would yield compounds insoluble 
in weak acetic acid. 

The AsH 3 evolved from Marsh's apparatus may be passed into 
fuming HNO 3 , evaporated to dryness, the arsenic acid dissolved in 
water (antimony if present is insoluble), then titrated cautiously 
with uranium in presence of free acetic acid and sodic acetate as 
above described. 

5. By Standard Silver as Arsenate. 

The principle of this method has been adopted by Pearca 
of the Colorado Smelting Company, and also by McCay (G, N. 
xlviii. 7). The authors, however, differ in the details of the 
process. The former prefers to separate the arsenic as silver 
arsenate, and, estimating the silver so combined, thence calculate 
the arsenic. The latter nses a known excess of standard silver, 
and estimates the combined silver residually. 

P e a r c e ' s Process. The finely-powdered substance for analysis is mixed 
in a large porcelain crucible with from six to ten times its weight of 
a mixture of equal parts of sodic carbonate and potassic nitrate. The mass 
is then heated with a gradually increasing temperature to fusion for a few 
minutes, allowed to cool, and the soluble portion extracted by warming with 
water in the crucible, and filtering from the insoluble residue. The arsenic 
is in the filtrate as alkaline arsenate. The solution is acidified with nitric 
acid and boiled to expel CO" 2 and nitrous fumes. It is then cooled to the 
ordinary temperature, and almost exactly neutralized as follows : Place 
a small piece of litmus paper in the liquid: it should show tin acid reaction. 
Now gradually add strong ammonia till the litmus turns blue, avoiding 
a great excess. Again make slightly acid with a drop or two of strong nitric 



47- ARSENIC. 153 

acid ; and then, by means of very dilute ammonia and nitric acid, added drop 
by drop, bring the solution to such a condition that the litmus paper, after 
having previously been reddened, will, in the course of half a minute, 
begin to show signs of alkalinity. The litmus paper may now be removed 
and washed, and the solution, if tolerably clear, is ready for the addition of 
silver nitrate. If the neutralization has caused much of a precipitate 
(alumina, etc.), it is best to filter it off at once, to render the subsequent 
filtration and washing of the arsenate of silver easier. 

A solution of silver nitrate (neutral) is now added in slight excess ; and 
after stirring a moment, to partially coagulate the precipitated arsenate, 
which is of a brick-red colour, the liquid is filtered, and the precipitate 
washed with cold water. The filtrate is then tested with silver and dilute 
ammonia, to see that the precipitation is complete. 

The object is now to determine the amount of silver in the precipitate, 
and from this to calculate the arsenic. The arsenate of silver is dissolved on 
the filter with dilute nitric acid (which leaves undissolved any chloride 
of silver), and the filtrate titrated, after the addition of ferric sulphate, with 
ammonic thiocyanate ( 43). 

From the formula 3Ag 2 O.As 2 5 , 648 parts Ag-150 parts As, 
or Ag: As =108: 25. , 
i 

McCay's Process. The preliminary fusion is the same as in the 
former method, but after acidulating with nitric acid and boiling 
off CO 2 , the liquid is evaporated to drynessand heated till no more 
acid fumes are given off. The residue is taken up with wa-ter, 
filtered, made up to a definite volume, and the arsenic determined 
in the following manner : 

The solution of arsenic acid or arsenate is heated to boiling, and excess of 
standard silver nitrate run in ; the liquid is then stirred briskly until the 
precipitate begins to settle and the liquid becomes clear, when the beaker is 
to be removed from the flame and left to cool to about 37 . Dilute ammonia 
is now carefully added until a cloudiness ceases to form. The solution 
should be well stirred before each successive addition, so as to obtain a clear 
liquid in order to observe the cloud formation more distinctly. The silver 
arsenate is finally filtered off and well washed ; the filtrate is acidulated with 
nitric acid ; ferric sulphate added ; and the silver titrated with ammonic 
thiocyanate ( 43). The amount of silver thus found deducted from the 
quantity taken gives the amount combined with the arsenic ; and from this 
datum the quantity of arsenic present is calculated. 

Of these two methods the preference must be given to the first 
on the score of accuracy, there being less probability of error from 
contaminating substances; both, however, are available for technical 
purposes. 

Owing to the large amount of arsenate of silver formed from 
a small quantity of arsenic (nearly six times by weight), it is 
not at all necessary or even desirable to work with large amounts 
of substance. 0'5 gm. is usually sufficient for the determination of 
the smallest quantity of arsenic ; and where the percentage is high, 
as little as O'l gm. may be taken with advantage. The method 
has been used with very satisfactory results on the sulphide of 
arsenic obtained in the ordinary course of analysis. 

Substances such as molybdic and phosphoric acids, which may 



154 VOLUMETRIC ANALYSIS. 49. 

behave similarly to arsenic under this treatment, interfere, of course, 
with the method. Antimony, by forming antimoniate of sodium, 
remains practically insoluble and without effect. 

The method has been used by Me Cay for the estimation of 
arsenic in the presence of alkaline earths, as occurring in some 
minerals, with success. 

BARIUM. 
Ba = 136-8. 

48. IN a great number of instances the estimation of barium 
is simply the Converse of the process for sulphuric acid ( 76), 
using either a standard solution of sulphuric acid or a neutral 
sulphate in a known excess, and finding the amount by residual 
titration. 

When barium can be separated as carbonate, the estimation 
is made as in 18.2. 

Precipitation as Baric Clare-mate. A clecinormal Solution of 
bichromate for precipitation purposes must differ from that used 
for oxidation purposes. In the present case the solution is made 
by dissolving 7 '37 gm. of pure potassic bichromate in water, and 
diluting to 1 liter. 

The barium compound, which may contain alkalies, magnesia, strontia, 
and lime, is dissolved in a good quantity of water, ammonia free from 
carbonate added, heated to 60 or 70 C., and the standard bichromate added 
cautiously, with shaking, so long as the yellow precipitate of baric chromate 
is formed, and until the clear supernatant liquid possesses a faint yellow 
colour. 1 c.c. y^ solution = 0'00684< gm. Ba. 

Titration of the Precipitate with Permanganate. In this case the 
precipitate of baric chromate is well washed, transferred to a flask, and mixed 
with an excess of double iron salt ; the amount of iron oxidized by the 
chromic acid is then estimated by titration with permanganate ; the quantity 
of iron changed to the ferric state multiplied by the factor 0'8187 = Ba. 

BISMUTH. 

Bi = 208. 

49. THE estimation of this metal or its compounds volume t- 
rically has occupied the attention of Pa tt ins on Muir, to whom 
we are indebted for several methods of gaining this end. Two of 
the best are given here, namely, (1) precipitation of the metal as 
basic oxalate, and titration with permanganate ; (2) precipitation 
as phosphate with excess of standard sodic phosphate, and titration 
of that excess by standard uranic acetate. 

1. Titration as Oxalate. 

Normal bismuth oxalate, produced by adding excess of oxalic 
acid to a nitric solution of the metal when separated by filtration, 



49. BISMUTH. 155 

and boiled with successive quantities of water for three or four 
times, is transformed into basic oxalate. The method of titratioii 
is as follows : 

The solution containing bismuth must be free fr om hydrochloric acid, as 
the basic oxalate is readily soluble in that acid. A large excess of nitric 
acid must also be avoided. Oxalic acid must be added in considerable 
excess. If the precipitate be thoroughly shaken up with the liquid, and the 
vessel be then set aside, the precipitate quickly settles, and the supernatant 
liquid may be poured off through a filter in a very short time. If the 
precipitate be boiled for five or "ten minutes with successive quantities of 
about 50 c.c. of water, it is quickly transformed into the basic salt. So soon 
as the supernatant liquid ceases to show an acid reaction, the transformation 
is complete. It is well to employ a solution of permanganate so dilute, that 
at least 50 c.c. are required for the titration (^ strength suffices). The basic 
oxalate may be dissolved in dilute sulphuric acid in place of hydrochloric ; 
it is more soluble, however, in the latter acid. If the solution contains but 
little hydrochloric acid, there is no danger of chlorine being evolved during 
the process of titration. 

In applying this process to the estimation of bismuth in a solution 
containing other metals, it is necessary, if the solution contain substances 
capable of acting upon, or of being acted on b} 7 permanganate, to separate 
the bismuth from the other metals present. This is easily done by 
precipitating in a partially neutralized solution with much warm water and 
a little ammonic chloride. The precipitate must be dissolved in nitric acid, 
and the liquid boiled down once or twice with addition of the same acid in 
order to expel all hydrochloric acid, before precipitating as oxalate. The 
liquid should contain just sufficient nitric acid to prevent precipitation of 
the basic nitrate before oxalic acid is added. 1 molecule oxalic acid corresponds 
to 1 atom bismuth, or 126 = 203. 

A shorter method, based on the same reactions, has been arranged 
by Muir and Robbs (J. C. S. I. xli. 1). In this case, however, 
the double oxalate of potassium and bismuth is the compound 
obtained, the excess of oxalate of potash being determined 
residually. Reis (Bericlite, xiv. 1172) has shown that when 
normal potassic oxalate is added to a solution of bismuth nearly 
free from mineral acid, but containing acetic acid, a double salt of 
the formula Bi' 2 (C 2 4 ) a , K 2 C 2 4 is precipitated. In applying this 
process for the estimation of bismuth in mixtures, it is necessary 
to separate the metal as oxychloride, and that it should be obtained 
in solution as nitrate with a small excess of nitric acid. This is 
done by evaporating off the greater part of the free acid, allowing 
just sufficient to remain that the bismuth may remain in solution 
while hot. A large excess of acetic acid is then added, it is made 
up to a definite measure, and an aliquot portion taken for titration. 

The solution of normal potassic oxalate standardized by perman- 
ganate must not be added in great excess. It is well, therefore, 
to deliver it into the bismuth liquid from a burette until the 
precipitation is apparently complete, then add a few extra c.c., and 
allow to remain for some time with shaking. It is then filtered 
through a dry filter, a measured portion taken, and the residual 
oxalic acid found by permanganate. 



156 VOLUMETBIC ANALYSIS. 50. 

2. Precipitation as Phosphate. 

The necessary standard solutions are 

(a) Standard sodic phosphate containing 35*8 gin. per liter. 
1 c.c. -0-0071 gm. P 2 5 . 

(b) Standard uramc acetate, corresponding volume for volume 
with the above, when titrated with an approximately equal amount 
of sodic acetate and free acetic acid. 

Success depends very much upon identity of conditions, as is 
explained in 72. 

The bismuth to be estimated must be dissolved in nitric acid ; bases other 
than the alkalies and alkaline earths must be absent. The absence of those 
acids which interfere with the determination of phosphoric acid by the 
uranium process (non-volatile, and reducing organic acids, sulphuretted 
hydrogen, hydriodic acid, etc.) must be assured. As bismuth is readily 
separated from other metals, with the exception of antimony and tin, by 
addition of much warm water and a little ammonic chloride to feebly acid 
solutions, a separation of the bismuth from those other metals which are 
present should precede the process of estimation. If alkalies or alkaline 
earths be alone present, the separation may be dispensed with. The pre- 
cipitated bismuth salt is to be washed, dissolved in a little strong nitric acid. 
and the solution boiled down twice with addition of a little more nitric acid, 
in order to remove the whole of the hydrochloric acid present. 

Such a quantity of a tolerably concentrated solution of sodic acetate is 
added as shall insure the neutralization of the nitric acid, and therefore the 
presence in the liquid of free acetic acid. If a precipitate form, a further 
addition of sodic acetate must be made. The liquid is heated to boiling ; 
a measured volume .of the sodic phosphate solution is run in ; the boiling 
is continued for a few minutes ; the liquid is passed through a ribbed filter. 
the precipitate being washed repeatedl}" with hot water ; and the excess of 
phosphoric acid is determined in the filtrate by titration with uranium. 
If the filtered liquid be received in a measuring flask, which is subsequently 
filled to the mark with water, and if the inverted uranium method be then 
employed, the results are exceedingly accurate. This method is especially to 
be recommended in the estimation of somewhat large quantities of bismuth, 
since it is possible that in such cases a large amount of sodic acetate will 
have been used, which, as is well known, has a considerable disturbing effect 
on the reaction of the indicator. 

If the bismuth solution contain a large excess of nitric acid, it is better to 
neutralize nearly with sodic carbonate before adding sodic acetate and titrating. 

Fuller details of both the above processes are contained in J. C. S. 
1877 (p. 674) and 1878 (p. 70). 



BROMINE. 

Br-80. 

50. THIS element, or its unoxidized compounds, can be 
estimated precisely in the same way as chlorine by -~ silver solution 
( 42), or alkalimetrically as in 32, or by thiocyanate ( 43), 
but these methods are seldom of any avail, since the absence of 
chlorine or its combinations is a necessary condition of accuracy. 

Bromine in aqueous solution, or as gas, may be estimated by 



50. BROMINE. 157 

absorption with solution of potassic iodide, in many cases by mere 
digestion, and in other cases by distillation, in any of the forms of 
apparatus given in 39, and the operation is carried out precisely 
as for chlorine ( 54). 1 eq. I = 1 eq. Br. or I found x O63 = Br. 

A process for the estimation of bromine in presence of chlorine 
is still much wanted in the case of examining kelp liquors, etc. 
Heine (Journ. f. pract. Cliem. xxxvi. 184) uses a colour method 
in which the bromine is liberated by free chlorine, absorbed by 
ether, and the colour compared with an ethereal solution of bromine 
of known strength. Fehling states that with care the process 
gives fairly accurate results. It is of course necessary to have an 
approximate knowledge of the amount of bromine present in any 
given solution. 

Reimann (Annal. d. CJiem. u. Pkarm. cxv. 140) adopts the 
following method, which gives tolerably accurate results, but 
requires skill and practice. 

The neutral bromine solution is placed in a stoppered vessel, 
together with a globule of chloroform about the size of a hazel nut. 
Chlorine water of known strength is then added cautiously from 
a burette, protected from bright light, in such a way as to insure 
first the liberation of the bromine, which colours the chloroform 
orange yellow ; then more chlorine water, until the yellowish white 
colour of chloride of bromine occurs (KBr + 2C1 = KC1 + BrCl). 

The operation may be assisted by making a w r eak solution of 
potassic chromate, of the same colour as a solution of chloride of 
bromine in chloroform, to serve as a standard of comparison. 

The strength of the chlorine water is ascertained by potassic 
iodide and -^ thiosulphate. 2 eq. Cl.=l eq. Br. 

In examining mother-liquors containing organic matter, they 
must be evaporated to dryness in presence of free alkali, ignited, 
extracted with water ; then neutralized with hydrochloric acid 
before titrating as above. 

Cavazzi (Gazz. Chim. Hal. xiii. 174) gives a method which 
answers well for estimating bromine in small quantity, when mixed 
with large proportions of alkaline chlorides. It is based on the 
fact that, when such a mixture is heated to 100 C. with baric 
peroxide and sulphuric acid, the whole of the bromine is liberated 
with a mere trace of chlorine ; the bromine so evolved is absorbed 
in any convenient apparatus, such as fig. 37. The distillation 
is made in a 350 c.c. flask with double-bored stopper; one bore 
contains an open tube reaching to the bottom of the flask, the 
other carries the delivery tube which is connected with the (J tubes. 
The first U tu ^ e i g empty; the second contains 20 c.c. of 
a standard solution of arsenious acid in hydrochloric acid, con- 
taining 0-005 gm. As 2 3 in each c.c., and is connected with an 
aspirator. The apparatus is arranged so that the flask and empty 
(J tube are immersed in boiling water, the vapours of H 2 2 are 
thus decomposed, and the stream regulated by the aspirator. 



158 VOLUMETRIC ANALYSIS. 50. 

The requisites used by the author are 

Baric peroxide, containing 63 % BaO 2 . 

Dilute sulphuric acid 1:2. 

Arsenious acid dissolved in dilute hydrochloric acid, 5 gm. of 
pure As 2 3 per liter. 

Standard permanganate, 3 '55 gm. per liter. 

It was found that the relative strengths of the arsenic and 
permanganate solutions, when titrated together, diluted, and boiling, 
were, 18*2 c.c. of the latter to 20 c.c. of the former. Therefore 
1 c.c. of permanganate by calculation = 0*00888 gm. Br. 

The author found that treating 2 gm. of KC1 in the apparatus, 
without bromine, always gave a faint trace of Cl, so that only 
18 c.c. of permanganate were required for the 20 c.c. of arsenic, 
instead of 18 '2 c.c. ; and this he regards as a constant for that 
quantity of material. The examples of analysis with from 
O'Oo to 0*2 gm. KBr, and all with the correction of 0*2 c.c., are 
satisfactory. 

Norman McCulloch (C. N. Ix. 259) has described a method, 
devised by himself, for the rapid and accurate estimation of bromine, 
in presence of iodine or chlorine, in any of the ordinary commercial 
forms or chemical combinations, free from oxidizing and reducing 
agents and metals forming bromides, insoluble in hydrochloric 
acid. The author's explanation of the principles upon which 
the method is based is complicated and voluminous, for which the 
reader is referred to the original article. I have not been able to 
verify the method, but as the author is . known to have practical 
experience, as well as theoretical knowledge, a short summary is 
given here. 

The requisites described by the author are 

Standard permanganate, 31 '9 grains of the salt in 10,000 grains 
of water (or 3*19 gm. per liter). 

Standard potassic iodide, 82 '78 grains of KI in 10,000 grains of 
water" (or 8 '2 7 8 gm. per liter.) 

The solutions should agree volume for volume, but it is pre- 
ferable to verify them by dissolving 3-5 grains of iodine in 
caustic soda, in a 5-oz. stoppered bottle, adding HC1 in good excess, 
cooling, then adding the permanganate from a burette, until nearly 
colourless. A little chloroform as indicator is then added, and the 
permanganate cautiously run in, with shaking until the violet 
colour of the iodine is discharged, owing to production of IC1, 
due to the reaction of Cl liberated by the permanganate from 
HC1. 

The iodine equivalent of the permanganate is calculated to 
bromine by the coefficient x 0'6713 and each decem of permanganate 
should represent about 0*04 grain Br (or each c.c 0*004 gm 
Br). 

The other reagents are purified chloroform, made by adding 
some permanganate, then HC1 till colour is discharged, then a little 



51. CADMIUM. 159 

KI and the I so liberated again discharged with permanganate, 
finally the chloroform is washed free from all acid. 

A three per cent, solution of hydrocyanic acid, made by decom- 
posing a solution of pure potassic cyanide, with excess of HC1, and 
adding permanganate till a faint pink colour remains. 600 grains 
of KC1ST in 13 J ounces of water (or 40 gm. in 400 c.c.) with 
2J- ounces of HC1 (or 70 c.c.) will give such a solution. Owing to 
its poisonous nature great caution must be used in making this 
solution, and to avoid as much as possible the evolution of prussic 
acid the temperature must be kept down by ice, or a freezing 
mixture of nitre and sal ammoniac. If the cyanide contains, as is 
often the case, some alkaline carbonate, this should be removed 
previously by Bad, as otherwise CO 2 will be liberated and a loss 
of HCX occur, finally the cool solution is rendered faintly pink 
with some permanganate. 

Solution of manganous chloride, made by dissolving half a pound 
of MnCl 2 + 4H 2 in 4 oz. of warm water (or 500 gm. in 250 c.c.), 
This solution is used to prevent the liberation of free chlorine 
from the HC1 in the analysis. 

Process : The weighed bromide, containing from 1 to 3 grains of Br 
(0'05 to 0'15 gm.), is dissolved in half an ounce (15 c.c.) of water in a 5 oz. 
stoppered bottle, and about an ounce (30 c.c.) of the manganese sohition 
added ; permanganate is then run in excess of the required quantity, and the 
bottle cooled rapidly to 50 P. (10 C.) by ice or a freezing mixture. When 
cooled, the bottle is shaken by a rotary motion, and about half an ounce 
(15 c.c.) of moderately strong HC1 slowly added, with motion of the bottle 
to dissolve the manganic hydroxide, 3 to 6 dm. (2 4 c.c.) 0f hydro- 
cyanic solution are then delivered in, the bottle closed and returned to the 
cooling mixture for about half an hour. The liquid is then titrated with 
the standard potassic iodide, until nearly decolorized from the decomposition 
of the manganic chloride, and then slightly coloured from liberation of 
free I. Lastly, the slight excess of iodide is estimated by adding a little 
chloroform, and the titration finished with permanganate. The bromine is 
calculated by taking the difference between the amounts of bromine, 
represented by total permanganate and iodide used. If iodine is present it 
is of course recorded as bromine, and its amount, if required, must be 
ascertained by some other method capable of its estimation in the presence 
of bromine. 

The author gives several very good results with pure sodic 
bromide, an example of which may be given. Each measure of 
permanganate=0-0392 Br. T032 grain Br was taken, and 40 '6 
measures of permanganate with 14*3 measures of iodide used, then 
40-6 -14-3=26-3 which multiplied by 0-0392=1-031 Br. 

CADMIUM. 

Cd=lll-6. 

51. THIS metal may be estimated, as is the case with many 
others, by precipitation as sulphide, and decomposing the sulphide 
with a ferric salt, the iron being reduced to the ferrous state in 
proportion to the amount of sulphide present. 



160 VOLUMETRIC ANALYSIS. 52. 

Follenius has found that when cadmium is precipitated as 
sulphide in acid liquids, the precipitate is apt to be contaminated 
with salts other than sulphide to a small extent. The separation 
as sulphide is hest made by passing H 2 S into the hot liquid which 
contains the cadmium, and which should be acidified with 10 per- 
cent, of concentrated sulphuric acid by volume. . From hydrochloric 
acid solutions the metal is only completely separated by H 2 S when 
the hot solution contains not more than 5 per cent, of acid of 
sp. gr. I'll, or 14 per cent, if the liquid is cold. 

Ferric chloride is to be preferred for the decomposition of the 
cadmium sulphide, and the titration is carried out precisely as in 
the case of zinc ( 81). 

P. von Berg (Z. a. C. xxvi. 23) gives a good technical process 
for the estimation of either cadmium or zinc as sulphides, by 
means of iodine, as follows : - 

The washed sulphide of zinc or cadmium is allowed to drain upon the 
filter, and then transferred, together with the filter, to a stoppered flask 
containing 800 c.c. of water deprived of air by boiling and the passage of 
carbonic acid gas. The whole is well shaken to break up the precipitate and 
bring it intfl the most finely divided condition possible, so that the sulphide 
may not be protected from the action of the iodine by separated sulphur. 
A moderate quantity of hydrochloric acid is added, there being no necessity 
to entirely dissolve the sulphide, and then an excess of iodine solution of 
known strength. The residual free iodine is then titrated with thiosulphate 
without loss of time. The whole operation, from the transference of the 
sulphide to the flask to the final titration, occupies about five minutes, and 
gives results varying between 98'8 and 100'2 per cent. The reaction proceeds 
according to the equation, ZnS+2HCl+2l = ZnCl 2 +2HI + S. 

Cadmium may also be estimated, when existing as sulphate or 
nitrate, by precipitation as oxalate, and titration of the washed 
precipitate by permanganate. The details are carried out precisely 
as in the case of estimating zinc as oxalate ( 81). 



CALCIUM. 

Ca40. 

1 c.c. y^ permanganate = 0'002S gm. CaO 

- 0-0050 gm. CaCO 3 
= 0-0086 gm. CaS0 4 + 
,, normal oxalic acid 0-0280 gm. CaO 

Cryst. oxalic acid x 0-444 = CaO 

Double iron salt x 0-07143 = CaO 

52. THE estimation of calcium alkalimetrically has already 
been given ( 18), but that method is of limited application, unless 
calcic oxalate, in which form Ca is generally separated from other 
bases, be converted into carbonate or oxide by ignition, and thus 
determined with normal nitric acid and alkali. This and the 
following method by Hemp el are as exact in their results as the 



52. CALCIUM. 161 

determination by weight ; and where a series of estimations have 
to be made, the method is very convenient. 

Titration with. Permanganate. The readiness with which calcium 
can be separated as oxalate facilitates the use of this method, so 
that it can be applied successfully in a great variety of instances. 
It is not necessary here to enter into detail as to the method 
of precipitation ; except to say, that it may occur in either 
ammoniacal or weak acetic acid solution ; and that it is absolutely 
necessary to remove all excess of ammonic oxalate from the 
precipitate by washing with warm water previous to titration. 

Process : When the clean precipitate is obtained, a hole is made in the 
filter, and the bulk of the precipitate is washed through the funnel into 
a flask ; the filter is then treated with small quantities of hot dilute sulphuric 
acid, and again washed into the flask. Hydrochloric acid in moderate 
quantity may be safely used for the solution of the oxalate, since there is 
not the danger of liberating free chlorine Avhich exists in the case of iron 
(Fleischer, Titrirmethode, p. 76), but the sulphuric is better. 

When the precipitate is completely dissolved, the solution is freely diluted 
with water, and further acidified with sulphuric acid, warmed to 60 or 70, 
and the standard permanganate cautiously delivered into the liquid with 
constant agitation until a faint permanent pink tinge occurs, precisely as in 
the case of standardizing permanganate with oxalic acid ( 34.2c). 

Process for Lime in Blast Furnace Slags : Place about 1 gm. of the very 
finely-ground slag into a beaker, cover with water, and boil gently, then 
add gradually strong HC1 until the whole is dissolved, including SiO' 2 . 
Dilute the liquid, nearly neutralize with ammonia, and add a solution of 
ammonic acetate. The silica and alumina form a flocculent precipitate 
which is easily washed on a filter. The filtrate and washings are concen- 
trated somewhat, and the CaO precipitated with oxalate of ammonia and 
free ammonia ; the precipitate is dissolved as before described in hot dilute 
sulphuric acid, and titrated with permanganate. If much manganese is 
present, the calcic oxalate must be re-dissolved and re-precipitated before 
the titration is made. 

In all cases where a clean oxalate precipitate can be obtained, 
such as mineral waters, manures, etc., very exact results are obtain- 
able ; in fact, quite a,s accurate as by the gravimetric method. 
Ample testimony on this point is given by Fresenius, Mohr, 
Hempel, and others. 

Tucker (Iron, ^"ov. 16, 1878) has given the results of many 
experiments made by him upon mixtures of Ca.with abnormal 
proportions of iron, magnesia, alumina, etc. ; and even here the 
numbers obtained did not vary more than 2 to 3 per cent, from the 
truth. In the case of large proportions of these substances it will 
be preferable to re-precipitate the oxalate, so as to free it from 
adhering contaminations previous to titration. 

o 

Indirect Titration. In the case of calcic salts soluble in water and 
of tolerably pure nature, the estimation by permanganate can be 
made by adding to the solution a measured excess of normal oxalic 
acid, neutralizing with ammonia in slight excess, and heating to 

M 



162 VOLUMETRIC ANALYSIS. 54 

boiling, so as to rapidly separate the precipitate. The mixture is 
then cooled, diluted to a measured volume, filtered through a dry 
filter, and an aliquot portion titrated with permanganate after 
acidifying with sulphuric acid as usual. A great variety of calcium 
salts may be converted into oxalic by a short or long treatment 
with oxalic acid or ammonic oxalate, including calcic sulphate, 
phosphate, tartrate, citrate, etc. 



CERIUM. 

Ce- 141-2. 

53. THE most exact method of estimating this metal is by 
precipitating as cerotis oxalate, then drying the precipitate, and 
strongly igniting in an open crucible, so as to convert it into eerie 
oxide. 

Stolba (Z. a. C. xix. 194) states that the moist oxalate may be 
titrated precisely as in the case of calcic oxalate with permanganate, 
and with accurate results. ~No examples or details, however, are 
given. 

CHLORINE. 

01=35-37. 

1 c.c. ~ silver solution^O'00^537 gm. Cl. 
=0-005837 sin. 



54. THE powerful affinity existing between chlorine and silver 
in solution, and the ready precipitation of the resulting chloride, 
seem to have led to the earliest important volumetric process in 
existence, viz., the assay of silver by the w r et method of Gay 
Lussac. The details of the process are more particularly described 
under the article relating to the assay of silver ( 73) ; the deter- 
mination of chlorine is just the converse of the process there 
described, and the same precautions, and to a certain extent the 
same apparatus, are required. 

The solutions required, however, are systematic, and for exactness 
and convenient dilution are of decinormal* strength as described in 
41. In many cases it is advisable to possess also centinormal 
solutions, made by diluting 100 c.c. of ~ solution to 1 liter. 

1. Direct Precipitation -with ^Q Silver. 

Very weak solutions of chlorides, such as drinking waters, are not easily 
examined for chlorine by direct precipitation, unless they are considerably 
concentrated by evaporation previous to treatment, owing to the fact that, 
unless a tolerable quantity of chloride can be formed, it Avill not collect 
together and separate so as to leave the liquid clear enough to tell on the 
addition of fresh silver whether a distinct formation of chloride occurs. 
The best effects are produced when the mixture contains chlorine equal to 
from H to 2 gm. of salt per 100 c.c. Should the proportion be much less 



54. . CHLORINE. 163 

than this, the difficulty of precipitation may be overcome by adding a 
quantity of freshly precipitated chloride, made by mixing 1 equal volumes of 
-j^r salt and silver solution, shaking vigorously, pouring off the clear liquid, 
and adding the chloride to the mixture under titration. The best vessel to 
use for the trial is a well-stoppered round white bottle, holding 100 to 150 
c.c., and fitting into a paper case, so as to prevent access of strong light 
during the titration. Supposing, for instance, a neutral solution of potassic 
chloride requires titration, 20 or 30 c.c. are measured into the shaking 
bottle, a few drops of strong nitric acid added (free acid must always be 
present in direct precipitation), and a round number of c.c. of silver solution 
added from the burette. The bottle is placed in its case, or may be enveloped 
in a dark cloth and vigorous!} 7 shaken for half a minute, then uncovered, 
and gently tapped upon a table or book, so as to start the chloride downward 
from the surface of the liquid \vhere it often swims. A quick clarification 
indicates excess of silver. The nearer the point of exact counterbalance the 
more difficult to obtain a clear solution by shaking, but a little practice soon 
accustoms the eye to distinguish the faintest precipitate. 

In case of overstepping the balance in any trial, it is only 
necessary to add to the liquid under titration a definite volume of 
~ salt solution, and finish the titration in the same liquid, 
deducting, of course, the same number of c.c. of silver as has been 
added of salt solution. . 

Fuller details and precautions are given in 73. 



2. Precipitation by Silver in Neutral Solution with Chromate 
Indicator (see 41, 2 t>). 

3. Titration with ^ Silver and ' Thiocyanate (see 43). 

This method gives very accurate results if, after the chlorine is 
precipitated with excess of silver, the silver chloride is filtered 
off, washed well, and the filtrate and washings titrated with 
thiocyanate for the excess of silver. . 

Process : The material to be titrated, such as water residues, beer ash, or 
other substances in which the chlorine is to be estimated being brought into 
clear solution, a known volume of ^ silver in excess is added, having previously 
acidified the mixture with nitric acid ; the mixture is well stirred, and the 
supernatant liquid filtered off through a small filter, the chloride well 
washed, and to the filtrate and washings 5 c.c. of ferric indicator ( 43.3) 
and the same volume of nitric acid ( 43.4) are added. The flask is then 
brought under the thiocyanate burette, and the solution delivered in with 
a constant gentle movement of the liquid until a permanent light-brown 
colour appears. If the silver chloride is not- removed from the liquid 
previous to titration a serious error may occur, owing to the read}^ solubility 
of the chloride in the thiocj-auate solution. 

4. By Distillation and Titration with Thiosulphate or Arsenite. 

In cases where chlorine is evolved direct in the gaseous form or 
as the representative of some other body (see 39), a very useful 
absorption apparatus is shown in fig. 37. The little flask a is used 
as a distilling vessel, connected with the bulb tubes by an india- 

M 2 



164 VOLUMETRIC ANALYSIS. 

rubber joint ;* the stoppers for the tubes are also of the same 
material, the whole of which should be cleansed from sulphur by 
boiling in weak alkali. A fragment of solid magnesite may with 
advantage be added to the acid liquid in the distilling flask ; in 
all other respects the process is conducted exactly as is described 
in 39. 

This apparatus is equally well adapted to the absorption of 
ammonia or other gases, and possesses the great recommendation 
that there is scarcely a possibility of regurgitation. 

Mohr's apparatus (fig. 38) is also serviceable for this method. 

CHLORINE G-AS AND BLEACHING- COMPOUNDS. 

1 c.c. yjj- arsenious or thiosulphate solution=0*003537 gm. CI. 
1 liter of chlorine at C., and 760 m.m., weighs 3'167 gm. 

55. CHLORINE water may be titrated with thiosulphate by 
adding a measured quantity of it to a solution of potassic iodide, 
then delivering the thiosulphate from a burette till the colour of 
the free iodine has disappeared; or by using an excess of the 
reducing agent, then starch, and titrating residually with iodine. 
When arsenious solution is used for titration, the chlorine water is 
delivered into a solution of sodic carbonate, excess of arsenious 
solution added, then starch and iodine till the colour appears, 
or iodized starch-paper may be used. 

Bleaching- Powder. The chief substance of importance among 
the compounds of hypochlorous acid is the so-called chloride of 
lime. The estimation of the free chlorine contained in it presents 
no difficulty when arsenious solution is used for titration. 

Commercial bleaching powder consists of a mixture in variable 
proportions of calcic hypochlorite (the true bleaching agent), calcic 
chloride, and hydrate; and in some cases the preparation contains 
considerable quantities of chlorate, due to imperfect manufacture or 
age. It is generally valued and sold in this country by its 
percentage of chlorine. In France it is sold by degrees calculated 
from the volume of gaseous chlorine: 100 Erench=31 < 78 per 
cent. English. 

1. Titration by Arsenious Solution (Penot). 

The first thing to be done in determining the value of a sample 
of bleaching powder is to bring it into solution, which is best 
managed as follows : 

The sample is well and quickly mixed, and 7'17 gm. weighed, put into 
a mortar, a little water added, and the mixture rubbed to a smooth cream ; 
more water is then stirred in with the pestle, allowed to settle a little while, 
then poured off into a liter flask; the sediment again rubbed with water, 

* India-rubber and specially vulcanized rubber is open to some objection in these 
analyses, and apparatus is now readily to be had with glass connections. 



55. BLEACHING POWDER. 165 

poured off, and so on repeatedly, until the whole of the chloride has been 
conveyed into the flask without loss, and the mortar washed quite clean. 
The flask is then filled to the mark with water, well shaken, and 50 c.c. of 
the milky liquid taken out with a pipette, emptied into a beaker, and the & 
arsenious solution delivered in from a burette until a drop of the mixture 
taken out with a glass rod, and brought hi contact with the prepared starch- 
paper ( 40) gives no blue stain. 

The starch-paper may be dispensed with by adding arsenious solution in 
excess, then starch, and titrating residually with & iodine till the blue 
colour appears. The number of c.c. of arsenic used shows direct percentage 
of available chlorine. 

A more rapid technical method can be adopted in cases where a series of 
samples has to be tested, as follows : 4'95 gm. of pure arsenious acid are 
finely powdered and dissolved by the aid of a gentle heat in about 15 c.c. 
of glycerine, then diluted with water to 1 liter ; 25 c.c. are measured into 
a flask, and 1 c.c. of indigo solution added. The turbid solution of bleaching 
powder is poured into a suitable burette, and before it has time to settle is 
delivered with constant shaking into the blue arsenious solution until the 
colour is just discharged : the percentage of chlorine is then found by a slight 
calculation. 

2. Bunsen's Method. 

10 or 20 c.c. of the chloride of lime solution, prepared as above, are 
measured into a beaker, and an excess of solution of potassic iodide added ; 
the mixture is then diluted somewhat, acidified with acetic acid, and the 
liberated iodine titrated with T ^ thiosulphate and starch; 1 eq. iodine so 
found represents 1 eq. chlorine. 

The presence of chlorate does not affect the result when acetic 
acid is used. If it be desired to estimate the amount of chlorate 
in bleach, the following method has been devised by R. Fresenius. 
It depends on the fact that hypochlorites are decomposed by lead 
acetate with formation of lead peroxide, whilst the chlorate which 
may be present is unaffected. 

Process ; 20 gm. of bleaching powder are ground up with water in 
repeated quantities and made up to a liter; after settling, 50 c.c. 1 gm. of 
bleach are filtered off through a dry filter, put into a flask, and mixed with 
a solution of lead acetate in some excess. There is formed at first a white 
precipitate of lead chloride and lead hydroxide, these being acted on by the 
hypochlorite become first yellow, then brown, with liberation of chlorine and 
passing into lead peroxide. After the precipitate has settled, more lead 
solution is added, to be sure that the conversion is complete. The mixture 
is allowed to stand in the open flask, with frequent shaking, till all smell of 
chlorine has disappeared, which occurs in from eight to ten hours. The 
precipitate is then filtered off and washed till the wash-water is free from 
acid. ' The washings are evaporated somewhat, added to the filtrate, and the 
whole mixed with sodic carbonate in slight excess, to precipitate the lead and 
lime as carbonates these are well Avashed, the filtrate and w r ashings 
evaporated nearly to dryness, then transferred to either a P r e s e n i u s or 
Mohr apparatus (fig. 37 or 38) and distilled with HC1 as directed in 39. 
1 cq. :I-1 eq. Cl-O 5 . 

3. Gasometric Process. 

This method has been devised by Lunge (Bericlde xix. 868, 
also J. S. C. I. ix. 22) and is both accurate and rapid. The 
instrument used for the analysis is preferably the improve^ 



166 VOLUMETRIC ANALYSIS. 

nitrometer, with patent tap and bulb (see Part VII.), winch permits 
the use of a larger weight of the sample than the ordinary 50 c.c. 
nitrometer. In both instruments for this class of analysis ordinary 
tap water may be used, instead of mercury, with equally accurate 
results. 

The reagent used for the decomposition of the bleach is 
hydrogen peroxide, and the reaction is CaOCl 2 + H 2 2 =CaCl 2 + 
H 2 + O 2 . Lunge's directions are as follows : 

It is not necessary to know the exact composition of the hydrogen peroxide, 
but as it is desirable not to employ too large an excess of it in this case, it is 
best to estimate its percentage by a preliminary test occupying but a few 
minutes, in which a certain yolume of H-O- is decomposed by an excess of 
bleach solution (the inverse of the titration of the latter). This need be 
done only quite roughly. For the ana-lysis of chloride of lime the hydrogen 
peroxide must be diluted before use so as not to give out more than 7 c.c. of 
oxygen per c.c., and it must be made alkaline by means of caustic soda 
solution up to the point where a flocculent precipitate appears. The alkaline 
reaction ought to be quite distinct, but any fjreat excess of alkali should be 
avoided. It is not necessary to shake much, and the reading ought to be 
made quickly, say five minutes after mixing the liquids, otherwise the results 
will be too high owing to the gradual evolution of more oxygen from the 
alkaline liquid. It might be thought that muddy solutions, such as arc 
regularly employed in testing commercial bleaching powder, would yield less 
reliable results, the solid matter favouring the evolution of oxygen from 
II-O- otherwise than through the action of CaOCl 2 ; but this is not so; 
muddy solutions can be tested by the nitrometer just as well as clear bleach 
liquors, provided the time of five minutes is not exceeded. As the reaction 
does not produce a sensible change of temperature, that time will quite 
suffice, provided that the operator has avoided raising the temperature of the 
flask in manipulating it, which he can do by handling it always by the neck 
with his thumb and forefinger only. 

In order to find the percentage of available chlorine by weight, that is, 
the English chlorometrical degrees, it should be borne in mind that every 
c.c, of gas evolved, reduced to and 760 m.m., represents 0'003167 gin. of 
chlorine. Hence, if the quantity of bleach employed is = 1 gin. (for 
instance, by dissolving 20 gm. in 500 c.c. of water, and employing 25 c.c. 
of the solution for each test), each c.c. of gas is = 3167 per cent, of 
available chlorine in the bleach. This involves the use of a bulb nitrometer 
holding 140 c.c. If only a 50 c.c. instrument is at hand, it will be necessary 
to take, say, 5 c.c. of the first-mentioned bleach solution, in which case every 
c.c. of gas represents 5xO'3167 = l'58 per cent, of chlorine. The most con- 
venient way is to dissolve 7'917 gm. of bleach in 250 c.c. of water, and 
emploj'ing 10 c.c. of the solution for each test, when each c.c. of oxygen 
evolved will directly indicate 1 per cent, of available chlorine, and a 50 c.c. 
nitrometer should be used. 

The general method of manipulating the nitrometer is described 
in Part VII. 

CHLORATES, IODATES, AND BROMATES. 

Chloric anhydride, C1 2 5 =150'74. lodic anhydride, I 2 5 =333. 
Bromic anhydride, Br 2 5 =239'5. 

The compounds of chloric, iodic, and bromic anhydrides may 
all be determined by distillation or digestion with excess of 



56. CHROMIUM. 167 

hydrochloric acid ; with chlorates the quantity of acid must be 
considerably in excess. 

In. each case 1 eq. of the respective anhydrides taken as 
monobasic or their compounds, liberates 6 eq. of chlorine, and 
consequently 6 eq. of iodine when decomposed in the digestion 
flask. In the case of distillation, however, iodic and bromic acids 
only set free 4 eq. iodine, while iodous and bromous chlorides 
remain in the retort. In both these cases digestion is preferable 
to distillation. 

Example : 0'2043 gm. pure potassic chlorate, equal to the sixth part of 
r " TJ J inr eq. was decomposed by digestion with potassic iodide and strong 
hydrochloric acid in the bottle shown in fig. 39. After the reaction was 
complete, and the bottle cold, the stopper was removed, and the contents 
washed out into a beaker, starch added, and 103 c.c. T ^ thiosulphate delivered 
in from the burette ; then again 23'2 c.c. of ^ iodine solution, to reproduce 
the blue colour ; this latter was therefore equal to 2'32 c.c. T ^- iodine, which 
deducted from the 103 c.c. thiosulphate gave 100'68 c.c., which multiplied by 
the factor 0'002043, gave 0'2056 gm., instead of 0'2043 gm. 



CHROMIUM. 

Cr=52-4. 
1. Ueduction by Iron. 

56. THE estimation of chromates is very simply arid success- 
fully performed by the aid of ferrous sulphate, being the converse 
of the process devised by Penny for the estimation of iron 
(see 37). 

Process : A very small beaker or other convenient vessel is partly or 
wholly filled, as may be requisite, with perfectly dry and granular double 
sulphate of iron and ammonia ; the exact weight then taken and noted. 
The chromium compound is brought into solution, not too dilute, acidified 
with sulphuric acid, and small quantities of the iron salt added from time to 
time with a dry spoon, taking care that none is spilled, and stirring with 
a glass rod, until the mixture becomes green, and the iron is in excess, best 
known by a small drop being brought in contact with a drop of red'prussiate 
of potash on a white plate; if a blue colour appears at the point of contact, 
the iron is in excess. It is necessary to estimate this excess, which is most 
conveniently done by ^ bichromate being added until the blue colour 
produced by contact with the red prussiate disappears. The vessel containing 
the iron salt is again weighed, the loss noted; the quantity of the salt 
represented by the ^ bichromate deducted from it, and the remainder 
multiplied by the factor required by the substance sought. A freshly made 
standard -solution of iron salt, well acidified with sulphuric acid, may be used 
in place of the dry salt. 

Example : 0'5 gm. pure potassic bichromate was taken for analysis, and to 
its acid solution 4'15 gm. double iron salt added. 33 c.c. of / bichromate 
Avere required to oxidize the excess of iron salt ; it was found that 0'7 gm. of 
the salt= 17'85 c.c. bichromate, consequently 3'3 c.c. of the latter were equal 
to 0'12985 gm. iron salt; this deducted from the quantity originally used 
left 4-02015 gm., which multiplied by 01255 gave 0'504 gm. instead of 
0'5 gm. 



168 VOLUMETRIC ANALYSIS. 56. 

In the case of lead chromate being estimated in this way, it is 
best to mix both the chromate and the iron salt together in 
a mortar, rubbing them to powder, adding hydrochloric acid, 
stirring well together, then diluting with water and titrating as 
before. ...Where 'pure double iron salt is not at hand, a solution of 
iron wire in sulphuric acid, freshly made, and of ascertained 
strength, may be used. 

2. Estimation of Chromates by Distillation with Hydrochloric Acid. 

When chromates are boiled with an excess of strong hydrochloric 
acid in one of the apparatus (fig. 37 or 38), every 1 eq. of chromic 
acid liberates 3 eq. chlorine. For instance, with potassic bichromate 
the reaction may be expressed as follows 

K 2 O 2 O r + 14HC1=2KC1 + Cr 2 Cl 6 + 7H 2 + 6C1. 

If the liberated chlorine is conducted into a solution of potassic 
iodide, 3 eq. of iodine are set free, and can be estimated by 
arsenite or thiosulphate. 3 eq. of iodine so obtained=379'5 
represent 1 eq. chromic acid 100 '40. The same decomposition 
takes place by mere digestion, as described in 39. 

3. Chrome Iron Ore, Ste'el, etc. 

The ore varies in quality, some samples being very rich, while 
others are very poor, in chromium. In all cases the sample is to 
be first of all brought into extremely fine powder. About a gram is 
rubbed tolerably fine in a steel mortar, then finished fractionally 
in an agate mortar. 

Christomanos recommends that the coarse powder should be 
ignited for a short time on platinum previous to powdering with 
the agate mortar ; after that it should be sifted through the finest 
material that can be used, and the coarser particles returned to the 
mortar for regrinding. 

Previous to analysis it should be again ignited, and the analysis 
made on the dry sample. 

O'Neill's Process. The very finely powdered ore is fused with 
ten times its weight of potassic bisulphate for twenty minutes, taking care 
that it does not rise over the edge of the platinum crucible ; when the fusion 
is complete, the molten mass is caused to flow over the sides of the crucible, 
so as to prevent the formation of a solid lump, and the crucible set aside to 
cool. The mass is transferred to a porcelain dish, and lixiviated with warm 
water until entirely dissolved (no black residue must occur, otherwise the 
ore is not completely decomposed) ; sodic carbonate is then added to the 
liquid until it is stro'ngly alkaline ; it is then brought on a filter, washed 
slightly, and the filter dried. When perfectly dry, the precipitate is 
detached from the filter as much as possible ; the filter burned separately ; 
the ashes and precipitate mixed with about twelve times the weight of the 
original ore, of a mixture of two parts potassic chlorate and three parts 
sodic carbonate, and fused in a platinum crucible for twenty minutes or so ; 
the resulting mass is then treated with boiling water, filtered, and the filtrate 
titrated for chromic acid as in 5 56.1. 



56. 



CHROMIUM. 169 



The ferric oxide remaining on the filter is titrated, if required, 
by any of the methods described in 63 and 64. 

Britton's Process. Reduce the mineral to the finest state of 
division possible in an agate mortar. Weigh off 0'5 gm., and add to it 
4 gm. of flux, previously prepared, composed of one part potassic chlorate 
and three parts soda-lime ; thoroughly mix the mass by triturating in a 
porcelain mortar, and then ignite in a covered platinum crucible at a bright- 
red heat for an hour and a half or more. 20 minutes is sufficient with 'the 
gas blowpipe. The mass will not fuse, but when cold can be turned out of 
the crucible by a few gentle taps, leaving the interior of the vessel clean 
and bright. Triturate in the mortar again and turn the powder into a tall 
4-oz. beaker, and add about 20 c.c. of hot water, and boil for two or three 
minutes ; when cold add 15 c.c. of HC1, and stir with a glass rod, till 
the solid matter, with the exception probably of a little silica in flakes,' 
becomes dissolved. Both the iron and chromium will then be in the highest 
state of oxidation Ee 2 O 3 and Cr' 2 O 3 . Pour the fluid into a white porcelain 
dish of about 20-oz. capacity, and dilute with washings of the beaker to 
about 3 oz. Immediately after, also, add cautiously 1 gm. of metallic iron 
of known purity, or an equivalent quantity of double iron salt, previously 
dissolved in dilute sulphuric acid, and further dilute with cold water to 
about 5-oz., to make up the volume in the dish to about 8 oz., then titrate 
with j permanganate the amount of. ferrous oxide remaining. The 
difference between the amount of iron found and of the iron weighed will 
be the amount oxidized to sesquioxide by the chromic acid. Every one part 
so oxidized will represent 0'320 of Cr"or 0'4663 of sesquioxide, Cr 2 O 3 , in 
which last condition the substance usually exists in the ore. 

If the amount of iron only in the ore is to be determined, the process is 
still shorter. After the fluxed mineral has been ignited and reduced to 
powder, as already directed, dissolve it by adding first, 10 c.c. of hot water 
and applying a gentle heat, and then 15 c.c. of HC1, continuing the heat to 
incipient boiling till complete decomposition has been effected; cool by 
immersing the tube in a bath of cold water, add pieces of pure metallic zinc 
sufficient to bring the iron to the condition of protoxide and the chromium 
to sesquioxide, and apply heat till small bubbles of hydrogen cease, and the 
zinc has become quite dissolved; then nearly fill the tube with cold water, 
acidulated with one-tenth of sulphuric acid, and pour the contents into the 
porcelain dish, add cold water to make up the volume to about 8 oz., and 
complete the operation with standard permanganate or bichromate. 

Sell's Process. This method is described in J. C. S. 1879 
(p. 292), and is carried out by first. fusing the finely ground ore 
with a mixture of sodic bisulphate and fluoride in the proportion 
of I mol. bisulphate, and 2 mol. fluoride, and subsequent titration 
of the chromic acid by standard thiosulphate and iodine. 

Prom O'l to 0'5 gm. of the ore is placed on the top of ten times its weight 
of the above-mentioned mixture in a large platinum crucible, and ignited for 
fifteen minutes ; an equal weight of sodic bisulphate is then added and well 
incorporated by fusion, and stirring with a platinum wire; then a further 
like quantity of bisulphate added in the same way. When complete 
decomposition has occurred, the mass is boiled with water acidulated with 
sulphuric acid, and the solution diluted to a definite volume according to the 
quantity of ore originally taken. 

To insure the oxidation of all the chromium and iron previous to titration, 
a portion, or the whole, of the solution is heated to boiling, and permanganate 
added until a permanent red colour occurs. Sodic carbonate is then added 
in slight excess, and sufficient alcohol to destroy the excess of permanganate ; 



170 VOLUMETRIC ANALYSIS. 56. 

the manganese precipitate is then filtered off, and the clear solution titrated 
with T ^j- thiosulphate and iodine. 

The author states that the analysis of an ore by this method 
may be accomplished in one hour and a half. 

For the oxidation of salts of chromium, the same authority 
recommends boiling with potash or sodic carbonate (to which 
a small quantity of hydrogen peroxide is added) for 1 5 minutes. 

For the preliminary fusion and oxidation of chrome iron ore, 
Dittmar recommends a mixture of two parts borax glass, and one 
and a half part each of sodic and potassic carbonate. These are 
fused together in a platinum crucible until all effervescence ceases, 
then poured out into a large platinum basin or upon a clean iron 
plate to cool, broken up, and preserved for use. 

Ten parts of this mixture is used for one part of chrome ore, 
and the fusion made in a platinum crucible, closed for the first five 
minutes, then opened for about forty minutes, frequently stirring 
with a platinum wire, and using a powerful Bunsen name. The 
gas blowpipe hastens this method considerably. 

The above described methods of treating the ores of chromium, 
so as to obtain complete decomposition, are apparently now super- 
seded to a great extent by the use of sodic peroxide, but the action 
of this agent is so energetic upon platinum, gold, silver, nickel, or 
porcelain that its use requires great care. Many well known 
authorities on the analysis of chrome ores use a basic mixture 
such as was first suggested by Clark, but modified by Stead, i.e., 
magnesia or lime four parts, potassic and sodic carbonates of each 
one part. Clark's original mixture of magnesia and caustic soda 
acts on platinum, but Stead's mixture does not. 

The fusion is made by mixing the very finely ground sample with ten 
times its weight of the basic mixture in a platinum crucible, and heating to 
bright redness at the back of a gas muffle for about an hour. When the 
crucible is removed and cool the mass is found sintered together. It is 
removed to a beaker, and the crucible washed out with water and dilute 
sulphuric acid. The decomposition i. generally complete, but if any black 
specks are found they must be separated by filtration, dried, and again fused 
with some of the basic mixture ; finally the whole is mixed with excess of 
ferrous salt, and the unoxidized iron titrated with bichromate as before 
described. 

Hi deal and Rosenblum (J. S. C. 1. xiv. 1017) give a series 
of experiments on the estimation of chromium in ores, steels, etc., 
and on the use of sodic peroxide, which latter they find has a 
most destructive effect 011 all kinds of vessels in which the 
decomposition is made nickel seems the best material if not 
exposed to too high a temperature, but they found also that a good 
deal of nickel was dissolved from the crucibles by the sulphuric 
acid used to dissolve the melt, and they therefore attach great 
importance to the filtration of the aqueous solution of the melt, se- 
as to remove nickel and iron oxides, which otherwise interfere with 
the titration by masking the colour of the indicator. 




56. CHROMIUM. 

Ferroclirome, Ciirommm Steel, etc. S puller and Ivalman 
(Chein. Zeit. xvii. 880 and 1207) describe a method which gives 
good results, but is unfortunately tedious in working. 

Process for Ferroclirome. 0'35 gm. of the finely-powdered sample first 
sifted through linen and then rubbed down in an agate mortar, is mixed 
with 4 gm. of sodium peroxide and 8 gm. of caustic soda, and heated in 
a silver dish over a slightly smoky flame. The temperature is gradually 
raised so that at the end of five minutes the edge of the mixture begins 
to fuse, and after a further period of ten minutes the whole mass has 
become liquid. The heating is continued for half an hour over the slightly 
smoky flame until the bottom of the dish is covered with soot. During the 
last quarter of an hour the melt is stirred with a silver spatula. The attack 
of the ferro-chromium is then complete if the heating has been conducted 
as described, and the sample has been powdered sufficiently fine. The basin 
Avith its contents is allowed to cool to 40 50 C., freed from soot, and 
digested, in a large hemispherical porcelain dish, with hot water. The dish 
is then removed and rinsed into the basin. The loss in weight of a silver 
dish Aveighing about 38 gm. may be as much as 0'04 0'05 gm. for a single 
fusion. The aqueous extract of the melt contains sodium manganate and 
ferrate as well as chromate. Only traces of sodium peroxide remain, as the 
bulk is decomposed during solution. Sodium manganate and ferrate are 
removed by the addition of successive small quantities of sodium peroxide, 
which reduces these salts, itself undergoing simultaneous reduction. A 
quantity of 0'3 0'6 gm. is usually requisite, and any excess that may be 
added is got rid of either by allowing the solution to stand while being 
kept warm for some hours, or preferably by passing CO 2 into the solution 
for an hour and heating it for fifteen minutes on a water or sand bath. 
By the latter treatment hydrogen peroxide is liberated from the sodium 
peroxide, and being unstable in alkaline solution is decomposed on heating. 
Sodium chromate is not affected by excess of the peroxide in alkaline 
solution. Clark and E/ideal both find that mere boiling for ten minutes, 
is sufficient to decompose the excess of peroxide. 

The aqueous solution of the melt is made up to 500 c.c., the contents 
of the flask allowed to stand and an aliquot portion (?.g. 100 c.c.) filtered 
from ferric oxide, etc., and the chromium in it determined by a permanganate 
solution of which 1 c.c. equals about O'OOS gm. of iron, and a solution of 
ferrous ammonium sulphate containing 7 gm. of the salt in 500 c.c. The 
chromium solution is diluted with 1 liter of cold Avater which has been 
previously boiled and acidified with 20 c.c. of sulphuric acid (1 : 5 by volume); 
100 c.c. of ferrous ammonium sulphate are added, and the mixture titrated 
back with permanganate. The strength of the ferrous solution is determined 
by a blank experiment under similar conditions. 

Process for Chromium Steel : The material is dissolved in dilute sulphuric 
acid, evaporated to dry ness and fused Avith caustic soda and sodium peroxide,, 
as above described. The mass is digested with Avater, and after removal of 
any alkaline manganate or ferrate with peroxide and decomposing excess of 
the latter by CO 12 or by simple boiling,. the' solution is diluted to a definite 
volume, and aliquot portions titrated as before mentioned. 

Rideal and llosenbluni have obtained excellent results with 
ferrochrome, by fusion with sodic peroxide alone. The manner of 
procedure was as follows : 

About 0'5 gm. of a very finely poAvdered ferrochrome was mixed with 
3 gm. of sodic peroxide and heated very gently in a nickel crucible, until 



172 VOLUMETRIC ANALYSIS. 56. 

the mass began to melt, and then to glow by itself. The heating was then 
continued for ten minutes, and after the mass was partially cooled 1 gm. of 
sodic peroxide was added and the heating continued for another five minutes. 

The crucible, when still moderately warm, was placed in a suitable 
porcelain basin, which was then half filled with hot water and covered with 
a clock glass. The melt easily dissolved in the hot water, the solution 
obtained being of a deep purple colour, due to sodic ferrate, which is 
abundantly formed during the fusion. The solution also contained sodic 
manganate, resulting from the oxidation of the manganese which is present 
in ferrochrome. 

To decompose both these salts a small quantity of sodic peroxide was 
ndded, on which the solution immediately lost its purple colour. The 
solution was then boiled for ten minutes to decompose the excess of sodic 
peroxide and the insoluble residue of iron, nickel, and manganese oxide was 
filtered off. An excess of sulphuric acid w r as then added to the solution and 
after cooling it was titrated in the usual manner. 

Galbraith's method, modified somewhat by Stead (Jour. Iron 
and Steel Institute, 1893, 153), is considered the most rapid method 
for the estimation of chromium in irons and steels. 

The sample is dissolved in dilute sulphuric acid, filtered, the solution 
diluted to about 300 c.c., and heated to boiling. Strong solution of potassic 
permanganate is now" added until the red colour is permanent for ten 
minutes, then 80 c.c. of 10 per cent, hydrochloric acid, and the liquid 
heated until decolorized ; 150 c.c. of water are added, about 100 c.c. boiled 
off to expel the chlorine; and the chromium is then titrated. The residue 
insoluble in dilute sulphuric acid is mixed with 0'5 gm. of the basic mixture 
previously mentioned, and heated to intense redness for half an hour ; the 
chromium is afterwards titrated in hydrochloric acid solution with ferrous 
sulphate and bichromate. 

Another process consists in dissolving 2 gm. of the sample in hydrochloric 
acid ; without filtering, the liquid is nearly neutralized with a 2 per cent, 
solution of caustic soda, and after diluting to 300 c.c., 10 c.c. of a 5 per cent. 
-solution of sodic phosphate and 30 gm. of sodic thiosulphate are added. After 
boiling to expel the SO 2 , 20 c.c. of a saturated solution of sodic acetate 
are added, and the boiling continued for five minutes ; the precipitated 
chromium phosphate is then washed with a 2 per cent, solution of ammonium 
nitrate, dried, calcined, and fused with the basic mixture. The melt, 
dissolved in 30 c.c. of hydrochloric acid and 150 c.c. of water, is boiled 
for ten minutes and titrated. The process may be used in presence of 
vanadium. In this case, the chromium must be titrated by means of ferrous 
sulphate and permanganate in presence of sulphuric acid. 

E ideal and Rosenbl urn's experiments appear to show that 
.sodic peroxide, if certain conditions be observed in its use, is 
.a very valuable agent for the analysis of chrome ore, ferro- 
chrome, and chrome steel, as it removes the two main defects 
of former methods, viz., the necessity of repeated fusion to effect 
complete decomposition and the inconvenient slowness of these 
processes. The conditions which should be observed are sum- 
marized by them as follows : 

(1) Great care should be taken to reduce the chrome ore or the ferro- 
chrome to an almost impalpable powder. This can be done without much 
difficulty if the ore or the alloy be crushed in a steel mortar until a powder 
is obtained which will pass through a linen bag. This powder is then 



57. COBALT. 

ground in an agate mortar to the required degree of fineness, a little water 
feeing added to facilitate the grinding. 

(2) The water solution of the melt, before acidulation, must be freed, 
from an excess of sodic peroxide. Whenever sodium ferrate or sodium 
manganate is formed during the fusion it must be decomposed in the water 
solution of the melt. 

(3) As the result of the analysis depends to a large extent upon the 
titration, and especially upon a clear perception of its final point, it is 
important that the solution in which the chrome is to be determined should 
be as free as possible from other metallic salts, as for instance, iron, 
manganese, and nickel salts. We have also observed that the ferricyanide 
solution which is used as an indicator is most satisfactory when it contains 
no more than 1 per cent, of ferricyanide. 

COBALT. 

Co=59. 
Estimation "by Mercuric Oxide and Permanganate (W inkier). 

57. IF an aqueous solution of cobaltous chloride or sulphate be- 
treated with moist finely divided mercuric oxide, no decomposition- 
ensues, but on the addition of permanganate to the mixturej 
hydrated cobaltic and manganic oxides are precipitated. It is 
probable that no definite formula can be given for the reaction, 
and therefore practically the working effect of the permanganate 
is best established by a standard solution of c'obalt of known 
strength, say metallic cobalt dissolved as chloride, or neutral 
cobaltous sulphate. 

Process : The solution, free from any great excess of acid, is placed in 
a flask, diluted to about 200 c.c., and a tolerable quantity of moist mercuric 
oxide (precipitated from the nitrate or perchloride by alkali and washed)' 
added. Permanganate from a burette is then slow T ly added to the cold solution- 
with constant shaking until the rose colour appears in the clear liquid above 
the bulky brownish precipitate. 

The appearance of the mixture is somewhat puzzling at the 
beginning, but as more permanganate is added the precipitate 
settles more freely, and the end as. it approaches is very easily 
distinguished. The final ending is when the rose colour is 
persistent for a minute or two; subsequent bleaching must not 
be regarded. 

The actual decomposition as between cobaltous sulphate and 
permanganate may be formulated thus 

GCoSO 4 + 5H 2 + 2MnK0 4 = K 2 S0 4 + 5H 2 S0 4 + 3Co 2 3 + 2Mn0 2 
but as this exact decomposition cannot be depended upon in all the 
mixtures occurring, it is not possible to accept systematic .numbers- 
calculated from normal solutions. 

Solutions containing manganese, phosphorus, arsenic, active 
chlorine or oxygen compounds, or organic matter, cannot be used 
in this estimation ; moderate quantities of nickel are of no 
consequence. 



174 VOLTJMET1UC ANALYSIS. 57. 

Norman McCulloch (C. N. lix. 51) has proved that cobaltic 
oxide, as cobalticyanide, is a stable compound, and makes use of 
this fact to establish a process which gives very good results, by 
conversion of cobaltocyanide to the higher state of oxidation, the 
estimation of the oxygen being the measure of the cobalt itself. 
The method is exact in the presence of nickel, manganese, lead, 
arsenic, zinc, antimony, uranium, etc., but not in that of iron or 
copper. 

The standard solutions required are the ordinary ~ potassic 
bichromate, 1 c.c. of which represents O0059 gm. of Co, and an 
acid solution of ammonio-ferrous sulphate, whose strength is known 
by titration with the bichromate. There is also required a 5 per 
cent, solution of pure potassic cyanide, and a solution of nickel 
sulphate. 

The apparatus required may be simply a 12-oz. flask, fitted with 
two-hole stopper, one for a thistle funnel and the other as an 
escape for vapour. The mouth of the funnel should be somewhat 
constricted, and the lower end must dip beneath the surface of the 
liquid in the flask. 

Process : The standard bichromate and cyanide solutions are conveyed in 
their proper quantities to the flask above described, a few drops of ammonia 
added for subsequent neutralization of any free acid in solution to be tested, 
and the whole diluted, if necessary, to a convenient bulk with Avater. 

The amount of bichromate taken need not greatly exceed the theoretical 
requirement for the greatest probable quantity of cobalt to be estimated, but, 
with the cyanide, an allowance is made also for the conversion to soluble 
double c} r anides of such other metals as may be present. 

The cork and thistle-funnel are now placed in position, and the.solution 
boiled to expel air from the flask. Tho hot solution to be tested, of con- 
venient bulk and not too acid, and free, of course, from oxidizing or reducing 
constituents, is now added, and the ensuing reaction is instantaneously 
complete. 

After this stage the continued use of the cork and thistle-funnel is 
necessary only in presence of manganese. 

The contents of the flask are now cautiously treated with excess of 
a moderately warm concentrated solution of ammonic chloride, and the 
ebullition sustained for about ten minutes longer to expel volatile cyanide 
.(an operation conducted in a fume chamber or in a draught of air to carry 
off poisonous fumes). 

It now remains, preceding the estimation of non-reduced chromic acid 
with ferrous salt, to throw down soluble cobaltocyanide and decompose 
potassium-nickel cyanide by the addition of nickel sulphate. This is to 
prevent the subsequent formation of ferrous cobaltocyanide and double 
cyanide of iron and nickel respectively compounds difficultly soluble in 
dilute acid and, consequently, low results. To effect the above precipitation, 
a weight of nickel is required at least equal to that of the nickel and cobalt 
'existing in the contents of the flask, but if such acids as arsenic and phosphoric 
are present more is needed, as their precipitation is involved. Simply, the 
solution of nickel is added until no further precipitate is formed, or until the 
precipitate settles in a peculiar manner, to be known by experience ; great 
-excess of nickel is thus avoided, which would tend to interfere with the 
!erric3'anide reaction in the subsequent operation. 

The contents of the flask are now poured into excess of a hot aqueous 



oS. COPPER. 175 

solution of standard ferrous salt contained in a basin, acidified with a few 
drops of hydrochloric acid, and titrated with bichromate in usual way. 

The cobalt is calculated by multiplying the difference between the number 
of c.c. of bichromate taken at the outset of the estimation and that found 
at the completion, by 0*0059, and correcting- this by a slight allowance for 
reducing action of the potassic cyanide and its impurities on the chromate. 
In the author's case this correction was taken from experiment, and it was 
deemed sufficiently near to accept the reducing action of the cyanide as 
simp]}' proportionate to the quantity of this reagent used in the estimation, 
although it is not altogether independent of the proportion and amount 
of the bichromate, the degree of dilution, length of time of boiling, etc. 
The result showed that 100 c.c. of the bichromate boiled for a few minutes 
with its own bulk of the cyanide, and then for about ten minutes more with 
addition of excess of ammonic chloride, lost in value to the extent of about 
one c.c., which was deducted from the amount of bichromate reduced by the 
cobaltocyanide in such estimations, using the above bulk of cyanide, a fifth 
of this for 25 or 30 c.c., and so on. It is, of course, advisable, where the 
highest accuracy is desired,. to determine the necessary correction by a blank 
experiment, and duplicating also the approximate quantity of cobalt. 

It is best to separate iron as well as copper, and in the case of a cobalt ore 
the author would dissolve the sample in aqua-regia, and evaporate to dryness. 
The nitric acid would then be destroyed by two or three evaporations to 
dryness with hydrochloric acid, and the copper precipitated from the solution 
of the residue by sulphuretted hydrogen. In the filtrate from sulphide 
the iron would be separated by the acetate of soda method, and the iron 
precipitate re-dissolved and re-precipitated in a similar way to separate any 
small portion of cobalt. The combined filtrates from the acetate precipitates 
would be evaporated to convenient bulk, and the excess of acid neutralized 
by sodic hydrate or carbonate. The solution so obtained would then be added 
to suitable amounts of bichromate and cyanide, as described above. 

Examples : 0*114 gm. Co taken and 25*4 c.c. respectively of bichromate 
and cyanide used. The volume of bichromate reduced, allowing for the 
correction, was 19 2 c.c.=l*113 gm. Co. Again, 0*114 gm. Co and 0*228 gm. 
Ni taken, 25 c.c. of bichromate and 50 c.c. of cyanide used, the volume of 
the former reduced was 19'1 c.c.=0*112 gm. Co. Equally good results were 
obtained with mixtures of manganese, lead, arsenic, etc. 



COPPER. 

Cu=63. 

1 c.c. ~ soliitioii=0*0063 gm. Cu. 
Iron x 1-125 =Cu. 

Double Iron Salt x O1607=Cu. 

1. Reduction by Grape Sugar and subsequent titration with Ferric 
Chloride and Permanganate (Schwarz). 

58. THIS process is based upon the fact that grape sugar 
precipitates cuprous oxide from an alkaline solution of the metal 
containing tartaric acid ; the oxide so obtained is collected and 
mixed with ferric chloride and hydrochloric acid. The result is 
the following decomposition : 

Cu 2 + Fe 2 Cl 6 + 2HCl=:2CuCl 2 + 2FeCl 2 + H 2 0. 
Each equivalent of copper reduces one equivalent of ferric to ferrous 



176 VOLUMETRIC ANALYSIS. 58. 

chloride, which is estimated by permanganate with due precaution. 
The iron so obtained is calculated into copper by the requisite factor. 

Process : The weighed substance is brought into solution by nitric or 
sulphuric acid or water, in a porcelain dish or glass flask, and most of the 
acid in excess saturated with sodic carbonate ; neutral potassic tartrate is then 
added in not too large quantity, and the precipitate so produced dissolved to 
a clear blue liquid by adding caustic potash or soda in excess ; the vessel is 
next heated cautiously to about. 50 C. in the water bath, and sufficient 
grape sugar added to precipitate the copper present ; the heating is continued 
until the precipitate is of a bright red colour, and the upper liquid is 
brownish at the edges from the action of the alkali on the sugar : the heat 
must never exceed 90 C. When the mixture has somewhat cleared, the 
upper fluid is poured through a moistened filter, and afterwards the precipitate 
brought on the same, and washed with hot water till thoroughly clean ; the 
precipitate which may adhere to the dish or flask is well washed, and the 
filter containing the bulk of the protoxide put with it, and an excess of 
solution of ferric chloride (free from nitric acid or free chlorine) added, 
together with a little sulphuric acid ; the whole is then warmed and stirred 
until the cuprous chloride is all dissolved. It is then filtered into a good- 
sized flask, the old and new filters being both well washed with hot water, to 
which at first a little free sulphuric acid should be added, in order to be 
certain of dissolving all the oxide in the folds of the paper. The entire 
solution is then titrated with permanganate in the usual way. Bichromate 
ma} 7 ' also be used, but the end of the reaction is not so distinct as usual, from 
the turbidity produced by the presence of copper. 

2. Reduction by Zinc and subsequent titration with Ferric Chloride 
and Permang-anate (Fleitmann). 

The metallic solution, free from nitric acid, bismuth, or lead, is 
precipitated with clean sticks of pure zinc ; the copper collected, 
washed, and dissolved in a mixture of ferric chloride and hydro- 
chloric acid : a little sodic carbonate may be added to expel the 
atmospheric air. The reaction is 

Cu + Fe 2 Cl 6 =:CuCl 2 + 2FeCP. 

When the copper is all dissolved, the solution is diluted and 
titrated with permanganate; 56 Fe=31'5 Cu. 

If the original solution contains nitric acid, bismuth, or lead, 
the decomposition by zinc must take place in an ammoniacal 
solution, from which the precipitates of either of the above metals 
have been removed by filtration ; the zinc must in this case be 
finely divided and the mixture warmed. The copper is all 
precipitated when the colour of the solution has disappeared. It 
is washed first with hot water, then with weak HC1 and water to 
remove the zinc, again with water, and then dissolved in the acid, 
and ferric chloride as before. 

3. Estimation as Cuprous Iodide (E. O. Brown). 

This excellent method is based on the fact that when potassic 
iodide is mixed with a salt of copper in acid solution, cuprous 
iodide is precipitated as a dirty white powder, and iodine set free.. 



58. COPPER. 

If the latter is then immediately titrated with thiosulphate and 
starch, the corresponding quantity of copper is found. 

The solution of the metal, if it contain nitric acid, is evaporated 
with sulphuric acid till the former is expelled, or the nitric acid is 
neutralized with sodic carhonate, and acetic acid added ; the 
sulphate solution must be neutral, or only faintly acid ; excess of 
acetic acid is of no consequence, and therefore it is always 
necessary to get rid of all free mineral acids and work only Avith 
free acetic acid. 

J. W. Westmoreland (/. S. C. I. v. 51), who has had very large 
experience in examining a variety of copper products, strongly 
recommends this process for the estimation of copper in its various 
ores, etc. The metal may very conveniently be separated from a hot 
sulphuric acid solution by sodic thiosulphate : this gives a flocculent 
precipitate of subsulphide mixed with sulphur, which filters readily, 
and can be washed with hot water. Arsenic and antimony, if 
present, are also precipitated ; tin, zinc, iron, nickel, cobalt, and 
manganese are not precipitated. On igniting the precipitate most 
of the arsenic and the excess of sulphur is expelled, an impure 
subsulphide of copper being left. Sulphuretted hydrogen may of 
course be used instead of the thiosulphate, but its use is objection- 
able to many operators, beside which, under some circumstances, 
a small amount of copper remains in the solution, and moreover 
iron in small quantity is also precipitated with the copper, and 
cannot be entirely removed by washing. If HAS is used it should 
be passed for some time, and the precipitate allowed to stand a few 
hours to settle after nitration and washing the CuS should be 
redissolved in HXO 3 and reprecipitated with -the gas, it is then quite 
free from iron. 

Standardizing- the Thiosulphate Solution. This may be done on 
pure electrotype copper, but this is not always pure, and the safest 
standard is high conductivity wire, dissolved first in nitric acid, 
boiling to expel nitrous fumes, diluting, neutralizing with sodic 
carbonate till a precipitate occurs, then adding acetic acid till clear. 
The liquid is then made up to a definite volume, and a quantity 
equal to about 0'5 gm. Cu taken in a flask or beaker, about ten 
times the copper weight of potassic iodide added, and when 
dissolved the thiosulphate is run in from a burette until the free 
iodine is nearly removed, add then some starch, and finish the 
titration in the usual way. The thiosulphate will of course need 
to be checked occasionally. 

If strictly - thiosulphate is used, each c.c. =0*0063 gm. Cu. 

Process : For estimating the copper in iron pyrites or burnt ore 5 gm. of 
the substance should be taken, 2 gm. for 30 40 / mattes or 1 gin. for 
60 /'o mattes, and with precipitates it is best to dissolve say 5 gm. and dilute 
to a definite volume, and take as much as would represent from 0'5 to 07 gm. 
of Cu for titration. The solution is made with nitric acid, to which hydrochloric 
is also added later on, and then evaporated to dryness with excess of sulphuric 

N 



178 VOLUMETRIC ANALYSIS. 58. 

acid to convert the bases into sulphates ; the residue is treated with warm 
water and any insoluble PbSO 4 , c., filtered off. The filtrate is heated to 
boiling and precipitated with sodic thiosulphate, this precipitate is filtered 
off, washed with hot water, dried, and roasted, the resulting copper oxide is 
then dissolved in nitric acid, and after the excess of acid is chiefly removed 
by evaporation sodic carbonate is added, so as to precipitate part of the 
copper and ensure freedom from mineral acid, acetic acid is added till 
a clear solution is obtained ; about ten parts of potassic iodide to one of 
copper, supposed to be present, are then added, and the titration carried out in 
the usual way. 

A modification of this process is adopted in tlie United States 
(Peters, Eng. and Min. Journ. lix. 124) as follows : 

In the treatment of ores 1 gin. is heated with hot, strong nitric acid, to 
which is then added strong hydrochloric acid. After boiling, strong- 
sulphuric acid is added, and the volatile acids evaporated off. After 
diluting, the PbSO 4 , &c., is filtered off, and the solution, which should not 
exceed 75 c.c., is run into a beaker, at the bottom of which is a strip of 
aluminium 3 in. long, 1^ in. wide, and turned up at the ends so that the 
body of the strip can lie flat. The copper is all precipitated after boiling for 
six or seven minutes. The liquid is filtered off, and the loose and adherent 
copper is all dissolved in a little nitric acid. To this is added half a gram 
of chlorate of potash, to fulty oxidize any arsenic present, and the solution 
boiled down to small bulk, but not sufficiently low to produce a basic salt 
of copper. The solution is then neutralized with ammonia, acidified with 
acetic acid, and titrated in the usual manner. 

This treatment removes all interfering impurities or renders them inert. 
Zinc is not such a good precipitant for the copper as aluminium, as some 
iron is also carried down even from strongly acid solutions. When aluminium 
is used, the precipitation may be effected without boiling by adding a little 
hydrochloric acid to the solution, but this is not so desirable as the method 
described. For the success of the titration it is essential that no free nitric 
acid or nitrate of copper be present. Cold ammonia in excess does not, 
apparently, entirely decompose the latter, hence the necessity for boiling. 
Care must be taken that the aluminium contains no copper, or if it does 
its quantity must be known. 

By either of the above methods there is no interference from 
arsenic or bismuth, so long as no free mineral acid is present. 

4. Estimation by Potassic Cyanide (P ; 'arkes and C. Itlohr). 

This well-known and much-used, process for estimating copper 
depends upon the decoloration of an ammoniacal solution of copper 
by potassic cyanide. The reaction (which is not absolutely uniform 
with variable quantities of ammonia) is such that a double cyanide 
of copper and ammonia is formed; cyanogen is also liberated, which 
reacts on the free ammonia, producing urea, oxalate of urea, 
ammonic cyanide and formate (Liebig). Owing to the influence 
exercised by variable quantities of ammonia, or its neutral salts, 
upon the decoloration of a copper solution by the cyanide, it has 
been suggested by Beringer to substitute some other alkali for 
neutralizing the free acid in the copper solution other than 
ammonia. The suggestion has been adopted by Da vies (C. N. 
Iviii. 131) and by Eessenden (C. N. Ixi. 131), who both 



COPPER. ] 79 

recommend sodic carbonate. My own experiments completely 
confirm their statement that none of the irregularity common to 
variable quantities of ammonia or its salts occurs with soda or 
potash. Suppose for example that copper has been separated as 
sulphide, and brought into solution by nitric acid, the free nitro- 
sulphuric acid is neutralized with JX r a 2 C0 3 , and an excess of it 
added to redissolve tlie precipitate. The cyanide solution is then 
cautiously ran into the light blue solution until the colour is just 
discharged. My own experience is, that it is impossible to 
redissolve the whole of the precipitate without using a very large 
excess of soda ; but there is no need to add such an excess, as 
the precipitate easily dissolves when the cyanide is added. 
I have used a modification of this method, which gives excellent 
results, viz., to neutralize the acid copper solution either with 
Na' 2 CO :J or NaHO, add a trifling excess, and then 1 c.c. of 
ammonia 0'960 sp. gr. ; a deep b^ue clear solution is at once given, 
which permits of very sharp end-reaction with the cyanide. 

J. J. and C. Beringer (C. N. xlix. iii.) have already adopted 
the method of neutralizing the acid copper solution with soda, 
then adding ammonia, but the proportion they recommend is larger 
than necessary. 

In standardizing the cyanide, it is advisable to arrange so that 
copper is precipitated with soda exactly as in the titration of 
a copper ore ; that is to say, free nitric or mtro-sulphuric acid 
should be added, then neutralized with slight excess of soda, 
cleared with 1 c.c. of ammonia, then titrated with cyanide. Large 
quantities of nitrate or sulphate of soda or potash, however, make 
very little difference in the quantity of cyanide used. 

It lias generally feeen thought that where copper and iron occur together, 
it is necessary to separate the latter before using the cyanide. P. Field, 
however, has stated that this is not necessary (C. N. i. 25) ; and I can fully 
eadorse his statement that the presence of the suspended ferric oxide is no 
hindrance to the estimation of the copper ; in fact, it is rather an advantage, 
as it acts as an indicator to the end of the process. 

While the copper is in excess, the oxide possesses a purplish-brown colour, 
but as this excess lessens, the colour becomes gradually lighter, until it is 
orange brown. If it be now allowed to settle, which it does very rapidly, the 
clear liquid above will be found nearly colourless. A little practice is of 
course necessary to enable the operator to hit the exact point. 

It is. impossible to separate the ferric oxide by filtration without 
leaving some copper in it, and no amount of washing will remove 
it. For example, 10 c.c. of a copper solution with 10 c.c. of ferric 
solution were directly titrated with cyanide after treatment with 
]N r aHO in slight excess and 1 c.c. of ammonia: The cyanide 
required was 12 c.c. Another 10 c.c. of the same copper and iron 
solutions were then precipitated with soda and ammonia in same 
proportions. This gave a complete solution of the copper with the 
ferric oxide suspended in it. The solution was filtered and the 
ferric oxide well washed with hot water, then the filtrate cooled and 

N 2 



180 VOLUMETRIC ANALYSIS. 58. 

titrated with cyanide, 9*5 c.c. only being required. On treating the 
ferric oxide on the filter with nitric acid, neutralizing with XaHO 
and NH 8 in proper proportions exactly, 2 '5 c.c. of cyanide were 
required, showing that the ferric oxide had retained 20 per cent, 
of the copper. 

I strongly recommend that operators who have to deal with 
copper determination upon samples containing much iron, should 
practise the use of the cyanide method in the presence of the iron, 
and accustom their eyes to the exact colour which the ferric oxide 
takes when the titration is finished, always, however, with this 
proviso, that the cyanide solution is standardized upon a known 
weight of copper in the presence of a moderate amount of iron. 

The solution of potassic cyanide should he titrated afresh at 
intervals of a few days. Further details of this process are given 
in 58.8. 

Dulin (Jour. Amer. Cliem. Soc.^vii. 346) advocates the cyanide 
process for copper ores as follows : 

Process : The ore is treated in the way described in 58.3 to obtain a solution 
of the copper practically free from silver and lead. The copper is then pre- 
cipitated upon aluminium foil as there mentioned. Should cadmium be 
present it is also precipitated to some extent, but only after the copper 
is thrown down. If care be taken to stop the boiling immediately after 
the copper is precipitated, which a practised eye will readily detect, the 
amount of cadmium precipitated is so small as to cause no sensible error. 
The liquid being decanted from the copper and foil, the latter are washed 
well with hot water, taking care to lose no metal ; when quite clean, dilute 
nitric acid is added and boiled till the copper is dissolved, the liquid then 
neutralized with excess of ammonia, and titrated with cyanide in the 
usual way. 

5. Estimation as Sulphide (Pelouze). 

It is first necessary to have a solution of pure copper of known 
strength, which is best made by dissolving 39*523 gm. of pure 
cupric sulphate in 1 liter of water ; each c.c. will contain 
0-01 gm. Cu. 

Precipitation in Alkaline Solution. This process is based on the 
fact that if an ammoniacal solution of copper is heated to from 
40 to 80 C., and a solution of sodic sulphide added, the whole 
of the copper is precipitated as oxysulphide, leaving the liquid 
colourless. The loss of colour indicates, therefore, the end of the 
process, and this is its weak point. Special practice, however, will 
enable the operator to hit the exact point closely. 

Example : A measured quantity (say 50 c.c.) of standard solution of copper 
is freely supersaturated with ammonia, and heated till it begins to boil. 
The temperature will not be higher than 80 C. in consequence of the 
presence of the ammonia ; it is always well, however, to use a thermometer. 
The sodic sulphide is delivered cautiously from a Molar's burette, until the 
last traces of blue colour have disappeared from the clear liquid above the 
precipitate. The experiment is repeated, and if the same result is obtained, 
the number of c.c. required to precipitate the amount of copper contained 



58. COPPER. 181 

in 50 c.c. 0'5 gin., is marked upon the alkaline sulphide bottle. As the 
strength of the solution gradually deteriorates, it must be titrated afresh every 
day or two. Special regard must be had to the temperature of the precipi- 
tation, otherwise the accuracy of the process is seriously interfered with. 

Casamajor (0. N. xlv. 167) uses instead of ammonia the alkaline 
tartrate solution same as for Fehling, adding a slight excess so as 
to make a clear blue solution. The addition of the sulphide gives 
an intense black brown precipitate, which is stirred vigorously till 
clear. The copper sulphide agglomerates into curds, and the 
reagent is added until no further action occurs with a drop of 
the sodic sulphide. This modification can also be used for lead. 
PbSO 4 is easily soluble in the tartrate solution, and can be estimated 
by the sodic sulphide in the same way as copper. 

The colour of the solution is not regarded, but the clotty 
precipitate of sulphide, which is easily cleared by vigorous stirring. 
Very good results may be gained by this modification. 

Copper can also be first separated by glucose, or as thiocyanate 
(Rivot), then dissolved in HXO 3 , and treated with the tartrate. 

Precipitation in Acid Solution. The copper solution is placed 
in a tall stoppered flask of tolerable size (400 or 500 c.c.), freely 
acidified with hydrochloric acid, then diluted with about 200 c.c. 
of hot water. 

The alkaline sulphide is then delivered in from a burette, 
the stopper replaced, and the mixture well shaken ; the precipitate 
of copper sulphide settles readily, leaving the supernatant liquid 
clear ; fresh sulphide solution is then at intervals added until no 
more precipitate occurs. The calculation is the same as in the case 
of alkaline precipitation, but the copper is precipitated as pure 
sulphide instead of oxysulphide. 

6. Estimation by Staniious Chloride (Weil). 

This process is based on the fact, that a solution of a cupric salt 
in large excess of hydrochloric acid at a boiling heat shows, even 
when the smallest trace is present, a greenish-yellow colour. If to 
such a solution stannous chloride is added in minute excess, a 
colourless cuprous chloride is produced, and the loss of colour 
indicates the end of the process. 

2CuCl 2 + SnCl 2 ==Cu 2 Cl 2 + SnCl 4 . 

The change is easily distinguishable to the eye, but should any 
doubt exist as to whether stannous chloride is in excess, a small 
portion of the solution may be tested with mercuric chloride. Any 
precipitate of calomel indicates the presence of stannous chloride. 

The tin solution is prepared as described in 37.2. 

A standard copper solution is made by dissolving pure cupric 
sulphate in distilled water, in the proportion of 39 '523 gm. per 
liter=10 gm, of Cu. 



182 VOLUMETRIC ANALYSIS. 58. 

Process for Copper alone. 10 c.c. of the copper solution O'l gm. of 
Cu are put into a Avhite-glass flask, 25 c.c. of pure strong hydrochloric acid 
added, placed on a sand-bath and brought to boiling heat ; the tin solution is 
then quickly delivered in from a burette until the colour is nearly destroyed, 
finally a drop at a time till the liquid is as colourless as distilled water. No 
oxidation will take place during the boiling, owing to the flask being filled 
with acid vapours. 

A sample of copper ore is prepared in the usual way by treatment Avith 
nitric acid, which is afterwards removed by evaporating with sulphuric acid. 
Silica, lead, tin, silver, or arsenic, are of no consequence, as when the solution 
is diluted with water to a definite volume, the precipitates of these substances 
settle to the bottom of the measuring flask, and the clear liquid may 
be taken out for titration. In case antimonic acid is present it will be 
reduced with the copper, but on exposing the liquid for a night in an open 
basin, the copper will be completely re-oxidized but not the antimony ; 
a second titration will then show the amount of copper. 

Process for Ores containing 1 Copper and Iron. In the case of copper 
ores where iron is also present, the quantity of tin solution required will of 
course represent both the iron and the copper. In this case a second titration 
of the original solution is made with zinc and permanganate, and the quantity 
so found is deducted from the total quantity; the amount of tin solution 
corresponding to copper is thus found. 

Example : A solution was prepared from 10 gm. of ore and diluted to 
250 c.c. : 10 c.c. required 26'75 c.c. of tin solution whose strength was 
16'2 c.c. for O'l gm. of Cu. 

10 c.c. of ore solution were diluted, warmed, zinc and platinum added till 
reduction Avas complete, and the solution titrated with permanganate Avhose 
quantity=0-0809 gm.-of Fe. 

The relative strength of the tin solution to iron is 18'34 c.c.=0'l gm. 
of Fe : thus : 

63 : 56 =0-1 : 0'0888. 

therefore O'l gm. of Cu=0'0888 gm. of Fe=16'2 c.c. of SnCP 
whence 0'0888 : 0'1=16'2 :" 18'34 
thus 0*0809 Fe (found above)=14'837 c.c. of SnCl 2 
O'l : 0809=18-34 : 14'837 hence 

Iron and copper = 26'750 c.c. SnCl 2 

Subtract for iron = 14'837 

Leaving for copper 1T913 

10 c.c. of ore solution therefore contained 16'2 : O'l : : 11-913=0'0735 gm. 
of Cu, and as 10 gm. of ore=250 c.c. contained T837 gm. of Cu=18'37 per 
cent. Aiiatysis by weight as a control gave 18'34 per cent. Cu. 
Fe voluinetrically 20'25 per cent., by Aveight 20' iO per cent. 

The method is specially adapted for the technical analysis of 
fahl-ores. 

Process for Ores containing Nickel or Cobalt. The ore is dissolved 
in nitric or nitre-hydrochloric acid, then nearly neutralized with sodic 
carbonate, diluted Avith cold water, and freshly precipitated baric carbonate 
and some ammouic chloride added ; the whole is Avell mixed together, 
producing a precipitate containing all the copper and iron, AAiiile the nickel 
or cobalt remains in solution ; the precipitate is first washed by decantation, 
collected on a filter, well washed, then dissolved in hydrochloric add, and 
titrated with stannous chloride as before described. 



COPPEK. 183 

Method for Copper, Iron, and Antimony. The necessary solutions 
arc: (I) Standard copper. 19'667 gm. of copper sulphate are dissolved 
in water to 500 c.c. (2) A similar solution containing 7'867 gin. of copper 
sulphate. (3) Standard tin solution. 4'5 to 5 gm. of stannous chloride, 
and 230 gm. of HC1, are made up to 500 c.c. with water. This solution is 
standardized with No. 1, 10 c.c. of which solution should be mixed with 
25 c.c. hydrochloric acid, boiled, and the tin solution to be standardized run 
in until the green colour disappears. 

Estimation of Copper. 5 gm. of substance are dissolved in HC1 or 
IT 2 S0 4 , and made up to 250 c.c. 10 c.c. of this solution are taken, 25 c.c. 
HC1 added, and then titrated as above. 

Estimation of Iron. When there are 2 vols. of free HCl to 1 vol. of 
the ferric solution no indicator is necessary, and the standard tin solution is 
run in until the iron solution is colourless ; in this way the quantity of iron 
is obtained in terms of copper. Of solutions containing 2 gm. of the sample 
in 250 c.c., 10 c.c. are evaporated in a porcelain capsule, with 10 c.c. of the 
copper solution (No. 2) ; to the concentrated mixed solution large excess 
(about 75 c.c.) of HC1 is added, and this is titrated with the tin solution as 
before. Of course the tin required for the copper used must be deducted. 
The copper is used as an indicator, and is not required with substances 
containing more than 2 per cent, of iron. 

Estimation of Iron and. Copper. 5 gm. of ore in 250 c.c. Titrate as 
before directed. In another 10 c.c. of solution, precipitate the copper with 
zinc, filter, reconvert the ferrous into ferric salt by means of permanganate, 
and titrate the iron again. 

Estimation of Antimony. In making up the 250 c.c. in this case, it is 
necessary to use aqueous solution of tartaric acid to prevent precipitation of 
antimony. The solution of antimonic chloride is mixed with No. 1 copper 
solution and a large excess of HCl, then titrated; the c.c. of standard tin 
solution used indicates the sum of the Cu and Sb. If the mixed solution of 
cuprous and antimonious chloride is allowed to remain some hours the Cu 
becomes re-oxidized, but the Sb does not, therefore a second titration gives 
the quantity of Cu only; this is scarcely required when the strength and 
volume of copper solution added is known. 

Antimony, Copper, and Iron, when together in same sample, are thus 
determined. 5 gm. substance are dissolved in nitric acid, evaporated down, 
and filtered. The filtrate contains iron and copper, which are determined as 
above directed. The precipitate contains all the antimony ; it is dissolved in 
HCl, treated with permanganate, and the antimonic chloride determined as 
directed. 

This process depends on the reducing action of stannous chloride. It is 
therefore necessary to get rid of extraneous oxidizing influences, such as 
free chlorine, nitric acid, or excess of permanganate, etc., before titration ; 
this is effected by evaporating to dryness, taking up with hydrochloric acid, 
and repeating, until the solution or vapour evolved on boiling ceases to turn 
iodized starch-paper blue. 

All the above described Weil methods must only be taken as 
approximately accurate, but sufficiently so for technical use. 

7. Volhard's method. 

The necessary standard solutions are described in 43, Each c.c, 
of ~Q thiocyanate represents 0*0063 gm. Cu, 



184 VOLUMETRIC ANALYSIS. 58. 

Process : The copper in sulphuric or nitric acid solution is evaporated to 
remove excess of acid, or if the acid is small in quantity neutralized with 
sodic carbonate, washed into a 300 c.c. flask, and enough aqueous solution of 
SO 2 added to dissolve the traces of basic carbonate and leave a distinct smell 
of SO 2 . Heat to boiling, and run in from a burette the thiocyanate until the 
addition produces no change of colour, add 3 or 4 c.c., and note the entire 
quantit} 7 ", allow to cool, fill to mark, and shake well. 100 c.c. are then filtered 
through a dry filter, 10 c.c. of ferric indicator with some nitric acid added, 
then titrated with T ^ silver till colourless : then again thiocyanate till the 
reddish colour occurs. The volume of silver solution, less the final correction 
with thiocyanate, deducted from the original thiocyanate, will give the 
volume of the latter required to precipitate the copper" 

The process is not accurate in presence of Fe, Ag, Hg, Cl, I or Br. 

8. Technical Examination of Copper Ores (Steinbeck's 
Process): 

In 1867 the Directors of the Mansfield Copper Mines offered 
a premium for the best method of examining these ores, the chief 
conditions being tolerable accuracy, simplicity of working, and the 
possibility of one operator making at least eighteen assays in the day. 

The fortunate competitor was Dr. Steinbeck, whose process 
satisfied completely the requirements. The whole report is con- 
tained in Z. a. C. viii. 1, and is also translated in C. JV. xix. 181. 
The following is a condensed ri'sunu'. of the process, the final 
titration of the copper being accomplished by potassic cyanide as 
in 58.4. A very convenient arrangement for filling the burette 
with standard solution where a series of analyses has to be made, 
and the burette continually emptied, is shown in fig. 40 ; it may 
be refilled by simply blowing upon the surface of the liquid. 

(a) The extraction of the Copper from the Ore. 5 gm. of pulverized 
ore are put into a flask with from 40 to 50 c.c. of hydrochloric acid (specific 
gravity 1*16), whereb} r all carbonates are converted into chlorides, while 
carbonic acid is expelled. After a while there is added to the fluid in 
the flask 6 c.c. of a special nitric acid, prepared by mixing equal bulks of 
water and pure nitric acid of 1'2 sp. gr. As regards certain ores, however, 
specially met with in the district of Mansfield, some, having a very high 
percentage of sulphur and bitumen, have to be roasted previous to being 
subjected to this process; and others, again, require only 1 c.c. of nitric acid 
instead of 6. The flask containing the assay is digested on a sand-bath for 
half an hour, and the contents boiled for about fifteen minutes; after which 
the whole of the copper occurring in the ore, and all other metals, are in 
solution as chlorides. The blackish residue, consisting of sand and schist, 
has been proved by numerous experiments to be either entirely free from 
copper, or to contain at the most only O'Ol to 0'03 per cent. 

(b) Separation of the Copper. The solution of metallic and earthy 
chlorides, and some free HC1, obtained as just described, is separated by 
filtration from the insoluble residue, and the fluid run into a covered beaker 
of about 400 c.c. capacity. In this beaker a rod of metallic zinc, weighing 
about 50 gm., has been previously placed, fastened to a piece of stout 
platinum foil. The zinc to be used for this purpose should be as much as 
possible free from lead, and at any rate should not contain more than from 
O'l to 0'3 per cent, of the latter metal. The precipitation of the copper in 







COPPER. 



185 



the metallic state sets in already during the nitration of the warm and 
concentrated fluid, and is, owing especially also to the entire absence of 
nitric acid, completely finished in from half to three-quarters of an hour after 
the beginning of the filtration. If the fluid be tested with SH 2 , no trace 
of copper can or should be detected; the spongy metal partly covers the 
platinum foil, partly floats about in the liquid, and in case either the ore 
itself or the zinc applied in the experiment contained lead, small quantities 
of that metal will accompany the precipitated copper. After the excess of 
zinc (for an excess must always be employed) has been removed, the metal is 
repeatedly and carefully washed by decantation with fresh water, and care 
taken to collect together every particle of the spongy mass. 




Pig. 40. 

(c) Estimation of the precipitated Copper. To the spongy metallic- 
mass in the beaker glass, wherein the platinum foil is left, since some of the 
metal adheres to it, 8 c.c. of the special nitric acid are added, and the copper 
dissolved by the aid of moderate heat in the form of cupric nitrate, which,, 
in the event of any small quantity of lead being present, will of course be 
contaminated with lead. 

"VYhen copper ores are dealt with containing above 6 per cent, of copper, 
which may be approximately estimated from the bulk of the spongy mass off 



186 VOLUMETRIC ANALYSIS. 58. 

precipitated metal, 16 c.c. of nitric acid, instead of 8, are applied for 
dissolving the metal. The solution thus obtained is left to cool, and next 
mixed, immediately before titration with potassic cyanide, with ]() c.c. of 
special solution of liquid ammonia, prepared by diluting 1 volume of liquid 
ammonia (sp. gr. 0'93) with 2 volumes of distilled water. 

The titration with cyanide is conducted as described in 58.4. 

In the case of such ores as } r ield over 6 per cent, of copper, and when a 
double quantity of nitric acid has consequent!}' been used, the solution i.? 
diluted with water, and made to occupy a bulk of .100 c.c.; this bulk is then 
exactly divided into two portions of 50 c.c. each, and each of these separately 
mixed with 10 c.c. of ammonia, and the copper therein volumetrically 
determined. The deep blue coloured solution only contains, in addition to 
the copper compound, ammouic nitrate ; any lead which might have been 
dissolved having been precipitated as hydrated oxide, which does not interfere 
with the titration with cyanide. The solution of the last-named salt is so 
arranged, that 1 c.c. thereof exactly indicates 0'005 gin. of copper (about 
21 gm. of the pure salt per liter). Since, for every assay, 5 gin. of ore have 
been taken, 1 c.c. of the titration fluid is equal to 0*1 per cent, of copper, 
it hence follows that, by multiplying the number of c.c. of cyanide solution 
used to make the blue colour of the copper solution disappear by O'l, the 
percentage of copper contained in the ore is immediately ascertained. 

Steinbeck tested tins method specially, in order to see what 
influence is exercised thereupon by (1) ammonic nitrate, (2) caustic 
ammonia, (3) lead. The copper used for the experiments for this 
purpose was pure metal, obtained by galvanic action, and was 
ignited to destroy any organic matter which might accidentally 
adhere to it, and next cleaned by placing it in dilute nitric acid. 
5 gm. of this metal were placed in a liter flask, and dissolved in 
266*6 c.c. of special nitric acid, the flask gently heated, and, after 
cooling, the contents diluted with water, and thus brought to a 
bulk of 1000 c.c. 30 c.c. of this solution were always applied to 
titrate one and the same solution of cyanide under all circumstances. 
When 5 gm. of ore, containing on an average 3 per cent, of copper, 
are taken for assay, that quantity of copper is exactly equal to 
0*150 gm. of the chemically pure copper. The quantity of nitric 
acid taken to dissolve 5 gm. of pure copper (266*6 c.c.) was 
purposely taken, so as to correspond with the quantity of 8 c.c. of 
special nitric acid which is applied in^ the assay of the copper 
obtained from the ore, and this quantity of acid is exactly met 
Avith in 30 c.c. of the solution of pure copper. 

The influence of double quantities of ammonic nitrate and free 
caustic ammonia (the quantity of copper remaining the same) is 
shown as follows : 

(a) 30 c.c. of the normal solution of copper, containing exactlj' O'lSO gm. 
of copper, were rendered alkaline with 10 c.c. of special ammonia, and were 
found to require, for entire decoloration, 29'8 c.c. of cyanide. A second 
experiment, again with 30 c.c. of copper solution, and otherwise under 
identically the same conditions, required 29 9 c.c. of cyanide. The average 
is 29'85 c.c. 

(6) When to 30 c.c. of the copper solution, first 8 c.c. of special nitric 
acid are added, and then 20 c.c. of special ammonia instead of only 8, whereby 



COPPER. 187 

the quantity of free ammonia and of amraonic nitrate is double what it was 
in the case of , there is required of the same cyanide 30*0 c.c. to produce 
decoloration. A repetition of. the experiment,' exactly under the same 
conditions, gave 30'4 c.c. of the cyanide ; the average is, therefore, 30'35 c.c. 
The difference amounts to only 0'05 per cent, of copper, which may be 
allowed for in the final calculation. 

When, however, larger quantities of ammoniacal salts are present 
in the fluid to be assayed for copper, by means of cyanide, and 
especially when ammonic carbonate, sulphate, and worse still, 
chloride are simultaneously present, these salts exert a very dis- 
turbing influence.* The presence of lead in the copper solution 
to be assayed has the effect of producing, on the addition of 10 c.c. 
of normal ammonia, a milkiness with the blue tint ; but this does 
not at all interfere with the estimation of the copper by means 
of the cyanide, provided the lead be not in great excess ; and a 
slight milkiness of the solution even promotes the visibility of the 
approaching end of the operation. 

Steinbeck purposely made some experiments to test this point, 
and his results show that a moderate quantity of lead has no 
influence. 

Experiments were also carefully made to ascertain the influence 
of zinc, the result of which showed that up to 5 per cent, of the 
copper present, the zinc had no disturbing action; but a considerable 
variation occurred as the percentage increased above that proportion. 
Care must therefore always be taken in washing the spongy copper 
precipitated from the ore solution by means of zinc. 

The titration must always take place at ordinary temperatures, 
since heating the ammoniacal solution while under titration to 40 
or 45 C. considerably reduces the quantity of cyanide required. 

9. Estimation of Copper by Colour Titration. 

This method can be adopted with very accurate results, as in the 
case of iron, and is available for slags, poor cupreous pyrites, 
waters, etc. (see Carnelly, C. N. xxxii. 308). 

The reagent used is the same as in the case of iron, viz., potassic 
ferrocyanide, which gives, a purple-brown colour with very dilute 
solutions of copper. This reaction, however, is not so delicate as 
it is with iron, for 1 part of the latter in 13,000,000 parts of water 
can be detected by means of potassic ferrocyanide ; while 1 part 
of copper in a neutral solution, containing ammonic nitrate, can 
only be detected in 2,500,000 parts of water. Of the coloured 
reactions which copper gives with different reagents, those with 
sulphuretted hydrogen and potassic ferrocyanide are by far the 
most delicate, both showing their respective colours in 2,500,000 
parts of water. 

* I have retained this technical process in its original form, notwithstanding the use 
of ammonia, because it is systematic, and the results obtained by it are all comparable 
among themselves. Of course soda or potash may be used in place of ammonia, if the 
cyanide is standardized with them. 



188 VOLUMETRIC ANALYSIS. 58. 

Of the two reagents sulphuretted hydrogen is the more delicate ; 
but potassic ferrocyanide has a decided advantage over sulphuretted 
hydrogen in the fact that lead, when not present in too large 
quantity, does not interfere with the depth of colour obtained, 
whereas to sulphuretted hydrogen it is, as is well known, very 
sensitive.'"' 

And though iron if present would, without special precaution 
being taken, prevent the determination of copper by means of 
ferrocyanide ; yet, by the method as described below, the amounts 
of these metals contained together in a solution can be estimated 
by this reagent. 

Ammonic nitrate renders the reaction much more delicate ; other 
salts, as ammonic chloride and potassic nitrate, have likewise the 
same effect. 

The method of analysis consists in the comparison of the 
purple-brown colours produced by adding to a solution of potassic 
ferrocyanide first, a solution of copper of known strength ; and, 
secondly, the solution in which the copper is to be determined. 

The solutions and materials required are as follows : - 

(1) Standard Copper solution. Prepared by dissolvingO'395gm. 
of pure CuSO 4 , 5H 2 in one liter of water. 1 c.c. = O'l m.gm. Cu. 

(2) Solution of Ammonic nitrate. Made by dissolving 100 gm. 
of the salt in one liter of water. 

(3) Potassic ferrocyanide solution. 1 : 25. 

(4) Two glass cylinders holding rather more than 150 c.c. each, 
the point equivalent to that volume being marked on the glass. 
They must both be of the same tint, and as colourless as possible. 

A burette, graduated to y 1 ^ c.c. for the copper solution; a 5 c.c. 
pipette for the ammonic nitrate ; and a small tube to deliver the 
ferrocyanide in drops. 

Process : Five drops of the potassic ferrocyanide are placed in each 
cylinder, and then a measured quantity of the neutral solution in which 
the copper is to be determined is placed into one of them, and both filled 
up to the mark with distilled water, 5 c.c. of the ammonic nitrate solution 
added to each, and then the standard copper solution ran gradually into 
the other till the colours in both cylinders are of the same depth, the 
liquid being well stirred after each addition. The number of c.c. used are 
then read off. Each c.c. corresponds to O'l m.gm. of copper, from which 
the amount of copper in the solution in question can be calculated. 

The solution in which the copper is to be estimated must be 
neutral ; for if it contain free acid the latter lessens the depth of 
colour, and changes it from a purple-brown to an earthy brown. 
If it should be acid, it is rendered slightly alkaline with ammonia, 
and the excess of the latter got rid of by boiling. The solution 
must not be alkaline, as the brown coloration is soluble in ammonia 

* In colour titrations of this character it is essential that the comparisons be made 
under the same circumstances as to temperature, dilution, and admixture of foreign 
substances, otherwise serious errors will arise. 



58. CYANOGEN. 189 

and decomposed by potash or soda ; if it be alkaline from ammonia, 
this is remedied as before by boiling it off; while free potash or 
soda, should they be present, are, neutralized by an acid, and the 
latter by ammonia, 

Lead, when present in not too large quantity, has little or no 
effect on the accuracy of the method. The precipitate obtained on 
adding potassic ferrocyanide to a lead salt is white ; and this, except 
when present in comparatively large quantity with respect to the 
copper, does not interfere with the comparison of the colours. 

When copper is to be estimated in a solution containing iron, 
the following method is adopted : 

A few drops of nitric acid are added to the solution in order to oxidize the 
iron, the liquid evaporated to a small bulk, and the iron precipitated by 
ammonia. Even when very small quantities of iron are present, this can be 
done easily and completely if there be only a very small quantity of fluid. 
The precipitate of ferric oxide is then filtered off, washed once, dissolved in 
nitric acid, and re-precipitated by ammonia, filtered and washed. The iron 
precipitate is now free from copper, and in it the iron can be estimated by 
dissolving in nitric acid, making the solution nearly neutral with .ammonia, 
and determining the iron by the method in 64.4. The filtrate from the 
iron precipitate is boiled till the ammonia is completely driven off, and the 
copper estimated in the solution so obtained as already described. 

When the solution containing copper is too dilute to give any 
coloration directly with ferrocyanide, a measured quantity of it 
must be evaporated to a small bulk, and filtered if necessary; 
.and if it contain iron, also treated as already described. 

In the determination of copper and iron in water, for which the 
method is specially applicable, a measured quantity is evaporated 
to dryness with a few drops of nitric acid, ignited to get rid of any 
organic matter that might colour the liquid, dissolved in a little 
boiling water and a drop or two of nitric acid ; if it is not all 
soluble it does not matter. Ammonia is next added to precipitate 
the iron, the latter filtered off, washed, re-dissolved in nitric acid, 
and again precipitated by ammonia, filtered off, and washed. The 
filtrate is added to the one previously obtained, the iron estimated 
in the precipitate, and the copper in the united filtrates. 

CYANOGEN. 

CIS T -=26. 

1 c.c. T ^ silver solution=0'0052 gin. 

Cyanogen. 
=0-0054 gm. 

Hydrocyanic acid. 
=0;01302 gm. 

Potassic cyanide. 
yjj iodine solution=0'003255 gm. 

Potassic cyanide. 



190 VOLUMETRIC ANALYSIS. 

1. By Standard Silver Solution (Lie big). 

59. THIS ready and accurate method of estimating cyanogen 
in prnssic acid, alkaline cyanides, 1 etc., was discovered by Liebig, 
and is fully described in Ann. der Ghem. und Pliarm. Ixxvii. 102. 
It is based on the fact, that when a solution of silver nitrate is 
added to an alkaline solution containing cyanogen, with constant 
stirring, no permanent precipitate of silver cyanide occurs until all 
the cyanogen has combined with the alkali and the silver, to form 
a soluble double salt (in the presence of potash, for example, 
KCy, AgCy). If the slightest excess of silver, over and above the 
quantity required to form this combination, be added, a permanent 
precipitate of silver cyanide occurs, the double compound being 
destroyed. If, therefore, the silver solution be of known strength, 
the quantity of cyanogen present is easily found ; 1 eq. of silver 
in this case being equal to 2 eq. cyanogen. 

So fast is this double combination, that, when sodic chloride is 
present, no permanent precipitate of silver chloride occurs, until 
the quantity of silver necessary to form the compound is slightly 
overstepped. 

Siebold, however, has pointed out that this process, in the case 
of free hydrocyanic acid, is liable to serious errors unless the 
following precautions are observed : 

() The solution of sodic or potassic hydrate should be placed in the 
beaker first, and the hydrocyanic acid added to it from a burette dipping 
into the alkali. If, instead of this, the acid is placed in the beaker first, 
and the alkaline hydrate added afterwards, there mny be a slight loss by 
evaporation, which becomes appreciable whenever there is any delay in the 
addition of the alkali. 

(5) The mixture of hydrocyanic acid and alkali should be largely diluted 
with water before the silver nitrate is added. The most suitable proportion 
of water is from ten to twenty times the volume of the officinal or of 
Scheele's acid. With such a degree of dilution, the final point of the- 
reaction can be observed with greater precision. 

(c) The amount of alkali used should be as exactly as possible that 
required for the conversion of the hydrocyanic acid into alkaline cyanide., 
as an insufficiency or an excess both affect the accuracy of the result. It is 
advisable to make first a rough estimation with excess of soda as a guide, 
then finish with a solution as neutral as possible. 

Caution. In using the pipette for measuring hydrocyanic acid, 
it is advisable to insert a plug of cotton wool, slightly moistened 
with silver nitrate, into the upper end, so as to avoid the danger 
of inhaling any of the acid ; otherwise it is decidedly preferable 
to weigh it. 

Example ivith Commercial Potassic Cyanide : The quantity of this sub- 
stance necessary to be taken for analysis, so that each c.c. or dm. shall be 
equal to 1 per cent, of the pure C3*auide. is 1'30 gm. or 13'0 grn. 13 grains, 
therefore, of the commercial article were dissolved in water, no further alkali 
being necessaiy, and 54 dm. yV silver required to produce the permanent 
turbidity. The sample therefore contained 54 per cent, of real cyanide. 



59. CYANOGEN. 191 

2. By Standard Mercuric Chloride (Hannay). 

This convenient method is fully described by the author (/. C. S. 
1878, 245), and is well adapted for the technical examination of 
commercial cyanides, etc., giving good results in the presence of 
cyanates, sulphocyanates, alkaline salts, and compounds of ammonia 
and silver. 

The standard solution of mercury is made by dissolving 13 '537 
gm. HgCl 2 in water, and diluting to a liter. Each c.c.=:0'00651 gm. 
of potassic cyanide or 0*0026 gm. Cy. 

Process : The cyanide is dissolved in water, and the beaker placed upon 
black paper or velvet ; ammonia is then added in moderate quantity, and 
the mercuric solution cautiously added with constant stirring until a bluish- 
white opalescence is permanently produced. With pure substances the 
reaction is very delicate, but not so accurate with impure mixtures occurring 
in commerce. 

3. By Iodine (Fordos and Gelis). 

This process, which is principally applicable to alkaline cyanides, 
depends on the fact, that when a solution of iodine is added to one 
of potassic cyanide, the iodine loses its colour so long as any 
imdecomposed cyanide remains. The reaction may be expressed 
by the following formula : - 



Therefore, 2 eq. iodine represent 1 eq. cyanogen in combination; so 
that 1 c.c. of -^ iodine expresses the half of T o-Jo-o eq. cyanogen 
or its compounds. The end of the reaction is known by the 
yellow colour of the iodine solution becoming permanent. 

Commercial cyanides are, however, generally contaminated with 
caustic or monocarbonate alkalies, which would equally destroy 
the colour of the iodine as the cyanide ; consequently these must 
be converted into bicarbonates, best done by adding carbonic acid 
water (ordinary soda water). 

Example : 5 gm. of potassio cyanide were weighed and dissolved in 500 c.c. 
water; then 10 c.c. (=0'1 gni. cyanide) taken with a pipette, diluted with 
about i liter of water, 100 c.c. of soda water added, then T ^ iodine delivered 
from the burette until the solution possessed a slight but permanent yellow 
colour; 25'5 c.c. were required, which multiplied by 0'003255 gave 0'08300 
gm. instead of O'l gm., or 83 per cent, real cyanide. Sulphides must of 
course be absent. 

4. By ]-0 Silver and Chromate Indicator. 

Yielhaber (Arch. PJtarm. [3] xiii. 408) has shown that weak 
solutions of prussic acid, such as bitter-almond water, etc., may be 
readily titrated by adding magnesic hydrate suspended in water 
until alkaline, adding a drop or two of cliromate indicator, and 
delivering in $ silver until the red colour appears, as in 



192 VOLUMETRIC ANALYSIS. 59. 

the case of titrating chlorides. 1 c.c. silver solution=0'0027 
gm. HCy. 

This method may be found serviceable in the examination of 
opaque solutions of hydrocyanic acid, such as solutions of bitter- 
almond oil, etc. ; but of course the absence of chlorine must be 
insured, or, if present, the amount must be allowed for. 

It is preferable to add the HCy solution to a mixture of 
magnesia and chromate, then immediately titrate with silver. 

5. Cyanides used in Gold Extraction. 

All interesting series of papers on this subject have been 
contributed by Glenn ell (G. N. Ixxvii. 227, and Eettel, idem 
286, 298). The experiments carried out by these chemists are 
far too voluminous to be reproduced here, but a short summary 
of the results may be acceptable for the technical examination of 
the original solutions and their nature, after partial decomposition 
and admixture with zinc and other impurities which naturally 
occur in the processes of gold extraction. The results of both 
chemists point to the fact that the estimation of cyanide in the 
weak solutions used in the MacArthur-Forrest process is much 
hampered by zinc double cyanide, by thiocyanates, also by ferro and 
ferricyanides, together with organic matters which occur in the 
liquors after leaching the ores. According to Glenn ell the presence 
of ferrocyanides gives too high a result when the silver process of 
Liebig is used, but is not of much consequence unless the cyanide 
is relatively small as compared with the ferrocyanide ; with the 
iodine process the interference of ferrocyanide is much less, and 
very fair technical results may be obtained in the presence of both 
ferro and ferri salts by this process. The silver process appears to 
be fairly serviceable where the quantity of ferrocyanide is not too 
large ; the reddish precipitate which forms at first from the ferri salt 
is soluble in the presence of excess of cyanide, and a definite end- 
reaction can be obtained. Thiocyanates render the silver process 
useless, but do not interfere with the iodine process. Ammonic 
carbonate interferes with the silver process unless potassic iodide is 
added so as to produce silver iodide, which is insoluble in the 
ammonia salt. Ferrocyanides, in the absence of other reducing 
.agents, may be accurately estimated, as in 60. 1 ; the presence of 
cyanides and ferricyanides does not seriously interfere. Ferri- 
cyanides may be estimated as in 60.2; ferrocyanides do not 
seriously interfere, but cyanides render the results somewhat low. 
These remarks apply to solutions not complicated by admixture of 
zinc or other matters which naturally occur in the cyanide liquors 
after they have been in contact with the ore. For the actual 
methods which have been found useful in examining the usual 
cyanide liquors the following processes, devised by Bettel, are 
given, not as being absolutely correct, but sufficiently so for 
technical purposes, and occupying little time in the working : 



59. CYANOGEN. 193 

It is necessary to state at the outset that the following remarks have 
reference to the MacArthur-Forrest working solutions containing zinc, 
an element which complicates the analysis in a truly surprising manner. 
Before dealing with the analysis proper, attention is drawn to the peculiarities 
of a solution of the double c} r anide of zinc and potassium, usually written 
K 2 ZnCy 4 . As is stated in works on chemistry, this cyanide is alkaline to 
indicators. Now here lies the peculiarity. To phenolphthalein the alkalinity, 
as tested by T ^ acid, is equal to 19' 5 parts of cyanide of potassium out of 
a possible 130'2 parts. "With methyl orange as indicator, the whole of the 
metallic cyanide may be decomposed by T \ acid, as under : 

K 2 ZnCy 4 +4HCl=ZnCl 2 +2KCl+4HCy. 

On titration with silver nitrate solution the end-reaction is painfully indefinite. 
If caustic alkali in excess (a few c.c. normal soda) be added to a known 
quantity of potassic zinc cyanide solution together with a few drops of 
potassic iodide, and standard silver solution added to opalescence, the reaction 
will indicate sharply the total cyanogen present in the double cyanide even 
in presence of ferrocyanides. If to a solution of potassic zinc cyanide 
be added a small quantity of ferrocyanide of potassium, and the silver 
solution added, the flocculent precipitate of what is supposed to be normal 
zinc ferrocyanide (Zn 2 FeCy 6 ) appears, the end-reaction is fairly sharp, and 
indicates 19' 5 parts of potassic cyanide out of the actual molecular contents 
of 130'2 KCy. If, however, an excess of ferrocyanide be present, the 
flocculeut precipitate does not appear, but in its place one gets an opalescence 
which speedily turns to a finely granular (sometimes slimy) precipitate of 
potassic zinc ferrocyanide, K 2 Zn 3 Fe 2 Cy 12 . This introduces a personal equation 
into the analysis of such a solution, for if the silver solution be added 
rapidly the results are higher than if added drop by drop, as this ferro- 
cyanide of zinc and potassium separates out slowly in dilute solutions 
alkaline or neutral to litmus paper. 

For the estimation of free hydrocyanic acid use is made of Sieb old's 
ingenious method for estimating alkalies in carbonates and bicarbonates, by 
reversing the process, adding bicarbonate of soda, free from carbonate, to 
the solution to be titrated for hydrocyanic acid and free cyanide. This is 
the one instance where hydrocyanic acid turns carbonic acid out of its 
combinations, and as such is interesting. 

2KHC0 3 +AgN0 3 +2HCy=KAgCy 2 +KN0 3 +2C0 2 +2H 2 0. 

The methods of analysis are as follows : 

1. Free Cyanide. 50 c.c. of solution are taken and titrated with silver 
nitrate to faint opalescence or first indication of a flocculeut precipitate. 
This will indicate (if sufficient ferrocyanide be present to form a flocculent 
precipitate of zinc ferrocyanide) the free cyanide, and cyanide equal to 7'9 
per cent, of the potassic zinc cyanide present. 

2. Hydrocyanic Acid. To 50 c.c. of the solution add a solution of 
alkaline bicarbonate, free from carbonate or excess of carbonic acid. Titrate 
as for free cyanide. Deduct the first from the second result 



=HCy 1 c.c. AgN0 3 ==:0-00829 / HCy. 

3. Double Cyanides: Add excess of normal caustic soda to 50 c.c. of 
solution and a few drops of a 10 per cent, solution of KI, titrate to opalescence 
with AgNO 3 . This gives 1, 2, and 3. Deduct 1 and 2=K 2 ZnCy 4 as KCy 
less 7'9 per cent. 

A correction is here introduced. The KCy found in 3 is calculated to 
K 2 ZnCy 4 . Factor : KCy (as K 2 ZnCy 4 ) x 0'9493=K ? ZnCy 4 . Add to this 
7'9 per cent, of total, or for every 92'1 parts of K 2 ZnCy 4 add 7'9 parts. 





194 VOLUMETRIC ANALYSIS. 59. 

If this fraction, calculated back to KCj, be deducted from 1, the true free 
cyanide (calculated to KCy) is obtained. 

4. Ferro cyanides and Thiocyanates. In absence of organic matters 
it is found that an acidified solution of a simple cyanide, such as KCy, or 
a double cyanide (as K'-ZnCy 4 ), i.e., solution of HCy, is not affected by 
dilute permanganate. On the other hand, acidified solutions of femxryanides 
and sulphocyanides are rapidly oxidizedthe one to ferrocyanide, the other 
to H 2 SO 4 +ilCy. 

If, now, the ferroc} r anogen be removed as Prussian blue, by ferric chloride 
in an acid solution, the filtrate will contain ferric and hydric thiocyanate, 
both of which are oxidized by permanganate as if iron were not present ; 
by deducting the smaller from the larger result, we get the permanganate 
consumed in oxidizing ferrocyanide, the remainder equals the permanganate 
consumed in oxidizing thiocyanate. 

The method of titratiou is as follows (in presence of zinc) : A burette 
is filled with the c} r anide solution for analysis, and run into 10 or 20 c.c. 
rjfo K 2 Mn 2 O 8 strongly acidified with H 2 SO 4 until colour is just discharged. 
Result noted (a). 

A solution of ferric sulphate or chloride is acidified with H 2 SO 4 and 
50 c.c. of the cyanide solution poured in. After shaking for about half 
a minute, the Prussian blue is separated from the liquid by filtration, and 
the precipitate and filter paper washed. The filtrate is next titrated Avith 
T^T K 2 Mn 2 8 (b). 

Let c c.c. permanganate required to oxidize ferrocyanide. 

Then ab = c. 

(c) 1 c.c. ^ K 2 Mn 2 8 = 0-003684 gm. K 4 FeCy 6 . 

(*) 1 c.c. y^- K 2 Mn 2 O 8 =0'0001618 gm. KCNS. 

5. Oxidizable Org-anic Matter in Solution. In treating spruit tail- 
ings, or material containing decaying vegetable matter., the following method 
is used for testing coloured solutions : 

Prepare a solution of a thiocyanate, so that 1 C.C.T^ K 2 Mn 2 O 8 . 

To 50 c.c. solution add sulphuric acid in excess, and then a large 
excess of permanganate, y^. Keep at 6070 C. for an hour. Then cool 
and titrate back with the KCNS solution. 

Result O consumed in oxidizing organic matter. 
O K'FeCy 6 . 

O KCNS. 

After estimating KCNS and K 4 FeCy 6 , a simple calculation gives the 
oxygen to oxidize organic matter. This result multiplied by 9 will give 
approximately the amount of organic matter present. 

In order to clarify such organically charged solutions, they are shaken 
up with powdered quicklime and filtered ; the solution is then of a faint 
straw colour, and is in a proper condition for analysis. In such clarified 
solution the oxidizable organic matter is no longer present, and the 
estimations are readily performed. 

6. Alkalinity. Potassic cyanide acts as caustic alkali, when neutralized 
by an acid; the end-reaction, however, is influenced to some extent by 
the hydrocyanic acid present, and is therefore not sharp. It is possible, 
however, to estimate 



With phenolphtMeinas indicator. 

}^^ 1 -'*"- '" 
Ey T ^- acid the K 2 O in ZnK 2 O 2 ... With phenolphthalein as indicator. 



59. FERRO- AND FERRI-CYANIDES. 195 

It will be necessary to point out the decompositions which result from 
adding alkali, or a carbonate of an alkali, to a working solution containing 
;zinc. 

K 2 ZnCy 4 + 4KHO=ZnK 2 O 2 + 4KCy . 
K 2 ZnCy 4 + 4Na 2 CO 3 + 2H 2 O=2KCy + 2NaCy + ZnNa 2 O 2 + 4NaHCO 3 . 

Bicarbonates have no action upon potassic or sodic zinc cyanide. 
Potassic or sodic zinc oxide (in solution as hydrate) acts as an alkali 
towards phenolphthalein and methyl orange. 

ZnK 2 O 2 + 4HC1 = 2KC1 + ZnCl 2 + 2H 2 O . 

Calcic and magnesic hydrates decompose the double salt of K 2 ZuCy 4 to 
some extent, but not completely, so that it is possible to find in one and the 
same solution a considerable proportion of alkalinity towards phenolphthalein, 
due to calcic hydrate in presence of K 2 ZnCy 4 . 

The total alkalinity as determined by T ^ acid with methyl orange as 
indicator gives, in addition to those before mentioned, the bicarbonates. 
If to a solution containing sodic bicarbonate and potassic zinc cyanide be 
added lime or lime and magnesia, the percentage of C3 r anide will increase, 
the zinc remaining in solution as zinc sodic oxide. 

Clennell (C. N. Ixxi. 93) gives a method for the approximate estimation 
of alkaline hydrates and carbonates in the presence of alkaline cyanides, 
as follows : 

(1) Estimation of the cyanide by direct titration with silver. 

(2) Estimation of the hydrate and half the carbonate of alkali on adding 
phenolphthalein to the previous solution (after titration with silver) by 
^ hydrochloric acid. 

(3) Estimation of the total alkali by direct titration, in another portion 
of the solution, with T ^ hydrochloric acid and methyl orange. 

7. Ferricyanide Estimation. This is effected by allowing sodium 
amalgam to act for fifteen minutes on the solution in a narrow cylinder, 
then estimating the ferrocyanide formed by permanganate in an acid 
solution. Deduct from the results obtained the ferrocyauide and thiocyanate 
previously found, 1 c.c. ^ permanganate^O'003293 gm. K 6 Fe 2 Cy 12 . 

8. Sulphides. It rarely happens that sulphides are present in a cyanide 
solution ; if present, however, shake up with precipitated carbonate of lead, 
filter, and titrate with T - - permanganate. The loss over the previous 
estimation (of 'K 4 FeCy 6 KCNS, &c.) is due to elimination of sulphides. 

1 c.c.' T K 2 Mn 2 O 8 =0'OOOl7 gm. H 2 S, or 0'00055 gm. K 2 S. 

The hydrogen alone being oxidized by dilute permanganate in acid solution 
where the permanganate is not first of all in excess. 

9. Ammonia. If sufficient silver nitrate be added to a solution (say 
10 c.c.) to wholly precipitate the cyanogen compounds and a drop or two of 
f HC1 be added, the Avhole made up to 100 c.c., and filtered ; then 10 c.c. 
distilled with about 150 c.c. of ammonia free water and Nesslerized in the 
usual way, the amount of ammonia may be ascertained. 

FERRO- AND FERRI-CYANIDES. 

Potassic Ferrocyanide. 



Metallic iron 7 -541= Crystallized Potassic ferrocyanide. 

Double iron salt x 1*077= 

o 2 



196 VOLUMETRIC ANALYSIS. 60. 

1. Oxidation to Ferricyanide by Permanganate (De Ha en). 

60. THIS substance may be estimated by potassic permanga- 
nate, which acts by converting it into red prussiate. The process 
is easy of application, and the results accurate. A standard 
solution of pure ferrocyanide should be used as the basis upon 
which to work, but may, however, be dispensed with, if the operator 
chooses to calculate the strength of his permanganate upon iron or its 
compounds. If the permanganate is decinormal, there is of course 
very little need for calculation (1 eq.=422 must be used as the 
systematic number, and therefore 1 c.c. of ^5- permanganate is 
equal to OO422 gm. of yellow prussiate). The standard solution 
of pure ferrocyanide contains 20 gm. in the liter : each c.c. will 
therefore contain 0'02 gm. 

Process : 10 c.c. of the standard prussiate solution are put into a white 
porcelain dish or beaker standing on white paper, and 250 c.c. or so of water 
added; it is then acidified pretty strongly with sulphuric acid, and the 
permanganate delivered from the burette until a pure uranium yellow colour 
appears ; it is then cautiously added until the faintest pink tinge occurs. 

Ferrocyanides in Alkali waste. Acidulate the solution with 
HC1, and add strong bleaching powder solution with agitation until 
a drop of the liquid gives no blue colour with ferric indicator. The 
liquid is then titrated with a solution of cupric sulphate, standardized 
on pure potassic ferrocyanide, using dilute ferrous sulphate as 
indicator; as soon as no more blue or grey colour occurs, but 
a faint reddening, the process is ended. 

Ferrocyanides in G-as Liquor. 250 c.c. are evaporated to dryness^. 
dissolved in water, the solution filtered, and the ferrocyanides 
precipitated as Prussian blue by ferric chloride. The blue is 
filtered off, w T ashed, and decomposed with caustic soda. The ferric 
hydroxide so obtained is, after filtering, washing, and dissolving in 
dilute H 2 S0 4 reduced with zinc, and titrated with permanganate.. 
Fe x 5-07-=(NH 4 ) 4 FeCy 6 . 

POTASSIC FERRICYANIDE. 

K 6 Cy 12 Fe 2 =658. 

Metallic iron x 5 '88 = Potassic ferricyanide. 

Double iron salt x 1*68 = ,, 

T N o- Thiosulphate x 0-0329 

2. By Iodine and Thiosulphate. 

This salt can be estimated either by reduction to ferrocyanide 
and titratioii with permanganate or bichromate as above, or by 
Lenssen's method, which is based upon the fact, that when 
potassic iodide and ferricyanide are mixed with tolerably concen- 
trated hydrochloric acid, iodine is set free. 



60. THIOCYANATES. 197 

K 6jVCy 12 + 2KI=2K*Cy 6 Fe + 1 2 

the quantity of which can be estimated by -^ thiosulphate and 
starch. This method does not, however, give the most satis- 
factory results, owing to the variation produced by working 
with dilute or concentrated solutions. C. Mohr's modification 
(see Zinc, 81) is, however, more accurate, and is as follows : 
The ferricyanide is dissolved in a convenient quantity of water, 
potassic iodide in crystals added, together with hydrochloric acid 
in tolerable quantity, then a solution of pure zinc sulphate in 
excess ; after standing a few minutes to allow the decomposition 
to perfect itself, the excess of acid is neutralized by sodic carbonate, 
.so that the latter slightly predominates. 

At this stage all the zinc ferricyanide first formed is converted 
into the ferrocyanide of that metal, and an equivalent quantity of 
iodine set free, which can at once be titrated with T ^ thiosulphate 
and starch, and with very great exactness. 1 c.c. ~ thiosalphate 
= 0'0329 gm. potassic ferricyanide. 

The mean of five determinations made by Mohr gave 10O21 
instead of 100. 

Another method consists in boiling with excess of potash, then 
cooling, and adding H 2 2 till the colour is yellow. The excess of 
the peroxide is then boiled off, H 2 S0 4 added, and titrated with 
permanganate. 

3. Reduction of Ferri- to Ferro-cyanide. 

This process is, of course, necessary when the determination by 
permanganate has to be made, and is best .effected by boiling the 
weighed ferricyanide with an excess of potash or soda, and adding 
small quantities of concentrated solution of ferrous sulphate until 
the precipitate which occurs possesses a blackish colour (signifying 
that the magnetic oxide is formed). The solution is then diluted 
to a convenient quantity, say 300 c.c., well mixed and filtered 
through a dry filter; 50 or 100 c.c. may then be taken, sulphuric 
acid added, and titrated with permanganate as before described. 

Kassner suggests the use of sodic peroxide for the reduction 
of ferri- to ferrocyanide (Arch. Pharm. ccxxxii. 226) as being 
rapid and complete. About 0'5 gm. in 100 c.c. water requires 
about. 0*06 gm. -of the peroxide ; the mixture is heated till all 
effervescence is over, acidified with sulphuric acid, cooled, and 
titrated with permanganate in the usual way. 

THIOCYANATES. 

For the estimation of thiocyanic acid in combination with the 
alkaline or earthy bases, Barnes and Liddle (/. S. C. I. ii. 122) 
have devised a method which is easy of application, and gives good 
technical results. It is not, however, available for gas liquors. 



198 VOLUMETRIC ANALYSIS. 61. 

The method depends upon the fact that when a solution of 
a cupric salt is added to a solution of a thiocyanate in presence 
of a reducing agent, as sodic bisulphite, the insoluble cuprous salt 
of thiocyanic acid is precipitated, the end of the reaction being 
ascertained by a drop of the solution in the flask giving a brown 
colouration when brought in contact with a drop of ferrocyanide,. 
The following reactions take place : 

2CuS0 4 + 2KSCX + Xa 2 S0 3 + H 2 = 



and 

2CuS0 4 + Ba(SC^s T ) 2 + !S T a 2 S0 3 + H 2 = 
Cu 2 S 2 C 2 JS T2 + BaSO 4 + 2 



The following solutions are required : 

1. A standard solution of Cupric sulphate containing 6 '2375 
gm. per liter, 1 c.c. of which is equivalent to '00 145 gm. SC^N". 

2. A solution of Sodic bisulphite of specific gravity 1*3. 

3. A solution of Potassic ferrocyanide (1 : 20). 

Process: About 3 gm. of the sample are weighed from a stoppered 
tube into a liter flask, dissolved in water, and made up to the mark. After 
well mixing, 25 c.c. are measured into a flask, about 3 c.c. of the bisulphite 
added, and the whole boiled. Whilst this is heating a burette is filled with 
the copper solution, and a white porcelain slab is dotted over with the 
ferrocyanide. When the liquid in the flask has reached the boiling point, 
20 c.c. of the copper solution are run in, well shaken, the precipitate allowed 
to settle for about a minute, a drop is taken out by means of a glass rod, and 
brought in contact with a drop of ferrocyanide, and should no brown 
colouration appear, more of the copper solution is run in, say 1 c.c. at 
a time, and again tested. This is continued until a drop gives an immediate 
colour. By this means an approximation to the truth is obtained. It will 
be observed, during a titration, that the mixed drops, after standing for 
a minute, or even less, produce a brown tint. It is of the utmost importance 
that the colouration be immediate. 

A second 25 c.c. of the thiocyanate solution are run into a clean flask, 
the bisulphite added, and boiled as before. 

Suppose that in the first experiment, after an addition of 27 c.c. of copper 
solution, no colour was formed with ferrocyanide, but that 28 c.c. gave an 
immediate colour ; then in the second experiment 27 c.c. are run in at once, 
and the liquid is again tested, when no colour should appear. The copper 
solution is then run in drop by drop until there is a slight excess of copper, 
as proved by the delicate reaction with the ferrocyanide. The second 
experiment is thus rendered more exact by the experience gained in the first. 



GOLD. 

Au- 196-5. 
1 c.c. or 1 dm. normal oxalic acid=0'0655 gm. or 0'655 grn. Gold. 

61. THE technical assay of gold for coining purposes is 
invariably performed by cupellation. Terchloride of gold is, 
however, largely used in photography and electro-gilding, and 



62. IODINE. 199 

therefore it may be necessary sometimes to ascertain the strength 
of a solution of the chloride, or its value as it occurs in commerce. 

If to a solution of gold in the form of chloride (free from nitric 
acid) an excess of oxalic acid be added, in the course of from 
eighteen to twenty-four hours all the gold will be precipitated in 
the metallic form, while the corresponding quantity of oxalic acid 
has been dissipated in the form of carbonic acid ; if, therefore, the 
quantity of oxalic acid originally added be known, and the excess, 
after complete precipitation of the gold, be found by permanganate, 
the amount of gold will be obtained. 

Example : A 15-grain tube of the chloride of gold of commerce was 
dissolved in water, and the solution made up to 300 decems. 20 dm. of 
normal oxalic acid were then added, and the flask set aside for twenty-four 
hours in a warm, dark place ; at the end of that time the gold had settled, 
and the supernatant liquid was clear and colourless. 100 dm. were taken 
out with a pipette, and titrated with T ^ permanganate, of which 25 dm. were 
required ; this multiplied by 3 gives 75 dm.=7'5 dm. normal oxalic acid, 
which deducted from the 20 dm. originally added, left 12'5 dm. ; this 
multiplied by ^ the equivalent of gold (1 eq. of gold chloride decomposing 
3 eq. oxalic acid)=0'655 gave 8'195 grn. metallic gold, or multiplied by 101 
(=1 eq. AuCl 3 ) gave 12'625 grn. ; the result was 84 per cent, of chloride of 
gold instead of 100. A more rapid method consists in boiling the gold 
solution with an excess of standard solution of potassic oxalate containing 
8'3 gm. of the pure salt per liter, and titrating back with a permanganate 
solution which has the same working strength as the oxalate. Each c.c. 
of oxalate solution decomposed represents 0'00855 gm. Au. 



IODINE. 

1=127-0. 
1. By Distillation. 



in potassic iodide, and titration with starch and -- thiosulphate, 
as described in 38.* 

Combined iodine in haloid salts, such as the alkaline iodides, 
must be subjected to distillation with hydrochloric acid, and some 
other substance capable of assisting in the liberation of free iodine, 
which is received into a solution of potassic iodide, and then 
titrated with ~ thiosulphate in the ordinary way. Such a 
substance presents itself best in the form of ferric oxide, or some 
of its combinations ; if, therefore, hydriodic acid, or what amounts 
to the same thing, an alkaline iodide, be mixed with an excess of 



62. FREE iodine is of course very readily estimated by solution 



* I would here again impress upon the operator's notice that it is of great importance 
to ascertain the exact strength of the standard solutions of iodine and thiosulphate as 
compared with each other. Both solutions constantly undergo an amount of change 
depending upon the temperature at which they are kept, their exposure to light, etc., 
and therefore it is absolutely necessary, to ensure exactness in the multifarious analyses 
which can be made by the aid of these two reagents, to verify their agreement by 
weighing a small portion of pure dry iodine at intervals, and titrating it with the 
standard thiosulphate, or checking the iodine with baric or sodic thiosulphate of 
known purity. 



OF THE 

UNIVERSITY 

CAI icrikBNliA* 



200 VOLUMETRIC ANALYSIS. 62. 

ferric oxide or chloride, and distilled in the apparatus shown in 
fig. 37 or 38, the following reaction occurs : 

Fe 2 3 + 2IH=2FeO + H 2 + 1 2 . 

The best form in which to use the ferric oxide is iron alum. 

The iodide and iron alum being brought into the little flask (fig. 38), 
sulphuric acid of about 1 '3 sp. gr. is added, and the cork carrying 
the still tube inserted. This tube is not carried into the solution 
of potassic iodide in this special case, but within a short distance 
of it ; and the end must not be drawn out to a fine point, as there 
represented, but cut off straight. The reason for this arrangement 
is, that it is not a chlorine distillation for the purpose of setting 
iodine free from the iodide solution, as is usually the case, but an 
actual distillation of iodine, which would speedily choke up the 
narrow point of the tube, and so prevent the further progress 
of the operation, 

As the distillation goes on, the steam washes the condensed 
iodine out of the tube into the solution of iodide, which must be 
present in sufficient quantity to absorb it all. When no more 
violet vapours are to be seen in the flask, the operation is ended ; 
but to make sure, it is well to empty the solution of iodine out 
of the condensing tube into a beaker, and put a little fresh iodide 
solution with starch in, then heat the flask again ; the slightest 
traces of iodine may then be discovered by the occurrence of the 
blue colour when cooled. In case this occurs the distillation is 
continued a little while, then both liquids mixed, and titrated 
with j~ thiosulphate as usual. 

It has been previously stated that the rubber joints to the 
special apparatus of Fresenius, Bunsen, or Mohr for iodine 
distillations are objectionable. Topf avoids this by fitting his 
apparatus together, so that although rubber is used, the reagents 
do not come in contact with it (Z. a. C. xxvi. 293). 

Another form of apparatus designed by Stortenbeker (Z. a. C. 
xxix. 273) is shown in fig. 41, in which rubber joints are entirely 
dispensed with, and glass connections used. The connection 
between the distilling tube and the absorbing apparatus is a water 
joint, the tube resting in a socket kept wet with water, the 
chloride of calcium tube is filled with glass pearls, moistened 
with concentrated solution of potassic iodide, and the connection 
with the absorbing apparatus is ground in like an ordinary stopper. 
The absorbing bulbs are immersed in water to the middle of the 
bulbs, and the iodide solution filled to the lower end of them. 

Ferric chloride may be used instead of the iron alum, but it 
must be free from nitric acid or active chlorine (best prepared 
from dry Fe 2 3 and HC1). 

The iodides of silver, mercury, and copper cannot be accurately 
analyzed in this way, but must be specially treated. They should 
be dissolved in the least possible quantity of sodic thiosulphate 



62. IODINE. 201 

solution, and precipitated boiling with sodic sulphide, then filtered ; 
the nitrate contains the whole of the iodine free from metal. The 
nitrate is evaporated to dryness and ignited, then dissolved in 
water, and distilled with a good excess of ferric salt (Mensel 
Z. a. C. xii. 137). 

2. Mixtures of Iodides, Bromides, and Chlorides. 

Don a th (Z. a. C. xix. 19) has shown that iodine may be 
accurately estimated by distillation in the presence of other halogen 
salts, by means of a solution containing about 2 to 3 per cent, of 
chromic acid, free from sulphuric acid. 

In the case of iodides and chlorides together the action is 
perfectly regular, and the whole of the iodine may be received into 
potassic iodide without any interference from the chlorine. 




Fig. 41. 

In the case of bromides being present, the chromic solution must 
be rather more dilute, and the distillation must not be continued 
more than two or three minutes after ebullition has commenced, 
otherwise a small amount of bromide is decomposed. 

The reaction in the case of potassic iodide may be expressed 
thus : 

6KI + 8Cr0 3 = I 6 + Cr 2 3 + 3K 2 OW. 

The distillation may be made in Mohr's appnratus (fig. 38), 
using about 50 c.c. of chromic solution for about 0'3 gm. I. 

The titration is made with thiosulphate in the usual way. 

A much less troublesome method of estimating iodine in the 
presence of bromides or chlorides has been worked out by Cook 
(/. C. S. 1885, 471), and depends on the fact that hydrogen 
peroxide liberates iodine completely from an alkaline base in the 



202 VOLUMETRIC ANALYSIS. 62. 

presence of excess of acetic acid, while neither bromine nor 
chlorine are affected. 

Hydrogen peroxide alone will only partially liberate iodine from 
potassic iodide, but with excess of a weak organic acid to combine 
with the alkaline hydroxide, the liberation is complete. Strong 
mineral acids must not be used, or bromine and chlorine, if present, 
would also be set free. 

Process : The solution is strongly acidified with acetic acid, and sufficient 
hydrogen peroxide added to liberate the .iodine (5 c.c. will suffice for 1 gm. 
KI). The mixture is allowed to stand from half an hour to an hour; the 
whole of the iodine separates, some being in the solid state if the quantity 
is considerable. Chloroform is now added in sufficient volume to dissolve 
the iodine, the solution syphoned off, and the globule repeatedly washed 
with small quantities of water to remove excess of peroxide, then titrated 
with thiosulphate, with or without starch, in the usual way. If the 
peroxide is not completely removed by washing, it will decompose the sodic 
iodide produced in the titration, and so liberate traces of iodine. 

The results obtained by Cook in mixtures of bromides, iodides, 
and chlorides, were about 99 per cent, of the iodine present. 

Gooch and Browning (Ainer. Jour. Science xxxix. March, 
1890, also C. N. Ixi. 279) publish a method of estimating iodine 
in halogen salts of the alkalies which gives excellent results, and 
which is based on the fact that arsenic acid in strongly acid solution 
liberates iodine, becoming itself reduced to arsenious acid, according 
to the equation 

IPAsO 4 + 2HI = HMsO 3 + H 2 + 21. 

A very careful series of experiments are detailed in the original 
paper, the outcome of the whole- being summarized in the following 
process : 

Process : The substance (which should not contain of chloride more 
than an amount corresponding to 0'5 gm. of sodic chloride, nor of bromide 
more than corresponds to 0'5 gm. of potassic bromide, nor of iodide much 
more than the equivalent of 0'5 gm. of potassic iodide) is dissolved in water 
in an Erlenmeyer beaker of 300 c.c. capacity, and to the solution are 
added 2 gm. of potassic binarseniate dissolved in water, and 20 c.c. of a 
mixture of sulphuric acid and water in equal volumes, and enough water to 
increase the total volume to 100 c.c. or a little more. A platinum spiral is 
introduced, a trap made of a straight two-bulb drying tube, cut off short, is 
hung with the larger end downward in the neck of the flask, and the liquid 
is boiled until the level reaches a mark put upon the flask to indicate a 
volume of 35 c.c. Great care should be taken not to press the concentration 
be}*ond this point on account of the double danger of losing arsenious 
chloride and setting up reduction of the arseniate by the bromide. On the 
other hand, though 35 c.c. is the ideal volume to be attained, failure to- 
concentrate below 40 c.c. introduces no appreciable error. The liquid 
remaining is cooled and nearly neutralized by sodic hydrate (ammonia is not 
equally good), neutralization is completed by potassic bicarbonate, an excess 
of 20 c.c. of the saturated solution of the latter is added, and the arsenious 
oxide in solution is titrated by standard iodine in the presence of starch. 

With ordinary care the method is rapid, reliable, and easily 



62. IODINE. 203 

executed, and the error is small. In analyses requiring extreme 
accuracy, all but accidental errors may be eliminated from the 
results by applying the corrections indicated. 

The indicated corrections are based on a long series of ex- 
periments, which cannot well be given here, but the results may 
be stated shortly as follows : 

When no chloride or bromide is present the iodine may be 
estimated with a mean error of 0*2 m.gm. in 0*5 gm. or so of the 
alkaline iodide. When sodic chloride is present there is a slight 
deficiency in iodine, which is proportional to the amount of iodide 
decomposed. For about 0*56 gm. of potassic iodide and 0*5 gm. 
of sodic chloride the deficiency in iodine amounted to 0*0011 gm. 
When the iodide is decreased, say to one-tenth or less, the deficiency 
falls to 0*0002 gm. The presence of potassic bromide liberates 
traces of bromine, and consequently increases the AsO 3 , and gives 
apparent excess of iodine, the mean error being 0*0008 gm. for 
0*5 gm. of bromide. 

The simultaneous action of the chloride and bromide tends of 
course to neutralize the error due to each. Thus, in a mixture 
weighing about 1*5 gm. and consisting of sodic chloride, potassic 
bromide, and potassic iodide in equal parts, the mean error amounts 
to -0*0003 gm. The largest error in the series is +0*0016 gm., 
when the bromide was at its maximum, and no chloride was 
present; and the next largest was - 0*0013 gm., when the chloride 
was at its maximum and no bromide was present. 

From a series of experiments detailed in the original paper, it 
was deduced that the amount of iodine to be added, in each case, 
may be obtained by multiplying the product of the weights in 
grams of sodic chloride and potassic iodide by the constant 0*004 ; 
and the amount to be subtracted, by multiplying the weight in 
grams of potassic bromide by 0*0016; but in order to make use 
of these corrections, the approximate amounts of these salts must 
be known. 

3. Titration with -j^ Silver and Thiocyanate. 

The thiocyanate and silver solutions are described in 43. 

The iodide is dissolved in 300 or 400 times its weight of water 
in a well-stoppered flask, and y^r silver delivered in from the burette 
with constant shaking until the precipitate coagulates, showing 
that silver is in excess. Ferric indicator and nitric acid are then 
added in proper proportion, and the excess of silver estimated 
by thiocyanate as described in 43. 

4. Oxidation of combined Iodine toy Chlorine (Golfier Besseyre 
and D u p r e) . 

This wonderfully sharp method of estimating iodine depend3 
upon its conversion into iodic acid by free chlorine. When a 



204 VOLUMETRIC ANALYSIS. 62. 

solution of potassic iodide is treated with successive quantities of 
chlorine water, first iodine is liberated, then chloride of iodine 
(IC1) formed. If starch, chloroform, benzole, or -bisulphide of 
carbon be added, the first will be turned blue, while any of the 
others will be coloured intense violet. A further addition of chlorine, 
in sufficient quantity, produces pentachloride of iodine (IC1 5 ), or 
rather, as water is present, iodic acid (I0 3 H). No colouration of 
the above substances is produced by these compounds, and the 
accuracy with which the reaction takes place has been made use of 
byGolfier Besseyre and Dupre, independently of each other, 
for the purpose of estimating iodine. The former suggested the use 
of starch, the latter chloroform or benzole, with very dilute chlorine 
water. Dupre 's method is preferable on many accounts. 

Example : 30 c.c. of weak chlorine water were put into a beaker with 
potassic iodide and starch, and then titrated with ^ thiosulphate, of which 
17 c.c. were required. 

10 c.c. of solution of potassic iodide containing O'OIO gm. of iodine were 
put into a stoppered bottle, chloroform added, and the same chlorine water as 
above delivered in from the burette, with constant shaking, until the red 
colour of the chloroform had disappeared ; the quantity used was 85'8 c.c. 
The excess of chlorine was then ascertained by adding sodic bicarbonate, 
potassic iodide, and starch. A slight blue colour occurred ; this was removed 
by T g- thiosulphate, of which 1*2 c.c. was used. Now, as 30 c.c. of the 
chlorine solution required 17 c.c., the 85'8 c.c. required 48'62 c.c. of thio- 
sulphate. From this, however, must be deducted the 1*2 c.c. in excess, 
leaving 47'42 c.c. T 77 =4'742 c.c. of r ^ solution, which multiplied by 0'00211, 
the one-sixth of Twinr e Q- (\ e( l- f iodic acid liberating 6 eq. iodine), gave 
0'010056 gin. iodine instead of O'Ol gm. 

Mohr suggests a modification of this method, which dispenses 
with the use of chloroform, or other similar agent. 

The weighed iodine compound is brought into a stoppered flask, and 
chlorine water delivered from a large burette until all yellow colour has 
disappeared. A drop of the mixture brought in contact with a drop of 
starch must produce no blue colour ; sodic bicarbonate is then added till 
the mixture is neutral or slightly alkaline, together with potassic iodide 
and starch ; the blue colour is then removed by f^ thiosulphate. The 
strength of the chlorine water being known, the calculation presents no 
difficulty. 

Mohr obtained by this means 0*010108 gm. iodine, instead of 
1-01 gm. 

5. Oxidation by Permanganate (Reinig-e). 

This process for estimating iodine in presence of bromides and 
chlorides gives satisfactory results. 

When potassic iodide and permanganate are mixed, the rose 
colour of the latter disappears, a brown precipitate of manganic 
peroxide results, and free potash with potassic iodide remains in. 
solution. 1 eq. I=l27 reacts on 1 eq. K 2 Mn 2 O s ==316, thus 

KI + K 2 Mn 2 8 ==KIO :5 + K 2 + 2Mn0 2 . 



62. IODINE. 205 

Heat accelerates the reaction, and it is advisable, especially with 
weak solutions, to add a small quantity of potassic carbonate to 
increase the alkalinity. Xo organic matter must be present. 

The permanganate and thiosulphate solutions required in the 
process may conveniently be of T ^- strength, but their reaction upon 
each other must be definitely fixed by experiment as follows : 
2 c.c. of permanganate solution are freely diluted with water, a few 
drops of sodic carbonate added, and the thiosulphate added in very 
small portions until the rose colour is just discharged. The slight 
turbidity produced by .the precipitation of hydrated manganic 
oxide need not interfere with the observation of the exact point. 

Process : The iodine compound being dissolved in water, and always 
existing only in combination with alkaline or earthy bases, is heated to 
gentle boiling, rendered alkaline with sodic or potassic carbonate, and 
permanganate added till in distinct excess, best known by removing the 
liquid from the fire for a minute, when the precipitate will subside, leaving- 
the upper liquid rose-coloured; the whole may then be poured into a 500-c.c. 
flask, cooled, diluted to the mark, and 100 c.c. taken out for titration with 
thiosulphate. The amount so used, being multiplied by 5, will give the 
proportion required for the whole liquid, whence can be calculated the 
amount of iodine. To prove the accuracy of the process in a mixture of 
iodides, bromides, and chlorides, with excess of alkali, the following experi- 
ment was made. 7 gm. commercial potassic bromide, the same of sodic 
chloride, with 1 gm. each of potassic hydrate and carbonate, were dissolved 
in a convenient quantity of water, and heated to boiling ; permanganate was 
then added cautiously to destroy the traces of iodine and other impurities 
affecting the permanganate so long as decolouration took place; the slightest 
excess showed a green colour (manganate). To the mixture was then added 
0'1246 gm. pure iodine, and the titration continued as described : the result 
was 0'125 gm. I. 

With systematic solutions of permanganate and thiosulphate- 

the calculation is as follows : 



1 c.c. solution=0-0127 m. I. 



6. By Nitrous Acid and Carbon Bisulphide (Fresenius). 
This process requires the following standard solutions : 

(a) Potassic iodide, about 5 gm. per liter. 

(b) Sodic thiosulphate, ^V normal, 12;4 gm. per liter, or there- 
about. 

(c) Nitrous acid, prepared by passing the gas into tolerably 
strong sulphuric acid until saturated. 

(d) Pure Carbon bisulphide. 

(e) Solution of Sodic bicarbonate, made by dissolving 5 gm. of 
the salt in 1 liter of water, and adding 1 c.c. of hydrochloric acid. 

The strength of the sodic thiosulphate in relation to iodine is 
first ascertained by placing 50 c.c. of the iodide solution into 
a 500 c.c. stoppered flask, then about 150 c.c. water, 20 c.c. 



206 VOLUMETRIC ANALYSIS. 63. 

carbon bisulphide, then dilute sulphuric acid, and lastly, 10 drops 
of the nitrous solution. The stopper is then replaced, and the 
whole well shaken, set aside to allow the carbon liquid to settle, 
and the supernatant liquid poured into another clean flask. The 
carbon bisulphide is then treated three or four times successively 
with water in the same way till the free acid is mostly removed, 
the washings being all mixed in one flask ; 10 c.c. of bisulphide 
are then added to the washings, well shaken, and if at all coloured, 
the same process of washing is carried on. Finally, the two 
quantities of bisulphide are brought upon a moistened filter, 
washed till free from acid, a hole made in the filter, and the 
bisulphide which now contains all the iodine in solution allowed 
to run into a clean small flask, 30 c.c, of the sodic bicarbonate 
solution added, then brought under the thiosulphate burette, and 
the solution allowed to flow into the mixture while shaking until 
the violet colour is entirely discharged. The quantity so used 
represents the weight of iodine contained in 50 c.c. of the standard 
potassic iodide, and may be used on that basis to ascertain any 
unknown weight contained in a similar solution. 

When very small quantities of iodine are to be titrated, weaker 
solutions and smaller vessels may be used. 

7. By ^5- Silver Solution and Starch Iodide (Pisani). 

The details of this process are given under the head of silver 
assay ( 73.2), and are of course simply a reversal of the method 
there given. This method is exceedingly serviceable for estimating 
small quantities of combined .iodine in the presence of chlorides 
and bromides, inasmuch as the silver solution does not react upon 
these bodies until the blue colour is destroyed. 

IRON. 

Fe = 56. 
Factors. 

1 c.c. ~ permanganate, bichromate, 

or thiosulphate = 0-0056 Fe 

= 0-0072 FeO 
= 0-0080 FeW 

ESTIMATION IN THE FERROUS STATE. 

1. Verification of the standard solutions of Permanganate or 

Bichromate. 

63. THE estimation of iron in the ferrous state has already 
been incidentally described in 34, 35, and 37. The present 
and following sections are an amplification of the methods there 
given, as applied more distinctly to ores and products of iron 
manufacture ; but before applying the permanganate or bichromate 



63. IRON. 207 

process to these substances, and since many operators prefer, with 
reason, to standardize such solutions upon metallic iron, especially 
for use in iron analysis, the following method is given as the 
Lest : 

A piece of soft iron wire, known as flower wire, is well cleaned with 
scouring paper, and about 1 gram accurately weighed ; this is placed into 
a, 250 c.c. boiling flask a, and 100 c.c. of dilute pure sulphuric acid (1 part 
concentrated acid to 5 of water) poured over it; about a gram of sodic 
carbonate in crystals is then added, and the apparatus fixed together as 
in fig. 42, the pinch-cock remaining open. The flask a is closed by a tight- 
fitting india-rubber stopper, through which is passed the bent tube. The 
flask c contains 20 or 30 c.c. of pure distilled water; the flask a being 
supported over a lamp is gently heated to boiling, and kept at this 
temperature until all the iron is dissolved; meanwhile about 300 c.c. of 
distilled water are boiled in a separate vessel to remove all air, and allowed to 
cool. As soon as the iron is dissolved, the lamp is removed, and the pinch- 
cock closed ; when cooled somewhat, the pinch-cock is opened, and the wash 
water suffered to flow back together with the boiled water, which is added 
to it until the flask is filled nearly to the mark. The apparatus is then 
disconnected, and the flask a securely corked with a solid rubber cork, and 
suffered to cool to the temperature of the room. Finally, the flask is filled 
exactly to the mark with the boiled water, and the whole well shaken and 
mixed. When the small portion of uudissolved carbon has subsided, 
SO c.c., equal to i the weight of iron taken, may be removed with the pipette 
for titration with the permanganate or bichromate. 

In the case of permanganate the 50 c.c. are freely diluted with freshly 
boiled and cooled distilled water, and the standard solution cautiously added 
from a tap burette, divided into T V c.c., until the rose colour is faintly 
perceived. 

In the case of bichromate the solution should be less diluted, and the 
titration conducted precisely as in 37. 

The amount of pure iron contained in the portion weighed for titration is 
found by the co-efficient 0'996, and from this is calculated the w r orking 
.strength of the oxidizing solution (see p. 122.) 




Pig. 42. 

Instead of the two flasks, many operators use a single flask, fitted 
^vith caoutchouc stopper, through which a straight glass tube is 
passed, fitted with an india-rubber slit valve (known as Bunsen's 
valve), which allows gas or vapour to pass out, but closes by 
atmospheric pressure when the evolution ceases. Another 
arrangement is described on p. 122. 



208 VOLUMETRIC ANALYSIS. 6 

A large number of technical operators do not trouble themselves 
to arrange any apparatus of the kind described, but simply dissolve 
a weighed quantity of wire of known ferrous contents in a conical 
beaker covered with a clock glass. If kept from draughts of cold 
air while dissolving so as to avoid convection, it is said that 
practically no oxidation takes place. 

The double iron salt (p. 122) is a most convenient material for 
adjusting standard solutions, but it must be most carefully made 
from pure materials, dried perfectly in the granular form, and kept 
from the light in small dry bottles, well closed. In this state it 
will keep for years unchanged, and only needs immediate solution 
in dilute H 2 S0 4 for use. Even in the case of the salt not being 
strictly free from ferric oxide, due to faulty preparation, if it be 
once thoroughly dried, and kept as above described, its actual 
ferrous strength may be found by comparison with metallic iron, 
and a factor found for weighing it in system. 

It should be borne in mind that ferrous compounds are much 
more stable in sulphuric than in hydrochloric acid solution, and 
whenever possible, sulphuric acid should be used as the solvent. 
When hydrochloric acid must be used, manganous or magnesia 
sulphate should be added unless the solution is very dilute. 

2. Reduction of Ferric Compounds to the Ferrous State. 

This may be accomplished by metallic zinc OP magnesium, for use with 
permanganate, or by stannous chloride or an alkaline sulphite for bichromate 
solution. The magnesium method is elegant and rapid but costty. In the 
case of zinc being used, the metal must either be free from iron, or if it 
contain any, the exact quantity must be known and allowed for ; and further, 
the pieces of zinc used must be entirely dissolved before the solution is- 
titrated.* The solution to be reduced by zinc should not contain more than 
0'15 gm. Fe. per 250 c.c., and for this quantity about 10 gin. of Zn. and 
25 c.c. H 2 SO 4 are advisable ; when the zinc is all dissolved, the whole 
should be boiled with exclusion of air, then cooled rapidly before titration 
with the permanganate. In the case of stannous chloride the solution must 
be clear, and is best made to contain 10 to 15 gm. per liter, as directed 
in 37.2. The point of exact reduction in the boiling hot and somewhat 
concentrated acid liquid may be known very closely by the discharge of 
colour in the ferric solution : but may be made sure by the use of a saturated 
aqueous solution of mercuric chloride as mentioned p. 127. Some operators 
use a few drops of solution of platinic chloride in addition to the mercury. 

It is exceedingly difficult to hit the exact point of reduction so- 
that there shall be neither excess of tin nor unreduced iron, and 

* Many operators now use amalgamated zinc in conjunction with platinum foil for 
the reduction, but a practical difficulty occurs from the platinum becoming also 
amalgamated through contact with the zinc and stopping the action. Beebe 
(C. N. liii. 269) suggests the following convenient arrangement : A strip of thin 
platinum foil, 1 in. square, is perforated with pin holes all over, then bent into 
a U form, and the ends connected with platinum wire so as to form a basket. In this 
is placed a piece of amalgamated zinc, and the whole suspended by a stout platinum 
wire in the reducing flask. When lowered into the solution, another strip of platinum 
foil, 2 in. square, is dropped in and leaned against the wire carrying the basket : a very 
free evolution of hydrogen is then obtained from the foil. When the reduction- 
is complete, the basket is lifted out and well washed into the beaker containing the 
liquid to be titrated. 



63. IRON. 209 

technical iron analysts now almost universally use mercuric chloride 
as a precaution against excess of tin solution. The general method 
of procedure is to dissolve the material in diluted hydrochloric acid 
(1 acid 2 water) in a conical beaker moderately heated over a rose 
burner; when solution is complete the sides of the vessel are 
washed down with hot water, the liquid brought to gentle boiling, 
and strong tin solution added from a dropping bottle until the 
colour of the iron solution is nearly discharged, a dilute tin 
solution is then dropped in until all colour has disappeared, and 
there is a decided slight excess of tin. Cold air-free water is then 
washed over the sides of the beaker, the vessel covered with 
a clock-glass placed in a bowl of cold water and allowed to cool, 
an excess of the mercuric solution is then added, and the 
titratioii with bichromate is at once completed in the usual way. 

Some technical operators prefer to use sodic sulphite or ammonic 
bisulphite for the reduction. In the case of using the sodic 
sulphite the solution of iron must not be too acid and should 
be dilute, say a volume of half a liter for J gm. of Fe, the 
sulphite is added and the flask gently heated till the liquid 
is colourless. It is then boiled briskly till all SO 2 is dissipated, 
when cooled it is ready for titration with bichromate. In the case 
of ores containing titanium it is preferable to avoid the use of zinc 
for reduction, as it reduces also more or less the titanium ; alkaline 
sulphite does not. 

The ammonic bisulphite is used as follows : (Atkinson C. N. xlvi. 217). 
To the acid solution of the ore or metal, diluted and filtered, ammonia 
is added until a faint precipitate of ferric oxide occurs. This is re-dissolved 
with a few drops of IIC1, and some strong solution of bisulphite added, in 
the proportion of about 1 c.c. for each O'l gm. of ore, or 0'05 gm. Fe. The 
mixture is well stirred, boiling water added, then acidified with dilute 
sulphuric acid, and boiled for half an hour : it is then ready for titration. 

I). J. Carnegie (J. C. 8. liii. 468) points out the value of zinc dust for 
the rapid reduction of ferric solutions, and suggests the following method 
of carrying it out. 

The bottom of a dry and narrow beaker is covered with zinc dust sifted 
through muslin. A known volume of ferric solution, previously nearly 
neutralized with ammonia, is placed in the beaker and shaken with the zinc 
dust ; then a known volume of dilute sulphuric acid is added and shaken for 
a few moments. The reduction is much more rapid in neutral than in acid 
solutions, but of course acid in this case must be present in excess to keep 
the iron in solution. Carnegie withdraws a portion of the reduced 
solution from the undissolved zinc by help of a filter, such as is described on 
p. 18, and as measured volumes have been used, an aliquot part taken with 
a pipette may be at once titrated, and the amount of iron found.* 

* Commercial zinc dust is probably a by-product in zinc manufacture, and cannot 
therefore be obtained pure. Samples examined by myself, and apparently others also, 
do not, however, contain much iron, but a good deal of zinc oxide with traces of 
cadmium and lead. Carnegie states that the oxide maybe removed by repeatedly 
digesting- with weak acid, and still better, by treatment with ammonic chloride and 
ammonia, the well-washed dust being 'finally dried on porous tiles in a vacuum. 
I find that by washing once with strong alcohol after the water, and finally with ether, 
the dust may be rapidly dried in good condition, and when a quantity of such purified 
dust is obtained, its amount of iron may easily be estimated once for all, and allowed 
for in titration. Good zinc dust is undoubtedly a valuable reagent in a laboratory for 
other purposes beside iron titrations. 

P 



210 VOLUMETRIC ANALYSIS. 64. 

Clemens Jones in a paper read before the American Institute of 
Mining Engineers, and which is reproduced in C. N. Ix. 93, adopts the plan 
suggested by Carnegie, and has designed a special apparatus for filtering 
the ferric solution through a column of zinc dust. This arrangement gives 
complete reduction in a very short period of time, and is serviceable where 
a large number of titrations have to be carried on. 



ESTIMATION OF IRON IN THE FERRIC STATE. 
1. Direct Titration of Iron by Stannous Chloride. 

64. THE reduction of iron from the ferric to the ferrous state 
by this reagent has been previously referred to ; and it will be 
readily seen that the principle involved in the reaction can be made 
available for a direct estimation of iron, being, in fact, simply 
a reversion of the ordinary process by permanganate and 
bichromate. 

Fresenius has recorded a series of experiments made on the 
weak points of this process, and gives it as his opinion that, with 
proper care, the results are quite accurate. The summary of his 
process is as follows : 

(a) A solution of ferric oxide of known strength is first prepared by 
dissolving 10*04 gm. of soft iron wire (=10 gm. of pure iron) in pure hydro- 
chloric acid, adding potassic chlorate to complete oxidation, boiling till the 
excess of chlorine is removed, and diluting the solution to 1 liter.* 

(6) A clear solution of stannous chloride, of such strength that about one 
volume of it and two of the iron solution are required for the complete 
reaction (see 37.2). 

(<?) A solution of iodine in potassic iodide, containing about O'OIO gm. 
of iodine in 1 c.c. (if the operator has the ordinary decinormal iodine solution 
at hand, it is equally applicable). The operations are as follows : 

(1) 1 or 2 c.c. of the tin solution are put into a beaker with a little starch, 
and the iodine solution added from a burette till the blue colour occurs ; 
the quantity is recorded. 

(2) 50 c.c. of the iron solution (-=0'5 gm. of iron) are put into a small 
flask with a little hydrochloric acid, and heated to gentle boiling (preferably 
on a hot plate) ; the tin solution is then allowed to flow in from a burette 
until the yellow colour of the solution is nearly destroyed ; it is then added 
drop by drop, waiting after each addition until the colour is completely 
gone, and the reduction ended. If this is carefully managed, there need be 
no more tin solution added than is actually required; however, to guard 
against any error in this respect, the solution is cooled, a little starch 
added, and the iodine solution added by drops until a permanent blue colour 
is obtained. As the strength of the iodine solution compared with the tin 
has been found in 1, the excess of tin solution corresponding to the iodine 
used is deducted from the original quantity, so that by this means the volume 
of tin solution corresponding to 0'5 gm. of iron is found. 

The operator is therefore now in a position to estimate any 

* A ferric standard may also be made, as suggested by French (C. N. Ix. 271), by 
dissolving a weighed amount of double iron salt in dilute sulphuric acid, adding- an 
excess of hydrogen peroxide, warming up, and finally boiling to dissipate the excess of 
the peroxide. 



64 IRON. 211 

unknown quantity of iron which may exist in a given solution, in 
the ferric state, by means of the solution of tin.* 

If the iron should exist partly or wholly in the state of ferrous 
oxide, it must be oxidized by the addition of potassic chlorate, and 
boiling to dissipate the excess of chlorine, as described in a, or 
with hydrogen peroxide. 

Example : 50 c.c. of iron solution, containing 0*5 gm. of iron, required 
25 c.c. of tin solution. 

A solution containing an unknown quantity of iron was then taken for 
analysis, which required 20 c.c., consequently a rule-of-three sum gave the 
proportion of iron as follows : 

25 : 0'50 gm. : : 20 : 0'40 gm. 

It must be remembered that the solution of tin is not permanent, conse- 
quently it must be tested every day afresh. Two conditions are necessary in 
order to ensure accurate results. 

(1) The iron solution must be tolerably concentrated, since the end of 
the reduction by loss of colour is more distinct ; and further, the dilution of 
the liquid to any extent interferes with the quantity of tin solution necessary 
to effect the reduction. Fresenius found that by diluting the 10 c.c. of 
iron solution with 30 c.c. of distilled water, O'l c.c. more was required than 
in the concentrated state. This is, however, always the case with stannous 
chloride in acid solution, and constitutes the weak point in Streng's method 
of analysis by its means. 

(2) The addition of the tin solution to the iron must be so regulated, that 
only a very small quantity of iodine is necessary to estimate the excess ; if 
this is not done another source of error steps in, namely, the influence which 
dilution, on the one hand, or the presence of great or small quantities of 
hydrochloric acid on the other, are known to exercise over this reaction. 
Practically, it was found that where the addition of tin to the somewhat 
concentrated iron solution was cautiously made, so that the colour was just 
discharged, the mixture then rapidly cooled, starch added, and then iodine 
till it became blue, the estimation was extremely accurate. 

2. Titration by Sodic Thiosulphate. 

Scherer first suggested the direct titration of iron by thio- 
sulphate, which latter was added to a solution of ferric chloride 
until no further violet colour was produced. This was found by 
many to be inexact, but Kremer ( Journ. f. Pract. Chem. Ixxxiv. 
339) has made a series of practical experiments, the result of which 
is that the following modified method can be recommended. 

The reaction which takes place is such as to produce ferrous 
chloride, sodic tetrathionate, and sodic chloride. 2S 2 3 Na 2 + 
Fe 2 Cl 6 + 2HC1 = S 4 6 H 2 + 4ffaCl + 2FeCl 2 . The thiosulphate, 
which may conveniently be of T ^- strength, is added in excess, 
and the excess determined by iodine and starch. 

* F. H.. Morgan (Journ. Anal. Chem. ii. 169) points out a very simple and useful 
method of finding the end-point in titrating iron solutions with stannous chloride 
without resorting to an indicator. It consists in using a round bottom white glass 
flask containing the boiling liquid under titration, fixed over a small bluish-coloured 
B u n s e n flame at a distance of 13 m.m. in a darkened room or a dark corner. So long 
as the slightest trace of unreduced iron exists, a distinct green colour appears when 
looking at the faint blue flame through the solution. It is stated that one part of iron 
as oxide may be recognized in 1,500,000 parts of solution by this means. 

p 2 



212 VOLUMETKIC ANALYSIS. 64V 

Process : The iron solution, containing not more than 1 per cent, of metal, 
which must exist in the ferric state without any excess of oxidizing material 
(best obtained by adding excess of hydrogen peroxide, then boiling till the 
excess is expelled), is moderately acidified with hydrochloric acid, sodic 
acetate added till the mixture is red, then dilute hydrochloric acid until the 
red colour disappears ; then diluted till the iron amounts to i or i per cent., 
and T ^5- thiosulphate added in excess, best known by throwing in a particle of 
potassic sulphocyanide after the violet colour produced has disappeared ; if 
any red colour occurs, more thiosulphate must be added. Starch and ^ 
iodine are then used to ascertain the excess. A mean of several experiments 
gave 100-06 Fe, instead of 100. 

Oudemanns' Method. A simpler process for the direct titra- 
tion of iron by thiosulphate has been devised by Oudemanns 
(Z. a. C. vi. 129 and ix. 342), which gives very good results. 

Process: To the dilute ferric solution, which should not contain more 
than O'l to 0'2 gm. Fe in 100 c.c. nor much free HC1, 3 c.c. of 1 per cent, 
solution of cupric sulphate are added, 2 c.c. of concentrated hydrochloric 
acid, and to about every 100 c.c. of fluid, 1 c.c. of a 1 per cent, solution 
of potassic thiocyanate. 

The mixture may with advantage be very slightly warmed, and the 
thiosulphate delivered in from the burette at first pretty freely. The red 
colour produced by the indicator gradually fades away; as this occurs, the 
thiosulphate must be added in smaller quantities, constantly agitating the 
liquid until it becomes as colourless as pure water. If any doubt exists as 
to the exact ending, a slight excess of thiosulphate may be added, and the- 
quantity found by T ^- iodine and starch. Greater accuracy will always be 
insured by this latter method. 

The accuracy of the process is not interfered with by the 1 
presence of salts of the alkalies, strontia, lime, magnesia, alumina,, 
or manganous oxide ; neither do salts of nickel, cobalt, or copper,, 
unless their -quantity is such as to give colour to the solution. 

The process is a rapid one, and with care gives very satisfactory 
results. 



3. Estimation by Iodine and Sodic Thiosulphate. 

When ferric chloride is digested with potassic iodide in excess,, 
iodine is liberated, which dissolves in the free potassic iodide 

Fed 3 + KI - Fed 2 + KC1 + L 

Process : The hydrochloric acid solution, which must contain no free- 
chlorine or nitric acid, and all the iron in the ferric state, is nearh r 
neutralized with caustic potash or soda, transferred to a well-stoppered flask, 
and an excess of strong solution of potassic iodide added ; it is then heated 
to 50 or 60 C. on the water bath, closely stoppered, for about twenty 
minutes ; the flask is then cooled, starch added, and titrated with thiosulphate 
till the blue colour disappears. This process gives very satisfactory results 
in the absence of all substances liable to affect the potassic iodide, such as 
free chlorine or nitric acid, and is particularly serviceable for estimating. 
small quantities of iron. 



' 64 IRON. 213 

4. Estimation of Iron by Colour Titration. 

These methods, which approach in delicacy the Nessler test 
for ammonia, are applicable for very minute quantities of iron, 
such as may occur in the ash of bread when testing for alum, water 
residues, alloys, and similar cases. 

It is first necessary to have a standard solution of iron in the ferric state, 
which can be made by dissolving 1*004 gm. of iron wire in nitro-hydrochloric 
acid, precipitating with ammonia, washing and re-dissolving the ferric oxide 
in a little hydrochloric acid, then diluting to 1 liter. 1 c.c. of this solution 
contains 1 milligram of pure iron in the form of ferric chloride. It may be 
further diluted, when required, so as to contain ^ milligram in a c.c./and 
this is the best standard to use.* The solution for striking the colour is 
either potassic ferrocyanide or thiocyanate dissolved in water (1 : 20). 

Example with Ferrocyanide : The material containing a minute unknown 
quantity of iron, say a w r ater residue, is dissolved in hydrochloric acid, and 
diluted to 100 c.c., or any other convenient measure. 10 c.c. are placed 
into a white glass cylinder marked at 100 c.c., 1 c.c. of concentrated nitric 
acid added (the presence of free acid is always necessary in this process), then 
diluted to the mark with distilled water, and well stirred. 

1 c.c. of ferrocyanide solution is then added, well mixed, and allowed to 
stand at rest a few minutes to develop the colour. 

A similar cylinder is then filled with a mixture of, say 1 c.c. of standard 
iron solution, 1 c.c. nitric acid and distilled water, and 1 c.c. ferrocyanide 
added ; if this does not approach the colour of the first mixture, other 
quantities of iron are tried until an exact similarity of colour occurs. The 
exact strength of , the iron solution being known, it is easy to arrive at the 
quantity of pure iron present in the substance examined, and to convert 
it into its state of combination by calculation. 

Carter Bell (J. S. C. I. viii. 175) adopts the following plan in 
the case of waters : 70 c.c. of the water are evaporated to 
dryrjess in a platinum dish, and gently ignited to burn off organic 
matters. 1 c.c. of dilute nitric acid, 50 c.c. of strong acid in 
a liter, is then poured over the residue from a pipette, and 
evaporated to dryness in the water bath ; the residue is then 
dissolved in 1 c.c. of a 10 per cent, hydrochloric acid, 5 or 10 c.c. 
of distilled water added, and the solution filtered and washed 
through a small filter, and made up to 50 c.c. in a Messier glass; 
and finally tested with 1 c.c. each of ferrocyanide solution and 
nitric acid. 

With Thiocyanate. Thomson (J. C. S. 1885, 493) recom- 
mends this method as being specially available in the presence of 
other ordinary metals and organic matters, silver, copper, and 
cobalt being the only interfering substances. The delicacy is said 
to be such, that 1 part of iron can be recognized in 50 million parts 
of water. The presence of free mineral acids greatly adds to the 

* A solution of this strength can also be made by weighing 0'7 gin. of pure ammonio- 
ferrous sulphate ( 34.2b), dissolving in water, acidifying- with sulphuric acid, adding 
sufficient permanganate solution to convert the iron exactly into ferric salt, then 
diluting to 1 liter. Hydrogen peroxide may also be used in place of permanganate, 
taking care to dissipate the excess by boiling. 



214 VOLUMETRIC ANALYSIS. 65. 

sensitiveness. The standard ferric solution may be the same as for 
ferrocyanide ; and in preparing the material for titration, the 
weighed quantity is dissolved in an. appropriate acid, evaporated 
nearly to dryness, taken up with water, converted into the ferric 
state by cautious addition of permanganate, then diluted with water 
to a measured volume, and an aliquot portion taken for titration. 

The standard iron used by Thomson y 1 ^ m.gm. per c.c. 
(0*7 gm. double iron salt [oxidized] per liter). 

Example : Into two colourless glass cylinders marked at 100 c.c. pour 
5 c.c. of nitric or hydrochloric acid (1 : 5), together with 15 c.c. of 
thiocyanate, and to one glass a measured volume of the solution to be tested : 
fill up both glasses to the mark with pure water. If iron be present, a blood 
red colour more or less intense will be produced. Standard iron is then 
cautiously added from a burette to the other glass till the colour agrees. 
The quantity of Pe taken should not require more than 2 or 3 c.c. of the: 
standard to equal it, or the colour will be too deep for comparison. 

If other metals are present which form two sets of salts, they 
must be in the higher state of oxidation, or the colour is destroyed. 
Oxalic acid also destroys it. Examples in the presence of a great 
variety of metals show very good results. 

IRON ORES. 

65. THE great desideratum in the analysis of iron ores is 
to get them into the finest possible state of division, and ten 
minutes' hard work with the agate mortar will often save hours 
of treatment of the material with acids. The operator of 
experience can generally tell if the ore to be examined will dissolve- 
in acids. Some clay iron stones and brown haematites contain 
organic matters, and they are best first roasted in an open platinum 
crucible, gradually raising the heat to redness; this course is 
advisable also when an ore contains pyrites ; this latter is easily 
converted to Fe 2 3 by roasting. The proportion in iron ores is 
generally under half a per cent. Some ores give a residue in any 
case by treatment with HC1, this should be separated by nitration and 
fused with sodic carbonate which will render all the iron in a soluble 
state. In the analysis of iron ores it is very often necessary to. 
determine not only the total amount of iron, but also the state in 
which it exists; for instance, magnetic iron ore consists of 
a mixture of the two oxides in tolerably definite proportions, and 
it is sometimes advisable to know the quantities of each. 

In order to prevent, therefore, in such cases, the further oxidation 
of the ferrous oxide, the little flask apparatus (fig. 43) adopted by 
Mohr is recommended, or fig. 42 is equally serviceable. 

The left-hand flask contains the weighed ore in a finely powdered state, to 
which, tolerably strong hydrochloric acid is added ; the other flask contains- 
distilled water only, the tube from the first flask entering to the bottom of the 
second. When the ore is ready in the flask and the tubes fitted, hydrochloric 
acid is poured in, and a few grains of sodic bicarbonate added to produce 



IRON ORES. 



215 



a flow of CO' 2 . The air of the flask is thus dispelled, and as the acid dissolves 
the ore, the gases evolved drive out in turn the CO-, which is partly absorbed 
by the water in the second flask. When the ore is all dissolved and the 
lamp removed, the water immediately rushes out of the second flask into the 
first, diluting and cooling the solution of ore, so that, in the majority 
of cases, it is ready for immediate titration. If not sufficiently cool or 
dilute, a sufficient quantity of boiled and cooled distilled water is added. 

When the total amount of iron present in any sample of ore has 
to be determined, it is necessary to reduce any peroxide present to- 
the state of protoxide previous to titration. 




Reduction to the Ferrous state may be done by sodic sulphite 
in dilute solution, but not so with stannous chloride, the latter 
must be used in a boiling and concentrated solution strongly 
acidified with hydrochloric acid. Most technical operators now 
use the tin method, which, by the help of mercuric chloride as 
described 63.2, is rendered both rapid and trustworthy. Both 
with the sulphite and tin method bichromate is the invariable 
titrating solution. When permanganate is to be used for titration 
the reduction is always best made with zinc or magnesium in 
sulphuric or very weak hydrochloric acid solution. With bichro- 
mate, the best agent is either pure sodic sulphite, ammonic 
bisulphite, or stannous chloride. 

1. Bed and Brown Haematites. Red haematite consists generally 
of ferric oxide accompanied with matters insoluble in acids. 
Sometimes, however, it contains phosphoric acid, manganese, and 
earthy carbonates. 

Brown haematite contains hydrated ferric oxide, often accompanied 



216 VOLUMETRIC ANALYSIS. 65. 

"by small quantities of ferrous oxide, manganese, and alumina ; 
sometimes traces of copper, zinc, nickel, cobalt, with lime, magnesia 
and silica ; occasionally also organic matters. 

In cases where the total iron only has to be estimated, it is 
advisable to ignite gently to destroy organic matters, then treat 
with strong hydrochloric acid at near boiling heat till all iron is 
dissolved, and in case ferrous oxide is present add small quantities 
of potassic chlorate, afterwards evaporating to dryness to dissipate 
free chlorine ; then dissolve the iron with hot dilute hydrochloric 
acid, filter, and make up to a given measure for reduction and 
titration. 

In some instances the insoluble residue persistently retains some 
iron in an insoluble form; when this occurs, resort must be had to 
fluxing the residue with sodic carbonate, followed by solution in 
hydrochloric acid. 

2. Magnetic Iron Ore. The ferrous oxide is determined first by 
means of the apparatus fig. 42 or 43. The ore is put into the 
vessel in a state of very fine powder, strong hydrochloric acid 
added, together with a few grains of sodic bicarbonate, and heat 
applied gently with the lamp until the ore is dissolved, then 
diluted if necessary, and titrated with bichromate or permanganate. 
Technical operators generally use only a covered beaker. 

Example : 0"5 gm. of ore was treated as above, and required 19"5 c.c. of 
xo- bichromate, which multiplied by 00)56 gave 0-1032 gin. of iron = Q-14D4 
gm. of ferrous oxide = 28"08 per cent. FeO. 

The ferric oxide was now found by reducing 0'5 gm. of the same ore, and 
estimating the total iron present: the quantity of bichromate required was 
59 c.c. T ^=0-3304 gm. total Fe 
Deduct 0-1092 gm. Fe as FeO 



Leaving 0'2212 gm. Fe as Fe-O 3 

The result of the analysis is therefore 

Ferrous oxide 28'08 per cent. 

Ferric oxide 63'20 

Difference (Gangue, etc.) ... 872 

100-00 

3. Spathose Iron Ore. The total amount of ferrous oxide in 
this carbonate is ascertained directly by solution in hydrochloric 
acid ; as the carbonic acid evolved is generally sufficient to expel 
all air, the tube dipping under water may be dispensed with. If 
the ore contains pyrites it should be first roasted, but this of course 
converts the ferrous carbonate into Fe 2 3 . 

As the ore contains, in most cases, the carbonates of manganese, 
lime, and magnesia, these may all be determined, together with the 
iron, as follows : 

A weighed portion of ore is brought into solution in hydrochloric acid, 
after ignition if pyrite is present, and filtered, if necessary, to separate 
insoluble silicious matter. 



Go. IRON ORES. 217 

The solution is then boiled, Avith a few drops of nitric acid to peroxidize 
the iron, diluted, nearly neutralized with ammonia, and a solution of 
ummonic acetate added, then boiled for two minutes and allowed to settle. 
The precipitate is collected on a filter and washed with boiling water, 
containing a little ammonic acetate. It is then dissolved off the filter in 
HC1 which also dissolves any A1 2 O 3 or P 2 O 5 which may be present. The 
liquid is then evaporated, reduced, and titrated as usual. " 

The filtrate from the above is concentrated by evaporation, cooled, 3 or 
4 c.c. of bromine added, and well mixed by shaking; when most of the 
bromine is dissolved the liquid is rendered alkaline by ammonia, and gently 
warmed till the Mn separates in large flocks as hydrated oxide, which is 
collected and titrated by one of the methods in 67. 

The filtrate from the last is mixed with ammonic oxalate to precipitate the 
lime, which is estimated by permanganate, as in 52. 

The filtrate from the lime contains the magnesia, which may be precipitated 
with sodic phosphate and ammonia, and the precipitate weighed as usual, or 
titrated with uranium solution. 

4. Estimation of Iron in Silicates. Wilbur and Wllittlesey 
(C. N. xxii. 5) give a series of determinations of iron existing in 
various silicates, either as mixtures of ferric and ferrous salts or of 
either separately, which appear very satisfactory. 

The very finely powdered silicate is mixed with rather more than its own 
weight of powdered fluor-spar or cryolite (free from iron) in a platinum 
crucible, covered with hydrochloric acid, and heated on the water-bath until 
the silicate is all dissolved. During the digestion either carbonic acid gas or 
coal gas free from H 2 S is supplied over the surface of the liquid so as to 
prevent access of air. When decomposition is complete (the time varying 
with the nature of the material), the mixture is diluted and titrated with 
permanganate in the usual way for ferrous oxide ; the ferric oxide can then 
be reduced by zinc and its proportion found. 

By Hydrofluoric Acid. 2 gm. of the finely powdered silicate are placed 
in a deep platinum crucible, and 4O c.c. of hydrofluoric acid (containing 
about 20 per cent. HF) added. The mixture is heated to near the boiling 
point and occasionally stirred with a platinum wire until the decomposition 
of the silicate is complete, which occupies usually about ten minutes. 10 c.c. 
of pure H 2 SO 4 diluted with an equal quantity of water are then added, and 
the heat continued for a few minutes. The crucible and its contents are 
then quickly cooled, diluted with fresh boiled water, and the ferrous salt 
estimated with permanganate or bichromate as usual. 

Leeds (Z. a. C. xvi. 323) recommends that the finely powdered 
silicate be mixed with a suitable quantity of dilute sulphuric 
acid, and air excluded by CO 2 during the action of the hydrofluoric 
acid. The titration may then at once be proceeded with when the 
decomposition is complete. 

If the hydrofluoric acid has been prepared in leaden vessels, it 
invariably contains SO 2 ; in such cases it is necessary to add to it, 
previous to use, some hydrogen peroxide (avoiding excess) so as to 
oxidize the SO 2 . 

The process is a rapid and satisfactory one, yielding much more 
accurate results than the method of fusion with alkaline carbonates 
or acid potassic sulphate. 



218 VOLUMETRIC ANALYSIS. 65. 

5. Colorimetric estimation of Carbon in Steel and Iron. The 
method devised by Eggertz, and largely adopted by chemists, for 
estimation of combined carbon, is well known, but is open to the 
objection that minute quantities of carbon cannot be discriminated 
by it, owing to the colour of the ferric nitrate present. Stead 
(G. N. xlvii. 285) in order to overcome this difficulty has devised 
a method described as follows : 

In some careful investigations on the nature of the colouring 
matter which is produced by the action of dilute nitric acid upon 
white iron and steel, it was found it had the property of being 
soluble in potash and soda solutions, and that the alkaline solution 
had about two and a half times the depth of colour possessed by 
the acid solution. This being so, it was clear that the colouring 
matter might readily be separated from the iron, and be obtained 
in an alkaline solution, by simply adding an excess of sodic 
hydrate to the nitric acid solution of iron, and that. the coloured 
solution thus obtained might be used as a means of determining 
the amount of carbon present. Upon trial this was found to 
be the case, and that as small a quantity as OO3 per cent, of 
carbon could be readily determined. 

The solutions required are : 

Standard solution of Nitric acid, 1 '20 sp. gr. 

Standard solution of Sodic hydrate, 1'27 sp. gr. 

Process : One gram of the steel or iron to be tested is weighed off and 
placed in a 200 c.c. beaker, and after covering with a watch-glass, 12 c.c. 
of standard nitric acid are added. The beaker and contents are then placed 
on a warm plate, heated to about 90 to 100 C., and there allowed to remain 
until dissolved, which does not usually take more than ten minutes. At the 
same time a standard iron containing a known quantity of carbon is treated 
in exactly the same way, and when both are dissolved 30 c.c. of hot water 
are added to each, and 13 c.c. soda solution. 

The contents are now to be well shaken, and then poured into a glass 
measuring-jar and diluted till they occupy a bulk of 60 c.c. After again 
well mixing and allowing to stand for ten minutes in a warm place, they are 
filtered through dry filters, and the filtrates, only a portion of which is used r 
are compared. This may be done by pouring the two liquids into two 
separate measuring tubes in such quantity or proportion that upon looking 
down the tubes the colours appear to be equal. 

Thus if 50 measures of the standard solution are poured into one tube, and if 
the steel to be tested contains say half as much as the standard, there will be 
100 measures of its colour solution required to give the same tint. The carbon 
is therefore inversely proportional to the bulk compared with the standard, 
and in the above assumed case, if the standard steel contained 0'05 per cent, 
carbon,, the following simple equation would give the carbon in the sample 
tested : 

0'05 x 50 

,,-.,-. = 0'02o per cent. 

J-Ul/ 

The proportions here given must be strictly adhered to in order to insure 
exactness. The colours from low carbon irons differ in tint from those in 
high carbon steels, and therefore a low standard specimen must be used for 
comparison. It is evident that the safest plan to insure absolute comparison 



65. IRON. 219 

is to weigh and dissolve a known standard steel or iron for each batch 
of tests. 

Stead has devised a special colorimeter for the process, but it is 
evident that any of the usual instruments may be used. 

Arnold (Steel Works Analysts, p. 46) gives the following 
conditions as necessary for the accurate working of the Eggertz 
test. 

(a) The standard steel should have been made by the same process as the 
sample. 

(b) The standard should be in the same physical condition, as far as this 
can be secured by mechanical means. 

(c) The standard should not differ greatly in the percentage of carbon. 

(d) The solution of the standard and the samples should be made at the 
same time, and under identical conditions, and the comparisons should be 
made without delay. 

(e) Above all, the standard should be above suspicion, its carbon contents* 
having been settled on the mean of several concordant combustions made on 
different weights of steel from a homogeneous bar. 

6. Estimation of Arsenic in Iron Ores, Steel, and Pig- Iron 
(J. E. Stead). The best method of separating arsenic from iron 
solutions is undoubtedly that of distilling with hydrochloric acid 
and ferrous chloride. 

Stead found after many trials and experiments, that if the 
distillation is conducted in a special manner, the whole of the 
arsenic may be obtained in the distillate, unaccompanied with any 
traces of chloride of iron, and that if the hydrochloric acid is 
nearly neutralized with ammonia, and finally completely neutralized 
with bicarbonate of soda, the arsenic can be determined 
volumetrically with a standard solution of iodine. 

The standard solutions required are : 

Arsenious oxide. O66 gin. (0'5 gm. metallic arsenic) of pure 
arsenious acid in fine powder is weighed and placed into a flask, 
with '2 gm. of sodic carbonate and 100 c.c. of boiling distilled 
water, and the liquid boiled till all the arsenious oxide has 
dissolved. When cool, 2 gm. of sodic bicarbonate are added 
and diluted to one liter : 1 c.c. = 0'0005 gm. As. 

Iodine solution. 1*275 gm. of pure iodine is dissolved in 2 gm. 
of potassic iodide and water, the solution diluted to one liter, then 
standardized by titrating 20 c.c. of the arsenious solution. If the 
iodine. has been pure, 20 c.c. of the solution should be required to 
just produce a permanent blue with starch indicator. 

These solutions keep fairly well without alteration for several 
months. It is advisable, however, to periodically ascertain the 
value of the iodine by titrating 20 or 30 c.c. of the arsenic 
solution. 

Process for Steel: Prom 1 to 50 gm. of the steel in drillings are 
introduced into a 30-ounce flask, and a sufficient quantity of equal parts of 
strong hydrochloric acid and water is added to dissolve it. The mouth of the 
flask is closed with a rubber cork carrying a safety tube, and a tube to 



220 VOLUMETRIC ANALYSIS. 65. 

convey the gas evolved into the Winkler's spiral absorption tubes, 
containing a strong saturated solution of bromine in water. 

The tube is filled to one-third of its length with the solution, and about 
i c.c. of free bromine is run in to replace the bromine which is consumed or 
carried out with the passing gas. 

The contents of the flask are now gently heated to such a degree that 
a steady but not rapid current of gas passes through the bromine solution. 

In about one hour the whole of the steel will be dissolved, and when no 
more evolution of hydrogen can be observed, the liquid in the flask is w r ell 
boiled, so as to completely drive all the gas into and through the bromine 
solution. 

The absorption tube is now disconnected, and the bromine solution 
containing that part of the arsenic which has passed off as gas is rinsed out 
into a small 100 c.c. beaker, and the excess of bromine is gently boiled off, 
and the clear colourless solution is poured into the flask. About 0'5 gm. of 
.zinc sulphide is now dropped into the iron solution and the contents are 
violently shaken for about three minutes, by which time the whole of the 
arsenic will be in the insoluble state, partly as sulphide and partly as a black 
precipitate of possibly free arsenic and arsenide of iron. 

It has been found that violent agitation for a few minutes is quite as 
efficacious in effecting the complete separation of arsenic sulphide as by the 
method of passing a current of CO 2 through the solution to remove the 
excess of hydric sulphide, or by allowing it to stand ten or twenty hours to 
settle out. 

The insoluble precipitate is now rapidly filtered through a smooth filter- 
paper, and the flask is rinsed with cold distilled water. The precipitate 
usually does not adhere to the filter, and in such cases the paper is spread 
out flat upon a porcelain slab, and the arsenic compounds are rinsed off with 
a fine jet of hot water into a small beaker. The precipitate is now dissolved 
in bromine water, and a drop or two of HC1. 

The bromine solution now containing all the arsenic is gently boiled to 
expel the bromine, and it is then poured into a 10-ounce retort and is 
distilled with ferrous chloride and hydrochloric acid. 

The apparatus used consists of an ordinary Liebig's condenser, but the 
retort has its neck bent to an angle of about 150, and this is attached to the 
condenser, so that any iron mechanically carried over may run back. By 
this device, the distillate w r ill never contain more than the very slightest 
trace of iron. 

The solution containing the arsenic having been run into the retort, the 
beaker is washed out and the washings are also poured in. If the solution 
is much above 20 c.c. in bulk, it is advisable to add a strong solution of 
ferrous chloride containing about 0'5 gm. of iron in the ferrous state, and 
for this purpose nothing answers so well as a portion of the steel solution 
remaining after separating the arsenic, which is first well boiled to free it 
from hydric sulphide, and should contain about 10 per cent, of soluble iron 
as ferrous chloride. 5 c.c. of this solution will contain the necessary amount 
of iron to add to the retort. After adding the chloride, it is best to boil 
down the solution to about 20 c.c. before adding any HC1, taking care, of 
course, to collect what liquid distils over. When the necessary concentration 
has been effected, 20 c.c. strong HC1 is run in, and the distillation is 
continued till all excepting about 10 c.c. has passed over. A further 
quantity of 20 c.c. mixed with 5 c.c. of water is run in, and this is all 
distilled over. At this point, as a rule, all the arsenic will have passed into 
the distillate, but it is advisable to make quite certain, and to add a third portion 
of acid and w r ater, and to distil it over. If the distillation has not been 
forced, the distillate will be quite colourless. The arsenic in the distillate 
will exist as arseaious chloride, accompanied with a large excess of 
hydrochloric acid. A drop of litmus is put into this solution, and strong 



65. 



IRON. 221 



ammonia is run in until alkaline. It is now made slightly acid with a few 
drops of HC1, and a slight excess of solid bicarbonate of soda is dropped in. 
The contents of the flask are now cooled by a stream of water, and, after 
adding a clear solution of starch, the standard iodine is run in from a burette 
till a deep permanent blue colouration is produced. 

If the steel or iron contains much arsenic, a smaller quantity, say, one or 
two gm., may be dissolved in nitric acid of T20 specific gravity and the 
solution evaporated to dryness, the residue being dissolved in hydrochloric 
acid, and the solution transferred to the retort, and distilled directly with 
ferrous chloride and hydrochloric acid, care being taken that the distillation 
is not forced, so as to avoid any of the iron solution passing over into the 
distillate. 

Process for Pig Iron : In testing pig irons, they may be dissolved in 
nitric acid and evaporated to dryness, or be treated in a flask with HC1 
exactly in the manner described above, but it is advisable, if the latter 
method is adopted, after treating the voluminous mass of silica and 
graphite, &c., with bromine and hydrochloric acid, to filter off the insoluble 
matter and distil the clear solution. 

Process for Iron Ores : In testing ores, it is only necessary to place the 
powdered ore directly into the retort, and distil at once with HC1 and 
ferrous chloride, taking care to place a few pieces of fire-brick also in the- 
vessel, to avoid bumping. 

If the ore contains much manganese, it is advisable to dissolve it in 
a separate vessel to liberate and expel the chlorine, and then to transfer it 
into the retort. 

The time taken to test iron or steel need not exceed two hours, and for 
iron or other ores not much more than half an hour. 

It is quite possible to accurately determine as small a quantity as 0'002 
per cent, of arsenic by this method. 

When dissolving steels in dilute HC1, if there is no rust on the sample or 
ferric chloride present in the acid, and the presence of air is carefully 
avoided, as a rule only about one-tenth of the total arsenic present passes off 
with the gas. 

Platten's method, alluded to on page 149, depends on the fact 
that when sulphide of arsenic (obtained by treating the arsenical 
distillate with H 2 S) is boiled with pure water, the gas escapes, and 
arsenious oxide remains in solution. This solution is then titrated 
with iodine in the usual way. 

Both methods have been proved to give identical results, when 
carried out by separate skilled operators on the same samples of 
material. 

7. Estimation of Phosphorus in Iron and Steel. Dudley and 
Pease (/. Anal. Cliem. vii. 108) adopt the following method: 

1 gm. of the sample is dissolved in an Erlenmeyer flask, in 75 c.c. of 
nitric acid of sp. gr. T15 ; when dissolved, it is boiled for a minute and 
mixed with 10 c.c. of a solution of potassic permanganate, and then again 
boiled until manganese dioxide begins to separate. The liquid is now 
cleared by the cautious addition of pure ferrous sulphate, heated to 85 C., 
and mixed with 75 c.c. of ammonium molybdate solution at 27 C. After 
shaking for five minutes in a whirling apparatus, the precipitate is washed 
with solution of ammonic sulphate until the washings give no colouration 
with ammonic sulphide, and then dissolved in a mixture of 5 c.c. of 
ammonia and 25 c.c. of water. The solution is now mixed with 10 c.c. of 



222 VOLUMETKIC ANALYSIS. 66. 

strong sulphuric acid, diluted to 200 c.c., and reduced with zinc. The 
solution is then titrated with permanganate. The volume of the latter 
which represents 1 gm. of Pe equals 0'0172444 gm. of P. 

8. Estimation of Sulphur in Iron and Steel. The necessary 
solutions for this method are 

Standard iodine, 1 c.c. of which equals 1 m.gm. of S, made by 
dissolving 7 '9 gm. of pure iodine with 16 gm. of potassic iodide 
in a liter of water. 

Standard sodic thiosulphate of corresponding strength this 
solution should also contain about 20 gm. of sodic bicarbonate in 
the liter. The two solutions are adjusted by titration with starch. 

Solution of caustic soda 280 gm. of good commercial hydrate 
are dissolved in a liter of water, and as the soda generally contains 
some substances capable of liberating iodine, a titration must be 
made for each lot of solution to find the constant for that 
particular solution. 

Process : 5 gm. of drillings are put into a 20 oz. flask, to which is fitted 
a rubber stopper with two holes, through one of which is a safety funnel 
and the other a blank pipette, the upper end of which is bent twice at right 
angles, and attached by a small rubber stopper to a f-in. bulb \J tube. 
Into this latter is put 7 c.c. of the caustic soda solution and the apparatus 
put together. 75 c.c. of dilute HC1 (2 acid 1 water) are poured through the 
funnel, and the flask placed over a burner and heated moderately until the 
steel is dissolved. The tube containing the soda solution is disconnected, 
poured and washed into a beaker containing 15 c.c. of dilute H-SO 4 (1 : 4) 
mixed with as much iodine solution as will be in excess of the sulphur 
expected. The mixture is diluted to about 200 c.c. and at once titrated 
with thiosulphate and starch. The number of c.c. of iodine required, plus 
the caustic soda constant, multiplied by 20 will give the percentage of 
sulphur. 

LEAD. 

Pb = 206-4. 

1 c.c. -3^- permanganate =0*01032 gin. Lead. 
1 c.c. normal oxalic acid = 0'1032 gm. ,, 
Metallic iron x 1'846= 

Double iron salt x 0'263 = 

66. THE accurate estimation of lead is in most cases better 
effected by weight than by measure ; th^re are, however, instances 
in which, the latter may be used with advantage. 

1. As Oxalate (Hem pel). The acetic lead solution, which must contain 
no other body precipitable by oxalic acid, is put into a 300 c.c. flask, and 
a measured quantity of normal oxalic acid added in excess, the flask filled to 
the mark with water, shaken, and put aside to settle ; 100 c.c. of the clear 
liquid may then be taken, acidified with sulphuric acid, and titrated with 

rmanganate for the excess of oxalic acid. The amount so found multiplied 
3, and deducted from that originally added, will give the quantity 
combined with the lead. 

Where the nature of the filtrate is such that permanganate cannot be used 



66. LEAD. 223 

for titration, the precipitate must be collected, well washed, dissolved in 
dilute nitric acid, with a considerable quantity of sodic acetate, sulphuric 
acid added, and titrated with permanganate. 

In neither case are the results absolutely accurate, owing to the slight 
solubility of the precipitate, but with careful manipulation the error need 
not exceed 1 per cent. The error is much increased in the presence of 
ammoniacal salts. 

The technical analysis of red lead is best made as follows : 

2*064 gm. ( 7 V eq. of Pb) are placed in a 300 c.c. porcelain basin, and 20 or 
30 c.c. nitric acid sp. gr. 1'2 poured over it, then warmed gently with stirring. 
In a few minutes the lead oxide is dissolved and the peroxide left insoluble. 
50 c.c. of oxalic acid are added and the mixture boiled : this decomposes 
and dissolves the peroxide, leaving undissolved uny adulterant such as baryta, 
lead sulphate, oxide of iron, gypsum, or sand. While still hot f permanganate 
is added in moderate portions until the colour is permanent for a few seconds. 
The volume of permanganate deducted from 50 gives direct the percentage 
of lead existing as peroxide. 

The total lead may be found in the same solution b}' removing the excess 
of permanganate with a drop or two of oxalic acid, neutralizing with ammonia, 
adding a good excess of ammonic or sodic acetate, and titrating with bichro- 
mate as described in this section. 

Lead acetates in crystals or in solution may readily and with 
tolerable accuracy be titrated direct with normal oxalic acid. The 
best effects are obtained however by adding the lead solution 
(diluted and rendered clear by a little acetic acid) from a burette 
into the oxalic acid contained in a flask or beaker, warmed by 
a water-bath. The addition of the lead solution is continued with 
shaking and warming until no further precipitation takes place. 

Another method for acetates is to precipitate the lead with 
a slight excess of normal sulphuric acid in a 300 c.c. flask, fill 
to the mark, estimate the excess of H 2 S0 4 in 100 c.c. by weight, 
then calculate the combined acid into lead ; then by titrating 
another portion for acidity with phenolphthalein, the proportion 
of acetic acid can be obtained by deducting the free H 2 S0 4 from 
the total acid found. 

2. As Chromate (Schwarz). The lead is precipitated as chromate, 
<well washed, and digested with a weighed excess of double iron salt and 
hydrochloric acid; the resulting solution contains ferric and chromic 
chlorides, together with lead chloride, and undecomposed iron salt. The 
quantity of the last is found by permanganate, and deducted from the 
original weight; the remainder, multiplied by the factor 0'263, will give the 
weight. of lead. 

The difficulty with this method is the end-point, owing to the 
confusion produced by the yellow chromate. Diehl (Z. a. C. 1880, 
-306) modified the process by precipitating the lead with excess 
of bichromate and estimating the excess by thiosulphate, but this 
again is open to the objection of an indistinct end-point. 

Cushman and Hayes Campbell (Journ. Amer. Chem. Sac. 
xvii. 901) have, however, modified the process so as to be 
workable. Their method consists in titrating the solution after 



224 VOLUMETRIC ANALYSIS. 66, 

filtering off the precipitated lead chromate, with a standard 
solution of ammonio-ferrous sulphate, using ferricyanide as an 
outside indicator, under exactly the same conditions observed in 
standardizing bichromate solutions. The bichromate solution is 
made up of convenient empirical strength, and standardized 
against a weighed amount of pure ammonio-ferrous sulphate. 
Slightly more than the equivalent weight of the latter salt is then 
weighed out and dissolved in a liter of water, with the addition 
of a few drops of sulphuric acid. The solution is transferred to- 
a stock bottle into which is immediately poured a sufficient 
quantity of some light paraffin oil to form a layer over the solution r 
thus protecting it from oxidation. The stock bottle is fitted with 
a syphon tube and pinch-cock, so that the solution can be drawn 
out when needed. With this arrangement change in strength 
of the ammonio-ferrous sulphate solution takes place very slowly,, 
while, as a few moments only are required to titrate it against the 
standard bichromate, its exact strength can be easily determined 
from day to day. 

Process : About 1 gm. of finely pulverized ore is digested in a casserole or 
evaporating dish with 15 c.c. of a mixture of two parts of nitric acid and 
one part sulphuric acid, until decomposition is complete. 10 c.c. more 
of sulphuric acid are now added, and the liquid evaporated until it fumes 
freely. Cool, dilute with 10 c.c. of dilute sulphuric acid (1 10), and then 
add gradually 40 c.c. of water. Heat to boiling, filter, and wash by 
decantation with dilute sulphuric acid (1 10), getting as little of the lead 
sulphate on the filter as possible. To the residue in the dish add 20 c.c. 
of strong ammonia, then make slightly acid with acetic acid. Boil until the 
lead sulphate is dissolved, then pour the liquid through the filter, having first 
moistened the paper with ammonia. Wash the filter with water containing 
ammonic acetate in solution, and finally once or twice with hot water. 
Cool the filtrate, and run in from a burette an excess of standard bichromate 
solution, stirring until the precipitate settles rapidly and the supernatant 
liquid has a yellow colour. Allow to settle for a few minutes, then filter, 
under pressure if possible ; wash a few times, and titrate the filtrate against 
the standard ammonio-ferrous sulphate. 

After a little practice the method can be carried out as above detailed 
in about thirty minutes. In case the ore is known to be free from bismuth 
and antimony, the method can be materially shortened. Instead of bringing 
the ore into solution with a mixture of nitric and sulphuric acids, nitric acid 
alone is used. After solution the acid is neutralized with an excess of 
ammonia, and "then made acid with acetic acid : this dissolves any lead 
sulphate that has been formed. This solution is then immediately titrated 
with the bichromate and ammonio-ferrous sulphate solutions exactly as 
described above. In general it may be said that the results are a trifle low. 
The mean of the amount of lead recovered in twenty determinations. was 
99'6 per cent, of that taken. 

3. Alkalimetric Method (M o h r) . The lead is precipitated as carbonate 
by means of a slight excess of ammonic carbonate, together with free 
ammonia : the precipitate well washed, and dissolved in a measured excess 
of normal nitric acid; neutral solution of sodic sulphate is then added 
to precipitate the lead as sulphate. "Without filtering, the excess of nitric 
acid is then estimated by normal alkali, each c.c. combined being equal to 
0'1032 gm. of lead. 




66. LEAD. 225 



4. As Sulphide (Casamaj or). The lead, if not in a state convenient 
for titration, is separated as sulphate, well washed, and while still moist is 
dissolved in alkaline tartrate solution exactly as in the case of copper (see 
58.5) ; the precipitated sulphide separates very freely, and if the operation 
is performed in a white basin the end-point is easy of detection. 

The chief drawback to the method is the instability of the sulphide 
solution, which necessitates a fresh standardizing with known quantities of 
metal every day. 

5. Estimation of Lead in presence of Tin. For technical purposes 
this may be readily done as follows : 

The alloy is treated with nitric acid, by which means the lead is dissolved, 
and the tin rendered insoluble as stannic acid. The excess of nitric acid is 
removed by a very faint excess of sodic hydrate, then slightly acidified with 
acetic acid. The solution is diluted, so that it contains not less than half 
a per cent, of lead. It is then titrated with a standard solution of potassic 
ferrocyanide containing 10'2 gm. per liter, Avhich has been standardized 
against a lead nitrate solution containing 15'987 gm. per liter, using drops 
of ferric chloride solution on a white plate as indicator. 

6. Colorimetric estimation. Where there is no other metal 
than lead present, simple addition of freshly made sulphuretted 
hydrogen water in the presence of weak acetic acid as suggested 
by Miller gives good results, comparison being made with 
a standard solution of lead acetate containing 0*1831 gm. per liter. 
Each c.c. = O'OOOl gm. lead. The estimation is made in colourless 
glass cylinders in the same way as described for copper or iron 
58, 64, taking care that the comparative tests are made under 
precisely the same conditions. 

7. Leadin Citric and Tartaric Acids, etc. Warington has worked 
out the best method of ascertaining the proportions of lead in 
these commercial acids, and shows that ammonium sulphydrate 
is to be preferred to sulphuretted hydrogen for the process, 
inasmuch as the tint produced is much more uniform throughout 
a long scale, and very free from turbidity. Warington's 
description of the method is as follows : 

The depth of tint produced for the same quantity of lead present is far 
greater in an ammouiacal tartrate or citrate solution than in the same 
volume of water ; it is quite essential, therefore, if equality of tint is to be 
interpreted as equality of lead, that all comparisons should be between two 
citrate and tartrate solutions, and not between one of these and water. 

To carry out the method it is necessary to have solutions of lead-free 
tartaric and citric acid supersaturated with pure ammonia ; these solutions 
should develop no colour when treated with ammonium sulphydrate. 
A convenient strength is 100 gm. of acid in 300 c.c. of final solution.* 

Of the tartaric or citric acid to be examined, 40 gm. are taken and dissolved 
in a little water ; warm water is most convenient for crystal and cold for 
powder ; the solution is best prepared in a flask. To the cold solution pure 
strong ammonia is gradually added till it is in slight excess ; the final point 
is indicated in the case of tartaric acid by the solution of the acid ammonium 

* Tlie standard lead solutions are made by dissolving T6 gin. of crystallized lead 
nitrate dried over sulphuric acid in a liter of water, each c.c. =0'001 gm. Pb. A weaker 
solution is also made by diluting 100 c.c. of this to a liter. 

Q 



226 VOLUMETRIC ANALYSIS. 67. 

tart rate first formed ; in the case of citric acid it is conveniently shown by 
a fragment of turmeric paper floating in the liquid. When an excess 
of ammonia is reached the liquid is cooled, diluted to 120 c.c., and filtered. 

As a preliminary experiment 10 c.c. are taken, diluted to 50 c.c. in the 
measuring cylinder, and placed in a Nesslerizing glass, one drop of ammonium 
sulphydrate solution added, and the whole well stirred ; the colour developed 
indicates what volume of solution should be taken for the determination, this 
volume may range from 5 c.c. to 50 c.c. If less than 50 c.c. are taken the 
volume is brought to 50 c.c. with water, and one drop of ammonium 
sulphydrate is then added. 

The tint thus adopted has now to be matched with the pure solutions. 
A volume of the pure ammoniacal tartrate or citrate, identical with that 
taken of the acid under examination, receives a measured quantity of lead 
solution from the burette, the -volume is brought to 50 c.c., it is placed 
in a Nesslerizing glass, and receives one drop of ammonium sulplrydrate ; 
the experiment is repeated till a match is obtained. As in the previous 
method, the best comparison of tints is obtained by making finally three 
simultaneous experiments, one with the acid under examination, the other 
two with pure acid containing slightly varying amounts of lead, the aim 
being that the tint given by the acid to bo analyzed shall lie within this 
narrow scale. In following this method, considerable use has to be made of 
the weaker of the two lead solutions alread} r mentioned. 

The whole time required for a determination of lead by this method now 
given is about 1| hour; this time will be somewhat shortened as the 
operator becomes familiar with the tints produced by varying proportions 
of lead. If traces of copper or iron are present, any interference on their 
part may be removed by adding to the alkaline solution a few drops of 
potassic cyanide solution. 

MANGANESE. 

Mn=55, MnO = 71, Mn0 2 =S7. 
Factors. 

Metallic iron x 0'63393=MnO. 

x 0-491 =Mn. 

x 0-7768 =Mn0 2 . 

Double iron salt 0-0911 =.MnO. 

Cryst. oxalic acid x 0-6916 =Mn0 2 . 

Double iron salt x 1 1 1 = MnO 2 . 

1 c.c. T ^ solutioii=0-00355 gm. MiiO or=0'00435 gm. MnO 2 . 

67. ALL the oxides of manganese, with the exception of the 
first or protoxide, when boiled with hydrochloric acid, yield 
chlorine in the following ratios : 



Mn 2 3 =l eq. 0= 2 eq. Cl. 
Mn 3 O=r.l eq. 0=2 eq. Cl. 
Mn 2 =1 eq. 0= 2 eq. Cl. 
Mn 3 =2 eq. 0= 4 eq. Cl. 
Mn 2 7 =5 eq. = 10 eq. Cl. 



The chlorine so produced can be allowed to react upon a known 
weight of ferrous salt; and when the reaction is completed, the 



67. M.VXGAXESE. 227 

unchanged amount of iron salt is found by permanganate or 
bichromate. 

Or, the chlorine may be led by a suitable arrangement into 
a solution of potassic iodide, there setting free an equivalent 
quantity of iodine, which is found by the aid of sodic thio- 
sulphate. 

Or, in the case of manganese ores, the reaction may take place 
with oxalic acid, resulting in the production of carbonic acid, 
which can be weighed as in Fresenius' and Wills' method, 
or the amount of unchanged acid remaining after the action can 
l)e found by permanganate.""' 

The largely increased use of manganese in the manufacture of 
steel has now rendered it imperative that some rapid and trust- 
worthy method of estimation should be devised, and happily this 
has been done simultaneously by two chemists, Pattinson and 
Kessler; both have succeeded in finding a method of separating 
manganese as dioxide, of perfectly definite composition. Pattinson 
found that the regular precipitation was secured by ferric chloride, 
and Kessler by zinc chloride. Wright and Menke have 
experimented on both processes with equally satisfactory results, 
but give a slight preference to zinc. Pattinson titrates the 
resulting MnO- with standard bichromate, and Kessler with 
permanganate. 

Pat tins on's method has been fully described (/. C. S. 1879, 
-365), and again with slight modifications in J. S. C. I. x. 337. 

1. Precipitation as MnO? and Titration with Bichromate 
(Pattinson). 

The author's own description of the method is as follows : 

This method depends upon the whole of the manganese being 
precipitated as hydrated dioxide by calcium carbonate, when 
chlorine or bromine is added to a solution of manganous salt 
containing also a persalt of iron or a salt of zinc, and under 
certain conditions of temperature, &c. We have reason to believe 
that this method is now adopted by many chemists both in private 
laboratories and in the laboratories of steel works; and we therefore 
think that the following description of it in its slightly modified 
form, as we now use it for determining manganese in manganiferous 
iron ores, manganese ores, spiegeleisen, ferromanganese, &c., will 
not be out of -place. 

* The literature of manganese compounds and their estimation has now become very 
-voluminous. The principal contributions to the subject are as follows : 
Wright and Menke J. C. JS. 1880, 22-48. 

Morawski and S t i n g 1 Jour. /. pract. Chem. xviii. 96. 
Volhard Anna! en, cxcviii. 318. 

Guyard Bull. Soc. Chim. [2] i. 88. 

Kessler Z. a. C. 1879, 1-14. 

Pattinson J. C. S. 1879, 365. 

Pattinson J. S. C. I. x. 337. 



228 VOLUMETRIC ANALYSIS. 67. 

Process: A quantity of the sample to be analyzed, containing not more 
than about 4 grains (0'25 gm.) of manganese, is dissolved in hydrochloric 
acid. In the case of spiegeleisen and ferromanganese, about 50 grains 
(3 4 c.c.) of nitric acid are afterwards added to oxidize the iron. In the 
case of manganese ores, ferromanganese, and manganese slags, which do not 
contain about as much iron as manganese, we add to the solution as much 
iron, in the form of ferric chloride, as will make the quantities of iron and 
manganese in the solution about equal. An excess of iron is no draw- 
back, except that a larger precipitate has afterwards to be filtered and 
washed. 

The excess of acid in the solution is then neutralized by the addition 
of calcium carbonate, which is added until a slight reddening of the solution 
is produced. The solution is then rendered very slightly acid by dropping 
into it just enough hydrochloric acid to remove the red colour. 

We then add in all cases 1 oz. (30 c.c.) of a solution of zinc chloride 
containing 7 grains (0'5 gm.) of metallic zinc per ounce. The liquid is then 
brought to the boiling point, and diluted with boiling water to about 10 oz. 
(300 c.c.). 

Two oz. (60 c.c.) of a solution of calcium hypochlorite, made by dissolving 
1500 grains of 35 per cent, bleaching powder in 100 oz. of water (about 33 gm. 
of bleaching powder per liter) and filtering, are then poured into the 
manganese solution; but we add to the hypochlorite solution, before pouring 
it into the manganese solution, just enough hydrochloric acid to give it 
a faint permanent greenish-fellow colour after gentle agitation. 

The object of this addition of acid is to prevent a precipitate forming 
when the hypochlorite is added, due to the alkalinity of this solution. 
When hydrochloric acid is added in this way to the solution of calcium 
hypochlorite, the manganese solution remains clear on the addition of the 
calcium hypochlorite, and any possible local precipitation of manganese in 
a lower state of oxidation than MnO' 2 is obviated. 

Finally, Ave add to the manganese solution about 45 grains (3 gm.) of 
calcium carbonate diffused in about half an ounce (15 c.c.) of boiling water. 
After the first evolution of carbonic acid has ceased, during which time the 
cover is kept on the beaker, the precipitate is stirred to make it collect 
together, and half a dram (2 c.c.) of methylated spirits of wine is added 
and it is again stirred. 

The precipitate is then thrown upon a large filter of English filtering paper 
and washed, at first with cold water until the greater part of the chlorine is 
removed, and afterwards, to make the washing more rapid, with warm \vater 
at about 150 P. (65 C.). It is washed until, after draining, a drop shaken 
down straight from the precipitate, by gently jolting the funnel, shows 
no indication of chlorine when tested with a strip of iodized starch-paper. 
As a matter of practice we always give two or three washings after there has 
ceased to be any indication of chlorine. 

By carrying out the process in the manner here described, the temperature 
of the liquid, immediately after the precipitation is complete, is about 
170 3 1\ (77 C.), and we find that the best and most constant results are 
obtained when the temperature after precipitation is near this point. 

1000 grains of an acidified solution of ferrous sulphate, containing about 
10 grains of iron 'per 1000 grains of the solution, and made by dissolving 
crystallized ferrous sulphate in a mixture of one part of monohydrated 
sulphuric acid and three parts of water, are then accurately measured off by 
a pipette and run into the beaker in which the precipitation was made. The 
precipitate, together with the filter paper, are then removed from the funnel 
and placed in the solution of ferrous sulphate in the beaker. The precipitate 
readily dissolves even in the cold (sometimes it may be necessary to add 
a little more acid to dissolve the ferric hydrate completely), the manganese 
dioxide converting its equivalent of ferrous sulphate into ferric sulphate. 



67. MANGANESE. 229 

A sufficient quantity of cold water is now added, and the ferrous sulphate 
still remaining is titrated with a standard solution of potassium dichromate. 

The exact amount of ferrous sulphate in 1000 grains of the ferrous 
sulphate solution is determined hy measuring off into a clean beaker another 
portion of 1000 grains, and titrating with standard dichromate solution. 
The difference between the amounts of dichromate solution required gives 
the quantity of ferrous sulphate oxidized by the manganese dioxide, and 
from this the percentage of manganese in the sample can be calculated. 

The ferrous sulphate solution should be standardized from day to day, as it 
undergoes slow oxidation on exposure to air. 

A solution of bromine in water may of course be used instead of the 
hypochlorite solution, in which case no acid is added to the bromine solution. 
"When using bromine a solution containing about 10 grains of bromine 
per ounce (about 22 gm. per liter) should be used, and 3 oz. of this solution 
(90 c.c.) used for precipitating about 4 grains of manganese. 

The unpleasantness of working with bromine may be mitigated, to some 
extent, by adding to the bromine solution before pouring it into the liquid 
containing the manganese, a few drops of a solution of sodium hydrate until 
nearly all, but not quite all, the bromine is taken up. If an excess of 
sodium hydrate were added to the bromine it would produce a precipitate on 
pouring it into the manganese solution, and this is to be avoided. 

We prefer to have both zinc and iron in solution with the manganese. 
When working with either of these alone we obtain all the manganese in 
the form of dioxide, but with iron alone there is a greater tendency to 
the formation of permanganate, than when zinc is also present. This point 
was also noticed by Wright arid Menke (J. C. S. Trans. 1880, 43). 
When zinc alone is present we have found that the precipitation of the 
dioxide does not take place so rapidly as when iron is also present. When 
both iron and zinc are used, there is very seldom any permanganate formed, 
if care is taken not to use an t unnecessarily large excess of chlorine or 
bromine, but occasionally there is a small quantity formed, especially if the 
precipitate is left to stand some considerable time before filtering. We have 
found that the addition of a very small quantity of alcohol immediately 
after the precipitation of the manganese is complete, entirely prevents the 
formation of. permanganate even when a large excess of chlorine has been 
used, and for this reason we make a practice of adding it. 

We find that when filtering paper has been wetted with the solution 
containing free chlorine or bromine and afterwards washed clean, it has no 
reducing action either upon potassium dichromate or upon ferric sulphate. 
The addition of the filter together with the precipitate to the solution 
'of ferrous sulphate therefore does not influence the result. 

We must point out that the presence of lead, copper, nickel, cobalt, and 
chromium in the substances under examination interferes with the accuracy 
of this method of testing manganese. 

It was found that so large a proportion as 1 per cent, of lead, copper, and 
nickel does not greatly interfere with the test, but the interference of cobalt 
and, especially of chromium, is serious. All these substances, except 
chromium, form, under the conditions of the test, higher oxides insoluble in 
water, which are precipitated with the manganese dioxide, and which 
oxidize ferrous sulphate to ferric sulphate ; whilst chromium forms some 
insoluble chromate which goes down with the manganese dioxide. 

Fortunately these metals rarely, if ever, occur in the ores of manganese 
or in spiegeleisen and ferromanganese in sufficient quantity to affect the 
practical accuracy of this test. 

This volumetric method cannot, however, be applied to the determination 
of manganese in alloys of these metals, such as ferrochrome or in ores 
containing these metals, without previously separating them from the 
solution containing the manganese. 



230 VOLUMETRIC ANALYSIS. G7. 

The method as above described is undoubtedly one of the best 
volumetric ones known for the estimation of manganese in various 
compounds and ores; but Saniter in criticising the method gives 
it credit for slightly low results, and advocates the standardizing 
of the bichromate, not upon iron, but upon a manganese oxide of 
known composition (/. S. C. I. xiii. 112). 

Atkinson ( J, S. C. I. v. 365) gives the following short descrip- 
tion of the method as practically in daily use in a large steel works. 

Weigh out 0'5 gm. or 0'6 gm. of an ore containing about 20 per cent, 
manganese, dissolve in 7 or 8 c.c. of strong HC1, and when dissolved, wash 
the whole, without filtering, into a large narrow-sided beaker. When cold 
it is neutralized with precipitated calcic carbonate, until the liquid assumes 
a reddish hue. 40 or 50 c.c. of saturated bromine water are added, and the 
mixture allowed to stand in the cold for half-an-hour. At the expiration of 
that time the beaker is nearly filled up with boiling water, and precipitated 
calcic carbonate added until there is no further effervescence, and part of the 
carbonate is evidently unacted upon. A small quantity of spirits of wine is 
then added, the whole well stirred, and the precipitate allowed to settle. The 
clear liquid is filtered off and fresh boiling water added to the residue in the 
beaker, a little spirits of wine being used to reduce any permanganate which 
is formed. The filtration and washing are repeated until the nitrate when 
cooled no longer turns iodized starch-paper blue. During the washing about 
1'9 to 2' 5 gm. of pure granular ferrous-ammonium sulphate are weighed 
out, washed into the beaker in which the precipitation took place, and about 
30 to 50 c.c. of dilute sulphuric acid added. The filter containing the pre- 
cipitated MnO 2 is then placed in the beaker, and the latter is quickty dissolved 
by the oxidation of a portion of the ferrous salt into ferric sulphate. The 
remaining ferrous iron is then titrated with potassic bichromate in the usual 
way. The difference in the number of c.c. of bichromate used from the 
number which the original weight of the ferrous-ammonium sulphate would 
have required if directly titrated, is a measure of the quantity of MnO- 
present. For rapidity and simplicity this volumetric process leaves nothing 
to be desired ; duplicate experiments agree within very narrow limits ; and 
if the assumption is accepted that the presence of ferric chloride enables the 
complete oxidation of the manganese to the state of peroxide, no other 
process can compete with it. 

Pattinson prefers to use bleach solution to bromine, because 
the formation of permanganate is more easily seen. In any case 
not more than a trace of permanganate should be formed, and if 
the first experiment shows this to be the case, another trial must 
be commenced with less oxidizing material. 

J. "W. Westmoreland, in a communication to me, describes 
a modified method which is designed to overcome some objections 
raised against the above processes. 

With ferro-manganese and ores containing about 50 to 60 c of Mn about 
0*4 gm. is taken ; ores with 40 % ' 5 g m - '> manganiferous iron ores, with 
say about 20 % each of Fe and Mn, 0'75 gm.; spiegeleisen and silicospiegels, 
with about 25 % Mn, the same. 

The material having been brought into solution by any of the methods 
described, is concentrated to a small bulk in a large conical beaker. 
A solution of ferric chloride, containing about the same amount of iron as 
there is approximately of Mn, is added, together with a solution of zinc 



67. MANGANESE. 231 

chloride, containing about 0'5 gm. of Zn. The excess of acid is then 
neutralized with caustic potash, so that the bulk of liquid is about 80 c.c., 
to this is added about 60 c c. of saturated bromine water, more for 
ferro-manganese, less for manganiferous iron ores, and zinc oxide emulsion * 
is graduall}' dropped in with shaking, until the Pe and Mn are pre- 
cipitated, care must be taken to avoid a large excess of zinc oxide, the 
beaker is then filled up with boiling tap-water, and the clear liquid poured 
through a filter, previously adding a few drops of alcohol. The beaker is 
then filled with boiling water five times in succession, the precipitate being 
stirred up with the hot water each time of washing and allowed to settle. 
It is then brought on the filter, and again freely washed with boiling 
distilled water. The filter and contents are then transferred to the beaker, 
an excess of acid solution of ferrous sulphate added, and when the precipitate 
is dissolved the liquid is diluted with cold distilled water, and the excess of 
ferrous iron estimated at once with permanganate. The value of the iron 
solution in metallic iron is found by titrating the same volume of iron 
solution as has actually been used for dissolving the Mn precipitate, and the 
Fe oxidized multiplied by 0'491 = Mn. 

It is absolutely necessary, in order to get accurate results, to 
wash the precipitate as thoroughly as mentioned. 

2. By Precipitation with Potassic Permanganate (G-uyard). 

If a dilute neutral or faintly acid solution of manganese salt be 
heated to 80 C. and permanganate added, hydrated MnO 2 is pre- 
cipitated, and the end of the reaction is known by the occurrence of 
the usual rose colour of permanganate in excess. The reaction is 
exact in neutral solutions. Any large excess of either HC1 or H' 2 S0 4 
causes irregularity, as also do ferric or chromic salts; nickel, cobalt, 
zinc, alumina, or lime, in moderate quantity are of no consequence. 

This method is of easy execution, and gives good results in cases 
where it can be properly applied, but such instances are few. 

Process : 1 or 2 gm. of the manganese compound are dissolved in aqua 
refjia, boiled a few minutes, the excess of acid neutralized with alkali, then 
diluted largely with boiling water (1 or 2 liters), kept at a temperature 
of 80 C., arid standard permanganate added so long as a brownish precipitate 
forms, and until the clear supernatant liquid shows a distinct rose colour. 
2 eq. of permanganate = 3 eq. of manganese, therefore 1 c.c. of T ^ solution = 
0-0016542 gm. of Mn. 

Volhard's method. This is now largely used by Continental 
chemists, the details of the original process being as follows : 

A quantity of material is taken so as to contain from 0'3 to 0'5 gm. Mn, 
dissolved in hydrochloric or nitric acid, evaporated in porcelain to dryness, 
first adding a little ammonia nitrate, then heated over the flame to destroy 
organic matter. The residue is digested with HC1, adding a little strong 
H-SO 4 , and again evaporated to dryness, first on the water-bath, then with 
greater heat till vapours of SO 3 occur. It is then washed into a liter flask 
and neutralized with sodic hydrate or carbonate : sufficient pure zinc oxide, 
made into a cream, is added to precipitate all the iron. The flask is filled to 

* The emulsion of zinc oxide may, of course, be readily made by rubbing down pure 
zinc oxide in water so as to be about the consistence of cream. Emmertpn (Trans. 
Antcr. Inst. Min. Eng. x. 201) suggests the following method of preparing this reagent. 
Dissolve ordinary zinc white in HC1, add the powder until there remains some 



232 VOLUMETRIC ANALYSIS. 67. 

the mark, shaken, and 200 c.c. filtered off into a boiling flask, acidified with 
2 drops of nitric acid, sp. gr. 1'2, heated to boiling, and titrated with 
T ^j- permanganate whilst still hot. 

Blair (Chem. Anal. Iron, 2nd edit.) for practical working recommends 
dissolving the material in HC1 and H 2 S0 4 , evaporate to dryness until fumes 
of the latter escape; allow to cool, add water, and heat till sulphates are 
dissolved. Wash into a 300 c.c. flask, add solution of Na 2 CO 3 until the 
precipitate, which at first forms, dissolves only with difficulty. Then add 
slowly the zinc oxide emulsion, shaking well after each addition, till the iron 
precipitate curdles ; after the precipitate has settled, there is left a slightly 
milky upper liquid. Fill the flask to the mark with water, and agitate well 
by pouring the contents of the flask back and forward into a dry beaker. 
Finally filter off 200 c.c. and titrate Avith permanganate as before described, 
first adding 2 drops of HNO 3 . The permanganate should be added slowly 
from the burette, shaking after each addition to facilitate the collection 
of the precipitate and avoid an excess of permanganate. If the solution 
during the process should cool too much, it should be heated up to near 
boiling again. 

Saniter recommends that any iron or chromium should be first separated 
by ammonia and ammonic acetate, and the manganese precipitated with 
bromine and ammonia. This precipitate is, after ignition, dissolved in 
hydrochloric acid, and neutralized with zinc oxide suspended in water, any 
excess of the latter being dissolved by adding nitric acid drop by drop. 

Another variation of this method is given (Jour. Ainer. Chem. 
Soc. xviii. 228) by G. E. Stone, in some criticisms on a previous 
paper by M. Auchy. 

The material taken should contain 0'05 to 0'15 Mn. If an alloy, dissolve 
in HNO 3 (sp.'gr. ri) ; if an ore or cinder, in HC1, and boil with a little 
KC1O 3 ; use but small excess of acid. Cool and wash into a 500 c.c. flask 
with cold water, then add zinc emulsion until precipitate curdles ; the 
change is sharp and distinct. Dilute to mark, shake well and pour into 
a beaker ; allow to settle ; measure 100 c.c. into a 4-inch casserole, dilute 
to about 200 c.c., heat nearly to boiling, and titrate with permanganate, 
1 c.c. of which=0 001 gin. Mn (1'99 gin. K-MnO 4 per liter). The greater part 
of the permanganate should be added at once with vigorous stirring. The 
Mn in spiegels is easily obtained in half an hour ; ores somewhat longer, 
as more difficult to dissolve. 

There are many other volumetric methods in use for estimating 
manganese either as binoxide or metal, among which may be 
mentioned that of Chalmers Harvey (C. N. xlvii. 2) by 
stannous chloride, and that of Williams (Trans. Amer. Inst. of 
Mining Engineers, x. 100), which consists in separating MnO 2 
from a nitric solution by potassic chlorate, dissolving in excess of 
standard oxalic acid, and estimating the excess by permanganate. 

A critical paper on this process, accompanied with the results of 
experiment, is contributed by Macintosh (C. N. 1. 75). Also 
another by Hintz (Z. a. C. xxiv. 421 438) reviewing a large 
number of volumetric methods for manganese, but as none of them 

undissolved, then add a little bromine water; heat the mixture, filter and precipitate 
the zinc oxide from the filter with the slightest possible excess of ammonia. Wash 
thoroughly by decantation, and finally wash into an appropriate bottle with enough 
water to give a proper consistence. By this method a very finely divided oxide is 
obtained, owing to its not being dried. 



67. MANGANESE. 233 

are more accurate or convenient than the methods here given, the 
details are omitted. 

3. Estimation of Manganese in small quantities (Chatard). 

This method depends upon the production of permanganic acid 
by the action of nitric acid and lead peroxide, originally used by 
Crum as a qualitative test. The accuracy of the process as 
a quantitative one can, however, only be depended on when the 
quantity of manganese is very small, such as exists in some 
minerals, soils, etc. 

The material to be examined is dissolved in nitric acid and 
boiled with lead peroxide, by which means any manganese present 
is converted to permanganate ; the quantity so produced is then 
ascertained by a weak freshly made standard solution of oxalic 
acid or ammonic oxalate. 

The process gives good results in determining manganese in 
dolomites and limestones, where the proportions amount to from 
yy to 2 per cent. In larger quantities the total conversion of the 
manganese cannot be depended on. 

Thorpe and Hambly ( J. C. S. liii. 182) found that the final 
point in the titration with ammonic oxalate was apt to be obscured 
by the precipitation of lead carbonate, and they suggest a modifi- 
cation which consists in using some dilute sulphuric acid with 
the lead peroxide and nitric acid during the oxidation of the 
manganese ; no lead then passes into solution, and the filtered 
liquid remains perfectly clear on titration. These operators found 
the method quite trustworthy for quantities of manganese below 
O'Ol gm., and carried out as follows : 

Process : To the manganese solution, which must be free from chlorine 
and not too dilute, say about 25 c.c., add 5 c.c. of nitric acid (sp. gr. 1'4), 
2 3 gm. of lead peroxide, and 1020 c.c. of dilute sulphuric acid (1 : 2). 
Boil gently for about four minutes, wash down the sides of the flask with 
hot water, and continue the boiling for half a minute. Allow the lead 
sulphate and peroxide to subside and filter at once (best with filter pump 
through asbestos, previously ignited and washed with dilute H 2 SO 4 ). Wash 
the residue in fl-ask with boiling water by decantation, heat the clear filtrate 
to 60 C., and titrate with T ^ ammonic oxalate. 

Peters avails himself of this method for estimating manganese 
in pig iron or steel, by weighing O'l gm. of the sample and boiling 
in 3 or 4 c.c. of nitric acid until solution of the metal is complete, 
adding O2 or 0'3 gm. PbO 2 , and again boiling for two or three 
minutes, without filtering off the insoluble graphite, if such should 
be present. The solution is then cooled, filtered through asbestos 
into a suitable graduated tube, and the colour compared with 
a standard solution of permanganate contained in a similar tube. 

The standard permanganate is best made by diluting 1 c.c. of 
Y^ solution with 109 c.c. of water; each c.c. will then represent 
'00001 gm. Mn. It has been previously mentioned that accurate 



234 VOLUMETRIC ANALYSIS. 67. 

results by this method can only be obtained by using very small 
quantities of material. Peters finds this to be the case, and hence 
recommends, that for irons containing from O10 to 0'35 per cent, 
of Mn Ol gm. should be operated upon ; when from 0~8 to 1 
per cent, is present, 0*1 gm. of the sample is weighed and one- 
fourth of the solution only treated with PbO' 2 ; in still richer 
samples proportionate quantities must be taken. As a guide, it is 
well to assume, that when the amount of iron taken yields a colour 
equal to 25 35 c.c. of the standard, the whole of the Mn is 
oxidized. The actual amount of manganese in any test should not 
exceed half a milligram (G. N. xxxiii. 35). 

4. Teslinical EXE mination of Manganese Ores used for Bleaching- 
Purposes, etc. 

The ore, when powdered and dried for analysis, rapidly absorbs 
moisture on exposing it to the air, and consequently has to be 
weighed quickly ; it is better to keep the powdered and dried 
sample in a small light stoppered bottle, the weight of which,, 
with its contents and stopper, is accurately known. About 1 or 2 
gm., or any other quantity within a trifle, can be emptied into- 
the proper vessel for analysis, and the exact quantity found by 
reweighing the bottle. 

A hardened steel or agate mortar must be used to reduce the- 
mineral to the finest possible powder, so as to insure its complete 
and rapid decomposition by the hydrochloric acid. 

Considerable discussion has occurred as to the best processes- 
for estimating the available oxygen in manganese ores, arising from 
the fact that many of the ores now occurring in the market contain 
iron in the ferrous state ; and if such ores be analyzed by the usual 
iron method with hydrochloric acid, a portion of the chlorine 
produced is employed in oxidizing the iron contained in the original 
ore. Such ores, if examined by Fresenius and Wills' method,, 
show therefore a higher percentage than by the iron method, since- 
no such consumption of chlorine occurs in the former process. 
Manufacturers have therefore refused to accept certificates of 
analysis of such ores when based on Fresenius and "Wills' 
method. This renders the volumetric processes of more importance,, 
and hence various experiments have been made to ascertain their 
possible sources of error. 

The results show that the three following methods give very 
satisfactory results (see Scherer and Eumpf, C. N. xx. 302; 
also Pattinson, Hid. xxi. 266; and Paul, xxi. 16). 

5. Direct Analysis by Distillation with Hydrochloric Acid. 

This is the quickest and most accurate method of finding the 
quantity of available oxygen present in any of the ores of manganese 
or mixtures of them. It also possesses the recommendation that the 



67. MANGANESE. 235 

quantity of chlorine which they liberate is directly expressed in the 
analysis itself ; and, further, gives an estimate of the quantity of 
hydrochloric acid required for the decomposition of any particular 
sample of ore, which is a matter of some moment to the manu- 
facturer of bleaching powder. 

The apparatus for the operation may be those shown in figs. 
37 and 38. For precautions in conducting the distillation 
see 39. 

Process : In order that the percentage of dioxide shall be directly 
expressed by the number of c.c. of r ^ thiosulphate solution used, 0'436 2:111. 
of the properly dried and powdered sample is weighed and put into the little 
flask ; solution of potassic iodide in sufficient quantity to absorb all the 
iodine set free is put into the large tube (if the solution containing T 2 7 eq. or 
33'2 gin. in the liter be used, about 70 or 80 c.c. Will in ordinary cases be 
sufficient) ; very strong hydrochloric acid is then poured into the distilling 
flask, and the operation conducted as in 39. Each equivalent of iodine 
liberated represents 1 eq. Cl, also 1 eq. MnO 2 . 

Instead of using a definite weight, it is well to do as before 
proposed, namely, to pour about the quantity required out of the 
weighed sample-bottle into the flask, and find the exact weight 
afterwards. 

Barlow (0. N. liii. 41) records a good method. of separating 
Mn from the metals of its own group as well as from alkalies and 
alkaline earths. 

For the quantitative estimation of Fe and Mn in the same 
solution as chlorides (other metals except Cr and Al may be present, 
but best absent), solution of iS r H 4 Cl is first added, then strong 
NH 4 HO in excess, boil, then hydrogen peroxide so long as a 
precipitate falls, boil for a few minutes, filter, wash with hot water, 
ignite, and weigh the mixed oxides together as Fe 2 3 + Mn 3 4 . 

The oxides are then distilled with HC1, and the amount of 
iodine found by thiosulphate. 

The weight of mixed oxides, minus the Mn 3 4 , gives the weight 
of Fe 2 3 . 

Pickering (/. 0. S. 1880, 128) has pointed out that pure 
manganese oxides, freshly prepared, or the dry oxides in very 
fine powder, may be rapidly estimated without distillation by 
merely adding them to a large excess of potassic iodide solution 
in a beaker, running in 2 or 3 c.c. of hydrochloric acid, when 
the oxides are immediately attacked and decomposed; the liberated 
iodine is then at once titrated with thiosulphate. Impure oxides, 
containing especially ferric oxide, cannot however be estimated in 
this way, since the iron would have the same effect as manganic 
oxide ; hence distillation must be resorted to in the case of all 
such ores, and it is imperative that the strongest hydrochloric acid 
should be used. 

Pickering's modified process is well adapted to the examination 
of the Weldon mud, for its available amount of manganese dioxide. 



236 VOLUMETRIC ANALYSIS. 67. 

6. Estimation by Oxalic Acid. 

The very finely powdered ore is mixed with a known volume of 
normal oxalic acid solution, sulphuric acid added, and the mixture 
heated and well shaken, to bring the materials into intimate contact 
and liberate the CO 2 . When the whole of the ore is decomposed, 
which may be known by the absence of brown or black sediment, 
the contents of the vessel are made up to a definite volume, say 
300 c.c., and 100 c.c. of the dirty milky fluid well acidified, 
diluted, and titrated for the excess of oxalic acid by permanganate. 
If, in consequence of the impurities of the ore, the mixture be 
brown or reddish coloured, this would of course interfere with the 
indication of the permanganate, and consequently the mixture in 
this case must be filtered ; the 300 c.c, are therefore well shaken 
and poured upon a large filter. When about 100 c.c. have passed 
through, that quantity can be taken by the pipette and titrated as 
in the former case. 

If the solution be not dilute and freely acid, it will be found 
that the permanganate produces a dirty brown colour instead of its 
well-known bright rose-red ; if the first few drops of permanganate 
produce the proper colour immediately they are added, the solution 
is sufficiently acid and dilute. 

If 4-357 gm. of the ore be weighed for analysis, the number of 
c.c. of normal oxalic acid will give the percentage of dioxide ; but 
as that is rather a large quantity, and takes some time to dissolve 
and decompose, half the quantity may be taken, when the per- 
centage is obtained by doubling the volume of oxalic acid. 

Example : The permanganate was titrated with normal oxalic acid, and it 
was found that 1 c.c.=0'25 c.c. of normal oxalic acid. 2'1T8 gm. of a rich 
sample of commercial manganese (pyrolusite) were treated with 50 c.c. of 
normal oxalic, together with 5 c.c. of concentrated sulphuric acid, until the 
decomposition was complete. The resulting solution was milky, but con- 
tained nothing to obscure the colour of the permanganate, and therefore 
needed no filtration. It was diluted to 300 c.c., and 100 c.c. taken for titra- 
tion, which required 6'2 c.c. of permanganate. A second 100 c.c. required 
G'3, mean 6'25, which multiplied by 3 gave 18'75 c.c.; this multiplied by 
the factor 0'25 to convert it into oxalic acid gave 4'68 c.c. normal oxalic, 
and this being deducted from the original 50 c.c. used, left 45'32 c.c.=90'64 
per cent, of pure manganic dioxide. 

This process possesses an advantage over the following, inasmuch 
as there is no fear of false results occurring from the presence of 
air. The analysis may be broken off at any stage, and resumed 
at the operator's, convenience. 

7. Estimation by Iron. 

The most satisfactory form of iron is soft " flower " wire, which 
is readily soluble in sulphuric acid. If a perfectly dry and un- 
oxidized double iron salt be at hand, its use saves time. 1 mol. 
-of this salt = 392, representing 43 '5 of MnO 2 , consequently, 1 gm. of 



67. MANGANESE. 2S7 

the latter requires 9 gm. of the double salt ; or in order that the 
percentage shall be obtained without calculation,, I'lll gm. of ore 
may be weighed and digested in the presence of free sulphuric acid, 
with 10 gm. of double iron salt, the whole of which would be 
required supposing the sample were pure dioxide. The. undecom- 
posed iron salt remaining at the end of the reaction is estimated by 
permanganate or bichromate ; the quantity so found is deducted 
from the original 10 gm., and if the remainder be multiplied by 10 
the percentage of dioxide is gained. 

Instead of this plan, which necessitates exact weighing, any 
convenient quantity may be taken from the tared bottle, as before 
described, and digested with an excess of double salt, the weight 
of which is known. After the undecomposed quantity is found by 
permanganate or bichromate, the remainder is multiplied by the 
factor O'lll, which gives the proportion of dioxide present, 
whence the percentage may be calculated. 

The decomposition of the ore may be made in any of the 
apparatus used for titrating ferrous iron. The ore is first put 
into the decomposing flask, then the iron salt and \vater, so as to 
dissolve the salt to some extent before the sulphuric acid is added. 
Sulphuric acid should be used in considerable excess, arid the 
flask heated by the spirit lamp till all the ore is decomposed ; 
the solution is then cooled, diluted, and the whole or part titrated 
with permanganate or bichromate. 

Example : 1 gin. of double iron salt was titrated with permanganate 
solution of which 21'4 c.c. were required. 

I'll I gm. of the sample of manganese was accurately weighed and digested 
with 8 gin. of iron salt, and sulphuric acid. After the decomposition, 8'8 c.c. 
of permanganate were required to peroxidize the undecomposed iron salt 
(=0'42 gm.), which deducted from the 8 gm. originally used left 7*58 gin. ; 
or placing the decimal point one place to the right, 75'8 per cent, of pure 
dioxide. 

In the case of using -^ bichromate for the titration, the following- 
plan is convenient : 100 c.c. of ^ bichromate = 3 '92 gm. of double 
iron salt (supposing it to be perfectly pure), therefore if 0'436 gm. 
of the sample of ore be boiled with 3 '92 gm. of the double salt 
and excess of acid, the number of c.c. of bichromate required 
deducted from 100 will leave the number corresponding to the 
percentage. 

Example : 0'436 gm. of the same sample as examined before was boiled 
with 3'92 gm. of double salt, and afterwards required 24 c.c. 'of T ^ bichro- 
mate, which deducted from 100 leaves 76 per cent, of dioxide, agreeing very 
closely with the previous examination. 

When using metallic iron for the titration (which in most cases is preferred) 
Pattinson proceeds as follows: 30 grn. of clean iron wire are placed in 
a suitable apparatus, with 3 oz. of dilute sulphuric acid, made by adding 
3 parts of water to one of concentrated acid. When the iron is quite dissolved, 
30 grn. of the finely powdered and dried sample of manganese ore to be 
tested are put into the flask, the cork replaced, and the contents again made 



238 VOLUMETRIC ANALYSIS. 68. 

to boil gently over a gas flame until it is seen that the whole of the black 
part of the manganese is dissolved. The Avater in the small flask is then 
allowed to recede through the bent tube into the larger flask, more distilled 
Avater is added to rinse out the small flask or beaker and bent tube, the cork 
well rinsed, and the contents of the flask made up to about 8 or 10 oz. with 
distilled water. The amount of iron remaining unoxidized in the solution 
is then ascertained by means of a standard solution of potassic bichromate. 
The amount indicated by the bichromate deducted from the total amount of 
iron used, gives the amount of iron which has been oxidized by the manga- 
nese ore, and from which the percentage of manganic dioxide contained in 
the ore can be calculated. Thus, supposing it were found that 4 grn. of 
iron remained unoxidized, then 30426 grn. of iron which have been 
oxidized by the 30 grn. of ore. Then, as 

5G : 43'5 : : 26 : 20'2 

the amount of dioxide in the 30 gru. of ore. The percentage is therefore 
67'33. Thus 

30 : 20-2 : : 100 : 67'33 

Grain weights are given in this example, but those who use the 
.gram system will have no difficulty in arranging the details 
.accordingly. 

MERCURY. 

Hg = 200. 

1 c.c. T \ solution = 0-0200 gm. Hg. 
-0-0208 gm. Hg 2 
= 0-0271 gm. HgCl 2 

Double iron salt x 0*5104 = Hg. 

x 0-6914 = HgCl 2 

1. Precipitation as Mercurous Chloride. 

68. THE solution to be titrated must not be warmed, and 
must contain the metal only in the form of protosalt. ~ sodic 
chloride is added in slight excess, the precipitate washed with the 
least possible quantity of water to ensure the removal of all the 
sodic chloride to the nitrate a few drops of chromate indicator are 
added, then pure sodic carbonate till the liquid is of a clear yellow 
colour, y 1 ^- silver is then delivered in till the red colour occurs. 
The quantity of sodic chloride so found is deducted from that 
originally used, and the difference calculated in the usual way. 

2. By Ferrous Oxide and Permanganate (Mohr). 

This process is based on the fact that when mercuric chloride 
(corrosive sublimate) is brought in contact with an alkaline solution 
of ferrous oxide in excess, the latter is converted into ferric oxide, 
while the mercury is reduced to mercurous chloride (calomel). 
The excess of ferrous oxide is then found by permanganate or 
^bichromate 

2HgCl 2 + 2FeCl 2 = Hg 2 Cl 2 + Fe 2 Cl 6 . 



68. MERCURY. 239 

It is therefore advisable in all cases to convert tlie mercury to be 
estimated into the form of sublimate, by evaporating it to dryness 
with nitro-liydrochloric acid ; this must take place, however, below 
boiling heat, as vapours of chloride escape with steam at 100 C. 
(Fresenius). 

Citric acid or free chlorine must be altogether absent during 
the decomposition with the iron protosalt, otherwise the residual 
titration will be inexact, and the quantity of the iron salt must be 
more than sufficient to absorb half the chlorine in the sublimate. 

Example : 1 gm. of pure sublimate was dissolved in warm water, and 
3 gm. of double iron salt added, then solution of caustic soda till freely 
ulkaline. The mixture became muddy and dark in colour, and was well 
shaken for a few minutes, then sodic chloride and sulphuric acid added, con- 
tinuing the shaking till the colour disappeared and the precipitate of ferric 
oxide dissolved, leaving the calomel white ; it Avas then diluted to 300 c.c. 
filtered through a dry filter, and 10D c.c. titrated with ^ permanganate, of 
which 13'2 c.c. were required 13'2 x 3=39'6, which deducted from 76'5 c.c. 
(the quantity required .for 3 gm. double iron salt), left 36'9 c.c.=r447 gm. 
of undecomposed iron salt, which multiplied by the factor 0'6914, gave 
1-0005 gm. of sublimate, instead of 1 gm., or the 36'9 c.c. may be multiplied 
by the ^ factor for mercuric chloride, which will give 1 gm. exactly. 



3. By Iodine and Thiosulphate (Hem pel). 

If the mercury exist as a protosalt it is precipitated by sodic 
chloride, the precipitate well washed and together with its filter 
pushed through the funnel into a stoppered flask, a sufficient 
quantity of potassic iodide added, together with ^ iodine solution 
(to 1 gm. of calomel about 2 '5 gm. of iodide, and 100 c.c. of ~ 
iodine), the flask closed, and shaken till the precipitate has 
dissolved 

Hg 2 Cl 2 + 6KI + 21 = 2HgK 2 P + 2KC1. 

The brown solution is then titrated with ^ thiosulphate till 
colourless, diluted to a definite volume, and a measured portion 
titrated with ~ iodine and starch for the excess of thiosulphate. 
1 c.c. T ^- iodine = 0'02 gm. Hg. 

Where the mercurial solution contains nitric acid, or the metal 
exists as peroxide, it may be converted into protochloride by the 
reducing action of ferrous sulphate, as in Mohr's method. The 
solution must contain hydrochloric acid or common salt in sufficient 
quantity to transform all the mercury into calomel. At least three 
times the weight of mercury present of ferrous sulphate in solution 
is to be added, then caustic soda in excess, the muddy liquid well 
shaken for a few minutes, then dilute sulphuric acid added in 
excess, and the mixture stirred till the dark-coloured precipitate 
lias become perfectly white. The calomel so obtained is collected 
on a filter, well washed, and titrated with T ^- iodine and thiosulphate 
as above. ' 



240 VOLUMETRIC ANALYSIS. 68. 

4. Direct Titration -with Sodic Thiosulphate (Scherer). 

The standard thiosulphate is made by dissolving -^ eq. = 12'4 
gm. of the salt in 1 liter of water. 

The reaction which takes place with thiosulphate in the case of 
mercurous nitrate is 



- Hg 2 S + Na 2 SO* + N*0 5 . 
With mercuric nitrate 

3Hg(N0 3 ) 2 + 2Na 2 S 2 3 - 2HgS.Hg(N0 3 ) 2 + 2Xa 2 80 4 + 23S' 2 5 . 
"With mercuric chloride 

SHgCl 2 + 2Na 2 S 2 3 + 2H 2 - 2HgS.HGl 2 + 2Xa 2 SO* + 4HC1. 

(a) Mercurous Salts. The solution containing the metal as a pro to- 
salt only is diluted, gently heated, and the thiosulphate delivered in from the 
burette at intervals, meanwhile well shaking until the last drop produces no 
brown colour. The sulphide settles freely, and allows the end of the reaction 
to be easily seen. 1 c.c. of thiosulphate 0'020 gm. Hg, or 0'0208 gm. 
Hg 2 0. 

(b) Mercuric Nitrate. The solution is considerably diluted, put into 
a stoppered flask, nitric acid added, and the thiosulphate cautiously delivered 
from the burette, vigorously shaken meanwhile, until the last drop produces 
no further } r ellow precipitate. Scherer recommends that when the greater 
part of the metal is precipitated, the mixture should be diluted to a definite 
volume, the precipitate allowed to settle, and a measured quantity of the 
clear liquid taken for titration ; the analysis may then be checked by a second 
titration of the clear liquid, if needful. 1 c.c. thiosulphate=0'015 gin. Hg, 
or 0-0162 gm. HgO. 

(c) Mercuric Chloride. With mercuric chloride (sublimate) the end of 
the process is not so easily seen. The procedure is as follows : The very 
dilute solution is acidified with hydrochloric acid, heated nearly to boiling, 
and the thiosulphate cautiously added so long as a white precipitate is seen 
to form ; any great excess of the precipitant produces a dirty -looking colour. 
Filtration is necessary to distinguish the exact ending of the reaction, for 
which purpose Beale's filter (fig. 23) is useful. 

Liebig's method is the reverse of that used for determining 
chlorides in urine, sodic phosphate being used as indicator in the 
estimation of mercury, instead of the urea occurring naturally in 
urine The method is capable of very slight application. 

5. As Mercuric Iodide (Personne), Compt. Rend. Ivi. 63. 

This process is founded on the fact that if a solution of mercuric 
chloride be added to one of potassic iodide, in the proportion of 
1 equivalent of the former to 4 of the latter, red mercuric iodide is 
formed, which dissolves to a colourless solution until the balance is 
overstepped, when the brilliant red colour of the iodide appears as 
a precipitate, which, even in the smallest quantity, communicates- 
its tint to the liquid. The mercuric solution must always be added 



68. MEUCUKY. 241 

to the potassic iodide ; a reversal of the process, though giving 
eventually the same quantitative reaction, is nevertheless much less 
speedy and trustworthy. The mercurial compounds to be estimated 
by this process must invariably be brought into the form of neutral 
mercuric chloride. 

The standard solutions required are decinormal, made as follows : 

Solution of Potassic iodide. 33*2 gm. of pure salt to 1 liter. 
1 c .c.=0-01 gin. Hg. or 0-01355 gm. HgCR 

Solution of Mercuric chloride 13 '537 gm. of the salt, with 
about 30 gm. of pure sodic chloride (to assist solution), are dissolved 
to 1 liter. 1 c.c. =0*1 gm. Hg. 

The conversion of various forms of mercury into mercuric chloride 
is, according to Personne, best effected by heating with caustic 
soda or potash, and passing chlorine gas into the mixture, which 
is afterwards boiled to expel excess of chlorine (the mercuric 
chloride is not volatile at boiling temperature when associated with 
alkaline chloride). The solution is then cooled and diluted to 
a given volume, placed in a burette, and delivered into a measured 
volume of the decinormal potassic iodide until the characteristic 
colour occurs. It is preferable to dilute the mercuric solution con- 
siderably, and make up to a given measure, say 300 or 500 c.c. ; 
and as a preliminary trial take 20 c.c. or so of iodide solution, 
and titrate it with the mercuric solution approximately with 
a graduated pipette ; the exact strength may then be found by 
using a burette of sufficient size. 

6. By Potassic Cyanide (Hannay). 

This process is exceedingly valuable for the estimation of almost 
all the salts of mercury when they occur, or can be separated, in 
a tolerably pure state. Organic compounds are of no consequence 
unless they affect the colour of the solution. 

The method depends on the fact that free ammonia produces 
a precipitate, or (when the quantity of mercury is very small) an 
opalescence in mercurial solutions, which is removed by a definite 
amount of potassic cyanide. 

The delicacy of the reaction is interfered with by excessive 
quantities of ammoniacal salts, or by caustic soda or potash ; but 
this difficulty is lessened by the modification suggested by Tuson 
and Xeison (J. C. S. 1877, 679). 

Chapman Jones (/. C. S. Ixi. 364) has further modified the 
process so as to make it easier to detect the end-point, and says of 
the method as worked by Tuson and Xeison: "Their general 
method consists in dissolving the mercury compound in acid, as 
may be convenient, adding a little ammonium chloride, and then 
potassic carbonate, until an opalescent precipitate appears. The 



242 VOLUMETKIC ANALYSIS. G8. 

cyanide solution is then added. They give experiments showing 
the trustworthiness of the process as applied to the nitrate, 
sulphate, acetate, oxalate, sebate, and citrate of mercury ; and 
state that the presence of nitrates, sulphates, chlorides, acetates, 
oxalates, citrates, and butyrates of potassium, sodium, calcium, 
and magnesium, and organic matter as far as tested, does not 
interfere with the accuracy of the method. 

From my experience, I cannot affirm that these methods of 
working are satisfactory. There is considerable uncertainty as to 
the end of the reaction, because less potassic cyanide will effect 
a clearance if longer time is allowed. 

These difficulties and uncertainties can, I find, be entirely 
eliminated, and the process reduced to a series of operations which 
are comparatively simple and rapid, by performing the titration in 
an entirely different manner from either variation suggested by 
the authors referred to. I employ a solution of mercuric chloride 
containing O'Ol gm. of metal per c.c., and a solution of crystallized 
potassic cyanide made by dissolving 7 gm. to the liter, the exact 
value of which is found by titrating it against the mercury solution. 
Strong ammonia diluted to ten times its bulk, and some diluted to 
fifty or a hundred times its bulk, are convenient. 

Process : If the mercury solution is not fit for titration, the metal is 
precipitated as sulphide, which, after washing, is washed off the filter and 
allowed to subside ; the clear water is then decanted off, and aqua regia 
added to the moist residue. The precipitate, with the paper it is on, might 
doubtless be treated direct with aqua regia, as Tuson and Neison found 
that organic matter, so far as ihey tried it, does not influence the result. 
To avoid the possibility of volatilizing the mercury salt, the aqua regia is 
allowed to act in the cold. In a few hours, sometimes in far less time, the 
residue is of a pure ^yellow colour, and the solution may be diluted and 
filtered. The solution, or an aliquot part of it, is then coloured distinctly 
with litmus, treated with successive small quantities of powdered potassic 
carbonate until alkaline, warming but slightly, and then rendered just acid 
with dilute hydrochloric acid, with subsequent boiling to remove the carbonic- 
anhydride. The mercury is not precipitated at all, unless the carbonic 
anhydride is boiled out before acidification. After cooling, the dilutest 
ammonia mentioned above is added, a drop at a time, until the litmus in the 
solution shows that the excess of acid is very slight, or in just insufficient 
quantity to produce a permanent precipitate. A quantity of cyanide 
solution, which is known to be in excess of that required, is added, and, as 
a guide for the first titration, the ammonia may be added until a slight 
precipitate is produced, and cyanide until the solution is cleared. Two or 
three drops (not more) of the 1 in 10 ammonia are introduced, and then 
more of the mercury solution is added until the permanent turbidity 
produced matches that obtained by adding O'l c.c. of the mercury solution 
to about the same bulk of water as the test, and containing approximately 
the same amounts of litmus and free ammonia. Each drop of the mercury 
solution added produces its maximum turbidity in a few seconds, and it can 
be seen at a glance, if the flasks are properly placed, whether this turbidity 
is equal to or less than the standard. In a few seconds more it is quite 
obvious whether the turbidity is permanent or is growing less. Too much 
free ammonia causes the precipitate to clot together, and so vitiates the 
result. The presence of the litmus tends, in my experience, to lessen the 



G9. NICKEL. 243 

error due to the variation in the state of aggregation of the precipitate 
when too much ammonia has been added. The turbidities so obtained will 
remain apparently unchanged for many hours. The (VI c.c. excess of 
mercury solution is of course allowed for in the calculation." 

NICKEL. 

69. THE estimation of this metal volumetrically has now 
become satisfactory, and we are indebted to T. Moore (C. N. Ixxii. 
92) for a much more perfect process than was given by him in the 
previous edition. The modified process consists in discarding the 
use of cupric ferrocyanide as the indicator, and substituting silver 
iodide in its place. Moore's own description of the method is as 
follows : 

"If to an ammoniacal solution of nickel containing Agl in 
suspension (silver iodide being almost insoluble in weak ammonia) 
there is added potassic cyanide, the solution will remain turbid so 
long as all the nickel is not converted into the double cyanide of 
nickel and potassium, the slightest excess of cyanide being indicated 
by the clearing up of the liquid, and, furthermore, this excess 
may be exactly determined by adding a solution of silver until the 
turbidity is reproduced. It is a fortunate circumstance that the 
complicated side-reactions existing in Parke's copper assay do not 
appear to take place with nickel solutions, at least not when the 
temperature is kept below 20 C. This is fully borne out by the 
fact that the potassic cyanide may be standardized on either silver 
or nickel solutions with equal exactness. In practice it has been 
found best to proceed in the following manner : 

Standard solution of Silver nitrate, containing about 3 gm. of 
silver per liter. The strength of this solution must be accurately 
known. 

Potassic iodide, 10 per cent, solution. 

Potassic cyanide, 22 to 25 gm. per liter. This solution must be 
tested every few days, owing to its liability to change. 

Standardizing- the Cyanide Solution. This may be accomplished in 
two ways: (a) on a solution of nickel of known metallic contents, or (6) on 
the silver solution. 

(a) First accurately establish the relation of the cyanide to the silver 
solution, by running into a beaker 3 or 4 c.c. of the former; dilute this with 
about 150 c.c. of water, render slightly alkaline with ammonia, and then add 
a few drops of the potassic iodide. Now carefully run in the silver solution 
until a faint permanent opalescence is produced, which is finally caused to 
disappear b\~ the further addition of a mere trace of cyanide. The 
respective volumes of the silver and cyanide solutions are then read off, and 
the equivalent in cyanide of 1 c.c. silver solution calculated. A solution 
containing a known quantity of nickel is now required. This must have 
sufficient free acid present to prevent the formation of any precipitate, on 
the subsequent addition of ammonia to alkaline reaction ; if this is not so, 
a little anmionic chloride may be added. A carefully measured quantity of 
the solution is then taken, containing about O'l gm. of nickel, and rendered 
distinctly alkaline with ammonia, a few drops of potassic iodide added, and 

R 2 



244 VOLUMETEIG ANALYSIS. 69. 

the liquid diluted to 150 or 200 c.c. A few drops of the silver solution are 
now run in, and the solution stirred to produce a uniform turbidity. The 
solution is now ready to be titrated with the potassic cyanide, which is 
added slowly and with constant stirring until the precipitate wholly 
disappears; a few extra drops are added, after which the beaker is placed under 
the silver nitrate burette, and this solution gently dropped in until a faint 
permanent turbidity is again visible ; this is now finally caused to dissolve 
by the mere fraction of a drop of the cyanide. A correction must now be 
applied for the excess of the cyanide added, by noting the amount of silver 
emplo^yed, and working out its value in cyanide from the data already 
found; this excess must then be deducted, the corrected number of c.c. 
being then noted as equivalent to the amount of nickel employed. 

(b) Having determined the relative value of the cyanide to the silver 
solution, and knowing accurately the metallic contents of the latter, then 
Ag x G'27196 gives the nickel equivalent. This method is quite as accurate 
as the direct titration. 

A modification of the above process, wherein one burette only 
is necessary, has been found very convenient, and lias given most 
excellent results. It is as follows : 

When a solution of potassic cyanide, containing a small quantity of 
silver cyanide dissolved in it, is added to an ammoniacal solution of nickel 
containing potassic iodide, it is seen that silver iodide is precipitated, and 
the turbidity thus caused in the solution continues to increase up to the 
point where the formation of the nickelo-potassic c} r anide is complete ; any 
further addition after this stage is reached will produce a clearing up of the 
liquid, until, at last, the addition of a single drop causes the precipitate to 
vanish. This final disappearance is most distinct, and leaves no room for 
doubt. Such a solution may be prepared by dissolving 20 to 25 gm. of 
potassic cyanide in a liter of water, adding to this about 0'25 gm. silver 
nitrate previously dissolved in a little water. For large quantities of nickel 
the quantity of silver may advantageously be diminished, and vice versa. 
The value of the cyanide is best ascertained in the manner already 
described, on a nickel solution. 

Small quantities of cobalt do not seriously affect the results, but 
It must be remembered that it will be estimated with the nickel ; 
its presence is at once detected by the darkening of the solution. 
Manganese or copper render the process valueless, so also does 
zinc ; the latter, however, in alkaline pyrophosphate solution 
exercises no influence. In the presence of alumina, magnesia, or 
ferric oxide, citric acid, tartaric acid, or pyrophosphate of soda 
may be employed to keep them in solution. The action of iron is 
somewhat deceptive, as the solution, once cleared up, often 
becomes troubled again on standing for a minute ; should this 
occur, a further addition of cyanide must be given until the liquid 
is rendered perfectly limpid. The temperature of the solution 
should never exceed 20 C. : above this the results become 
irregular. The amount of free ammonia has also a disturbing 
influence ; a large excess hinders or entirely prevents the reaction ; 
the liquid should, therefore, be only slightly but very distinctly 
alkaline. A word of caution must be given regarding the potassic 
cyanide, as many of the reputed pure samples are very far from 



70. NITRATES. 245 

being so. The most hurtful impurity is, however, sulphur, as it 
gives rise to a darkening of the solution, owing to the formation of 
the less readily soluble silver sulphide ; to get rid of the sulphur 
impurity it is necessary to thoroughly agitate the cyanide liquor 
with oxide of lead, or, what is far preferable, oxide of bismuth. 

As regards the exactness of the methods, it may be said, that, 
after a prolonged experience, extending over many thousands of 
estimations, they have been found to be more accurate and reliable 
than either the electrolytic or gravimetric methods, and when time 
is a consideration the superiority is still more pronounced. The 
employment of organic acids or sodic pyrophosphate in the case 
when iron, zinc, etc., are present, allows the operator to dispense 
with the tedious separation which their presence otherwise entails-; 
and this is a matter of considerable importance in the assay of 
nickel mattes or German silver." 



NITROGEN AS NITRATES AND NITRITES. 

Nitric Anhydride. 

:N T2 5 =108. 

Nitrous Anhydride. 



Normal acid x 0-0540 = ]S T2 5 

Ditto x 0-1011 =KN T 3 

Metallic iron x 0-3750 = HNO 3 

Ditto x 0-601 8 = KN0 3 

Ditto x 0-3214 = N 2 5 

70. THE accurate estimation of nitric acid in combination. 
presents great difficulties, and can only be secured by indirect 
means ; the methods here given are sufficient for most purposes. 
Very few of them can be said to be simple, but it is to be feared 
that no simple process can ever be obtained for the determination 
of nitric acid in many of its combinations. 



1. Gay L.US sac's Method modified by Abel (applicable only 
to Alkaline Nitrates). 

This process depends upon the conversion of potassic or sodic 
nitrates into carbonates by ignition with carbon, and the titration 
of the carbonate so obtained by normal acid. The number of c.c. 
of normal acid required multiplied by O'lOl will give the weight 
of pure potassic nitrate in grams ; by 0'085, the weight of sodic 
nitrate in grams. 

The best method of procedure is as follows : 

The sample is finely powdered and dried in an air bath, and 1 gm., or- an 
equivalent quantity in grains, weighed, introduced into a platinum crucible. 



246 VOLUMETRIC ANALYSIS. 

and mixed with a fourth of its weight of pure graphite (prepared by 
Erodie's process), and four times its weight of pure ignited sodic chloride. 
The crucible is then covered and heated moderately for twenty minutes over 
a B un sen's burner, or for eight or ten minutes in a muffle (the heat must 
not be so great as to volatilize the chloride of sodium to any extent). If 
sulphates are present they will be reduced to sulphides ; and as these would 
consume the normal acid, and so lead to false results, it is necessary to 
sprinkle the fused mass with a little powdered potassic chlorate, and heat 
again moderately till all effervescence has ceased. The crucible is then set 
aside to cool, warm water added, the contents brought upon a filter, and 
washed with hot water till the washings are no longer alkaline. The filtrate 
is then titrated with normal acid in the ordinary way. 



2. Estimation of Nitrates by Distillation with Sulphuric Acid. 

This method is of very general application, but particularly so 
with the impure alkaline nitrates of commerce. The process needs 
careful manipulation, but yields accurate results. 

There are two methods of procedure. 

(a) To bring the weighed nitrate into a small tubulated retort with 
a cooled mixture of water and strong sulphuric acid, in the proportion of 10 
c.c. of water and 5 c.c. of sulphuric acid for 1 gin. of nitrate. The neck of 
the retort is drawn out to a point and bent downward, entering a potash or 
other convenient bulb apparatus containing normal caustic alkali. The 
retort is then buried to its neck in the sand-bath, and heated to 170 C. 
(338 Fahr.) so long as any liquid distils over ; the heat must never exceed 
175 C. (347 Fahr.), otherwise traces of sulphuric acid will come 1 over with 
the nitric acid. The quantity of acid distilled over is found by titrating the 
fluid in the receiver with normal acid as usual. 

(b) Distillation in a Partial Vacuum (Finkener).- By this 
arrangement there is no danger of contaminating the distillate with 
sulphuric acid, inasmuch as the operation is conducted in a water 
bath, and when once set going needs no superintendence. 

The retort is the same as before described, but the neck is not drawn out 
or bent; the stopper of the tubulure must be well ground. The receiver is 
a 200-c.c. flask with narrow neck, containing the requisite quantity of normal 
alkali diluted to about 30 c.c. The receiver is bound, air-tight, to the neck 
of the retort (which should reach nearly to the middle of the flask) by 
means of a vulcanized tube; the proportions of acid and water before 
mentioned are introduced into the retort with a tube funnel. The stopper 
of the retort is then removed, and the contents, both of the receiver and 
retort, heated by spirit or gas lamp to boiling, so as to drive out the air ; the 
weighed nitrate contained in a small tube is then dropped into the retort, 
the stopper inserted, the lamps removed, and the retort brought into the 
water bath, while the receiver is kept cool with wet tow, or placed in cold 
water. The distillate is titrated as before. 1 or 2 gm. of saltpetre require 
about four hours for the completion of the process. 

Finkener obtained very accurate results by this method. 
"When chlorides are present in the nitrate, a small quantity of 
moist oxide of silver is added to the mixture before distillation. 



70. 



NITRATES. 



247 



3. Estimation by conversion into Ammonia (Schxilze and 
Vernon Harcourt). 

The principle of this method is based on the fact that when 
a nitrate is heated with a strong alkaline solution, and zinc added, 




Fig. 44. 

ammonia is evolved ; when zinc alone is used, however, the 
quantity of ammonia liberated is not a constant measure of the 
nitric acid present. Vernon Ha r court and Sie we rt appear to 
have arrived independently at the result that by using a mixture 
of zinc and iron the reaction was perfect (/. C. S. 1862, 381 ; An. 
diem. u. Phar. cxxv. 293). 

A convenient form of apparatus is shown in fig. 44. 

The distilling flask holds about 200 c.c., and is closely connected by a bent 
tube Avith another smaller flask, in such a manner that both may be placed 
obliquely upon a sand-bath, the bulb of the smaller flask coming just under 
the neck of the larger. The oblique direction prevents the spirting of the 
boiling liquids from entering the exit tubes, but as a further precaution, 
these latter are in both flasks turned into the form of a hook ; from the second 
flask, which must be somewhat wide in the mouth, a long tube passes through 
aLiebig's condenser (which may be made of wide glass tube) into an 
ordinary tubulated receiver, containing normal sulphuric acid coloured with 
an indicator. The end of the distilling tube reaches to about the middle of 
the receiver, through the tubulure of which Harcourt passes a bulb 
apparatus of peculiar form, containing also coloured normal acid ; instead of 
this latter, however, a chloride of calcium tube, filled with broken glass, and 
moistened with acid, will answer the purpose. The distilling tube should be 
cut at about two inches from the cork of the second flask, and connected by 
means of a well-fitting vulcanized tube ; by this means water may be passed 
through the tube when the distillation is over so as to remove any traces of 
ammonia which may be retained on its sides. All the corks of the apparatus 
should be soaked in hot paraffine, so as to fill up the pores. 

All being ready, about 50 gm. of finely granulated zinc (best made by 
pouring molten zinc into a warm iron mortar while the pestle is rapidly 
being rubbed round) are put into the larger flask with about half the 
quantity of clean iron filings which have been ignited in a covered crucible 
(fresh iron and zinc should be used for each analysis) ; the weighed nitrate 
is then introduced, either in solution, or with water in sufficient quantity to 



2-iS VOLUMETRIC ANALYSIS. 70. 

dissolve it, strong solution of caustic potash added, and the flask immediately 
connected with the apparatus, and placed on a small sand-bath, which can 
be heated by a gas-burner, a little water being previously put into the 
second flask. Convenient proportions of material are i gm. nitre, and 
about 25 c.c. each of water, and solution of potash of spec, grav. .1/3. 
The mixture should be allowed to remain at ordinary temperature for about 
an hour (Eder). 

Heat is now applied to that part of the sand-bath immediately beneath 
the larger flask, and the mixture is gradually raised to the boiling point. 
"When distillation has actually commenced, the water in the second flask is 
made to boil gently ; by this arrangement the fluid is twice distilled, and any 
traces of fixed alkali which may escape the first are sure to be retained in the 
second flask. The distillation with the quantities above named will occupy 
about an hour and a half, and is completed when hydrogen is pretty freely 
liberated as the potash becomes concentrated. The lamp is then removed, 
and the whole allowed to cool, the distilling tube rinsed into the receiver, 
also the tube containing broken glass ; the contents of the receiver are then, 
titrated with ^ caustic potash or soda as usual. 

Eder recommends that an ordinary retort, with its beak set upwards, 
should be used instead of the flask for holding the nitrate, and that an 
aspirator should be attached to the exit tube, so that a current of air may be 
drawn through during and after the distillation. 

Chlorides and sulphates do not interfere with the accuracy 
of the results. Harcourt, Eder, and many others, including 
myself, have obtained very satisfactory results by this method. 

Siewert has suggested a modification of this process. The dis- 
tilling apparatus is a 300 350 c.c. flask with tube leading to two 
small flasks connected together as wash bottles, and containing 
standard acid. For 1 gm. of nitre, 4 gm. of iron, and 10 gm. 
of zinc filings, with 16 gm, of caustic potash, and 100 c.c. of 
alcohol of sp. gr. 0*825 are necessary. After digesting for half an 
hour in the cold or in slight warmth, a stronger heat may be 
applied to drive out all the ammonia into the acid flasks. Finally. 
10 15 c.c. of fresh alcohol are admitted to the distilling flask, 
and distilled off to drive over the last traces of ammonia, and the 
acid solution then titrated residually as usual. The alcohol is 
used to prevent bumping, but this is also avoided in the original 
process by adopting the current of air recommended by Eder. 

The copper-zinc couple devised by Gladstone and Tribe has 
been used by Thorp for the reduction of nitrates and nitrites 
occurring in water residues, etc. (/. C. S. 1873, 545). The 
resulting ammonia is distilled into weak hydrochloric acid, and an 
aliquot portion then JS T esslerized in the usual way. 

M. W. Williams (J, C. S. 1881, 100) has shown that this 
reduction, in the case of small quantities of nitric or nitrous acids, 
may be carried on by mere digestion with a properly arranged 
couple at ordinary temperatures, and may safely be hastened by 
increasing the temperature to about 25 C. in the presence of 
certain saline or acid substances ; alkaline substances, on tho 
contrary, retard the action. The details are further described ii\ 
Part VI, 



TO. NITRATES. 249 

4. By Oxidation of Ferrous Salts (Pelouze). Not available in the 
presence of Organic Matter. 

The principle upon which this well-known process is based 
is as follows : 

(a) When a nitrate is brought into contact with a solution of 
ferrous oxide, mixed with free hydrochloric acid, and heatedj part 
of the oxygen contained in the nitric acid passes over to the iron, 
forming a persalt, while the base combines with hydrochloric acid, 
and nitric oxide (NO 2 ) is set free. 3 eq. iron= 168 are oxidized 
by 1 eq. nitric acid 63. If, therefore, a weighed quantity of the 
nitrate be mixed with an acid solution of ferrous chloride or 
sulphate of known strength, in excess, and the solution boiled, to 
expel the liberated nitric oxide, then the amount of unoxidized 
iron remaining in the mixture found by a suitable method of 
titration, the quantity of iron converted from ferrous into ferric 
oxide will be the measure of the original nitric acid in the propor- 
tion of 168 to 63 ; or by dividing 63 by 168, the factor 0'375 is 
obtained, so that if the amount of iron changed as described be 
multiplied by this factor, the product will be the amount of nitric 
acid present. 

This method, though theoretically perfect, is in practice liable to 
serious errors, owing to the readiness with which a solution of 
ferrous oxide absorbs oxygen from the atmosphere. On tins- 
account accurate results are only obtained by conducting hydrogen 
or carbonic acid gas through the apparatus while the boiling is- 
carried on. This modification has been adopted by Fresenius 
with very satisfactory results. 

The boiling vessel may consist of a small tubulated retort, supported 
in such a manner that its neck inclines upward : a cork is fitted into the 
tubulure, and through it is passed a small tube connected with a vessel for 
generating either carbonic acid or hydrogen. If a weighed quantity of pure 
metallic iron is used for preparing the solution, the washed carbonic acid or 
hydrogen should be passed through the apparatus while it is being dissolved ; 
the solution so obtained, or one of double sulphate of iron and ammonia of 
known strength, being already in the retort, the nitrate is carefully introduced, 
and the mixture heated gently by a small lamp, or by the water bath, for ten 
minutes or so, then boiled until the dark-red colour of the liquid disappears,, 
and gives place to the brownish-yellow of ferric compounds. The retort is 
then suffered to cool, the current of carbonic acid or hydrogen still being 
kept up, then the liquid diluted freely, and titrated with & permanganate. 

Owing to the irregularities attending the use of permanganate 
with hydrochloric acid, it is preferable, in case this acid has been 
used, to dilute the solution less, and titrate with bichromate. Two 
grams of pure iron, or its equivalent in double iron salt, 0'5 gm. 
of saltpetre, and about 60 c.c. of strong hydrochloric acid, are 
convenient proportions for the analysis. 

Eder (Z. a. C. xvi. 267) has modified Fresenius' improve- 
ments as follows : 



250 VOLUMETRIC ANALYSIS. 

1*5 gm. of very thin iron wire is dissolved in 30 to 40 c.c. of pure fuming 
hydrochloric acid, placed in a retort of about 200 c.c. capacity ; the beak of 
the retort points upwards, at a moderately acute angle, and is connected with 
ti U-tube, which contains water. Solution of the iron is hastened ~by appty- 
ing a small flame to the retort. Throughout the entire process a stream of 
CO 2 is passed through the apparatus. When the iron is all dissolved the 
solution is allowed to cool, the stream of CO 2 being maintained ; the weighed 
quantity of nitrate contained in a small glass tube (equal to about 0'2 gm. 
HNO 3 ) is then quickly passed into the retort through the neck ; the heating 
is continued under the same conditions as before, until the liquid assumes 
the colour of ferric chloride. The whole is allowed to cool in a stream of 
CO 2 ; water is added in quantity, and the unoxidized iron is determined l>y 
titration with permanganate. The results are exceedingly good. 

If the CO 2 be generated in a flask, with a tube passing down- 
wards for the reception of the acid, air always finds its way into the 
retort, and the results are unsatisfactory. Eder recommends the 
use of Kipp's CO 2 apparatus. By carrying out the operation 
exactly as is now to be described, he has obtained very good results 
with ferrous sulphate in place of chloride. 

The same apparatus is employed ; the tube through which CO 2 enters the 
retort passes to the bottom of the liquid therein, and the lower extremity of 
this tube is drawn out to a fine point. The bubbles of CO 2 are thus reduced 
in size, and the whole of the nitric acid is removed from the liquid by the 
passage of these bubbles. The iron wire is dissolved in excess of dilute 
sulphuric acid (strength 1 : 3 or 1 : 4). When the liquid in the retort has 
become cold, a small tube containing the nitrate is quickly passed, by means 
of a piece of platinum wire attached to it, through the tubulus of the retort, 
and the cork is replaced before the tube has touched the liquid ; CO 2 is again 
passed through the apparatus for some time, after which, by slightly loosening 
the cork, the tube containing the nitrate is allowed to fall into the liquid. 
The Avhole is allowed to remain at the ordinary temperature for about an 
hour this is essential after which time the contents of the retort are heated 
to boiling, CO 2 being passed continuously into the retort, and the boiling 
continued till the liquid assumes the light yellow colour of ferric sulphate. 
After cooling, water is added (this maybe omitted with bichromate), and the 
unoxidized iron is determined by permanganate. 

Eder also describes a slight modification of this process, allowing 
of the use of a flask in place of the retort, and of ammonio-ferrous 
sulphate in place of iron wire. Although the titration with per- 
manganate is more trustworthy when sulphuric acid is employed 
than when hydrochloric acid is used, he nevertheless thinks that the 
use of ferrous chloride is generally to be recommended in preference 
to that of ferrous sulphate. When the chloride is employed, no 
special concentration of acid is necessary ; the nitric oxide is more 
readily expelled from the liquid, and the process is finished in 
a shorter time. 

The final point in the titration with permanganate, when the 
sulphate is employed, is rendered more easy of determination by 
adding a little potassic sulphate to the liquid. 

& Direct titration of the resulting- Ferric salt by Stanncms 



70. NITRATES. 251 

Chloride. Fresenius has adopted the use of stannous chloride for 
titrating the ferric salt with very good results. 

The following plan of procedure is recommended by the same 
authority. 

A solution of ferrous sulphate is prepared by 'dissolving 100 gm. of the 
crystals in 500 c.c. of hydrochloric acid of spec. grav. 1*10 ; when used for 
the analysis, the small proportion of ferric oxide invariably present in it 
is found by titrating with stanuous chloride. The nitrate being Aveighed 
or measured, is brought together with 50 c.c. (more or less, according to the 
quantity of nitrate) of the iron solution into a long-necked flask, through 
the cork of which two glass tubes are passed, one connected with a CO' 2 
apparatus, and reaching to the middle of the flask, the other simply an outlet 
for the passage of the gas. When the gas has driven out all the air, the flask 
is at iirst gently heated, and eventually boiled, to dispel all the nitric oxide. 
The CO 2 tube is then rinsed into the flask, and the liquid, while still boiling 
hot, titrated for ferric chloride, as in 64.1. 

The liquid must, however, be suffered to cool before titrating 
with iodine for the excess of stannous chloride. While cooling, 
the stream of CO 2 should still be continued. The quantity of iron 
changed into peroxide, multiplied by the factor O375, will give the 
amount of nitric acid. 

Example : (1) A solution of stannous chloride was used for titrating 
10 c.c. of solution of pure ferric chloride containing 0*215075 gm. Fe. 
25'65 c.c. of tin solution were required, therefore that quantity was equal 
to 0*0807 gm. of HNO 3 , or 0*069131 gm. of N 2 O 5 - 

(2) 50 c.c. of acid ferrous sulphata were titrated with tin solution for 
ferric oxide, and 0*24 c.c. was required. 

(3) 1 c.c. tin solution=3*3 c.c. iodine solution. 

(4) 0*2177 gm. of pure nitre was boiled, as described, with 50 c.c. of the 
acid ferrous sulphate, and required 45*03 c.c. tin solution, and 4*7 c.c. iodine 

4*7 c.c. iodine solution =1*42 c.c. SnCl' 2 

The peroxide in the protosulphate solution=0*24 c.c. 

f66 

45-03 1-66=43-37, therefore 25'65 : 0'069131=43'37 : ^,=0*1169 N 2 O 5 
instead of 0'1163, or 53*69 per cent, instead of 53*41. A mean of this, with 
three other estimations, using variable proportions of tin and iron solutions, 
gave exactly 53*41 per cent. The process is therefore entirely satisfactory in 
the case of pure materials. 

The above process is slightly modified by Eder. About 10 gm. 
of ammonio-ferrous sulphate are dissolved in a flask, in about 50 c.c. 
of hydrochloric acid (sp. gr. 1 -07) in a stream of CO 2 . The tube 
through which the CO 2 enters is drawn to a point ; an exit-tube, 
somewhat trumpet-shaped, to admit of any liquid that may spirt 
rinding its way back into the flask passes downwards into water. 
After solution of the double salt, the nitrate is dropped in with 
the precautions already detailed, and the liquid is boiled until the 
nitric oxide is all expelled. The hot liquid is diluted with twice 
its own volume of water, excess of standard stannous chloride 
solution is run in, the whole is allowed to cool in a stream of CO 2 , 
and the excess of tin is determined by means of standard iodine. 

^JtS 

f OF THE 

(UNIVERSITY 
V r^,. 



9n9 




VOLUMETEIC ANALYSIS. 

(c) Holland's Modification of the 
Pelouze Process. The arrangement of 
apparatus shown in fig. 45 obviates the 
use of an atmosphere of H or CO 2 . A is 
a long-necked assay flask drawn off at B, so 
as to form a shoulder, over which is passed 
a piece of stout pure india-rubber tube, D, 
about 6 centimeters long, the other end 
terminating in a glass tube, F, drawn off 
so as to leave only a small orifice. On 
the elastic connector D is placed a screw 
clamp. At c, a distance of 3 centimeters 
Fig. 45. from the shoulder, is cemented with 

a blow-pipe a piece of glass tube about 

2 centimeters long, surmounted by one of stout elastic tube rather 
more than twice that length. The elastic tubes must be securely 
attached to the glass by binding with wire. After binding, it is as 
well to turn the end of the conductor back, and smear the inner 
surface with fused caoutchouc, and then replace it to render the 
joint air-tight. 

Process : A small funnel is inserted into the elastic tube at c, the clamp 
at D being for the time open ; after the introduction of the solution, 
followed by a little Avater which washes all into the flask, the funnel is 
removed, and the flask supported by means of the wooden clamp, in the 
inclined position it occupies in the figure. The contents are now made to 
boil so as to expel all air and reduce the volume of the fluid to about 4 or 
5 c.c. When this point is reached a piece of glass rod is inserted into the 
elastic tube at c, which causes the water vapour to escape through F. 

Into the small beaker is put about 50 c.c. of a previously boiled solution of 
ferrous sulphate in hydrochloric acid (the amount of iron already existing 
as persalt must be known). 

The boiling is still continued for a moment to ensure perfect expulsion of 
air from F, the lamp is then removed, and the caoutchouc connector slightly 
compressed with the first finger and thumb of the left hand. As the flask 
cools the solution of iron is drawn into it ; when the whole has nearly receded 
the elastic tube is tightly compressed with the fingers, whilst the sides of the 
beaker are washed with a jet of boiled water, which is also allowed to pass 
into the flask. The washing may be repeated, taking care not to dilute more 
than is necessary or admit air. Whilst F is still full of water, the elastic- 
connector previously compressed with the fingers is now securely closed with 
the clamp, the screw of which is worked with the right hand. Provided 
the clamp is a good one, F will remain full of water during the subsequent 
digestion of the flask. 

After heating in a water bath at 100 for half an hour, the flask is removed 
from the water bath and cautiously heated with a small flame, the fingers at 
the same time resting on the elastic connector at the point nearest the 
shoulder ; as soon as the tube is felt to expand, owing to the pressure from 
within, the lamp is removed and the screw clamp released, the fingers main- 
taining a secure hold of the tube, the gas-flame is again replaced, and when 
the pressure on the tube is again felt, this latter is released altogether, thus 
admitting of the escape of the nitric oxide through F, -which should be 
below the surface of water in the beaker whilst these manipulations are 
performed. The contents of the flask are now boiled until the nitric oxide 



NITRATES. 



253 



is entirely expelled, and the solution of iron shows only the brown colour of 
the perchloride. At the completion of the operation the beaker is first 
removed, and then the lamp. 

It now only remains to transfer the ferric solution to a suitable vessel, and 
determine the perchloride with staunous chloride as in b. 

A mean of six experiments for the percentage determination of 
X-0 5 in pure nitre gave 53'53 per cent, instead of 53*41. The 
process is easy of execution, and gives satisfactory results. The 
point chiefly requiring attention is that the apparatus should be 
air-tight, which is secured by the use of good elastic tubes and 
clamp. 



5. S chlos in g-'s Method (available in the presence of Organic 

Matter). 

The solution of nitrate is boiled in a flask till all air is expelled, 
then an acid solution of ferrous chloride drawn in, the mixture 
boiled, and the nitric oxide gas collected over mercury in a balloon 
filled with mercury and milk of lime ; the gas is then brought, 
without loss, in contact with oxygen and water, so as to convert it 



again into nitric acid, then titrated with -f^ alkali as usual. 




This method was devised by Schlosing for the estimation of 
nitric acid in tobacco, and is especially suitable for that and similar 
purposes, where the presence of organic matter would interfere with 



254 VOLUMETttlC ANALYSIS. 

the direct titration of the iron solution. "Where the quantity of 
nitric acid is not below O15 gin. the process is fairly accurate, but 
needs a special and rather complicated arrangement of apparatus, 
the description of which may be found in the original paper in 
Annal. de Chim. [3] xl. 479, or in Fresenius' Quant. AnaL, sixth 
edition. 

An arrangement of apparatus, dispensing with the use of mercury, 
has been devised by Wildt and Scheibe (Z. a. C. xxiii. 151), 
which simplifies the analysis and gives accurate results with not 
less than 0*25 gin. N 2 5 . With smaller quantities the results are 
too low. Fig. 46 shows the apparatus used. 

A is an Erlenmeyer's flask of 250 c.c. capacity, containing 
the solution to be analyzed. B is a round-bottomed flask of 
250 300 c.c. capacity, half filled with caustic soda, to absorb any 
HC1 which might be carried over from A. C is an Erlenmeyer's 
flask of 750 c.c. capacity, containing a little water to absorb the 
nitric acid. D is a tube, containing water to collect any nitric 
acid not absorbed by the water in C. The tube d is bent, as shown 
in the diagram, and drawn out to a point, to diminish the size of 
the bubbles. The tube e is wide, and cut obliquely to prevent 
water collecting and passing into C. 

Process : The clip b is closed and c opened, and the tube e disconnected 
from f. The solutions in A and B are then boiled for 20 minutes to remove 
all oxygen. The tubes e and /are again connected, the clip c is closed, the 
flame under B increased to prevent the liquid in C from being drawn 
back, and the clip b is opened. As soon as steam issues from the tube a, it 
is dipped into a conical glass containing 50 c.c. of ferrous chloride prepared 
according to Schldsing's directions, and the flume under A is removed. 
when the ferrous chloride enters the flask. The clip b is regulated with the 
finger and thumb, so as to prevent the entry of air into the flask. The 
conical vessel is rinsed two or three times with water, and this is allowed 
to enter the flask, and the clip b is then closed, and the vessel A heated. 
The liquid in A turns brown in a short time, and nitric acid is evolved. 
The clip c is opened slightly from time to time until the pressure is high 
enough, when it is opened entirely. The flames must be regulated so that 
a slow current of gas bubbles through the water in C. The hydrochloric 
acid is removed by the caustic soda in B, and the nitric oxide on coming 
in contact with the air in C. is oxidized, and the nitric acid absorbed by 
the water. In case the current of gas is too rapid, the escaping nitric 
acid is absorbed in I). After an hour the tubes e and / are disconnected, 
while the solutions in A and B are still boiling, and the nitric acid is 
titrated with dilute caustic soda (about i normal). The vessel C must 
be well cooled during the whole experiment, which occupies about an 
hour and a half. 

Good results were obtained with nitrates of potash and soda, 
both alone and mixed with ammonium sulphate, superphosphate, 
and amido compounds. With superphosphate the solution should 
be made slightly alkaline, to prevent the liberation of nitric acid. 

AVarington (J. C. S. 1880, 468) has made a series of experi- 
ments on the original Schlosing process, for the purpose of 
testing its accuracy, when small quantities of nitric acid have to be 



70. 



NITRATES. 



determined in the presence of organic substances, such for instance 
as in soils, the sap of beet-root, etc. ; but instead of converting the 
nitric oxide back into nitric acid as in the original method, he 
collected the gas either over caustic soda as recommended by 
Reich ardt, or over mercury, and ascertained its amount by 
measurement in Frankland's gas apparatus. The results obtained 
by AVarington plainly showed that even on the most favourable 
circumstances the method as usually worked in Germany, either by 
the alkalimetric titration or by measurement of the gas, invariably 
gave results much too low, especially if the quantity of nitrate 
operated on was small, say 5 or 6 centigrams of nitre ; moreover,, 




Fig. 47. 

when sugar or similar organic substance was present the resulting 
gas was very impure, and the distillates were highly coloured from 
the presence of some volatile products.. The nitric oxide also 
suffered considerable diminution of volume, when left for any time 
in contact with the distillate, especially when over caustic soda. 
This being the case, the following modification originally recom- 
mended by Schlosing was adopted, in which CO 2 was employed, 
both to assist in expelling the air from the apparatus, and to chase 
out the nitric oxide produced. 

The form of apparatus adopted by Warington is shown in 
fig. 47. The vessel in which the reaction takes place is a small 



256 VOLUMETRIC ANALYSIS. 70. 

tubulated receiver, the tubulure of which has been bent near its 
extremity to make a convenient junction with the delivery tube, 
which dips into a trough of mercury on the left. The long supply 
tube attached to the receiver is of small bore, and is easily filled by 
a J c.c. of liquid. The short tube to the right is also of small bore, 
and is connected by a caoutchouc tube and clamp with an apparatus 
for the continuous production of carbonic acid. 

In using this apparatus the supply tube is first filled with 
strong HC1, and CO 2 is passed through the apparatus till a portion 
of the gas collected in a jar over mercury is found to be entirely 
absorbed by caustic potash. The current of gas is then stopped 
by closing the clamp to the right. A chloride of calcium bath at 
140 is next brought under the receiver, which is immersed one- 
half or more in the hot fluid ; the temperature of the bath is 
maintained throughout the operation by a gas burner placed beneath 
it. By allowing a few drops of HC1 to enter the hot receiver, the 
CO 2 it contains is almost entirely expelled. A jar filled with 
mercury is then placed over the end of the delivery tube, and all 
is ready for the commencement of a determination. 

The nitrate, which should be in the form of a dry residue in 
a small beaker or basin, is dissolved in about 2 c.c.* of strong 
ferrous chloride solution, 1 c.c. of strong HC1 is added, and the 
whole is then introduced into the receiver through the supply tube, 
being followed by successive rinsings with HC1, each rinsing not 
exceeding a J c.c., as the object is to introduce as small a bulk of 
liquid as possible. The contents of the receiver are in a few 
minutes boiled to dryness ; a little CO 2 is admitted before dryness 
is reached, and again afterwards to drive over all remains, of nitric 
oxide. If the gas will not be analyzed till next day, it is advisable 
to use more CO 2 , so as to leave the nitric oxide diluted with several 
times its volume of that gas. As soon as one operation is concluded 
the apparatus is ready for another charge. 

This mode of working presents the following advantages : 

(1) The volume of liquid introduced into the apparatus is much 
diminished, and with this of course the amount of dissolved air 
contributed from this source. 

(2) By evaporation to dryness a complete reaction of the nitrate 
.and ferrous chloride, and a perfect expulsion of the nitric oxide 
formed, is as far as possible attained. 

(3) The nitric oxide in the collecting jar is left in contact with 
.a much smaller volume of acid distillate, and its liability to 
absorption is greatly diminished by its dilution with CO-. 

The results obtained with this apparatus by Warington on 
small quantities of nitre alone, and mixed with variable quantities 

* Supposing the ferrous chloride to contain 2 gm. of iron per 10 c.c., then 1 c.c. of 
the solution will be nearly equivalent to 0'12 gm. of nitre, or 0'0166 gin. of nitrogen. 
A considerable excess of iron should, however, always be used. 



70. NITRATES. 257 

of ammonic salts and organic substances including sugar, showed 
a marked improvement upon the method as usually carried out. 

A further improvement has been made in this method by 
Warington (/. C. S. 1882, 345), and described by him as 
follows : 

The apparatus now employed is quite similar to that shown in fig. 47, with 
the only difference that the bulb retort in which the reaction takes place is 
now only 1 inch in diameter, thus more exactly resembling the. form 
employed by Sch losing. A bulb of this size is sufficient for the analysis 
of soil extracts ; for determinations of nitrates in vegetable extracts a larger 
bulb is required. 

The chief improvement consists in the use of CO 2 as free as possible from 
oxygen. The generator is formed of two vessels. The lower one consists of 
a bottle with a tubulurc in the side near the bottom; this bottle is supported 
in an inverted position, and contains the marble from which the gas is 
generated. The upper vessel consists of a similar bottle standing upright ; 
this contains the HC1 required to act on the marble. The two vessels are 
connected by a glass tube passing from the side tubulure of the upper vessel 
to the inverted mouth of the lower vessel ; the acid from the upper vessel 
thus enters below the marble. CO 2 is generated and removed at pleasure by 
opening a stop-cock attached to the side tubulure of the lower vessel, thus 
allowing HC1 to descend and come in contact with the marble. The 
fragments of marble used have been previously boiled in water. The boiling 
is conducted in a strong flask. After boiling has proceeded some time, 
a caoutchouc stopper is fixed in the neck of the flask, and the flame removed ; 
boiling will then continue for some time in a partial vacuum. The lower 
reservoir is nearly filled with the boiled marble thus prepared. The HC1 
has been also well boiled, and before it is introduced into the upper reservoir 
it has dissolved in it a moderate quantity of cuprous chloride. As soon as 
the acid has been placed in the upper reservoir it is covered by a layer of 
oil. The apparatus being thus charged is at once set in active work by 
opening the stop-cock of the marble reservoir ; the acid descends, enters the 
marble reservoir, and the CO 2 produced drives out the air which is necessarily 
present at starting. As the acid reservoir is kept on a higher level than the 
marble reservoir, the latter is always under internal pressure, and leakage of 
air from without cannot occur. 

The presence of the cuprous chloride in the hydrochloric acid not only 
ensures the removal of dissolved oxygen, but affords an indication to the eye 
of the maintenance of this condition. So long as the acid remains of 
an olive tint, oxygen will be absent ; but should the acid become of a clear 
blue-green, it is no longer certainly free from oxygen, and more cuprous 
chloride must be added. 

A further slight improvement adopted consists in the use of freshly-boiled 
reagents, which are employed in as small a quantity as possible. When 
boiling the hydrochloric acid it is well to add a few drops of ferrous chloride, 
in order more certainly to remove any dissolved oxygen. 

The mode of operation is as follows : The apparatus is fitted together, the 
long funnel tube attached to the bulb retort being filled with water. 
Connection is made with the glass stop- cock of the CO 2 generator by means 
of a short stout caoutchouc tube, provided with a pinch-cock. The pinch- 
cock being opened, the stop-cock is turned till a moderate stream of bubbles 
rises in the mercury trough ; the stop-cock is left in this position, and the 
admission of gas is afterwards controlled by the pinch-cock, pressure on 
which allows a few bubbles to pass at a time. The heated chloride of calcium 
bath is next raised, so that the bulb retort is almost submerged; the 
temperature, shown by a thermometer which forms part of the apparatus, 

s 



258 VOLUMETRIC ANALYSIS. 70. 

should be 130 140. By boiling small quantities of water or hydrochloric 
acid in the bulb retort in a stream of CO 2 the air present is expelled; the 
supply of gas must be stopped before the boiling has ceased, so as to leave 
little in the retort. Previous to very delicate experiments it is advisable to 
introduce through the funnel tube a small quantity of nitre, ferrous chloride, 
and hydrochloric acid, rinsing the tube Avith the latter reagent ; any trace of 
oxygen remaining in the apparatus is then consumed by the nitric oxide 
formed, and after boiling to d^ness, and driving out the nitric oxide with 
CO 2 , the apparatus is in a perfect condition for a quantitative experiment. 

Soil extracts may be used without other preparation than concentration. 
Vegetable juices, which coagulate when heated, require to be boiled and 
filtered, or else evaporated to a thin syrup, treated with alcohol and filtered. 
A clear solution being thus obtained, it is concentrated over a water-bath to 
the smallest volume, in a beaker of smallest size. As soon as cool, it is mixed 
with 1 c.c. of a cold saturated solution of ferrous chloride and 1 c.c. HC1, 
both reagents having been boiled and cooled immediately before use. In 
mixing with the reagents care must be taken that bubbles of air are not 
entangled; this is especial^ apt to occur with viscid extracts. The quantity 
of ferrous chloride mentioned is amply sufficient for most soil extracts, but 
it is well perhaps to use 2 c.c. in the first experiment of a series; the 
presence of a considerable excess of ferrous chloride in the retort is thus 
ensured. With bulky vegetable extracts more ferrous chloride should be 
employed ; to the syrup from 20 gm. of mangel sap should be added 5 c.c. 
of ferrous chloride, and 2 c.c. of hydrochloric acid. 

The mixture of the extract with ferrous chloride and HC1 is introduced 
through the funnel tube, and rinsed in with three or four successive ^ c.c. 
of HC1. The contents of the retort are then boiled to dry ness, a little CO- 
being from time to time admitted, and a more considerable quantity used at 
the end to expel any remaining nitric oxide. The most convenient tem- 
perature is 140, but in the case of vegetable extracts it is well to commence 
at 130, as there is some risk of the contents of the retort frothing over. 
The gas is collected in a small jar over mercury. As soon as one operation 
is completed, the jar is replaced by another full of mercury, and the 
apparatus is ready to receive a fresh extract. A series of five determinations, 
with all the accompanying gas analyses, may be readity performed in one 
day. The bulb retort becomes encrusted with charcoal when extracts rich 
in organic matter are the subject of analysis ; it is best cleaned first with 
water, and then by heating oil of vitriol in it. 

Mercury, contrary to the statement in most text-books, is gradually 
attacked by hj'drochloric acid in the presence of air ; the mercury in the 
trough is thus apt to become covered with a grey chloride, and it is quite 
necessary to keep the store of mercury in contact with sulphuric acid to 
preserve its mobile condition. 

The gas analysis is of a simple character; the gas is measured after 
absorption of the CO 2 by potash, and again after absorption of the nitric 
oxide, the difference giving the amount of this gas. For the absorption of 
nitric oxide, a saturated solution of ferrous chloride was for some time 
employed. This method is not, however, perfectly satisfactory when the 
highest accuracy is required, the nitric oxide being generally rather under- 
estimated, except the process of absorption is repeated with a fresh portion 
of ferrous chloride. The error is greater in proportion to the quantity of 
unabsorbed gas present. ThuSj with a mixture of nitrogen and nitric oxide 
containing little of the former, absorption of the nitric oxide by successive 
treatment with oxygen and pyrogallol over potash showed 97' 8 per cent, of 
nitric oxide ; while the same gas, analyzed by a single absorption with ferrous 
chloride (after potash), showed 97' 5 per cent, of nitric oxide. With a mixture 
containing more nitrogen, the oxygen method showed 65'9 per cent, of nitric 
oxide ; while one absorption with ferrous chloride gave 64'2 per cent., and 



70. NITRATES. 259 

a second absorption, in which the ferrous chloride was plainly discoloured, 
66'2 per cent. The use of ferrous chloride as an absorbent for nitric oxide 
has now been given up, and the oxygen method substituted. All the 
measurements of the gas are now made without shifting the laboratory 
vessel ; the conditions are thus favourable to extreme accuracy^ 

The chief source of error attending the oxygen process lies in the 
small quantity of carbonic oxide produced during the absorption with 
pyrogallol ; this error becomes negligible if the oxygen is only used 
in small excess. The difficulty of using the oxygen in nicely 
regulated quantity may be removed by the use of Bischof's gas 
delivery- tube. This may be made of a test-tube, having a small 
perforation half an inch from the mouth. The tube is partly 
filled with oxygen over mercury, and its mouth is then closed by 
a finely-perforated stopper, made from a piece of wide tube, and 
fitted tightly into the test-tube by means of a covering of 
caoutchouc. When this tube is inclined, the side perforation 
being downwards, the oxygen is discharged in small bubbles from 
the perforated stopper, while mercury enters through the side 
opening. Using this tube, the supply of oxygen is perfectly 
under control, and can be stopped as soon as a fresh bubble ceases 
to produce a red tinge in the laboratory vessel. The trials made 
with this apparatus have been very satisfactory. If nitrites are to 
be estimated by this method, it is necessary first to convert them 
into nitrates, with excess of hydrogen peroxide, which is entirely 
destroyed by the subsequent evaporation to dryness. 



Technical method for Alkaline Nitrates and Nitrated Manures. 

Wagner uses a simple arrangement of apparatus, which gives 
fairly good results, and permits of rapid working. 

A 200 c.c. flask is fitted with a two-hole rubber stopper. One hole carries 
an ordinary gas delivery tube, and the other a thistle funnel, having a stop- 
cock below the funnel. The end of this tube is narrowed, and does not 
quite reach the liquid in the flask. 

A solution of 200 gm. of iron wire in hydrochloric acid is made and 
diluted to 1 liter. 40 c.c. of this solution are placed in the flask, and the air 
expelled by boiling. 10 c.c. of a standard solution of sodic nitrate, con- 
taining 33 gm. per liter, are then placed in the funnel, and allowed gradually 
to drop into the boiling solution of iron. A gas tube graduated to 100 c.c. 
is filled with boiled and cooled distilled water, and the nitric oxide collected 
in the usual way. When the nitre solution is nearly all dropped in, the 
funnel is filled with 20 per cent. HC1, and run down ; this is repeated, the 
liquid being still kept gently boiling. 10 c.c. of the solution to be tested 
are now put into the funnel, taking care that not more than 100 c.c. of gas 
will result. The gas is collected as before in a fresh tube precisely as in the 
case of the pure nitrate. In this manner five or six estimations can be 
made with the one and the same ferrous solution. Finally, a fresh test is 
made with standard nitre solution ; the readings of the tubes are taken, and 
as they will all be of same temperature and pressure no correction is 
necessary, all being allowed to cool to the same point. 

s 2 



260 VOLUMETRIC ANALYSIS. 

The calculation is easy. Suppose that the pure nitre gave 90c.c. 
of gas, this volume = 6-33 gm, of XaXO 8 , or 1 c.c. = 0*00366 
gin. = 0-000604 gm. X. 

Technical use of the Pelouze Process for Manures. Vincent 
Edwards adopts the following method for manures containing 
nitrates together with ammonia and other matters (C. N. Ixxi. 307). 
The solutions required are : 

Standard Potassic bichromate, 14*742 gm. per liter. 1 c.c. = 
0-0085 gm. XaXO 3 or 0-0101 gm. KXO 3 . 

Ferrous Sulphate. 100 gm. of crystallized salt with 100 c.c. of 
concentrated H 2 S0 4 per liter. 

The exact working strength of these two solutions in practice, is 
found by boiling 50 c.c, of the iron solution till it becomes thick 
in a stout well annealed glass flask, preferably of Jena glass, which 
is fitted with a Buns en valve, made by cutting the rubber tube 
with a sharp razor, the glass tube to which it is fitted passing- 
through a light fitting rubber stopper ; after boiling the flask is set 
aside to cool, then 100 c.c. or so of water are added, and the 
titration made with bichromate in the usual way with fresh 
solution of ferricyanide as indicator. 

Process : 1020 gm. of the nitrated manure, according to its richness, are 
exhausted with water and the liquid made up to 200 c.c. 

20 c.c. of this solution are placed in the boiling flask together with 50 c.c. 
of the iron solution, the stopper with valve is then inserted, and the mixture 
boiled until it becomes thick, and semi-solid drops are splashed against the 
sides of the flask ; the flask is then enveloped in a cloth, and removed 
to cool ; when this has occurred, 100 c.c. or so of water are run into the flask, 
well shaken, then titrated with the bichromate as in the case of the blank 
experiment. 

Example : The blank titration showed that c.c. of iron solution 
required 54 c.c. of bichromate. 20 c.c. of the manure solution = 1 gm. 
manure were treated as above described, and required 31 c.c. of bichromate, 
therefore 5431 = 23 c.c. which multiplied by 0'0085 = 0'1955 or 19'55 % 
of XaXO 3 in the manure. The manure was known to be a mixture of 
20/ of nitrate of soda, of 95'5% strength, with 80 per cent, of an 
ammouiacal guano. 

This technical process is, of course, chiefly valuable where the 
nitrate is required to be estimated apart from the ammonia, 

6. By the Kj eldahl Process. 

By the modified method described on page 85, it is now quite 
possible to estimate the nitrogen in commercial nitrates with great 
accuracy and very little personal attention. 

7. lodometric Estimation of Nitrates. 

F. A. Goocli and H. Gruener (Amer. J. Sci. xliv. 117) 
recommend distilling the nitrate (about 0'2 gm.) with 20 c.c. of 



70. NITRATES. 261 

a saturated solution of crystallized manganous chloride in strong 
hydrochloric acid, in a current of CO' 2 . The products of the 
distillation are passed into a solution of potassic iodide, and the 
liberated iodine is afterwards titrated by means of sodic thiosulphate. 
3 mols. of iodine correspond with 2 mols. of nitric acid 



Process : The apparatus employed consists of a bent pipette, serving 
instead of a retort, which is connected with a Kipp's apparatus evolving 
CO 2 . The other goose-neck-like end is sealed to a Will and Varrentrap 
nitrogen bulb, the exit tube of which is drawn out, so that it may be pushed 
well within the inlet tube of a Will and Varrentrap absorption flask. 
A third receiver simply acts as a trap to exclude air from the absorption 
apparatus proper. The titration should be completed immediately after the 
distillation, during which the nitrogen bulbs should be immersed in cold 
w r ater; otherwise, traces of dissolved nitric oxide might get oxidized and 
liberate more iodine. 

Another method worked out by H. Gruener consists in 
distilling the nitrate with potassic iodide and phosphoric acid 
(Amer. J. Sd. xlvi., July, 1883.) 

Process : The nitrate, not to exceed in amount 0'05 gm. of potassio nitrate, 
is introduced into a retort, together with ten times its weight of potassic 
iodide, and 17 to 20 c.c. of phosphoric acid, of specific gravity 1'43. All 
water used should be recently boiled. CO 2 is passed from a proper apparatus. 
The neck of the retort passes into a receiver containing a known amount 
of TV arsenious oxide, alkaline with a good excess of sodic' bicarbonate, and 
diluted to a convenient bulk. To this flask is attached for additional safety 
a simple trap containing water. The solution in the retort is boiled until it 
is clear that no more iodine remains, when the receiver, after proper washing 
and addition of the liquid in the trap, is titrated with iodine to find the 
amount of arsenious oxide still left. This gives the measure of the iodine 
evolved and consequently of the nitrate present. 

These processes are simply mentioned here, as perhaps being 
available under particular circumstances, but the author has had 
no experience of them. The test examples given by the operators 
are fairly satisfactory, especially the first. 



8. G-asometric estimation as Nitric Oxide. 

This method of estimating nitrogen existing as nitric and nitrous 
acids, either separately or together, is an exceedingly delicate one, 
and capable of great accuracy under proper manipulation. 

It is now best known as the Crum-Frankland method, the 
original idea emanating from Crum, and afterwards improved in 
detail of manipulation by Frankland and Armstrong, in their 
well-known method of water analysis. 

So far as the use of the method for water analysis is concerned, 
the process is given in Part VI., where the shaking tube which is 
used for the decomposition of the nitrogen compounds by mercury 
and sulphuric acid is figured, and the details of the process as 
applied to waters fully described. 



262 VOLUMETRIC ANALYSIS. 70. 

The method there given, however, requires the use of a gas 
apparatus. This method obviates that necessity, and though the 
results cannot be said to be absolutely as exact, they are very 
satisfactory for some purposes, such as the examination of nitrous 
vitriol, raw commercial nitrates, manures, etc. 

The apparatus used is Lunge's nitrometer, a figure of which is 
given in the section on technical gas analysis, accompanied with 
a description of the method of using it. The 'application of the 
instrument to the estimation of nitrous and nitric acids in vitriol 
and other substances is explained in the same section. 

The volume of the nitric oxide obtained can be read off to -^ c.c. ; 
it is reduced by Buns en's tables to and 760 m.m., and the 
percentage of the acid calculated from it. Each c.c. of XO, 
measured at and 760 m.m., corresponds to 1*343 m.gm. XO, or 
1-701 m.gm. K 2 :} , or 2-417 m.gm. X 2 5 , or 4-521 KXCF, or 
3-805 m.gm. K"aN0 3 . By this process, of course, nitric and nitrons 
acids cannot be distinguished, but are always estimated together. 

The principle of the reaction is explained in the section on Water 
Analysis (Estimation of titrates and Nitrites), and the satisfactory 
nature of the method for vitriol-testing has been amply demonstrated 
by Watts, by Davis (C. N. xxxvii. 45), and many others. The 
instrument itself has been made in several modified ways, but the 
principle of its construction is the same. 

Allen (Analyst v. 181) recommends the use of this instrument 
for the estimation of nitrates and nitrites in water residues ; and 
to obviate the difficulty in reading the volume Avhich sometimes 
arises from the mercurial froth, he uses two nitrometers side by 
side, in one of which is worked a pure standard nitrate solution, 
and in the other the material for analysis under precisely the same 
conditions of temperature, pressure, etc. If the apparatus 
containing the comparative test is free from leakage, it may be 
retained for a long period for the purpose of comparison. 

9. Colorimetric Methods. 

Phenol Method (Spr eng-el). Both this and the carbazol method 
are applicable chiefly to waters where only small proportions of 
nitric acid are to be estimated. The solutions required are 

Standard Potassic nitrate. 0-7215 gm. of IvXO 3 is dissolved 
in a liter of water. 1 c.c. of this solution = -f^ m.gm. of X, or one 
part X in 100,000. 100 c.c. of it should be diluted to a liter for 
use in the actual analysis, and 10 c.c. taken, to avoid the possible 
error resulting from measuring only 1 c.c. 

Phenol Sulphomc acid. 80 c.c. of liquefied pure phenol are 
poured into 200 c.c. of pure concentrated sulphuric acid in 
a flask, and kept on a boiling water bath for eight hours. The 
mixture is cooled, and 140 c.c. of pure hydrochloric acid with 
420 c.c. of water added. The solution is then ready for use. 



70. NITRATES. 263 

Process : 10 c.c. of the water under examination and 10 c.c. of the 
standard potassic nitrate are pipetted into two small beakers and placed near 
the edge of a hot plate. When nearly evaporated they are removed to the 
top of the water-oven and left there till they are evaporated to complete 
dryness. As this operation usually takes about an hour and a half, it is 
better, when time is an object, to evaporate to dryness in a platinum dish 
over steam. The residue in each case is then treated with 1 c.c. of the 
phenolsulphuric acid, and the beakers are placed on the top of the water- 
oven. If the water under examination contain a large quantity of nitrates 
the liquid speedily assumes a red colour, which, in a good water, will not 
appear for about ten minutes. After standing for fifteen minutes the beakers 
are removed, the contents of each washed out successively into a 100 c.c. 
measuring glass, a slight excess (about 20 c.c. of 0'96) of ammonia added, 
the 100 c.c. made up by the addition of water, and the yellow liquid 
transferred to a Nessler glass. The more strongly coloured liquid is then 
partly transferred to the measuring glass again and the tints compared 
a second time. In this way the tints are adjusted, and when, as far as 
possible, matched, the liquid that has been partially removed is made up to 
the 100 c.c. mark with water, and, after well mixing, finally compared, If 
not exactly the same, a new liquid can at once be made up, probably of 
exactly the same tint, as the first experiment gives very nearly the number 
of c.c. of the one equivalent to the 100 c.c. of the other. A. E. Johnson 
in his very useful Analyst's Laboratory Companion (p. 50) has given 
a table for obtaining the nitrogen in parts per 100,000, and also in grains 
per gallon, by this method. 

In the case of very good waters, 20, 50, or more c.c. should be evaporated 
to a small bulk, rinsed into a small beaker, and evaporated to dryness and 
treated as above only 5 c.c. of the standard potassic nitrate ( = 0'5 N in 
100,000) being taken. In the case of very bad waters, 10 c.c. should be 
pipetted into a 100 c.c. measuring flask and made up to the mark with 
distilled water, then 10 c.c. of the well mixed liquid (=1 c.c. original water) 
withdrawn and treated as above. 

A. II. Gill (Tech. Quarterly vii., 1894, 5562) has studied 
this method, and says : The phenolsulphonic acid used should be 
the pure disulphonic acid (C 6 H 3 (OH) S0 3 H 2 ), which, with nitric 
acid, gives picric acid even in the cold (Kekule, Lehrbucli iii. 236.) 
To prepare it, 3 gm. of pure phenol and 37 gm. (20 '1 c.c.) of pure 
sulphuric acid of 1 '84 sp. gr. are mixed in a flask and heated for 
six hours to 100 in a water bath. The acid, as thus prepared, 
may crystallize out on standing, but may be brought into solution 
again by reheating for a short time. 

Process : The author takes 1 or 2 c.c. of the water (diluted if necessary), 
containing about - 0007 m.gm. of nitrogen as nitrate, and rapidly evaporates 
over a steam bath, in a 2 inch porcelain dish, the dish being removed as 
soon as dry, or, preferably, when just a drop remains. With "ground 
waters/' 10 c.c. of a portion which has been decolourized by alumina in the 
cold are evaporated. The residue is treated in the dish with enough of the 
acid to cover it, 10 drops (=0'7 c.c.) being usually sufficient, and by stirring 
with a glass rod every part of the residue is moistened. Seven c.c. of water 
are added and stirred, and then ammonia in excess, and the solution again 
stirred. The colour is compared with the standard, either in a similar dish, 
or both are poured into tubes If inch deep and f inch internal diameter. 

The standard solution of potassic nitrate is made by dissolving 
0'720 gm. KNO :s in water, diluting to 1 liter, evaporating 10 c.c. in vacua 



264 VOLUMETRIC ANALYSIS. 70. 

over sulphuric acid, treating the residue -with phenolsulphonic acid, as above, 
and diluting to 1 liter. One c.c. of this solution contains O'OOL m.gm. 
nitrogen. A measured volume of it is made alkaline with ammonia as 
required. 

The author concludes from his experiments that : 

1. The pure disulphonic acid gives the best results. 

2. No advantage is gained by treating the water residue with the acid at 
100, as Sprengel directs; equally good results are obtained in the cold ; 
but if the temperature be as low as 0, decidedly low results are obtained. 

3. The amount of acid used makes very little difference so long as there is 
enough used. 

4. There is a loss of nitrogen during evaporation, which is least if the 
evaporation take place in vacua over sulphuric acid, or rapidly in an open 
dish at 100 ; slower evaporation, at 65, caused more loss, and the dry 
residues, if further heated, lose nitrogen. The addition of sodium carbonate 
does not prevent the loss. 

5. Chlorine does not interfere if less than two parts per 100.000 be 
present ; if more be present, evaporation should be conducted in vacua ; 
but if the chlorine exceed seven parts per 100,000 it should be removed 
by pure silver sulphate before evaporation. 

6. In comparing the colours the most accurate estimations are made 
when the intensity of the colour does not exceed that produced by 1 c.c. of 
a water containing about 0'05 part nitrogen per 100,000. The colour 
produced by O'lO part per 100,000 is very difficult to match accurately. 

7. The process does not estimate the nitrogen as liitrite, as the action of 
nitrous acid results in the formation of nitrosophenol CH 4 (NO) (OH), 
which is colourless in dilute solutions. 

The Carbazol Method. The standard potassic nitrate and pure 
sulphuric acid, as above, are required as well as the following 
special reagents : 

(a) Silver sulphate solution containing 4 '3945 gm. per liter; 
1 c.c. will precipitate one part of chlorine per 100,000 from 
100 c.c. of water. 

(b) Aluminium sulphate solution free from chlorides and iron, 
5 gm. per liter. 

(c) Carbazol Solution. 0'6 gm. carbazol is dissolved in glacial 
acetic acid, and the solution made up to 100 c.c. with the glacial 
acid. For use, 1 c.c. of this solution is withdrawn by a pipette 
and mixed w r ith 15 c.c. of pure re-distilled sulphuric acid. 

It is advisable to prepare a series of solutions containing - 03, 
0'05, 0'07, etc., parts of nitrogen per 100,000 from the standard 
nitrate solution by diluting with water. 

Process : To 100 c.c. of the water, the amount of chlorides in which has 
first been ascertained, sufficient of the silver sulphate solution is added from 
a burette to precipitate all the chlorides. To this solution, containing the 
silver chloride in suspension, 2 c.c. of the aluminium sulphate solution are 
added, and the whole made up to a convenient bulk, 110 c.c. in the case of 
waters containing 1 to 6 parts of chlorine per 100,000. The solution is then 
filtered, and 2 c.c. of this filtrate are then taken for the nitrate estimation, 
and, of course, the amount found must be calculated from the diluted bulk 
of the solution. To the 2 c.c. of the filtered water contained in a test-tube, 
4 c.c. concentrated sulphuric acid are added, and the mixture well cooled, 



70. 



NITRITES. 



265 



1 c.c. of the carbazol solution in sulphuric acid as above described is then 
added, and a bright green colour appears in a few moments if nitrates are 
present. The amount of nitrate is roughly gauged from the colour 
produced, and 2 c.c. of the standard nitrate solution, considered to be equal 
to it, are placed in a second test-tube, and the operation repeated with it and 
a fresh 2 c.c. of the water under examination at the same time. If the 
tints are not similar a fresh comparison must be made, and in every case it is 
necessary to repeat the operation with a fresh quantity of the water, so that 
the colours may be developed as nearly as possible simultaneously. 

The author states that 0*0008 m.gm.of nitrogen as nitrate maybe detected 
by the carbazol method. The removal of chlorides is necessary for accurate 
results, but the nitration does not take much time when aluminium sulphate 
solution is added as described. 

Other special methods for the estimation of nitrates in water 
will be given in the section on Water Analysis. 





Fig. 48. 



NITRITES. 
1, lodometric method. 

Dunstan and Dymond (Pliarm. Journ. 
[3] xix. 741) have devised a method for the 
estimation of N 2 3 in organic and inorganic 
combination which is both simple in operation 
and accurate in results. The authors point 
out that although the inorganic nitrites may 
be accurately analyzed by gasometric methods, 
or by permanganate 1 , it is impossible to use 
such methods for the organic compounds or 
their alcoholic solutions. The reaction upon 
w T hich the method depends is not new, being 
based on the following equation 

2HI + 2ffis T 2 m 2H 2 + 2NO + 1 2 . 

The liberated iodine is titrated with - thio- 
sulphate in the usual way. The chief merit 
in the process is the simple form of apparatus 
used, and which is shewn in fig. 48. 

A stout glass flask, having a capacity of 
about 100 c.c., is closed by a tightly fitting 
rubber stopper, through which passes a piece of 
rather wide glass tubing (C), one end of which 
(that within the flask) is cut. off obliquely, so 
that liquid may flow freely through it. The 
other end of the tube is connected by means 
of a piece of thick rubber tubing with 
a large glass tube, which forms a lipped funnel 
(A). A steel screw clamp (B) regulates com- 
munication between the funnel and the tube, 
and the short interval of rubber which is not 
occupied by glass tubing forms a hinge upon 



2G6 VOLUMETRIC ANALYSIS. 70. 

which the flask may be moved into a position at right angles to- 
the funnel, in order to mix by agitation the liquids which are 
introduced into the apparatus. The absence of any leak in the 
apparatus is ascertained by boiling about 50 c.c. of water in the 
flask until steam has continuously issued from the funnel for some 
few minutes, when the screw clip is quickly closed and simul- 
taneously the source of heat is removed. A little water is now- 
placed in the funnel and the flask is cooled by immersion in water. 
On sharply inverting the flask the " click " of the water against 
the airless flask should be quite distinct. ]N T o water should be 
drawn from the funnel or from any of the joints into the flask, 
and no diminution in the intensity of the " click " should be 
observed after the apparatus has been standing, neither when the 
flask is inverted and the funnel empty should any bubbles of air 
pass through into the liquid. Having thus proved the absence 
of any leak in the apparatus, it is ready for use. The flask is now 
free from all but mere traces of oxygen. A conclusive proof of 
this is obtained by boiling in the flask a solution of potassic 
iodide, acidified with diluted sulphuric acid, and then, after the 
closed flask has been cooled, the funnel removed and its place 
taken by a smaller glass tube filled with air-free water, the 
apparatus is connected with a reservoir of pure nitric oxide. 
When the clamp is unscrewed nitric oxide is drawn into the flask,, 
and should any oxygen be present nitrous acid will be produced, 
and consequently iodine will be set free. This experiment has 
often been made by the authors, who have failed to observe any 
but an insignificant trace of liberated iodine. 

Process : 5 c.c. of a 10 per cent, solution of potassic iodide, 5 c.e. of 
a 10 per cent, solution of sulphuric acid, and 40 c.c. of water are introduced 
into the flask, which is securely fitted with the cork carrying the funnel and 
tube. The screw clip being open, and a free passage left for the escape of 
steam, the liquid is boiled. After a few minutes, when a,r\y iodine which 
may have been liberated has been expelled, and the upper part of the flask is 
completely filled with steam, which is also freely issuing from the funnel, the 
clip is tightly closed, and at the same moment the source of heat is removed. 
A little water is now put into the funnel, and also on the rim of the flask, as 
a safeguard against a possible minute leakage, and the vessel is cooled, by 
immersion in water. A solution containing a known weight of the nitrite 
(equivalent to about O'l gm. of nitrous acid) is placed in the funnel, and 
slowly drawn into the flask by cautiously unscrewing the clip. The liquid 
which adheres to the funnel is washed into the flask with recently boiled and 
air-free water, care being taken that during this operation no air is admitted 
into the flask. When experiments are being made with organic nitrites 
which are insoluble in water, they are dissolved in alcohol, and alcohol is also 
used to wash the funnel. When the nitrite is very volatile, a little cold 
alcohol should be put in the funnel, and the point of the pipette containing 
the nitrite should be held at the bottom of the funnel beneath the alcohol, 
and the liquid quickly drawn from the pipette into the flask. The nitrate 
having been introduced, the flask is well shaken and the liberated iodine is 
titrated with a standard solution of sodic thiosulphate, small quantities of 
which are delivered from a burette into the funnel and gradually drawn into- 



NITRITES. 267 

the flask ; the screw clip renders it quite easy to admit minute quantities of 
the solution. As soon as the iodine is decolorized any standard solution 
remaining in the funnel is returned to the burette. Or the funnel may, 
before the titration is commenced, be replaced by the burette itself, and the 
standard solution delivered direct into the flask. Starch may be used as an 
indicator, but it is usually quite easy to observe the complete disappearance of 
the yellow colour of the dissolved iodine. From the volume of the standard 
solution used, the amount of nitrous acid is calculated from the equation 
before given. 

It is obvious that the apparatus might be improved in several 
respects, as, for example, by constructing it entirely of glass, with 
a ground stopper and tap, as well as by the use of a graduated 
funnel to deliver the standard solution, and also in other ways. 

The authors quote numerous experiments, comparing the method 
with careful estimations of sodic and ethyl nitrites, gasometrieally 
shelving excellent results. 

As a further test of the accuracy of the process, experiments 
were made with various organic nitrites of known purity. In 
each instance a solution of the nitrite was made by weight, and 
a weighed quantity was used for the estimation. To prevent any 
loss of these volatile nitrites the experiments were conducted in 
the following manner : A well-stoppered bottle half filled with 
the alcohol corresponding to the nitrite'"' to be estimated was 
weighed. Sufficient of the nitrite was now introduced by means 
of a pipette to constitute approximately a 2 per cent, solution, and 
the liquid again weighed. The exact strength of the solution 
having been thus determined, the contents of the bottle were well 
mixed, and the neck and stopper of the bottle dried. The bottle 
was now re-weighed, and about 2 c.c. of the solution removed by 
a pipette, care being taken not to wet the neck of the bottle. The 
liquid having been introduced into the flask without exposure to 
air, in the manner which has been previously described, the bottle 
containing the solution was again weighed. The results obtained 
with ethyl nitrite were : 

Taken. Found. 

O'OSS gin. 0'089 gm. 

0-176 0-179 

0-113 . 0-115 , 



2. Analysis of Alkaline Nitrites by Permanganate. 

Kinnicutt and Xef have experimented on the following 
method, and obtained very fair results. 

The sample of nitrite is dissolved in cold water in the proportion of about 
1 to 300 : to this liquid T ^ permanganate is added drop by drop, till it has 

* The corresponding alcohol was employed to prevent loss consequent on the occurrence 
of a reverse chemical change, which takes place when a lower homologous alcohol is 
mixed with the nitrite corresponding to a higher homologous alcohol ; for example, 
a solution of ainyl nitrite in ethyl alcohol soon becomes a solution of ethyl nitrite in 
amyl alcohol, from which the ethyl nitrite rapidly volatilizes. 



268 VOLUMETRIC ANALYSIS. 70. 

a permanent red colour ; then 2 or 3 drops of dilute H-SO 4 , and immediately 
afterwards a known excess of the permanganate. The liquid, which should 
now be of a dark red colour, is strongly acidified with pure H-SO 4 , heated 
to boiling, and the excess of permanganate determined by means of freshly 
prepared T ^ oxalic acid. 1 c.c. permanganate-=0'0345 gm. ]S T aNO' 2 , or 
0-0425 gm. KNO 2 . 

Of course there must be no other reducing substance than the 
nitrite present in the material examined, and, to ensure accuracy, 
a blank experiment should be made with the like proportions of 
H 2 S0 4 and oxalic acid. 



3. Gasometric method. 

Percy Frankland (/". C. S. liii. 364) adopts this method for 
the estimation of nitrous acid in small quantity, but too large for 
colorimetric estimation, and where also ammonia, organic matters, 
and nitrates may co-exist. It is based on the fact that when 
nitrous acid, together with excess of urea, is mixed with sulphuric 
acid in the cold, the reaction is 

2CO(JS T H 2 ) 2 + X 2 :5 - CO(jS T ETO) 2 + CO 2 + 2X 2 . 

The decomposition is made in the Cr urn-Frank land shaking tube, 
described and figured in Part VI., and the evolved nitrogen gas 
measured in the usual gas apparatus. The ordinary nitrometer may 
also be used for larger quantities of XO 2 by the same method. 

In the case of an ordinary alkali nitrite, the dry substance, or 
its solution evaporated to dryness, is mixed with excess of 
crystallized urea, and dissolved in about 2 c.c. of boiling water in 
a beaker, then transferred, with the rinsings, to the cup of the 
apparatus, and passed into the tube. A few c.c. of dilute 
sulphuric acid (1:5) are then passed in. A vigorous evolution of 
gas takes place, and continues for some five minutes ; the gas is 
a mixture of nitrogen and carbonic anhydride. The decomposition 
is complete in fifteen minutes. A solution of pure sodic hydrate 
(1 : 3) is now added through the cup, and the mixture violently 
shaken, until the CO 2 is absorbed. The gas and liquid are then 
transferred, by means of another mercury trough, to the laboratory 
vessel, and the gas, which is double the volume of the X existing 
as ]S 2 3 , measured in a gas ajiparatus, and its weight calculated in 
the usual way. 

Example : A solution of sodic nitrite was made and standardized with 
permanganate, the result being that 10 c.c.=0'001346 gm. N. 10 c.c. of the 
same solution were evaporated to dryness in a small beaker, about 0'2 gm. of 
urea added, the whole dissolved in 2 c.c. of hot water, which, with the 
rinsings, were transferred through the cup into the tube, treated with 
sulphuric acid and caustic soda, then transferred to the gas apparatus with 
the following results: Volume of N, 13'79 c.c.; mercurial pressure, 127'5 
m.m. ; temperature, 17'7 C. The weight of N thus found, after the 
necessary corrections, was 0'0013645 gm. 



71. OXYGEN. 269 

The Crum -Frank land mercury method, described in the 
section on Water Analysis, and in which the same shaking tube is 
used, does not distinguish between nitric and nitrous nitrogen ; 
but Percy Frank land required a method for the estimation of 
nitrous acid in a mixture of nitrates, peptones, sugar, and various 
salts occurring in a solution used for cultivation of micro-organisms, 
and the experiments carried out by him showed that when such 
a mixture was evaporated to dryness the loss of HNO 2 was consider- 
able, and the results came out much too low. Further experiment, 
however, showed that the addition of a slight excess of caustic 
potash during evaporation prevented the loss of any HNO 2 ; and 
on the other hand the addition of a slight excess of ammonic chloride 
entirely destroyed it. Therefore by a combination of the mercury 
and the urea methods, the estimation of nitric and nitrous acids 
may be satisfactorily accomplished, the destruction of the HXO 2 
on the one hand being effected by excess of NH 4 C1, whilst on the 
other hand all loss of HJSTO 2 may be avoided by evaporation with 
caustic alkali. The mode of procedure has the advantage over all 
differential methods, in that each acid is determined individually 
and independently of the other. 

4. Mixtures of Alkaline Sulphites, Thiosulphates, and Nitrites. 

Lunge and Smith (J. S. C. I. ii. 465) have shown that the only 
satisfactory method of completely oxidizing sulphites and thio- 
sulphates by permanganate is to add to the solution a large excess 
of permanganate, more than sufficient for complete oxidation, and 
Avith formation of MnO 2 . Excess of FeSO 4 is then added, and 
again permanganate till pink. When such a mixture contains 
nitrites, they will of course be oxidized to nitrates. 

To find the amount of nitrites present, therefore, the following 
method is adopted : 

The solution of the substance in not too large quantity is 
exactly oxidized as described, a known volume of standard ferrous 
sulphate is added, together with a large excess of strong H 2 SO 4 . 
The mixture is boiled nearly to dryness in a flask with slit valve, 
diluted, and, when cool, titrated with permanganate. The difference 
between the volume then required and that required by the original 
Fe 2 SQ 4 , represents the nitric acid which has been reduced and 
escaped as NO. 

The exceedingly delicate colorimetric method of estimating 
nitrites originally devised by Griess, and improved by others, will 
be described in the section on Water Analysis. 

OXYGEN. 

0=16. 

71. THE volumetric determination of the dissolved oxygen in 
water, .is an operation of some importance in water analysis. It is. 



270 VOLUMETRIC ANALYSIS. 71. 

well known that organic and bacterial contamination generally 
exist side by side ; the organic matter offering a suitable nidus for 
the growth of bacterial life. Water thus contaminated is 
de-oxygenated by the living organisms, which consume oxygen 
during their growth ; hence the importance of the estimation of 
dissolved oxygen in water, as a means of ascertaining the 
co-existence of the two kinds of impurity. 

In brewing also a knowledge of the state of aeration of the wort 
is sometimes of importance, especially at the fermentation stage of 
the process. 

Several methods have been proposed for carrying out the 
estimation. Mo hr's method, depending on the oxidation of ferrous 
compounds, with subsequent titration by permanganate, has not 
come greatly into use. Winkler (Bericlite, 1888, 2851) has quite 
recently proposed to take advantage of the oxidation of manganous 
hydroxide* by dissolved oxygen, the higher oxide formed being 
decomposed by sulphuric acid and potassic iodide with liberation 
of iodine, which is estimated by titration with sodic thiosulphate. 
This method is disturbed by the presence of nitrites, which also 
liberate iodine from acidified potassic iodide ; great organic con- 
tamination also interferes, inasmuch as the impurities present take 
up a portion of the liberated iodine. 

Schiitzenberger's method,! fully described in the last edition 
of this book, has received great attention from many operators, 
some of whom have reported favourably, whilst others find the 
process unreliable. The reason for the anomalies apparent in the 
reports of the various experimenters is shown in the results of an 
interesting critical investigation of the process carried out by 
Koscoe and Lunt (/. C. S. 1889, 552). They show that an 
important disturbing influence had been overlooked, and explain 
many previously ill-understood points in the process. 

Schiitzenberger's original process depends on the reducing 
action of sodic hyposulphite Na 2 S0 2 , prepared by the action of 
zinc dust on a saturated solution of sodic bisulphite, containing 
an excess of sulphurous acid. The estimation was originally 
carried out in a large "Woullf 's bottle, of about two liters capacity, 
filled with pure hydrogen. About 20 30 c.c. of water were 
introduced, and slightly coloured blue by indigo-carmine solution. 
The blue colour was then cautiously discharged by the careful 
dropping in of hyposulphite solution. To the yellow reduced 
liquid thus produced, the water to be examined was added from 
.a pear-shaped vessel holding about 250 c.c. The dissolved oxygen 
restored the blue colour by oxidation, and the amount of hypo- 
. sulphite required to again decolorize the liquid was noted. 

Schiitzenberger showed that when a small amount of indigo 

* Obtained by mixing solutions of a manganous salt and caustic alkali. 

t See Fermentation by P. S c h il t z e u b e r g e r (International Scientific Scries). 



71. OXYGEN. 271 

was employed in the estimation, the' yellow colour produced when 
the titration was completed quickly returned to blue, and this 
when decolorized again turned blue, and so on for some time, until 
double the first amount of hyposulphite had been used. He 
showed also that by using a much larger amount of indigo the 
double portion of hyposulphite was required at once. 

By titrating an ammoniacal solution of copper sulphate with the 
hyposulphite used he arrived at a value (though an erroneous one) 
for the hyposulphite employed in his experiments, and concluded 
that, at the first yellow colour produced in a titration where 
a small amount of indigo was used, only half the oxygen actually 
present had been obtained. The other half he accounted for by 
saying that the reaction between hyposulphite and dissolved oxygen 
is such, that one-half the oxygen becomes latent as hydrogen 
peroxide, which slowly gives up half its oxygen. He thus accounted 
for the return of the blue colour, as well as his observation that 
only half the oxygen was at once obtained. To explain the 
observation, that when a large amount of indigo was employed 
the wliole, of the dissolved oxygen was found, he assumed that 
a different reaction takes place, one between dissolved oxygen 
and reduced indigo, in which the peroxide of hydrogen is not 
formed. 

Ramsay and Williams (/. C. S. 1886, 751), whilst agreeing 
with Schiitzenberger and with Dupre,* that the process gives 
reliable results, throw a doubt on the chemical explanation given 
of the above experiments. 

Instead of the ratio 1 : 2, they find 3 : 5 to be the ratio 
between the first and total quantity of hyposulphite required when 
-a small amount of indigo is employed, but give it only as the mean 
expression of the varying ratios they obtain, and add, " but it is 
difficult to devise an equation which will in a rational manner 
account for this partition of oxygen" into two stages of the 
process. Eoscoe and Lunt's investigation (J". C. S. 1889, 552) 
lias thrown a new light on these experiments. They show (1) that 
a series of fifteen estimations carried out with every care in 
improved apparatus, and under apparently identical conditions, 
gave discordant results, varying between 4 '55 and 6*50 c.c. of 
hyposulphite for the same volume of water, showing a difference 
of 0'35 per cent, of the moan value. (2) The rapidity of titration 
has a great influence on the result. The mean of a series of ten 
estimations carried out drop by drop was 5 '47, whilst ten 
experiments with the same sample of water gave a mean of 7 '12 
when the titration was performed quickly. (3) Not only is 
a low result obtained by a slow titration and a high result by 
a quick one, but by varying the time of titration still more, extreme 
variations in the result are obtained; any value between 1 and 100 

* Analyst x. 156. 



272 



VOLUMETRIC ANALYSIS. 



71. 



per cent, of the total oxygen- present being shown to be possible. 
(4) The ratio between the first reading and the total quantity 
of hyposulphite required is not a constant one, and is shown to 
be capable of an infinite range of variation. 




Fig. 49. 

The key to the explanation of these remarkable results is given 
by the authors as follows: "The conclusion" from their experi- 
ments "was, that when aerated water is introduced into an 
atmosphere of pure hydrogen, it immediately begins to lose oxygen 



71. OXYGEN. 273 

by diffusion into the hydrogen until an equilibrium is established." 
By the recognition of this disturbing influence, the previous 
anomalies are easily explainable on the following data. 

(1) Discordant results are obtained from the same water, 
because the several titrations are not performed in exactly the same 
time, therefore, varying amounts of oxygen diffuse, and leave 
a vary ing 'residue for titration. 

(2) The high results of a quick titration are accounted for by 
the fact that a large amount of oxygen is titrated and fixed before 
it has had time to diffuse, whilst the slow titration gives a low 
result, because a large amount of oxygen has already diffused 
from the liquid before the titration is completed. JS'o greater 
proof of the rapidity with which the water under examination lost 
oxygen by the old process need be given than the fact, that 
Schiitzenberger's results show that half the oxygen had left the 
liquid by diffusion before the estimation could be completed. 

(3) The return of the blue colour is due to the re-absorption 
of the diffused oxygen by the sensitive yellow liquid, oxidation by 
gaseous oxygen producing the blue colour, which is thus not due 
to a reaction -within the liquid. 

(4) The whole of the oxygen is obtained when a large amount 
of indigo is used, because when reduced it is capable of at once 
fixing the whole of the dissolved oxygen and thus prevents 
diffusion. The use of so large a quantity of indigo, necessary to 
effect this result, however, so disturbs the end-reaction that " it is 
difficult to fix the point at which the last trace of blue has been 
discharged with any degree of accuracy" (Dupre loc. cit.). Hence 
a new method must be resorted to in which diffusion is eliminated, 
and Roscoe and Lunt have devised the following method to 
satisfy the conditions of the case. The apparatus employed by 
them is shown in fig. 49. 

It consists essentially (1) of an apparatus for the continuous 
generation and purification of hydrogen, by the action of dilute 
sulphuric acid on zinc ; (2) a 200 c.c. wide-mouthed bottle, fitted 
with three burettes with glass taps, inlet and outlet tubes for 
a current of hydrogen, and an outlet tube for the titrated liquid ; 
(3) Winchester stock bottles of hyposulphite, indigo (not shown), 
and water (sample), communicating with their respective burettes 
by glass* syphons. The hydrogen generated in A passes through 
two wash-bottles containing caustic potash, thence through two 
E miner ling's tubes filled with glass beads, moistened with an 
alkaline solution of potassic pyrogallate, an arrangement being 
made whereby the beads may be re-moistened with fresh pyrogallate 
from the bottles beneath, the liquid being forced up by hydrogen 
pressure. Pure hydrogen is supplied continuously (1) to the 

* India-rubber tubing must not be used for the conveyance of the hyposulphite 
solution (or the water under examination), as atmospheric oxygen rapidly diffuses 
through the india-mbber and affects the strength of the solution. 

T 



274 VOLUMETKIG ANALYSIS. 71. 

stock bottle of hyposulphite, (2) to the hyposulphite burette, and 
(3) to the titration bottle. 

Preparation of the Reagents. The reagents required arc 
Hyposulphite solution. 
Indigo-carmine solution. 
Standard aerated distilled water. 

The Hyposulphite solution is prepared by dissolving 125 gm. of 
sodic bisulphite in 250 c.c. of water, and passing a current of SO 2 
through the solution until saturation is effected. The solution is 
poured into a stoppered bottle of about 500 c.c. capacity, containing 
50 gm. of zinc dust, the bottle is almost filled up with water, and 
the mixture well shaken for five minutes, after which the bottle is 
placed beneath a running tap to cool. The mixture is again 
agitated after a quarter of an hour and left to deposit the excess of 
zinc. The clear liquid is poured off from the sediment into 
a Winchester quart bottle half full of water. Milk of lime is 
added in excess, and the solution made up to fill the bottle almost 
completely. The mixture is now thoroughly shaken and allowed 
to stand (best overnight) until clear. 

The solution thus obtained is much too strong for use. 200 c.c. 
of this may be poured into a "Winchester quart bottle of water 
(never into a bottle filled with air) and well shaken with as little 
air as possible. The approximate strength of this dilute solution 
must now be found by titrating good tap water in the apparatus 
already described. The strength should be such that 100 c.c. of 
water require about 5 c.c. of hyposulphite, and the solution should 
be made up approximately to this value. It slowly loses strength 
on keeping, even in hydrogen, and its value should be determined 
daily as required to be used. 

The Indigo-carmine solution is prepared by shaking up 200 gm. 
of indigo-carmine in a Winchester quart bottle of water, and 
filtering the blue solution, which must be diluted to such a strength 
that 20 c.c. require about 5 c.c. of the above hyposulphite solution 
for decolorization. 

Standard Aerated Distilled "Water. Two Winchester quart bottles 
half filled with freshly distilled water are vigorously agitated for 
five minutes, and the air renewed several times by filling up one 
bottle with the contents of the other, and again dividing into two 
portions, which are repeatedly shaken with fresh air. Finally, one 
bottle being filled, the temperature of the water is taken, and also 
the barometric pressure, after which the bottle is allowed to stand 
stoppered for half an hour, to get rid of minute air-bubbles. The 
following table, due to Eoscoe and Lunt, gives the volume of 
oxygen contained in this standard aerated water, and the results 
show that Buns en's co-efficients, previously used, are inaccurate. 



OXYGEN. 275 

Oxyg-en Dissolved by Distilled Water. 530 C. 







| . 






Temp. 
C. 


c.c. Oxygen 
N.T.P. 
per liter Aq. 


Diff. for ' Temp. 
0'5 C. C. 


c.c. Oxygen 
N.T.P. 
per liter Aq. 


Diif. for 
0-5 C. 


5-0 


8-68 


18'0 


6-54 


0-07 


5-5 


8-58 


o-io 


18'5 


6-47 


0-07 


6-0 


8-49 


0'09 19'0 


6-40 


006 


6-5 


8'40 


0'09 


19-5 


6'34 


0-06 


7-0 


8'31 


0-09 20-0 


6-28 


0-06 


7'5 


8-22 


0-09 


20-5 


6-22 


0-06 


8'0 


8-13 


0-09 21-0 


6-16 


0-06 


8'5 


8-04 


0'09 21-5 


6-10 


0-06 


9-0 


7-95 


0-09 22-0 


6*04 


0-05 


9-5 


7-86 


0'09 


22-5 


5-99 


0-05 


10-0 


777 


0-09 


23-0 


5'94 


0'05 


10-5 


7-68 


0-08 23-5 5-89 


0-05 


ll'O 


7-60 


0-08 24-0 5-84 


O'Ol 


11-5 


7'52 


0-08 


24-5 


5-80 


0-04 


12-0 


744 


0-08 


25'0 


5-76 


0-04 


12'5 


7'36 


0-08 


25'5 5-72 


0-04 


13-0 


7'28 


0'08 


28-0 5'68 


0-04 


13-5 


7-20 


0'08 


26'5 5'64 


0'04 


14-0 


7-12 


0'08 


27'0 5'60 


0'03 


14-5 


7-04 


0-08 


27'5 5'57 


0'03 


15'0 


6-96 


0-08 


28-0 5-54 


0-03 


15-5 


6-89 


0-07 28'5 5-51 


0-03 


16-0 


6-82 


0-07 29-0 5-48 


0-03 


16-5 


675 


0'07 29'5 5-45 


0-02 


17'0 


668 


0'07 30'0 5-43 




17'5 


6-61 


0'07 


i 



In this table the results are calculated for aeration at an observed 
barometric pressure of 760 m.m. When the observed pressure is below 
760 m.m. T Vth the value must be subtracted for every 10 m.m. diff. The 
same^value must be added when the pressure is above 760 m.m. 

The Estimation : The burettes having been filled, and a preliminary trial 
made 

(1) 20 c.c. of the water are introduced into the small bottle and about 
3 c.c. of indigo solution added. 

(2) A moderite current of hydrogen is passed through the blue liquid by 
a very fine jet for three minutes to free both water and supernatant gas 
from free oxygen. 

(3) Hyposulphite is now carefully added, during the flow of hydrogen, 
until the change from blue to yellow occurs, taking care not to overstep this 
point. 

(4) A further measured quantity of hyposulphite is now added (say 10 c.c.) 
sufficient to combine with all the dissolved oxygen in the volume of water 
(50 100 c.c.) proposed to be used in the estimation. 

(5) The important point is, that the water is now quickly run in from 
a burette by a capillary tube passing beneath the surface of the liquid to the 
bottom of the vessel. The water is thus introduced into a liquid which will 
at once fix the free oxygen and thus prevent its diffusion on coming in 
contact with the hydrogen, the reduced indigo acting as an indicator for the 
complete oxidation of the hyposulphite. The liquid is kept in constant 
motion during the addition of the water, which is shut off the moment 
a permanent blue colour appears. 

T 2 



276 VOLUMETRIC ANALYSIS. 71. 

(6) The blue is decolorized by a further slight addition of hyposulphite. 
The volume of water used and the total hyposulphite, minus the first 
addition, are noted and the estimation repeated for confirmation. 

When the water contains very little oxygen the second addition 
of hyposulphite may be omitted, the reduced indigo-carmine being 
-sufficient to take up all the dissolved oxygen. In this case, care 
must be taken that the oxygen added should require not more 
than half the hyposulphite first added to decolorize the indigo- 
carmine. 

Standardizing: the Hyposulphite. In order to complete the 
estimation it is necessary to know the strength of the hyposulphite 
solution employed, and for this purpose the bottle of standard 
aerated distilled water is titrated. This method has the great 
advantage that it is a titration carried out under almost the same 
conditions as the examination of the sample. The result of an 
estimation is easily obtained by the following formula 

d x hs x Od 

r~y = # c.c. O per liter of water 
8 x lid 

where d and s = the volumes of distilled water and sample 
respectively used, lid and hs the hyposulphite required for the 
distilled water and sample respectively, arid Od the volume of 
dissolved oxygen contained in one liter of the standard water. 

Standardizing- the Indigo. When once the hyposulphite has 
been carefully standardized by distilled water, the rather trouble- 
some aeration may be avoided by finding the oxygen-value of the 
indigo-carmine solution. This solution remaining constant may be 
used for the subsequent standardizing of the hyposulphite. 

It is only necessary to take a suitable quantity of indigo solution, 
diluted with water if necessary, free it from all dissolved oxygen 
by a current of pure hydrogen continued for five minutes, then 
carefully decolorize with hyposulphite, the value of which has 
been found by using aerated distilled water. 

The authors show that Schutzenberger's method of standard- 
ization, depending on the decolorization of ammoniacal copper 
sulphate, gives inaccurate results. 

Free acids or alkalies greatly disturb the process. Bicarbonates 
have no effect. Of course when other substances than oxygen, 
which decompose hyposulphite, are present, the accuracy of the 
method is proportionately disturbed. The authors have applied the 
process to waters of very varied character, and containing widely 
different amounts of oxygen, and show that the method is capable 
of giving good results, compared with the actual volume of oxygen 
found by extracting the gases by boiling in vacua. 

The delicacy of the reaction is such that one part of oxygen in 
two million parts of water is easily detected. 



71. 



OXYGEN. 



277 



The following numbers were obtained from five different samples 
of London tap- water collected on five different days. 





(1) 


(2) 


(3) 


(4) 


(5) 


Nitrogen 


c.c. 
13*22 
5-15 

7'98 


c.c. 
13-95 
5-91 
9-29 


c.c. 
13-36 
5-38 
6-70 


c.c. 
13-43 
6-31 
7-35 


c.c. 
13-49 
5-80 
8-11 


Oxygen 


Carbonic acid 
Total o-as . . 


26-35 


29-15 


25-44 


27-09 


27-40 




Oxygen by the new 
volumetric method . . . 
Gas obtained 


5-52 
5-15 


6-13 
5-91 


5-64 
5-38 


6-41 
6'31 


6-24 
5-80 


Difference . 


0-37 


0-22 


0-26 


o-io 


0-44 




Mean difference 0'28 c.c. oxygen per liter of water. 



The oxygen values obtained by the two methods show close 
agreement, considering the possible experimental error in so 
complex a comparison. 

M. A. Adams describes and figures a very convenient arrange- 
ment for carrying out this process (J. C. S. Ixi. 310), which is 
well adapted for technical work, and less cumbrous than the 
apparatus here described. 

lodometric Method. 

A simpler method than the foregoing has been proposed by 
Thresh (/. C. S. Ivii. 185), which by comparison with Roscoe 
and Lunt's method appears to give satisfactory results when 
aerated distilled water was under titration, the differences occurring 
only in the second decimal place. The author was led to 
investigate the method by observing the. large amount of iodine 
which a very minute quantity of a nitrite caused to be liberated, 
when potassic iodide and dilute sulphuric acid were added to water 
containing it. The amount of iodine liberated varies with the 
length of exposure to air. If air is excluded no increase of free 
iodine occurs after the first few minutes, and if the water is 
previously boiled and cooled in an air-free space still less iodine is 
liberated. In this latter case the action is represented by the 
equation 

2HI + 2HX0 2 = I 2 + 2H 2 + 2X0. 

When oxygen has access to the solution, the nitric oxide acts as 
a carrier, and more hydrogen iodide is decomposed, the nitric oxid 



2/8 VOLUMETRIC ANALYSIS. 71. 

apparently remaining unaffected, and capable of causing the 
decomposition of an unlimited quantity of the iodide. 

This reaction is the one utilized in the process devised by 
Thresh for estimating the oxygen dissolved in water. As 16 parts 
by weight of oxygen will liberate 254 parts of iodine, thus 



and as the latter element admits of being accurately estimated, 
theoretically the oxygen should be capable of very precise 
determination. Practically such is the case ; the oxygen dissolved 
in drinking waters admits of being estimated both rapidly and 
with precision. It is only necessary to add to a known volume of 
the water a known quantity, of sodic nitrite, together with excess 
of potassic iodide and acid, avoiding access of air, and then to 
determine volumetrically the amount of iodine liberated. After 
deducting the proportion due to the nitrite used, the remainder 
represents the oxygen which was dissolved in the water and in the 
volumetric solution used. 

The following are the reagents required : 

(1) Solution of sodic nitrite and potassic iodide : 

Sodic nitrite 0'5 gm. 

Potassic iodide 20'0 gm. 

Distilled water 100 c.c. 

(2) Dilute sulphuric acid : 

Pure sulphuric acid 1 part. 

Distilled water 3 parts. 

(3) A clear fresh solution of starch. 

(4) A volumetric solution of sodic thiosulphate : 

Pure crystals of thiosulphate, 7 '7 5 gm. 

Distilled water to 1 liter. 

1 c.c. corresponds to 0'25 milligram of oxygen. 

The apparatus required is very simple, and can readily be fitted 
up. It consists of a wide-mouthed white glass bottle (A, fig. 50) 
of about 500 c.c. capacity, closed with a caoutchouc stopper having 
four perforations. Through one passes the tube B, drawn out at 
its lower extremity to a rather fine point, and connected at the 
upper end, by means of a few inches of rubber tubing, with the 
burette C, containing the thiosulphate. Through another opening 
passes the nozzle of a separatory tube D, having a stopper and 
stopcock. The capacity of this tube when full to the stopper 
must be accurately determined. Through the third opening passes 
a tube E, which can be attached to an ordinary gas supply. Through 
the last aperture is passed another tube, for the gas exit, and to 
tin's is attached a sufficient length of rubber tubing to enable the 



71. 



OXYGEN. 



279 



cork G at its end to be placed in the neck of the tube D when the 
stopper is removed. A small piece of glass tube projects through 
the cork, to allow of the escaping gas being ignited. 

The apparatus is used in the following manner : The bottle A 
being cleaned and dry, the perforated bung is inserted, the burette 
charged, and the tube B fixed in its place. E is connected with 
the gas supply. The tube D is filled to the level of the stopper 
with the water to be examined, 1 c.c. of the solution of sodic 
nitrite and potassic iodide added from a I c.c. pipette, then 1 c.c. 
of the dilute acid, and the stopper instantly fixed in its place, 
displacing a little of the water, and including no air. If the 
pipette be held in a vertical position with its tip just under the 
surface of the water, botli the saline solution and the acid, being 
much denser than the water, flow in a sharply defined column to 
the lower part of the tube, so that an infinitesimally small quantity 




(if any) is lost in the water which overflows when the stopper is 
inserted. The tube is next turned upside down for a few seconds 
for uniform admixture to take place, and then the nozzle is pushed 
through the bung of the bottle, and the whole allowed to remain 
at rest for 15 minutes, to enable the reaction to become complete. 
A rapid current of coal gas is now passed through the bottle A, 
until all the air is displaced and the gas burns at G with a full 



280 VOLUM ETHIC ANALYSIS. 71. 

luminous flame ; the flame is now extinguished, the stopper of D 
removed, and the cork G rapidly inserted. On turning the stop- 
cock, the water flows into the bottle A. The stopcock is turned 
off, the cork G removed, and the supply of gas regulated so that 
a small flame only is produced when this gas is ignited at G. 
Thiosulphate is now run in slowly until the colour of the iodine is 
nearly discharged. A little solution of starch is then poured into 
D, and about 1 c.c. allowed to flow into the bottle by turning the 
stopcock. The titration with thiosulphate is then completed. 
After the discharge of the blue colour, the latter returns faintly in 
the course of a few seconds, due to the oxygen dissolved in the 
volumetric solution ; after standing about two minutes, from 0*05 
to O'l c.c. of thiosulphate must be added to effect the final 
discharge. The amount of volumetric solution used must now be 
noted. This will represent a, the oxygen dissolved in the water 
examined, + &, the nitrite in the 1 c.c. of solution used, and the 
oxygen in the acid and starch solution + c, a portion of the dissolved 
oxygen in the volumetric solution. To find the value of a, it is 
obvious that I and c must be ascertained. This can be effected in 
many ways, and once known does not require re-determination 
unless the conditions are changed. 

To Find the Value of 1>. Probably the best plan is to complete 
a determination as above described, and then, by means of the 
stoppered tube, introduce into the bottle in succession 5 c.c. of 
nitrite solution, dilute acid, and starch solution. After standing 
a few minutes, titrate. One-fifth of the thiosulphate used will be 
the value required. 

To Find the Value of c. This correction is a comparatively 
small one, and admits of determination with sufficient accuracy if 
it is assumed that the thiosulphate solution normally contains as 
much dissolved oxygen as distilled water saturated at the same 
temperature. Complete a determination as above described, then 
remove the stoppered tube, and insert a tube similar to that 
attached to the burette, and drop in from it 10 or 20 c.c. of 
saturated distilled water exactly as the thiosulphate is dropped in. 
Allow to stand a few minutes and titrate. One-tenth or one- 
twentieth of the volumetric solution used, according to the 
number of c.c. of water added, will represent the correction for 
each c.c. of volumetric solution used. Call this value d. 

Let e be the number of c.c. of thiosulphate used in an actual 
determination of the amount of oxygen in a sample of water ; 

/= the capacity in c.c. of the tube employed -2 c.c., the volume 
of reagents added ; 

17 = the amount of oxygen in milligrams dissolved in 1 liter of 
the water ; 

1000, 7 ,, 
then V = -. . (e-l- e<?) 



71. OXYGEN. 



With a tube made to hold exactly 250 c.c., the most convenient 



quantity to use, becomes unity, and 



In the author's experiments two nitrite solutions were used; 
in the first l = 2'l c.c., in the second 3'1 c.c. A number of 
determinations of d were made, at temperatures varying from 
40 to 60 F. The value of d was found to vary between 0'03 
and 0*031 5. In all the author's recent experiments d was taken 
as 0-031. 

When e = 3 c.c. the reaction seems to be complete in five 
minutes, but, to be on the safe side, it is better to fix the minimum 
at fifteen minutes. 

The use of coal-gas is recommended by the author without 
passing it over alkaline pyrogallol or otherwise treating it before 
allowing it to pass through the apparatus. 

The results obtained, however, can be made to vary, the extreme 
limit being less than 0'5 milligram of oxygen per liter of water, 
using 250 c.c. for the estimation. To quote an extreme case. In 
one experiment (1), after the air had been wholly expelled from 
the bottle A, no more gas was passed through, and the titration 
was effected in the closed apparatus, the volumetric solution being 
run in as rapidly as possible. The end-reaction was not well 
defined. In the second experiment (2), the volumetric solution 
was run in very slowly drop by drop, and a brisk current of gas 
was kept passing through the apparatus. End-reaction well 
defined. 

Volume of water. Thiosulphate. Oxygen per liter. 

(1) ...... 322 c.c. 15-35 c.c. ' 9 '14 milligrams. 

(2) ...... 322 14-9 8-80 

The difference is probably due to nearly all the oxygen dissolved 
in thiosulphate being used up in the first case, and being lost by 
diffusion in the second. 

In the examination of waters from various sources, and making 
the experiments in pairs, using tubes of different sizes, the author 
found that exceedingly concordant results could easily be 
obtained. 

In estimating the oxygen in distilled water saturated with air,. 
the author found that the results at 25 and 30 C. were higher 
than those obtained by Roscoe and Lunt, whilst at the lower 
temperatures they were almost identical, and it occurred to him that 
the difference was probably due to the mode of saturation. The 
agitation in a couple of Winchesters was done as directed by 
them, but the water used had been previously saturated at the 
lower temperatures, and probably was slightly super-saturated. 
A further series of experiments were then made with freshly- 



282 VOLUMETRIC ANALYSIS. 71. 

distilled water, which was not agitated with air until it had 
attained the desired temperature. The results proved that this 
surmise was correct. Probably some such explanation accounts for 
the uniformly higher results obtained by Dittmar. 

No doubt there will be exceptional cases in which the process 
cannot be used, and others in which some modification may be 
required. A water containing nitrites will require the amount of 
the nitrous acid to be determined if the utmost accuracy is 
required. (A water containing 1 part of HNO 2 in 1,000,000, will 
affect the results + '17 milligram of oxygen per liter, 94 parts 
of the acid corresponding to 16 of oxygen). Where nitrites are 
present in sufficient quantity to interfere, the amount may be 
determined by any of the ordinary processes, but the author 
prefers the following method : 

To 250 c.c. of the water to be examined, rendered faintly 
alkaline if not already so, add a few drops of strong solution of 
potassic iodide, and boil vigorously for a few minutes. Then 
transfer to the bottle A used in the oxygen determination, and 
allow to get quite cold in a slow current of coal gas. Then add 
a few drops of dilute sulphuric acid and solution of starch, and 
titrate with the thiosulphate. The correction to be made in the 
oxygen determination is thus ascertained. One or two experi- 
mental results may be quoted. 



Quantity Thiosulphate p.,^ ar -\ Milligrams of 
of water. used. CoiiectecL oxygen per liter. 



Tap water 232T> 13'2 9'7 10'43 

Tap water + 5 milli- ^ 

grams commercial C 232' 5 15'95 9' 55 10'27 

sodic nitrite ) 

Tap water + 10 milli- ) 000 . K i n-i Q 

1 -j "* <_>_ O Jo O 7 ~rO J-V J-/ 

grams sodic nitrite ) 

In number 2, the thiosulphate used by 250 c.c. of the boiled 

water was 2*8 c.c. 

In number 3, the thiosulphate used by 250 c.c. of the boiled 
water was 5 '45 c.c. 

The results are fairly satisfactory, even with such large pro- 
portions of nitrite, proportions far larger than are likely to be met 
with in practice. 

Nitrates do not interfere, even when present in large quantities ; 
but fresh urine, when present to the extent of 1 per cent., has 
a small but very appreciable effect. 

The following is an example of the method at ordinary 
temperature : 




71. HYDROGEN PEROXIDE. 283 

Temperature 15 C. 

Quantity of Thiosulphate | , _ fe _ , d \ Milligrams of Difference 

water taken. used. ' Oxygen per liter, from mean. 



1... 

2... 



322-0 15-45 12-87 9'99 0'04 

322-0 15-55 12-97 10'07 + 0'04 



3... 232-5 11-90 9-43 10'14 +0'11 

4... 232-5 11-70 9-23 9'92 O'll 



Mean... - 10'03 



Barometer reading 30 in. 
10-03 milligrams=7'02 c.c. at N.P.T. 
Eoscoe and Lun't found 6'96 Difference + 0'06. 

Hydrog-en Peroxide. 
IPO 2 =34. 

This substance is now largely used in commerce, and is sold 
as containing 5, 10, or 20 volumes of oxygen in solution. This 
should mean that the specified number of volumes can be obtained 
from the solution itself, but preparations are sent into the market 
under false pretences. A so-called 10 volume solution gives, it is 
true, 10 volumes of when decomposed gasometrically with 
permanganate, but 5 volumes of the comes from the per- 
manganate itself, and therefore such a solution is really only 5 
volume. A true 10 volume solution should yield from itself, when 
fully decomposed, ten times its volume of O, and contain by weight 
3 '04 per cent, of H 2 2 or 1'43 per cent, by weight of 0. 

Kingzett (J. C. S. 1880, 792) has clearly shown that the best 
and most rapid estimation of the hydrogen peroxide, contained in 
.any given solution of it, is made by iodine and thiosulphate in the 
presence of a tolerably large excess of sulphuric acid, the reaction 
being 



The function performed by the sulphuric acid is difficult of ex- 
planation, but the want of uniformity in the reaction experienced 
by many operators no doubt has arisen from the use of insufficient 
acid. 

Process : Kingzett's consists in mixing 10 c.c. of the peroxide solution 
to be examined with about 30 c.c. of dilute sulphuric acid (1 : 2) in a beaker, 
adding crystals of potassic iodide in sufficient quantity, and after standing 
five minutes titrating the liberated iodine with ^ thiosulphate and starch. 
The peroxide solution should not exceed the strength of 2 volumes; if 
stronger, it must be diluted proportionately before the analysis. 

In the case of a very weak solution it will be advisable to titrate with 
x-iju- thiosulphate. 

1 c.c. & thiosulphate = 0'0017 gm. H-O 2 or 0*0016 gm. O. 

The estimation of this substance may also be readily made in the 



284 VOLUMETRIC ANALYSIS. 72. 

absence of organic or other reducing matters by weak standard per- 
manganate in the presence of free sulphuric acid, the permanganate 
being added until a faint rose colour occurs : the reaction is 

2KMn0 4 + 5H 2 2 + 3IPS0 4 = K 2 S0 4 + 2MnS0 4 + 8H 2 + 50 2 . 

Process : To about 500 c.c. of water in a white porcelain dish there is 
added 5 c.c. of dilute H-SO 4 , and then sufficient permanganate to give 
a faint persistent pink colour. 5 c.c. of the peroxide solution are then 
pipetted into the mixture, and standard permanganate containing 2'625 gm. 
per liter run in until the colour no longer disappears. The number of c.c. 
used, divided by ten, gives the volume of oxygen liberated by each c.c. of 
the hydrogen peroxide. 

Carpenter and Nicholson (Analyst ix. 36) report a series 
of experiments on the analysis of hydrogen peroxide, both by the 
iodine and permanganate methods. 

The conclusion they arrive at is, that the process of Kingzett 
is accurate, but in their hands somewhat tedious, owing to slow 
decomposition towards the end. Kingzett however states that if 
a volume of strong sulphuric acid equal to the peroxide taken be 
used, and especially if the dilute solution be slightly wanned, the 
reaction is complete in a few minutes, and this is my own 
experience. 

Soclic Peroxide. 

L. Archbutt (Analyst xx. 5) gives the results of some 
experiments on the estimation of the oxygen contained in this 
substance, and found that a near approximation to the truth could 
be obtained by simple titration with permanganate, the peroxide 
(one or two decigrams) being added to cold water acidified with 
H 2 S0 4 contained in a white dish, and j permanganate dropped in 
with stirring, until the colour became permanent ; but a more 
exact method would be to add a known weight of the peroxide to 
an excess of ~ permanganate, previously mixed with dilute 
H 2 S0 4 , and titrate for the excess of permanganate with -f^ oxalic 
acid. Archbutt, however, prefers to use the nitrometer, and 
recommends the following procedure : about 0*25 gm. of the 
substance is placed in the dry tube of the nitrometer flask, and in 
the flask itself about 5 c.c. of pure water, containing in suspension 
a few milligrams of precipitated cobalt sesqui-oxide, this latter 
reagent brings about a rapid and complete decomposition of the 
peroxide, the volume of oxygen evolved being the available oxygen 
in the sample. 

PHOSPHORIC ACID AND PHOSPHATES. 



72. THE estimation of phosphoric acid volu metrically may 
be done with more or less accuracy by a variety of processes, among 



72. PHOSPHORIC ACID. 285 

which may be mentioned that of Mohr as lead phosphate, the 
indirect method as silver phosphate (the excess of silver being found 
by thiocyanate), by standard uranium nitrate or acetate, by 
P ember ton's method as phospho-molybdate, or when existing 
only as monocalcic phosphate, by standard alkali, as recommended 
by Mo 11 end a or Emm er ling. These processes are mainly useful 
in the case of manures, or the raw phosphates from which manures 
are manufactured, and for P 2 5 in urine, etc. For the purpose 
mentioned, that is to say, when in combination with alkaline or 
earthy alkaline bases and moderate quantities of iron or alumina, 
phosphoric acid may be estimated volumetrically with very fair 
accuracy, and with much greater rapidity than by gravimetric 
means as usually carried out. This remark, however, can only be 
applied to uranium or molybdenum methods ; therefore only these 
will be described. 

1. Precipitation as Uranic Phosphate in Acetic Acid Solution. 

This method is based on the fact that when uranic acetate or 
nitrate is added to a neutral solution of tribasic phosphoric acid, 
such, for instance, as sodic orthophosphate, the whole of the 
phosphoric acid is thrown down as yellow uranic phosphate Ur 2 3 , 
P 2 5 + Aq. Should the solution, however, contain free mineral 
acid, it must be neutralized with an alkali, and an alkaline acetate 
added, together with excess of free acetic acid. In case of using 
ammonia and ammonic acetate, the whole of the phosphoric acid 
is thrown down as double phosphate of uranium and ammonia, 
having a light lemon colour, and the composition Ur 2 :j 
2(XH 4 0), P 2 5 + Aq. When this precipitate is washed with hot 
water, dried and burned, the ammonia is entirely dissipated leaving 
uranic phosphate, which possesses the formula Ur 2 3 , P 2 5 , and 
contains in 100 parts 80 '09 of uranic oxide and 19*91 of phosphoric 
acid. In the presence of fixed alkalies, instead of ammonia, the 
precipitate consists simply of uranic phosphate. By this method 
phosphoric acid may be completely removed from all the alkalies 
and alkaline earths ; also, with a slight modification, from iron ; 
not, however, satisfactorily from alumina when present in any 
quantity. 

The details of the gravimetric process were fully described by 
me (C. N. i. 97 122), and immediately after the publication of 
that article, while employed in further investigation of the subject, 
I devised the volumetric method now to be described. Since that 
time it has come to my knowledge that jS'eubauer* and Pincusf 
had independently of each other and myself arrived at the same 
process. This is not to be wondered at, if it be considered how 
easy the step is from the ordinary determination by weight to that 

* Archiv. f&r wissenschuftliclie Heillcunde, iv. 228. 
f Journal fur Prald. Chem. Ixxvi. 104. 



286 VOLUMETRIC ANALYSIS. 7 '2. 

by measure, when the delicate reaction between uranium and 
potassic ferrocyanide is known. Moreover, the great want of a 
really good volumetric process for phosphoric acid in place of those 
hitherto used has been felt by all who have anything to do with it, 
and consequently the most would be made of any new method 
possessing so great a claim to accuracy as the gravimetric estimation 
of phosphoric acid by uranium undoubtedly does. 

Conditions under -which, accxiracy may be insured. Objections 
have been urged, not without reason, that this process is inaccurate, 
because varying amounts of saline substances have an influence 
upon the production of colour with the indicator. Again, that 
very different shades of colour occur with lapse of time. This 
is all true, and the analysis is unfortunately one of that class which 
requires uniform conditions; but when the source of irregularity is 
known, it is not difficult to obviate them. Therefore it is absolutely 
essential that the standardizing of the uranium solution should be 
done under the same conditions as the analysis. For instance, a 
different volume of uranium will be required to give the colour in 
the presence of salts of ammonia to that which would be necessary 
with the salts of the fixed alkalies or alkaline earths. But if the 
standard solution is purposely adjusted with ammonia salts in about 
the same proportion, the difficulties all vanish. Fortunately this 
can be easily done, and as the chief substances requiring analysis 
are more or less ammoniacal in their composition, such as urine, 
manures, etc., no practical difficulty need occur. 

Excessive quantities of alkaline or earthy salts modify the colour, 
but especially is it so with acetate or citrate of ammonia. For this 
reason it is necessary to ensure the complete washing of the citro- 
magnesian precipitate, where that method of separating P 2 5 is 
adopted previous to titration. 



2. Estimation of Phosphoric Acid in combination with Alkaline 
Bases, or in presence of small quantities of Alkaline Earths. 

The necessary materials are 

(a) A standard solution of Uranium, 1 c.c. 0*005 gm. P 2 5 . 

(&) A standard solution of tribasic Phosphoric acid. 

(c) A solution of Sodic acetate in dilute acetic acid, made by 
dissolving 100 gm. of sodic acetate in water, adding 50 c.c. of 
glacial acetic acid, and diluting to 1 liter. Exact quantities are 
not necessary. 

(d) A freshly prepared solution of Potassic ferrocyanide, or 
some finely powdered pure crystals of the same salt. 

Standard Solution of Uranium. This solution may consist 
either of uranic nitrate or acetate. An approximate solution is 
obtained by using about 35 gm. of either salt to the liter. 
In using uranic nitrate it is imperative that the sodic acetate 



72. PHOSPHORIC ACID. 287 

should be added in order to avoid the possible occurrence of free 
nitric acid in the solution. With acetate, however, it may be 
omitted at the discretion of the operator, but it is important 
that the method used in standardizing the uranium be invariably 
adhered to in the actual analysis. The solution should be perfectly 
clear and free from basic salt. Whether made from acetate or 
nitrate, it is advisable to include about 50 c.c. of pure glacial acetic, 
or a corresponding quantity of weaker acid to each liter of 
solution ; exposure to light has then less reducing action. 

My own practice is to use in all cases acetate solution, and 
dispense entirely with the addition of sodic acetate. 

3. Titration of the Uranium Solution. 

Standard Phosphoric Acid. When the uranium solution is not 
required for phosphate of lime, it may be titrated upon ammonio- 
sodic phosphate (microcosmic salt) as follows : 5'886 gm. of the 
crystallized, non-effloresced salt (previously powdered and pressed 
between bibulous paper to remove any adhering moisture) are 
weighed, dissolved in water, and diluted to 1 liter. 50 c.c. of this 
solution will represent O'l gm. of P-0 5 .* 

Process : 50 c.c. of this solution are measured into a small beaker, 5 c.c. sodic 
acetate solution added if uranic nitrate is to be used, and the mixture heated to 
90 or 100 C. The uranium solution is then delivered in from a burette, 
divided into T V c.c., until a test taken shall show the slight predominance of 
uranium. This is done by spreading a drop .or two of the hot mixture upon 
a clean white level plate, and bringing in contact with the middle of the drop 
a small glass rod moistened with the freshly made solution of f errocyanide, 
or a dust of the powdered salt. The occurrence of a faint brown tinge shows 
an excess of uranium, the slightest amount of which produces a brown 
precipitate of uranic f errocyanide. 

A second or third titration is then made in the same way, so as 
to arrive exactly at the strength of the uranium solution, which 
is then diluted and re-titrated, until exactly 20 c.c. are required to 
produce the necessary reaction with 50 c.c. of phosphate. 

Suppose 18*7 c.c. of the uranium solution have been required to 
produce the colour with 50 c.c. of phosphate solution, then every 
18 '7 c.c. will have to be diluted to 20 c.c. in order to be of the 
proper strength, or 935 to 1000. After dilution, two or three 
fresh trials must be made to insure accuracy. 

It is of considerable importance that the actual experiment for 
estimating phosphoric acid by means of the uranium solution 
should take place with about the same bulk of fluid that has 
been used in standardizing the solution, and with as nearly as 

* "W. B. Giles, who has had great experience in the determination of phosphoric acid 
in various forms, has called my attention to dihydric potassic phosphate, KH-'PO 4 , as an 
excellent form of salt for a standard solution. The sample sent to nie was in beautifully 
formed crystals which do not alter on exposure to the air, and makes a solution which 
keeps clear. Every one knows how unsatisfactory sodic phosphate is, both as to its 
state of hydration and its keeping qualities in solution : the microcosmic salt is better, 
but is open to objection on the score of indefinite hydration. If the potassium salt ia 
used, a standard solution of the proper strength is made by dissolving 3'83 gm. in a liter. 



288 VOLUMETRIC ANALYSIS. 72. 

possible the same relative amount of sodic acetate, and the 
production of the same depth of colour in testing. Hence the 
proportions here recommended have been chosen, so that 50 c.c. of 
liquid shall contain O'l gm. P 2 5 . 

Standard Phosphoric Acid corresponding- volume for volume with 
Standard TJianium. This solution is obtained by dissolving 
14 '7 15 gm. of microcosmic salt in a liter, and is two and a half 
times the strength of the solution before described ; it is used for 
residual titration in case the required volume of uranium is over- 
stepped in any given analysis. 

A little practice enables the operator to tell very quickly the 
precise point ; but it must be remembered that when the two drops 
are brought together for the production of the chocolate colour, 
however faint it seems at first, owing to the retarding action of the 
sodic acetate and acetic acid upon the formation of uranic 
ferrocyanide, if left for some little time the colour increases con- 
siderably ; but this has no effect upon the accuracy of the process, 
since the original standard of the solution has been based on an 
experiment conducted in precisely the same way. 

Process : In estimating unknown quantities of P 2 5 , it is necessary to have 
an approximate knowledge of the amount in any given material, so as to 
fulfil as nearly as possible the conditions laid down above ; that is to say, 
50 c.c. of solution shall contain about O'l gm. P 2 O 5 , or whatever other pro- 
portion may have been used in standardizing the uranium. 

The compound containing the P 2 5 to be estimated is dissolved in water ; 
if no ammonia is present, 1 c.c. of 10 per cent, solution is dropped in and 
neutralized with the least possible quantity of acetic acid (also 5 c.c. of sodic 
acetate if uranic nitrate has to be used), and the volume made up to about 
50 c.c., then heated to about 90 C. on the water bath, and the uranium 
solution delivered in cautiously, with frequent testing as above described, 
until the faint brown tinge appears. 

The first trial will give roughly the amount of solution required, and 
taking that as a guide, the operator can vary the amount of liquid and sodic 
acetate for the final titration, should the proportions be fonnd widely 
differing from those under Avhich the strength of the uranium was 
originally fixed. 

Each c.c. of uranium solution=0'005 gm. P 2 O 5 . 

. Estimation of Phosphoric Acid in combination with Lime and 
Magnesia (Bones, Bone Ash, Soluble Phosphates, and other 
Phosphatic Materials, free from Iron and Alumina). 

The procedure in these cases differs from the foregoing in two 
Tespects only ; that is to say, the uranium solution is preferably 
standardized by tribasic calcic phosphate ; and in the process of 
titration it is necessary to add nearly the full amount of uranium 
required before heating the mixture, so as to prevent the precipita- 
tion of calcic phosphate, which is apt to occur in acetic acid 
solution when heated; or the modification adopted by Fresenius, 
Xeubauer, and Luck, may be used, which consists in reversing 
"the process by taking a measured volume of uranium, and delivering 



72. PHOSPHORIC ACID. 289 

into it the solution of phosphate until a drop of the mixture ceases 
to give a brown colour with ferrocyanide. This plan gives, how- 
ever, much more trouble, and possesses no advantage on the score 
of accuracy, because in any case at least two titrations must occur, 
and the first being made somewhat roughly, in the ordinary way, 
shows within 1 or 2 c.c. the volume of standard urani am required ; 
and in the final trial it is only necessary to add at once nearly the 
quantity, then heat the mixture, and finish the titration by adding 
a drop or two of uranium at a time until the required colour is 
obtained. 

This reversed process is strongly advocated by many operators, 
but except in rare instances I fail to see its superiority to the direct 
method for general use. The best modification to adopt in the 
reverse process is to use invariably an excess of uranium, and to 
titrate back with standard phosphate solution till the colour 
disappears ; this avoids all the trouble of preparing and cleaning 
a burette for the solution to be analyzed, and if a standard phosphate 
is made to correspond volume for volume with the uranium, an 
analysis may always be brought into order at any stage. 

Standard Calcic Phosphate. It is not safe to defend upon the 
usual preparations of tricalcic phosphate by weighing any given 
quantity direct, owing to uncertainty as to the state in which the 
phosphoric acid may exist ; therefore, in order to titrate the 
uranium solution with calcic phosphate, it is only necessary to 
take rather more than 5 gin. of precipitated pure tricalcic phosphate 
such as occurs in commerce, dissolve it in a slight excess of dilute 
hydrochloric acid, precipitate again with a slight excess of ammonia, 
re-dissolve in a moderate excess of acetic acid, then dilute to 
a liter ; by this means is obtained a solution of acid monocalcic 
phosphate, existing under the same conditions as occur in the 
actual analysis. In order to ascertain the exact amount of tribasic 
phosphoric acid present in a given measure of this solution, two 
portions of 50 c.c. each are placed in two beakers, each holding 
about half a liter. A slight excess of solution of uranic acetate 
or nitrate is then added to each, together with about 10 c.c. of the 
acetic solution of sodic acetate ; they are then heated to actual 
boiling on a hot-plate or sand-bath, the beakers filled up with 
boiling distilled water, and then set aside to settle, which occurs 
very speedily. The supernatant fluid should be faintly yellow 
from excess of uranium. When perfectly settled, the clear liquid 
is withdrawn by a syphon or poured off as closely as possible with- 
out disturbing the precipitate, and the beakers again filled up with 
boiling water. The same should be done a third time, when the 
precipitates may be brought on two filters, and need very little 
further washing. 

"When the filtration is complete, the filters are dried and ignited 
separate from the precipitate, taking care to burn off all carbon. 

u 



290 VOLUMETRIC ANALYSIS. 72. 

Before being weighed, however, the uranic-phosphate must be 
moistened with strong nitric acid, dried perfectly in the water bath 
or oven, and again ignited ; at first, very gently, then strongly, so 
as to leave a residue when cold of a pure light lemon colour. This 
is uranic phosphate Ur 2 3 , P 2 5 , the percentage composition of 
which is 80 '09 of uranic oxide, and 19 '91 of phosphoric acid. 

The two precipitates are accurately weighed, and should agree to 
within a trifle. If they differ, the mean is taken to represent the 
amount of P 2 0. 5 in the given quantity of tricalcic phosphate, from 
which may be calculated the strength of the solution to be used as 
a standard. Of course any other accurate method of determining 
the P 2 5 may be used in place of this. 

The actual standard required is 5 gm. of pure tricalcic phosphate 
per liter ; and it should be adjusted to this strength by dilution, 
after the actual strength has been found. In this way is obtained 
a standard which agrees exactly with the analysis of a super- 
phosphate or other similar manure. 

Standard Uranium Solution. This is best adjusted to such 
strength that 25 c.c. are required to give the faint chocolate colour 
with ferrocyaiiide, when 50 c.c. of the standard acetic solution of 
calcic phosphate are taken for titration. Working in this manner 
each c.c. of uranium solution represents 1 per cent, of soluble 
tricalcic phosphate, when 1 gm. of manure is taken for analysis, 
because 50 c.c. of the calcic phosphate will contain monocalcic 
phosphate equal to 0'25 gm. of Ca :j P 2 8 and will require 25 c.c. 
of uranium solution to balance it. 

These standards are given as convenient for manures, but they 
may be modified to suit any particular purpose. 

Process in case of Superphosphate free from Fe and Al, except 
in mere traces : 10 gm. of the substance are weighed, placed in a small glass 
mortar and gently broken down by the pestle, coid water being used to bring 
it to a smooth cream. The material should not be ground or rubbed hard, 
which might cause the solution of some insoluble phosphate in the 
concentrated mixture. The creamy substance is washed gradually without 
loss into a measuring flask marked at 503'5 c.c., the 3'5 c.c. being the space 
occupied by the insoluble matters in an ordinary 25 to 30 per cent, 
superphosphate. The flask is filled to the mark with cold water, and shaken 
every few minutes during about half-an-honr. A portion is then filtered 
through a dry filter into a dry beaker, and 50 c.c.==i gm. of manure 
measured into a beaker holding about 100 c.c. Sufficient 10 per cent, ammonia 
is then added to precipitate the monocalcic phosphate in the form of 
Ca 3 P 2 O 8 (in all ordinary superphosphates there is enough Ca present as 
sulphate to ensure this, and four or five drops of ammonia generally suffice 
to effect the precipitation). Acetic acid is then added in just sufficient 
quantity to render the liquid clear. Should traces of gelatinous A1PO 4 or 
FePO 4 occur at this stage, the liquid will be slightly opalescent ; but this 
may be disregarded if only slight, as the subsequent heating will enable the 
uranium to decompose it. If more than traces occur, the method will not 
be accurate, and recourse must be had to separation by the citro-magnesic 
solution. 

While the liquid is still cold, a measured volume of the standard uranium 



72. PHOSPHORIC ACID. 291 

is run in with stirring, and occasional drops are taken out with a glass rod, 
and put in contact with some ferrocyanide indicator sprinkled on a white 
plate until a faint colour occurs. The beaker is then placed in the water- 
bath for a few minutes, and again the mixture tested with the indicator : 
ufter heating in this way the testing ought to show no colour. More 
uranium is then added with stirring, and drop by drop till the proper reaction 
occurs. This titration is only a guide for a second, which maybe made more 
accurate by running in at once very nearh r the requisite volume of uranium. 

This operation may be reversed, if so desired, by making the 
clear solution of phosphate up to a definite volume (say 60 c.c.), 
and running it into a measured volume of uranium until a test 
taken shows no colour. 

5. Estimation of Phosphoric Acid in Minerals or other substances 
containing- Iron, Alumina, or other disturbing- matters. 

In order to make use of any volumetric process for this purpose, 
the phosphoric acid must be separated. As has been already 
described, this may be ^lone either by the molybdic precipitation 
followed by solution in !N"H 3 , again precipitated with ordinary 
magnesia mixture, or direct separation by the citro-magnesia 
mixture described below. In either case the ammonio-magnesic 
salt is dissolved in the least possible quantity of nitric or 
hydrochloric acid, neutralized with ammonia, acidified with acetic 
acid, and the titration with uranium carried out as before described. 

6. Joulie's Method. 

This differs somewhat from the foregoing, and may be summarized 
as follows (Munro, C.N. Hi. 85). 

Joulie applies the citro-magnesia method to all phosphates, 
whether containing iron and alumina or not, and prefers nitrate to 
acetate of uranium. 

1 to 10 gm. of the sample are dissolved in HC1. Some chemists use 
nitric acid with a view of leaving as much ferric oxide as possible undissolved. 
This course is condemned by the author, because the presence of ferric salts 
in no way interferes with the process, and because HC1 is a much better 
solvent of mineral phosphates than nitric acid, and leaves a residue free from 
iron, by the whiteness of which one may judge of the completeness of the 
attack. In the case of phosphates containing a little pyrites, nitric acid 
should be used in conjunction with hydrochloric. The removal of silica by 
evaporation to dry ness is necessary only in those cases where the sample 
contains silicates decomposable by HC1, with separation of gelatinous silica. 
The sample is boiled with the acid in a measuring flask until the residue is 
perfectly white, the contents are cooled, made up to the mark with cold 
water, mixed, filtered through a dry filter, and such a fraction of the filtrate 
withdrawn by a pipette as contains about 50 m.gm. of P-0 5 . The sample 
being delivered from the pipette into a small beaker, 10 c.c. of citro-magnesic 
solution are added, and then a large excess of ammonia. If this quantity 
of citro-magnesic solution is sufficient, no precipitate will form until the lapse 
of a few moments ; should an immediate precipitate form, it is iron or 
aluminium phosphate. In this case a fresh sample must be pipetted off, and 
20 c.c. of citro-rnagnesic solution are added ; it is of no use adding another 10 c.c. 

u 2 



292 VOLUMETRIC ANALYSIS. 72. 

of the citric solution to the original sample, as the precipitated phosphates 
of iron and aluminium do not readily redissolve when once formed. 

Citro-Magnesic Solution. 27 gm. of pure maguesic carbonate are added 
by degrees to a solution of 270 gm. of citric acid in 350 c.c. of warm water ; 
when all effervescence is over and the liquid cool, about 400 c.c. of solution 
of ammonia are added, containing 10 per cent, of NH 3 (about 0'96 sp. gr.), 
or if other strength is used, enough to ensure decided excess of NH 3 :" the 
whole is then diluted to a liter, and preserved in a well-stoppered bottle. 

The old plan of adding first citric acid and then " magnesia mixture " to 
the solution under analysis frequently leads to incomplete precipitation of 
the phosphoric acid, because . the ammonio-magnesic phosphate is slightly 
soluble in ammonic citrate unless a sufficient excess of magnesium salt is 
present, and therefore the quantity of magnesium salt should be increased 
pari passu with the citric acid required, which is best done when they are in 
solution together. The liquid after precipitation is allowed to stand from 2 
to 12 hours (covered to prevent evaporation of ammonia), and then decanted 
through a small filter. The precipitate remaining in the beaker is washed 
Avith weak ammonia by decantation, and then on the filter until the filtrate 
gives no precipitate with sodic phosphate. Dilute nitric acid is next poured 
into the beaker to dissolve the precipitate adhering to the glass, thence on to 
the precipitate on the filter. The nitric solution is received in a beaker 
holding about 150 c.c. and marked at 77 c.c. After two or three washings 
with acidulated water the filter itself is detached from the funnel and added 
to the contents of the beaker, as the paper is found to retain traces of P-O 5 
even after many washings. Dilute ammonia is next added until a slight 
turbidity is produced, which is removed by the addition of one or two drops 
of dilute nitric acid, the liquid is heated to boiling, 5 c.c. of the sodic acetate 
solution added ( 72.2c.), and the titration Avith uranic nitrate immediately 
proceeded with. 

The Standard Uranic Nitrate is made by dissolving about 40 gm. of the 
pure crj^stals in 800 c.c. water, adding a few drops of ammonia to produce 
a slight turbidity, then acetic acid until cleared, and diluting to 1 liter. 
Acetate of uranium should not be used, as it impairs the sensibility of the 
end-reaction. The uranium solution is titrated with 10 c.c. of a standard 
solution of acid ammonic phosphate containing 8'10 gm. of the pure dry salt 
per liter (1 c.c.=0'005 gm. P-O 5 ). The ammonic phosphate solution is- 
verified by evaporating a measured quantity (say 50 c.c.) of it to dryness 
with a measured quantity of a solution of pure ferric nitrate containing an 
excess of ferric oxide, and calcining the residue. The difference in weight 
between this calcined residue and that from an equal volume of ferric nitrate 
solution evaporated alone, is the weight of phosphoric anhydride contained 
in the 50 c.c. of ammonic phosphate solution. The actual verification of the 
uranic nitrate is performed by measuring accurately 10 c.c. of the ammonic 
phosphate into a beaker marked at 75 c.c., adding 5 c.c. of the sodic- 
acetate, making up with water to about 30 c.c., and heating to boiling. 
9 c.c. uranium are then run in from a burette, and the liquid tested in the 
usual way with ferrocyanide. From, this point the uranium is added two or 
three drops at a time, until the end-reaction just appears, the burette being 
read off at each testing. As soon as the faintest colouration appears, 
the beaker is immediately filled to the mark with boiling distilled Avater, 
and another test made. If the operation has been properly conducted no 
brown colour will be detected, owing to the dilution of the liquid, and one 
or two drops more of the uranium solution must be added before the colour 
becomes evident, and the burette is finally read off. A constant correction is 
subtracted from all readings obtained in this way : it is the quantity of 
uranium "found necessary to give the end-reaction Avith 5 c.c. of the sodic 
acetate solution alone, diluted to 75 c.c. Avith boiling Avater as above described. 
The end-point must always be verified by adding three or four drops o 



8 72. PHOSPHORIC ACID. 293 

o 

uranium in excess, and testing again, when a strongly marked colour should 
be produced. The standard uranium is made of the same strength as the 
standard ammonic phosphate, in order to eliminate the error caused by 
changes in the temperature of the laboratory. The actual analysis is made 
in the same way as the titration of the standard uranium, except that 
a slight error is introduced by the number of tests that have to be made 
abstracting a small fraction of the assay. To correct this, a second estimation 
should always be made, and nearly the whole of the uranium found 
necessary in the first trial should be added at once. Tests are then made at 
intervals of two or three drops, and the final and correct result should 
slightly exceed that obtained in the first trial. 

7. Pemberton's Original Molybdic Method. 

This process, with all the steps that led to its adoption, and the 
difficulties involved, is described in a paper read before the chemical 
section of the Franklin Institute in 1882 (C. N. xlvi. 4). 

The process is based on the fact that, if a standard aqueous 
solution of ammonic molybdate be added to one of phosphoric 
acid, in the presence of a large proportion of ammonic nitrate, 
accompanied with a small excess of nitric acid, and heat applied to 
the mixture, the whole of the P 2 5 is immediately and completely 
carried down as phospho-molybdate quite free from MoO 3 . A small 
excess of the precipitant renders the supernatant liquid clear and 
colourless, and the ratio of molybdic trioxide to phosphoric 
anhydride is always the same. 

The weak part of the method is the difficulty in finding the 
exact point at which the precipitation is ended, because the 
yellow precipitate does not settle in clots like silver chloride, 
and hence filtration is necessary, in order to obtain a portion of 
clear liquid for testing with a drop of the molybdate. Very good 
results may be obtained with some little patience and practice by 
using the Bcale filter (fig. 23). When the precipitation is thought 
to be nearly complete, the filter is dipped into the hot liquid, so 
as to obtain 2 c.c. or so in a clear condition : this is transferred 
to a clean test tube or small short beaker, and a drop or two 
of the precipitant added, then heated in the bath to see if a yellow 
colour occurs ; if it does, the filter and beaker are washed again 
into the bulk with hot water in very small quantities from a small 
wash-bottle. A second titration ought to result in "a very near 
approximation, and a third will be exact. A convenient small 
suction asbestos filter is figured and described by Professor 
C a Id well as well adapted to this process (C. N. xlviii. 61). 
As each titration can be made in a very short time, the process 
may be made valuable for technical purposes in the absence of 
either iron or alumina except in mere traces. , 

It is, however, imperative here, as it is in the usual molybdic 
process, to avoid the presence of soluble silica, organic matter, and 
organic acids, also iron and alumina. Chlorides in moderate 
quantity do not interfere. 



294 VOLUMETRIC ANALYSIS. 72. 

The necessary solutions and reagents are 

Standard Ammonic molybdate. 89'543 gin. of the pure crystallized 
salt are dissolved in about 900 c.c. of water ; if not quite clear, 
a very few drops of ammonia may be added to ensure perfect 
solution ; the flask is then filled to the liter mark. The weight of 
salt used is based on the proportion of 24 MoO :J to 1 of P 2 5 , and 
each c.c. precipitates 3 m.gm. P 2 5 . If any doubt exists as to the 
purity of the molybdate, the solution should be standardized with 
a solution of P 2 5 of known strength. In any case this is to be 
recommended. 

Ammonic nitrate in granular form and neutral. 

Nitric acid, sp. gr. not less than 1'4; or if of less strength, 
a proportionate increase must be used in the titration. 

Process : The phosphate to be titrated is taken in quantity containing not 
over O'l gm. P 2 5 or 0'15 gm. at the utmost. If silica is present, the 
solution is evaporated to dryness. In presence of organic matter ignite 
gently and evaporate to dryness twice with HNO 3 . There is no advantage 
in filtering off the SiO' 2 . The solution is transferred to a beaker of 100 to 
125 c.c., using as little water as possible to prevent unnecessary dilution 
and is just neutralized with NH 4 HO, i.e., until a slight precipitate is 
formed. 

If much iron is present the ammonia is added until the yellow colour 
begins to change to a darker shade. 2 c.c. of nitric acid are added. Care 
must be taken that the sp. gr. of the acid is not less than T4, otherwise more 
must be added. 10 gm. of granular nitrate of ammonia are now added. 
After a little experience the quantity can be judged with sufficient accuracy 
by the eye without the trouble of weighing. The solution is now heated to 
140 P. or over and the molybdate solution run in (most conveniently from 
a Gay Lussac burette), meanwhile stirring the liquid. The beaker is now 
left undisturbed for about a minute on the water-bath or hot plate until the 
precipitate settles, leaving the supernatant liquid not clear but containing 
widely disseminated particles, in which the yellow cloud can easily be seen 
on the further addition of the molybdate. This addition is continued as 
long as the precipitate is thick and of a deep colour. But as soon as it 
becomes rather faint and thin, a little of the solution, about 2 to 3 c.c., after 
settling of the precipitate, is filtered into a very small beuker, and this is 
heated on a hot plate and 4 or 5 drops of the molybdate added. If 
a precipitate is produced, the whole is poured back into the large beaker, and 
a further addition of the molybdate (1, 2, or 3 c.c.) added, according to the 
quantity of the precipitate in the small beaker. After stirring and settling, 
another small quantity is filtered and again tested. If the mark has been 
overstepped and too much molybdate added, a measured quantity of P-O" 
solution of known strength is added, and the corresponding amount of 
P 2 O 5 deducted. The results may be checked by adding 1 c.c. of standard 
P 2 O 5 solution, and then again testing. This can be repeated as often as 
desired. The portion that finally produces a cloud is the end-point ; from 
this is deducted 0'5 c.c. (for neutralizing the solvent action of the nitric 
acid), the remainder multiplied by 3 gives the weight of P 2 5 in milligrams. 
O'l gm. of P 2 5 gives about 275 gm. of the yellow precipitate, and the 
accuracy of the method is largely due to the low percentage of P 2 O 5 . 

8. P ember ton's new Molybiic Method. 

This method, a full description of which is given in Jour. 
jimer. Cliem. Soc. 1894, 278, is one which requires great delicacy 



72. PHOSPHOEIC ACID. 295 

of manipulation, but gives excellent results with all the alkaline 
or earthy phosphates, but unfortunately is practically useless with 
the phosphates of iron or alumina, or with materials containing 
more than mere traces of these substances. For superphosphates 
it is available, unless the amount of iron or alumina or both exist 
in more than ordinary proportion, and also for the raw phosphates 
from which they are made. One great recommendation of the 
method is that it occupies little time, the whole operation may be 
performed in less than an hour in the case of a raw phosphate of 
lime. With superphosphates there has of course to be the extraction 
of the soluble phosphate, but once this is done the determination 
of the soluble P 2 5 may readily be done in half-an-hour, and 
moreover two or three determinations may be carried on simul- 
taneously with the expenditure of very little extra time. 

The method is based on the fact, which has been proved by 
numerous experiments, that if a pure yellow phospho-molybdate 
be titrated with alkali and a proper indicator, so much of it as 
contains one molecule of P 2 5 will exactly represent 23 molecules 
of XjiHO. Of course it is of the greatest importance that in the 
method a pure phosphomolybdate should be obtained, and hence 
the difficulty where such bases as iron or alumina are present, as it 
seems impossible to prevent their being carried down with the 
yellow precipitate even in presence of much nitric acid. As 
has been already said, the process is one of great delicacy of 
treatment, and cannot be satisfactorily used by inexperienced 
operators. The most suitable alkali for the standard is caustic 
potash which should be free from CO 2 , and the most delicate 
indicator is phenolphthalein. Further, the quantity of material 
taken for the titration must be very small, preferably containing not 
more than Ol gin. of P 2 5 . It will readily be seen that if an 
error is made it becomes a serious matter, when results are 
calculated into percentages. 

The solutions required are : 

Ammonic molybdate. 1 c.c. of which will precipitate 3 m.gm. 
of P 2 5 . This is made by dissolving 90 gm. of the pure salt in 
about 700 c.c. of water, and allowing to stand a few hours, if 
then quite clear it may be diluted at once to a liter, but if a slight 
precipitate of molybdic acid occurs the clear liquid is decanted, 
the precipitate dissolved in a few drops of ammonia, and the 
whole made up to the liter. The strength of this solution need 
not be absolutely exact. 

Standard Caustic Potash. Made by diluting 323 '7 c.c. of 
strictly normal solution (free from CO 2 ) to a liter. 

Standard Sulphuric Acid. Made to correspond exactly with 
the standard alkali, using phenolphthalein as the indicator in the 
cold. The phenolphthalein solution is the same as described on 
page 37, and not less than 0'5 c.c. should be used for each titration. 



296 VOLUMETRIC ANALYSIS. 72. 

There are also required a saturated aqueous solution of ammonic 
nitrate and nitric acid of about 1*4 sp. gr. 

Process for raw Phosphates of Lime : 1 gm. of the phosphate is dissolved 
in nitric acid, an excess of which can be used with impunity, and the 
solution filtered into a 250 c.c. flask and made up to the mark. The solution 
can even be poured into the flask without filtering, since the presence of 
a little insoluble matter does not interfere in the least with the titration. 
Moreover, since most phosphate rocks seldom contain over 10 per cent, 
of insoluble matter, and as this has the specific gravity of, at least, 2, it 
occupies a volume of about 0'05 c.c., an amount so small that it may be 
neglected. 

After the clear solution has been poured off, it is well to treat the 
sand, etc., at the bottom of the beaker, with a c.c. or so of HC1, in the 
warmth, to insure complete solution. 

It is not necessary to evaporate to dryness. Isbert and Stutzer have 
shown (Z. A. C. xxvi. 584), that when the yellow precipitate is washed with 
water, the soluble silica is removed, and that evaporation (to render the 
silica insoluble) is superfluous. In the event of its being desirable to 
remove silica by evaporation for any purpose, the evaporation should be 
performed over a water-bath, or, if on an iron plate, with great care, since, 
otherwise, meta- or pyrophosphates are formed, with results that are 
correspondingly low. 

25 c.c. of the solution (equal to O'l gm.) are now measured out and 
delivered into a beaker holding not more than 100 to 125 c.c. A large 
beaker requires unnecessary washing to remove the free acid in washing the 
yelloAV precipitate. The solution is neutralized with ammonia until 
a precipitate just begins to form and 5 c.c. of nitric acid of sp. gr. 1*4 
added ; 10 c.c. of the ammonic nitrate solution are poured in, and the entire 
bulk of the mixture made up to 60 or 70 c.c. by adding water. 

Heat is now applied, and the solution brought to a full boil. It is then 
removed from the lamp, no more heat being applied, and treated at once 
with. 5 c.c. of the aqueous solution of ammonium molybdate, which is run into 
it slowly from a 5 c.c. pipette, the solution being stirred as the precipitate is 
added. The beaker is now allowed to rest quietly for about one minute, 
during which time the precipitate settles almost completely. The 5 c.c. 
pipette is filled with the molybdate solution, and a part of its contents 
allowed to drop in, holding the beaker up to the light. If a formation of 
a yellow cloud takes place it is at once perceptible in which case the 
remainder of the pipetteful is run in, the solution stirred and allowed to 
settle. A third pipetteful is now added as before. Should it cause no 
further cloud, only about one-half of its contents are added. 

It is seldom that more than 15 c.c. of the molybdate have to be added. 
Since each c.c. precipitates 3 m.gm. of P 2 O 5 , 15 c.c. will precipitate 45 m.gm. 
of P 2 O 5 . This is equivalent to 45 per cent, on the O'l gm. taken for 
analysis, and it is not often that any material to be examined contains over 
this percentage. This is not strictly true, for the reason that a small 
quantity (something over 1 c.c.) of the molybdate is required to neutralize 
the solvent action of the nitric acid. Therefore, in very high grade 
phosphates a fourth 5 c.c. pipetteful may be required. In this process the 
point at Avhich sufficient of the precipitant has been added is easily seen. 
No molybdic acid separates, because, in the first place, no great excess of 
molybdate is added; and because, in the second place, the solution is 
filtered immediately, or as soon as it has settled, which requires only 
a minute or two. The time required from the first addition of the molybdate 
to the beginning of the filtration is never over ten minutes, and is generally 
less. The filtrate and washings from the precipitate when treated with 
additional molybdate solution, give, on standing on a hot plate for an hour 



72. SILVER. 297 

or so, a snow-white precipitate of molybdic acid, showing that all of the 
phosphoric acid has been precipitated. 

The yellow precipitate is now filtered through a filter 7 c.m. in diameter, 
decanting the clear solution only. This is repeated three or four times, 
washing down the sides of the beaker, stirring up the precipitate, and 
washing the filter and sides of the funnel above the filter each time. The 
precipitate is then transferred to the filter and washed there. When the 
precipitate is large it cannot be churned up by the wash water, and cannot 
be washed down to the apex of the filter. This is generally the case when 
there is over 10 to 15 per cent, of phosphoric acid present in the substance 
analyzed. In such an event, the precipitate is washed back into the beaker, 
and the funnel tilled with water above the level of the filter, this being done 
two or three times, then the precipitate washed back into the filter. It is 
not necessary to transfer to the filter the precipitate adhering to the sides of 
the beaker. 

During the washing no ammonia must be present in the atmosphere of 
the laboratory. Inasmuch as the beaker, funnel, filter and precipitate are 
small, the washing does not take long to perform. It requires, in fact, from 
ten to fifteen minutes, even when large precipitates (=30 to 40 per cent. 
P-O 5 ) are handled. The precipitate and filter are now transferred together 
to the beaker. The standard alkali is run in until the precipitate has 
dissolved, phenolphthalein then added, and the acid run in without delay 
until the pearly colour disappears and the solution is colourless. The 
presence of the filter paper does not interfere in the least. The reaction of 
the indicator is not so sharp as when only acid and alkali are used, but it is 
easy to tell with certainty the difference caused by one drop of either acid 
or alkali. After deducting the volume of acid used from that of the alkali, 
the remainder gives the percentage of P-O 5 directly, each c.c. being equal to 
1 per cent. P 2 O 5 . Thus, if there are 28'3 c.c. of alkali consumed, the 
material contains 28'3 per cent. P 2 O 5 when one decigram is taken for 
analysis. From the time the 25 c.c. are measured out until the result is 
obtained, from thirty to forty minutes are required. 

Process for soluble P 2 5 in Superphosphates : A measured portion of the 
clear aqueous solution of the material according to its grade, and representing 
not more than 0'05 gm. P 2 O 5 , are pipetted into a small beaker and treated 
exactly as described above. 

B. W. Kilgore (Jour. Amer. Cliem. Soc. 1894, 765) states that 
good results in general were obtained by him in using this method, 
but that occasionally too high figures for P 2 5 were obtained. 
This is also stated by other operators. The variations in this 
direction are generally caused by the deposition of molybdic acid, 
but .they may, of course, be also caused by imperfect washing of 
the precipitate. Kilgore prefers to use the ordinary official acid 
molybdic solution, and to precipitate at 50 or 60 C. instead of 100 
C. The official molybdic solution is made by dissolving 100 gm. 
of molybdic acid in 417 c.c. of ammonia, sp. gr. 0*96, and 
pouring this into 1250 c.c. of nitric acid, sp. gr. 1*2, then filtering 
before use. 

SILVER. 

Ag=: 107-66. 

1 c.c. or 1 dm. - sodic chloride- 0'010766 gm. or 0'10766 grn. 
Silver; also 0*016966 gm. or 0*16966 grn. Silver nitiate. 



298 VOLUMETRIC ANALYSIS. 73. 

3. Precipitation with ^ Sodic Chloride. 

73. THE determination of silver is precisely the converse of 
tl;e operations described under chlorine ( 54, 1 and 2), and the 
process may either be concluded by adding the sodic chloride till 
no further precipitate is produced, or potassic chromate may be 
used as an indicator. In the latter case, however, it is advisable 
to add the salt solution in excess, then a drop or two of chromate, 
and titrate residually with silver, till the red colour is produced, 
for the excess of sodic chloride. 

2. By Ammonic Sulphocyanate (Thiocyanate) . 

The principle of this method is fully described in 43, and 
need not further he alluded to here. The author of the method 
(Volhard) states, that comparative tests made by this method 
and that of Gay Lussac gave equally exact results, both being 
controlled by cupellation, but claims for this process that the end 
of the reaction is more easily distinguished, and that there is no 
labour of shaking, or danger of decomposition by light, as in the 
case of chloride. My own experience fully confirms this. 

3. Estimation of Silver, in Ores and Alloys, by Starch Iodide 
(Method of Pisani and P. Field). 

If a solution of blue starch-iodide be added to a neutral solution 
of silver nitrate, while any of the latter is in excess, the blue colour 
disappears, the iodine entering into combination with the silver ; 
as soon as all the silver is thus saturated, the blue colour remains- 
permanent, and marks the end of the process. The reaction is 
very delicate, and the process is more especially applicable to the 
analysis of ores and alloys of silver containing lead and copper, but 
not mercury, tin, iron, manganese, antimony, arsenic, or gold in 
solution. 

The solution of starch iodide, devised by Pisani, is made by 
rubbing together in a mortar 2 gm. of iodine with 15 gm. of starch 
and about 6 or 8 drops of water, putting the moist mixture into 
a stoppered flask, and digesting in a water bath for about an hour, or 
until it has assumed a dark bluish-grey colour ; water is then added 
till all is dissolved. The strength of the solution is then ascertained 
by titrating it with 10 c.c. of a solution of silver containing 1 gm. 
in the liter, to which a portion of pure precipitated calcic carbonate 
is added; the addition. of this latter removes all excess of acid, and 
at the same time enables the operator to distinguish the end of the 
reaction more accurately. The starch iodide solution should be of 
such a strength that about 50 c.c. are required for 10 c.c. of the 
silver solution ( = O'Ol gm. silver). 

F. Field (C. N. ii. 17), who discovered the principle of this 
method simultaneously with Pisani, uses a solution of iodine in 
potassic iodide with starch. Those who desire to make use of 



73. SILVER. 299 

this plan can use the T ^ and T gy solutions of iodine described 
in 38. 

In the analysis of silver containing copper, the solution must be 
considerably diluted in order to weaken the colour of the copper ; 
a small measured portion is then taken, calcic carbonate added, and 
starch iodide till the colour is permanent. It is best to operate 
with about from 60 to 100 c.c., containing not more than 0'02 gni. 
silver; when the quantity is much greater than this, it is preferable 
to precipitate the greater portion with sodic chloride, and to 
complete with starch iodide after filtering off the chloride. When 
lead is present with silver in the nitric acid solution, add sulphuric 
acid, and filter off the lead sulphate, then add calcic carbonate to 
neutralize excess of acid, filter again if necessary, then add fresh 
carbonate and titrate as described above. 

4. Assay of Commercial Silver (Plate, Bullion, Coin, etc.). Gray 
Lussac's Method modified by J. G-. Mulder. 

For more than thirty years Gay Lussac's method of estimating 
silver in its alloys has been practised intact, at all the European 
mints, under the name of the " humid method," in place of the old 
system of cupellation. During that time it has been regarded as one 
of the most exact methods of quantitative analysis. The researches 
of Mulder, however, into the innermost details of the process 
have shown that it is capable of even greater accuracy than has 
hitherto been gained by it. 

The principle of the process is the same as described in 41, 
depending on the affinity which chlorine has for silver in preference 
to all other substances, and resulting in the formation of chloride 
of silver, a compound insoluble in dilute acids, and which readily 
separates itself from the liquid in which it is suspended. 

The plan originally devised by the illustrious inventor of the 
process for assaying silver, and which is still followed, is to consider 
the weight of alloy taken for examination to consist of 1000 parts, 
and the question is to find how many of these parts are pure silver. 
This empirical system was arranged for the convenience of commerce, 
and being now thoroughly established, it is the best plan of 
procedure. If, therefore, a standard solution of salt be made of 
such strength that 100 c.c. will exactly precipitate 1 gm. of silver, 
it is manifest that each yy c.c, will precipitate 1 in.gm. or y^cr 
part of the gram taken ; and consequently in the analysis of 
1 gm. of any alloy containing silver, the number of y 1 ^ c.c. 
required to precipitate all the silver out of it would be the number 
of thousandths of pure silver contained in the specimen. 

In practice, however, it would not do to follow this plan precisely, 
inasmuch as neither the measurement of the standard solution nor 
the ending of the process would be gained in the most exact 
manner ; consequently, a decimal solution of salt, one-tenth the 
strength of the standard solution, is prepared, so that. 1000 c.c. 



SOO VOLUMETRIC ANALYSIS. 73. 

will exactly precipitate 1 gm. of silver, and, therefore, 1 c.c. 
1 m.gm. 

The silver alloy to be examined (the composition of which must 
be approximately known) is weighed so that about 1 gm. of pure 
silver is present ; it is then dissolved in pure nitric acid by the aid 
of a gentle heat, and 100 c.c. of standard solution of salt added 
from a pipette in order to precipitate exactly 1 gm. of silver ; the 
bottle containing the mixture is then well shaken until the chloride 
of silver has curdled, leaA^ing the liquid clear. 

The question is now : Which is in excess, salt or silver ? A drop 
of decimal salt solution is added, and if a precipitate be produced 
1 c.c. is delivered in, and after clearing, another, and so on as long 
as a precipitate is produced. If on the other hand the one drop of 
salt produced no precipitate, showing that the pure silver present 
was less than 1 gm., a decimal solution of silver is used, prepared 
by dissolving 1 gm. pure silver in pure nitric acid and diluting to 
1 liter. This solution is added after the same manner as the salt 
solution just described, until no further precipitate occurs; in either 
case the quantity of decimal solution used is noted, and the results 
calculated in thousandths for 1 gm. of the alloy. 

The process thus shortly described is that originally devised by 
Gay Lussac, and it was taken for granted that when equivalent 
hemical proportions of silver and sodic chloride were brought thus 
in contact, that every trace of the metal was precipitated from the 
solution, leaving sodic nitrate and free nitric acid only in solution. 
The researches of Mulder, however, go to prove that this is not 
strictly the case, but that when the most exact chemical proportions 
of silver and salt are made to react on each other, and the chloride 
has subsided, a few drops more of either salt or silver solution will 
produce a further precipitate, indicating the presence of both silver 
nitrate and sodic chloride in a state of equilibrium, which is upset 
on the addition of either salt or silver. Mulder decides, and no 
doubt rightly, that this peculiarity is owing to the presence of sodic 
nitrate, and varies somewhat with the temperature and state of 
dilution of the liquid. 

It therefore follows that when a silver solution is carefully 
precipitated, first by concentrated and then by dilute salt solution, 
until no further precipitate appears, the clear liquid will at this 
point give a precipitate with dilute silver solution ; and if it be 
added till no further cloudiness is produced, it will again be 
precipitable by dilute salt solution. 

Example : Suppose that in a given silver analysis the decimal salt solution 
has been added so long as a precipitate is produced, and that 1 c.c. (=20 drops 
of Mulder's dropping apparatus) of decimal silver is in turn required to 
precipitate the apparent excess, it would be found that when this had been 
done, 1 c.c. more of salt solution would be wanted to reach the point at which 
110 further cloudiness is produced by it, and so the changes might be rung 
time after time ; if, however, instead of the last 1 c.c. (=20 drops) of salt, 
half the quantity be added, that is to say 10 drops (=| c.c.), Mulder's 



73. SILVEB. 301 

so-called neutral point is reached ; namely, that i n which, if the liquid be 
divided in half, both salt and silver will produce the same amount of 
precipitate. At this stage the solution contains silver chloride dissolved 
in sodic nitrate, and the addition of either salt or silver expels it from* 
solution. 

A silver analysis may therefore be concluded in three ways 

(1) By adding decimal salt solution until it just ceases 1x> 
produce a cloudiness. 

(2) By adding a slight excess of salt, and then decimal silver; 
till no more precipitate occurs. 

(3) By finding the neutral point. 

According to Mulder the latter is the only correct method, and 
preserves its accuracy at all temperatures up to 56 C. ( = 133 
Fahr.), while the difference between 1 and 3 amounts to | a m.gm., 
and that between 1 and 2 to 1 m.gm. on 1 gm. of silver at, 
16 C. (= 60 Fahr.), and is seriously increased by variation of 
temperature. 

It will readily be seen that much more trouble and care is 
required by Mulder's method than by that of Gay Lussac, but,, 
as a compensation, much greater accuracy is obtained. 

On the whole it appears to me preferable to weigh the alloy so, 
that slightly more than 1 gm. of silver is present, and to choose the- 
ending No. 1, adding drop by drop the decimal salt solution until 
just a trace of the precipitate is seen, and which, after some practice, 
is known by the operator to be final. It will be found that the- 
quantity of salt solution used will slightly exceed that required by 
chemical computation; say lOO'l c.c. are found equal to 1 gm. of 
silver, the operator has only to calculate that quantity of the salt 
solution in question for every 1 gm. of silver he assays in the form 
of alloy, and the error produced by the solubility of silver chloride 
in sodic nitrate is removed. 

If the decimal solution has been cautiously added, and the 
temperature not higher than 17 C. (62 Fahr.), this method of 
conclusion is as reliable as No. 3, and free from the possible errors^ 
of experiment ; for it requires a great expenditure of time and 
patience to reverse an assay two or three times, each time 
cautiously adding the solutions drop by drop, then shaking and' 
waiting for the liquid to clear, besides the risk of discolouring the 
chloride of silver, which would at once vitiate the results. 

The decimal silver solution, according to this arrangement, would, 
seldom be required ; if the salt has been incautiously added, or the 
quantity of alloy too little to contain 1 gm. pure silver, then it is 
best to add once for all 2, 3, or 5 c.c., according to circumstances, 
and finish with decimal salt as iNo. 1, deducting the silver added. 

The Standard Solutions and Apparatus. 

() Standard Salt Solution. Pure sodic chloride is prepared by treating;- 
a concentrated solution of the whitest table-salt first with a solution of 



VOLUMETRIC ANALYSIS, 73. 

caustic baryta to remove sulphuric acid and magnesia, then with a slight 
excess of bodic carbonate to remove baryta and lime, warming and allowing 
the precipitates to subside, then evaporating to a small bulk that crystals 
may form ; these are separated by a filter, and slightly washed with cold 
distilled water, dried, removed from the filter, and heated to dull redness, 
and when cold preserved in a well-closed bottle for use. The mother-liquor 
is thrown away, or used for other purposes. Of the salt so prepared, or of 
chemically pure rock-salt (Steinsalz, a substance to be obtained freely in 
Germany), 5'4145 gui. are to be weighed and dissolved in 1 liter of distilled 
water at 16 C. 100 c.c. of this solution will precipitate exactly 1 gm. of 
silver. It is preserved in a well-stoppered bottle, and shaken before use. 

(b) Decimal Salt Solution. 100 c.c. of the above solution are diluted to 
exactly 1 liter with distilled water at 16 C. 1 c.c. will precipitate O'OOl gm. 
of silver. 

(c) Decimal Silver Solution; Pure metallic silver is best prepared by 
galvanic action from pure chloride ; and as clean and seeure a method as any 
is to wrap a lump of clean zinc, into which a silver wire is melted, with 
a piece of wetted bladder or calico, so as to keep any particles of impurity 
contained in the zinc from the silver. The chloride is placed at the bottom 
of a porcelain dish, covered with dilute sulphuric acid, and the zinc laid in 
the middle ; the silver wire is bent over so as to be immersed in the chloride. 
As soon as the acid begins to act upon the zinc the reduction commences in 
the chloride, and grows gradually all over the mass ; the resulting finely- 
divided silver is well washed, first with dilute acid, then with hot water, till 
all acid and soluble zinc are removed. 

The moist metal is then mixed with a little sodic carbonate, saltpetre, 
and borax, say about an eighth part of each, dried perfectly, then melted. 
Mulder recommends that the melting should be done in a porcelain crucible 
immersed in sand contained in a common earthen crucible; borax is sprinkled 
over the surface of the sand so that it may be somewhat vitrified, that in 
pouring out the silver when nielted no particles of dirt or sand may fall into 
it. If the quantity of metal be small it may be melted in a porcelain crucible 
over a gas blowpipe. 

The molten metal obtained in either case can be poured into cold water 
and so granulated, or upon a slab of pipe-clay, into which a glass plate has 
been pressed when soft so as to form a shallow mould. The metal is then 
washed well with boiling water to remove accidental surface impurities, and 
rolled into thin strips by a goldsmith's mill, in order that it may be readily 
cut for weighing. The granulated metal is, of course, ready for use at once 
without any rolling. 

1 gm. of this silver is dissolved in pure dilute nitric acid, and 
diluted to 1 liter; each c.c. contains O'OOl gm. of silver. It should 
be kept from the light. 

(<i) Dropping- Apparatus for Concluding- the Assay.- Mulder 
constructs a special affair for this purpose, consisting of a pear- 
shaped vessel fixed in a stand, with special arrangements for 
preventing any continued flow of liquid. The delivery tube has an 
opening of such size that 20 drops measure exactly 1 c.c. The 
vessel itself is not graduated. As this arrangement is of more 
service to assay than to general laboratories, it need not be further 
described here. A small burette divided in -^ c.c. with a conve- 
nient dropping tube, will answer every purpose, and possesses the 
further advantage of recording the actual volume of fluid delivered. 



73. SILVEE. 303 

The 100-c.c. pipette, for delivering the concentrated salt solution, 
must be accurately graduated, and should deliver exactly 100 gm. 
of distilled water at 16 C. 

The test bottles, holding about 200 c.c., should have their 
stoppers well ground and brought to a point, and should be fitted 
into japanned tin tubes reaching as high as the neck, so as to pre- 
serve the precipitated chloride from the action of light, and, when 
shaken, a piece of black cloth should be covered over the stopper. 

(e) Titration of the Standard Salt Solution. From what has 
been said previously as to the principle of this method, it will be 
seen that it is not possible to rely absolutely upon a standard 
solution of salt containing 5 '4 145 gm. per liter, although this is 
chemically correct in its strength. The real working power must 
be found by experiment. From 1*002 to 1*004 gm. of absolutely 
pure silver is weighed on the assay balance, put into a test bottle 
with about 5 c.c. of pure nitric acid, of about 1'2 sp. gr., and gently 
heated in the water or sand bath till it is all dissolved, The 
nitrous vapours are then blown from the bottle, and it is set aside 
to cool down to about 16 C. or 60 Fahr. 

The 100 c.c. pipette, which should be securely fixed in a support, 
is then carefully filled with the salt solution, and delivered into 
the test bottle contained in its case, the moistened stopper inserted, 
covered over with the black velvet or cloth, and shaken con- 
tinuously till the chloride has clotted, and the liquid becomes clear ; 
the stopper is then slightly lifted, and its point touched against the 
neck of the bottle to remove excess of liquid, again inserted, and 
any particles of chloride washed down from the top of the bottle 
by carefully shaking the clear liquid over them. The bottle is 
then brought under the decimal salt burette, and J c.c. added, the 
mixture shaken, cleared, another | c.c. put in, and the bottle lifted 
partly out of its case to see if the precipitate is considerable * 
lastly, 2 or 3 drops only of the solution are added at a time until 
no further opacity is produced by the final drop. Suppose, for 
instance, that in titrating the salt solution it is found that 1 *003 gm. 
of silver require 100 c.c. concentrated, and 4 c.c. decimal solution, 
altogether equal to 100 '4 c.c. concentrated, then 

.1-003 silver : 100*4 salt : : 1*000 : x. =100*0999. 

The result is within -jo-J^yo" ^ 100*1, which is near enough for the 
purpose, and may be more conveniently used. The operator 
therefore knows that 100*1 c.c. of the concentrated salt solution 
at 16 C. will exactly precipitate 1 gm. silver, and calculates 
accordingly in his examination of alloys. 

In the assay of coin and plate of the English standard, namely, 
11*1 silver and 0*9 copper, the weight corresponding to 1 gm. of 
silver is 1*081 gm., therefore in examining this alloy 1*085 gm. 
may be weighed. 



304 \ 7 OLUMET11IC ANALYSIS. 73. 

When the quantity of silver is not approximately known, 
a preliminary analysis is necessary, which is best made by 
dissolving J or 1 gm. of the alloy in nitric acid, and precipitating 
very carefully with the concentrated salt solution from a ^ c.c. 
burette. Suppose that in this manner 1 gm. of alloy required 
45 c.c. salt solution, 

100-1 salt : 1-000 silver : : 45 : x. = 0-4495. 
Again 0-4495 : 1 : : 1-003 : x = 2'23l. 



r> 



2'231 gm. of this particular alloy are therefore taken for the 
assay. 

Where alloys of silver contain sulphur or gold, with small 
quantities of tin, lead, or antimony, they are first treated with 
a small quantity of nitric acid so long as red vapours are disengaged., 
then boiled with concentrated sulphuric acid till the gold has 
become compact, set aside to cool, diluted with water, and titrated 
as above. 

Assaying- on the Grain System. 

It will be readily seen that the process just described may quite 
as conveniently be arranged on the grain system by substituting 10 
grains of silver as the unit in place of the gram ; each decem of 
concentrated salt solution would then be equal to ~ of a grain 
of silver, and each decem of decimal solution to -$ of a grain. 



5. Analysis of the Silver Solutions used in Photography. 

The silver bath solutions for sensitizing collodion and paper 
frequently require examination, as their strength is constantly 
lessening. To save calculation, it is better to use an empirical 
solution of salt than the systematic one described above. 

This is best prepared by dissolving 43 grains of pure sodic 
chloride in 10,000 grains of distilled water. Each decem ( = 10 grn.) 
of this solution will precipitate 0-125 grn. (i.e, \ grn.) of pure 
silver nitrate ; therefore, if one fluid drachm of any silver solution 
be taken for examination, the number of decems of salt solution 
required to precipitate all the silver Avill be the number of grains 
of silver nitrate in each ounce of the solution. 

Example : One fluid drachm of an old nitrate bath was carefully measured 
into a stoppered bottle, 10 or 15 drops of pure nitric acid and a little 
distilled water added ; the salt solution was then cautiously added, shaking- 
well after each addition until no further precipitate was produced. The 
quantity required was 26'5 dm. 26| grains of silver nitrate in each ounce 
of solution. 

Crystals of silver nitrate may also be examined in the same way, 
by dissolving say 30 or 40 grn. in an ounce of water, taking one 
drachm of the fluid and titrating as above. 



74 SUGAR. 305 

In consequence of the rapidity and accuracy with which silver 
may be determined, when potassic chromate is used as indicator, 
some may prefer to use that method. It is then necessary to have 
a standard solution of silver, of the same chemical power as the 
salt solution : this is made by dissolving 125 grains of pure and 
dry neutral silver nitrate in 1000 dm. of distilled water; both 
solutions will then be equal, volume for volume. 

Suppose, therefore, it is necessary to examine a silver solution 
used for sensitizing paper. One drachm is measured, and 
if any free acid be present, cautiously neutralized with a weak 
solution- of sodic carbonate ; 100 dm. of salt solution are then 
added with a pipette. If the solution is under 100 grn. to 
the ounce, the quantity will be sufficient. 3 or 4 drops of 
chromate solution are then added, and the silver solution delivered 
from the burette until the red colour of silver chromate is just 
visible. If 25'5 dm. have been required, that number is deducted 
from the 100 dm. of salt solution, which leaves 74'5 dm., or 
74J- grains to the ounce. 

This method is- much more likely to give exact results in the 
hands of persons not expert in analysis than the ordinary plan by 
precipitation, inasmuch as, with collodion baths, containing as they 
always do silver iodide, it is almost impossible to get the supernatant 
liquid clear enough to distinguish the exact end of the analysis. 



SUGAR. 

74. SUGARS belong to the large class of organic bodies known 
as " carbo-hydrates," of which there are three main classes, viz. : 

(1) The Glucoses, C 6 H 12 0, the principal members of which 
are glucose, dextrose, or grape sugar, occurring in the urine in 
Diabetes mellitus, and with levulose in most sweet fruits and in 
honey ; levulose or fruit sugar ; galactose. 

(2) The Di-saccharides, C 12 H 22 O n , the chief members of which 
are cane sugar or sucrose, occurring in the juice of the sugar 
cane, beet root, and maple ; milk sugar or lactose, occurring in the 
milk of mammals and in various pathological secretions ; malt 
sugar or maltose, formed by the action of malt diastase upon 
starch. 

(3) The Poly-saccharides, or starches and gums (C 6 H 10 5 X of 
which the most important members are starch, glycogen (found in 
the liver), dextrine, and cellulose or wood-fibre. 

The di- and poly-saccharides are "inverted" or " hydrolyzed " 
by being boiled with dilute acids, or by the action of unorganized 
ferments like diastase and pepsin, and those contained in yeast 
and saliva ; i.e., they become converted into glucoses. Cane sugar 
on inversion yields equal parts of dextrose and levulose (invert 
sugar), milk sugar yields dextrose and galactose, maltose yields 

x 



306 VOLUMETRIC ANALYSIS. 74 

dextrose ; starch, glycogen, dextrose, and cellulose all yield dextrose 
as the final product. 

The methods in general use for the quantitative estimation of 
the various kinds of sugar are the fermentation method, 
estimating the final density of the saccharine solution, and 
the amount of CO 2 evolved; the optical method, by the polarimeter ; 
gravimetrically, by the reduction of an alkaline copper solution ; 
volumetrically, by reduction of copper or mercury solutions. 

All the glucoses reduce the alkaline copper solution, known as 
Fehling's, more or less readily; maltose and lactose reduce it in 
a less degree ; starch, cane sugar, dextrine, and cellulose rrot at all. 
Other substances besides sugars reduce Fell ling's solution, e.g., 
chloroform, salicylic and uric acids, creatinine and phenyl- 
hyclrazine. 

The volumetric method of estimating glucose by Fehl ing's 
copper solution has for a long time been thought open to question 
on the score of accuracy, and the extensive and elaborate experi- 
ments of Soxhlet have clearly shown, that only under identical 
conditions of dilution, etc., can concordant results be obtained. 
The high official position of this chemist, together with the evident 
care shown in his methods, leave no doubt as to the general 
accuracy of his conclusions. His rather sweeping statement, how- 
ever, that the accurate gravimetric estimation of glucose by 
Fehling's solution is impossible, is strongly controverted by 
Brown and Heron, whose large experience leads them to 
a different conclusion. It is probable, however, that both 
authorities are right from their own points of view, and that 
Brown and Heron do obtain concordant results when working in 
precisely the same way; whereas Soxhlet is equally correct in 
stating that the gravimetric estimation, as usually performed under 
varying conditions, is open to serious errors. 

Kjeldahl maintains that Fehling's solution, however pure its 
constituents, always undergoes a slight reduction on prolonged 
heating, especially in strong solution, and he fixes the limit of 
time for which the liquid should be exposed to the temperature of 
boiling water at twenty minutes. 

The Solution of Sugar. For all the processes of titration this 
must be so diluted as to contain J or at most 1 per cent, of sugar : 
if on trial it is found to be stronger than this, it must be further 
diluted with a measured quantity of distilled water. 

If the sugar solution to be examined is of dark colour, or likely 
to contain extractive matters which might interfere with the 
distinct ending of the reaction, it is advisable to heat a measured 
quantity to boiling, and add a few drops of milk of lime, allow the 
precipitate to settle, then filter through purified animal charcoal, 
and dilute with the washings to a definite volume. In some 
instances cream of alumina or basic lead acetate may be used to 



74. SUGAR. 307 

clarify highly coloured or impure solution, Imt no lead must be 
left in the solution.'* 

From thick mucilaginous liquids, or those which contain a large 
proportion of albuminous or extractive matters, the sugar is best 
extracted by Graham's dialyser. 

The Fell ling method may be applied directly to fresh diabetic 
urine (see Analysis of Urine), as also to brewer's wort or distiller's 
mash. Dextrine does not interfere, unless the boiling of the 
liquid under titration is long continued. 

]. Inversion of Various Sugars into Glucose. 

Ordinary cane sugar is best inverted by heating to about 70 C. 
a dilute solution (in no case should the concentration exceed 25 per 
cent.) of the sugar with 10 per cent, of fuming hydrochloric acid 
for 15 minutes. Dilute sulphuric acid is preferred by some 
operators. If the mixture is boiled, the inversion occurs in from 
5 to 10 minutes. The inversion of milk sugar takes longer time 
than cane sugar. 

Maltose or malt sugar takes a much longer time than milk 
sugar, but may be done by the addition of 3 c.c. of concentrated 
sulphuric acid to 100 c.c. of wort, and heating for 3 hours in 
a boiling water bath ; if dextrine is present, it is also inverted at 
the same time. 

The inversion of the slowly changing sugars may be hastened 
considerably by heating at increased atmospheric pressure, although 
some authorities condemn the process. 0' Sullivan however 
states that a good result with maltose or dextrine is obtained by 
heating 30 gm. of the substance in 100 c.c. of water containing 
1 c.c. of H 2 S0 4 for 20 minutes, at a pressure of one additional 
atmosphere (Allen's Organic Analysis i. 217). 

Allen also gives a handy means of carrying out this method, 
which consists in using a soda water bottle with rubber stopper 
through which passes a long glass tube bent at right angles, and 
immersed to a depth of 30 inches in mercury contained in 
a vertical tube of glass or metal. The rubber stopper must be 
secured by wire, and the bottle heated to boiling in a saturated 
solution of sodic nitrate, which gives a temperature corresponding 
to an extra atmosphere. Of course in all cases where acid has 
been used for the inversion of sugar, it must be neutralized before 
the copper titration takes place ; this may be done either with 
sodic or potassic hydrates or carbonates, or calcic carbonate may be 
used. 

* Although traces of lead are of no great consequence when clarifying sugars for the 
polariscope, it is of great importance to remove all lead in the volumetric method. In 
order to do this it is best to treat a measured quantity of the sugar solution which has 
been clarified by lead with a strong solution of sulphurous acid until no further 
precipitate occurs, then add a few drops of alumina hydrate suspended in water, 
dilute to a definite volume and filter. In many cases concentrated solution of sodic 
carbonate will suffice to remove all lead. These methods of clarification are highly 
necessary in the case of albuminous or gelatinous liquids, as otherwise the copper oxide 
will not settle readily, and it becomes difficult to tell when the end-reaction occurs. 

x 2 



308 VOLUMETEIC ANALYSIS. 74. 

Starch from various sources may be inverted in the same way 
as the sugars, but it needs a prolonged heating with acid. For 
approximate purposes 1 gm. of starch should be mixed to a smooth 
cream w r ith about 30 c.c. of cold water, then 1 c.c. of strong 
hydrochloric acid added, and the mixture kept at a boiling 
temperature in an obliquely fixed flask for 8 or 10 hours, replacing 
the evaporated water from time to time to avoid charring the 
sugar, and testing with iodine to ascertain when the inversion is 
complete. The product is glucose. 

For the estimation of the starch itself a number of processes 
were tried by Ost (Cliem. Zeit. 1895, xix. 1501), the one which 
was found to answer best being that of Sachsse (Cliem. CentralM. 
viii. 732), slightly modified. In this modification 3 gm. of the 
starch are heated with 200 c.c. of water and 20 c.c. of hydrochloric 
acid, specific gravity 1'125 ( - 5*600 gm. of HC1), for two to three 
hours in a boiling water bath, using the factor 0*925 to calculate 
the glucose found in the starch. Longer heating gives results too 
low, and two hours on the water bath are not sufficient. Slightly 
higher yields of glucose (89*8 instead of 89*5 per cent.) can be 
obtained by heating for a much longer period with less starch and 
acid, but there is no advantage to be gained by the alteration. 
Oxalic acid gives no better results. Dextrine may be determined 
in the same manner also maltose, if 1 gm. of the latter be heated 
for five hours with 100 c.c, of 1 to 2 per cent, hydrochloric acid 
as before. 

100 parts of grape sugar, found by Fehling's process, represent 
90 parts of starch or dextrine. When dextrine is present with 
grape sugar, care must be taken not to boil the mixture too long 
with the alkaline copper solution, as it has been found that a small 
portion of the copper is precipitated by the dextrine (Rumpf and 
Heintzerling, Z. a. C. ix. 358). 

An inversion of starch may be produced more rapidly, and at 
lower temperature, by using some form of diastase in place of 
acid. An infusion of malt is best suited to the purpose, but the 
temperature must not exceed 71 C. (160 Fahr.). The digestion 
may vary from fifteen minutes to as many hours. The presence of 
unchanged starch may be found by occasionally testing with iodine. 
If the digestion is carried beyond half an hour, a like quantity of 
the same malt solution must be digested alone, at the same 
temperature, and for the same time, then titrated for its amount of 
sugar, which is deducted from the total quantity found in the 
mixture. O'Sullivan (J. C. S. 1872, 579) has, however, clearly 
shown that the effect of the so-called diastase is to produce maltose, 
which has only the power of reducing the copper solution to the 
extent of about three-fifths that of dextrose or true grape sugar, 
the rest being probably various grades of dextrine. Brown and 
Heron's experiments clearly demonstrate that no dextrose is 
produced from starch by even prolonged treatment with malt 



74 SUGAR. 309 

extract ; the only product is maltose. Sulphuric or other similar 
acids cause complete inversion. 

For the exact estimation of starch in grain of various kinds 
O'Sullivan gives very elaborate directions, involving the treat- 
ment of the substance with alcohol and ether, to remove fatty and 
other constituents previous to digestion with diastase. The same 
authority also gives special directions for the preparation of the 
proper kind of diastase, all of which may be found in J. C. S. 
xlv. 1. 

2. Estimation of Glucose by Fehling's Solution. 

Preparation of the Standard Solutions. Fehling's Standard 
Copper Solution. Crystals of pure cupric sulphate are powdered 
and pressed between unsized paper to remove adhering moisture ; 
69 '28 gm. are weighed, dissolved in water, about 1 c.c. of pure 
sulphuric acid added, and the solution diluted to 1 liter. 

Alkaline Tartrate Solution.- 350 gm. of Rochelle salt (sodio- 
potassic tartrate) are dissolved in about 700 c.c. of water, and the 
solution filtered, if not already clear ; there is then added to it 
a clear solution of 100 gm. of caustic soda (prepared by alcohol) 
in about 200 c.c. of water. The volume is made up to 1 liter. 

These solutions are prepared separately, and when mixed in 
exactly equal proportions form the original Fell ling solution, 
each c.c. of which should contain 0*03464 gm. of cupric sulphate, 
and represents 0*005 gm. of pure anhydrous grape sugar, if the 
conditions of titration laid down below are adhered to.'" The 
method is based on the fact that although Fehling's solution 
may be heated to boiling without change, the introduction into it 
of the smallest quantity of grape sugar, at a boiling temperature, 
at once produces a precipitate of cuprous oxide, the ratio of 
reduction being uniform if the conditions of experiment are 
always the same. 

The Titration of Glucose with Fehling's Solution. 5 c.c. each of 
standard copper and alkaline tartrate solutions are accurately measured into 
a thin white porcelain basin, 40 c.c. of water added, and the basin quickly 
heated to boiling on a sand-bath or by a small flame. No reduction or 
change of colour should occur; it' it does, the alkaline tartrate solution 
is probably defective from age. This may probably be remedied by the 
addition of a little fresh caustic alkali on second trial, but it is advisable to 
use a new solution. The % or 1 per cent, sugar solution is then delivered in 
from a burettef in small quantities at a time, with subsequent boiling, unti 

* If pure cupric sulphate has been used, and the solutions mixed only at the time of 
titration, there need be very little fear of inaccuracy ; nevertheless it is advisable to 
verify the mixed solutions from time to time. This may be done by weighing and 
dissolving 0'95 gm. of pure cane sugar in about 500 c.c. of water, adding 2 c.c. of. 
hydrochloric acid, and heating to 70 C. for ten minutes. The acid is then neutralized 
with sodic carbonate and diluted to a liter. 50 c.c. of this liquid should exactly 
reduce the copper in 10 c.c. of Fell ling's solution. A standard solution of inverted 
sugar, which will keep good for many months, may be made in the foregoing manner : 
it should be of aboiit 20 per cent, strength, and rendered strongly alkaline with soda or 
potash. 

t The instrument should be arranged as described on page 12. 



310 VOLUMETRIC ANALYSIS. 74. 

the blue colour of the copper solution is just discharged, a point which is 
readily detected by inclining the basin, so that the colour of the clear 
supernatant fluid may be observed against the white sides of the basin. 
Some operators use a small thin boiling flask instead of the basin. 

It is almost impossible to hit the exact point of reduction in the 
first tit ration, but it affords a very good guide for a more rapid and 
exact addition of the sugar solution in a second trial, when the 
sugar may be added with more boldness, and the time of exposure 
of the copper solution to the air lessened, which is a matter of 
great importance, since prolonged boiling has undoubtedly a 
prejudicial effect on the accuracy of the process.'"' 

When the exact point of reduction is obtained, it is assumed 
that the volume of sugar solution used represents O05 gm. of 
grape sugar or glucose, for 10 c.c. Feh ling's solution contain 
O'll gm. cupric oxide, and 5 molecules CuO (396) are reduced 
to cuprous oxide by 1 molecule of glucose (180), therefore 
396 : 180 = 0*11 : 0'05, i.e. 0*05 gm. glucose exactly reduces 
10 c.c. Feh ling's solution. 

With this assumption, however, Soxhlet does not agree, but 
maintains from the results of his experiments on carefully prepared 
standard sugars, that the accuracy of the reaction is interfered 
with by varying concentration of the solutions, duration of the 
experiment, and the character of the sugar. 

For example, he found that the reducing power of glucose, 
invert sugar, and galactose was in each case lowered by dilution of 
the Fehling's solution, whilst that of maltose was raised, and that 
of milk sugar was not affected. 

The remarks which Soxhlet appends to his experiments are 
thus classified : 

(1) The reducing power of inverted sugar, for alkaline copper solution, is 
importantly influenced by the concentration of the solutions : a smaller 
quantity of sugar being required to decompose Fehling's solution in the 
undiluted state than when it is diluted Avith 1, 2, 3, or 4 volumes of water. 
It is immaterial whether the sugar solution be added to the cold or boiling 
copper reagent. 

(2) If inverted sugar acts on a larger quantity of copper solution than it 
is just able to reduce, its reducing power will be increased, the increment 
var}dng according to the amount of copper in excess and the concentration 
of the cupric liquid ; in the previous experiments the equivalents varied 
from 1 : 97 to 1 : 12*6, these numbers being by no means the limit of 
possible variation. 

(3) In a volumetric estimation of inverted sugar by means of Fehling's 
solution, the amount of copper reduced by each successive addition of sugar 
solution is a decreasing quantity ; the results obtained are therefore perfectly 
empirical, and are only true of that particular set of conditions. 

(4) The statement that 1 equivalent of inverted sugar reduces 10 

* It has been proposed to use an excess of copper, and to estimate the excess 
iodometrically or with cyanide ( 58) in view of the alleged uncertain ending in the 
ordinary Fehling process. My experiments with these methods show that the 
errors are g reater than the one they are siipposed to cure. Moreover, in practised 
hands the true ending presents no difficulty. 



74. SUGAR. 311 

equivalents of cupric oxide is not true, the hypothesis that 0'5 gm. inverted 
sugar reduces 100 c.c. of Fehling's solution being shown to be incorrect; 
the real amount under the conditions laid down by Fehling (1 volume of 
alkaline copper solution. 4 volumes of water, sugar solution fc 1 per cent.) 
being 97 c.c., the results obtained under this hypothesis are, therefore, 3 per 
cent, too low. Where, however, the above conditions have been fulfilled, the 
results, although not absolutely, are relatively correct; not so, however, those 
obtained by gravimetric processes, since the interference of concentration 
and excess has not been previously recognized. 

These facts, however, do not vitiate the process as carried out 
under the well recognized conditions insisted on in the directions 
for titration that were given above. If these are adhered to it is 
found the sugars have the following reducing powers 

10 c.c. Feh ling solution are completely reduced by 
O05 gm. glucose, levulose, galactose 
0'0475 gm. cane sugar (after inversion) 
O067S gm. milk sugar 
0*0807. gm. maltose 
0'045 gm. starch (after inversion). 

Lowe, and more recently Haines, have advocated the sub- 
stitution of an alkaline solution of glycerine for the alkaline tartrate 
in F,e hi ing's solution. This solution is said to keep indefinitely, 
but it is riot so delicate a test as Fehling's. 

3. Estimation of Glucose by Mercury. 

Knapp's Standard Mercuric cyanide. 10 gm. of pure dry 
mercuric cyanide are dissolved in about 600 c.c. of water ; 100 c.c. 
of caustic soda solution (sp. gr. 1'145) are added, and the liquid 
diluted to 1 liter. 

Sachsse's Standard Mercuric iodide. 18 gm. of pure dry 
mercuric iodide and 25 gm. of potassic iodide are dissolved in 
water, and to the liquid is added a solution of 80 gm. of caustic 
potash ; the mixture is finally diluted to 1 liter. 

These solutions, if well preserved, will hold their strength 
unaltered for a long period. 

These solutions are very nearly, but not quite, the same in 
mercurial strength, Knapp's containing 7'9365 gm. Hg in the 
liter, Sachsse's 7 '92 95 gm. 100 c.c. of the former are equal to 
100-1 c.c. of the latter. 

Indicators for the Mercurial Solutions. In the case of Fehling's 
solution, the absence of blue colour acts as a sufficient indicator, 
but with mercury solutions the end of reaction must be found by 
an external indicator. In the case of Knapp's solution the end 
of the reaction is found by placing a drop of the clear yellowish 
liquid above the precipitate on pure white Swedish filter paper, 
then holding it first over a bottle of fuming HC1, then over strong 
sulphuretted hydrogen water ; the slightest trace of free mercury 
shows a light brown or yellowish-brown stain. The indicator best 



312 VOLUMETEIC ANALYSIS. 74 

adapted for Sachsse's solution is a strongly alkaline solution of 
stannous chloride spotted on a porcelain tile. An excess of 
mercury gives a brown colour. 

The Titration : 40 c.c. of either solution are placed in a porcelain basin 
or a flask, diluted with an equal bulk of water, and heated to boiling. The 
solution of sugar of i per cent, strength is then delivered in until all the 
mercur} 7 - is precipitated, the theory being in either case that 40 c.c. should 
be reduced by O'l gm. of dextrose. 

The results of Soxhlet's experiments show that this estimate 
is entirely wrong""" ; nevertheless, it does not follow that these 
mercurial solutions are uselessi It is found that, using them l>y 
comparison with Feh ling's solution, it is possible to define to 
some extent the nature of mixed sugars, on the principle of indirect 
analysis. 

Knapp's solution is strongly recommended by good authorities 
for the estimation of diabetic sugar in urine. The method of using 
it is described in the section on Urinary Analysis. 

The behaviour of the sugars with alkaline mercury solutions was tested 
by Soxhlet both with Knapp's solution and Sachsse's solution. 

He found that different results are obtained from Knapp's solutions, 
according as the sugar solution is added gradually, or all at once ; when 
gradually added more sugar being required; with Sachsse's, however, the 
reverse is the case. 

To get comparable results the sugar must be added all at once, the solution 
boiled for two or three minutes, and the liquid tested for mercury, always 
using the same indicator ; in using the alkaline tin solution as indicator, 
0'200 0'202 gm. of grape sugar was always required for 100 c.c. Knapp, 
in a large number of experiments. It is remarkable that these two solutions, 
although containing almost exactly the same amount of mercury, require 
very different quantities of sugar to reduce equal volumes of 'them. This is 
shown to be due, to a great extent, to the different amounts of alkali present 
in them. 

The various sugars have different reducing powers for the 
alkaline mercury solutions, and there is no definite relation between 
the amount of Knapp's and Sachsse's solutions required by 
them; the amount of Sachsse's solution, to which 100 c.c. 
Knapp's correspond, varying from 54*7 c.c. in the case of galactose, 
to 7 4 '8 c.c. in the case of invert sugar. 

The two mercury methods have no advantage in point of 
accuracy or convenience over Fehlin g's method, the latter having 
the preference on account of the great certainty of the point at 
which the reduction is finished. 

The mercury methods are, however, of great importance, both 
for the identification of a sugar and for the estimation of two 
sugars in presence of each other, as proposed by Sachsse. 
For instance, in the estimation of grape and invert sugars in 
presence of each other, there are the two equations: ax + by = ~F,, 
ex + dy = S. 

* Careful experiment shows that 40 c.c. of Sachsse's solution is redrc3d by 0'1342. 
gin. dextrose or 0'1072 gm. invert sugar. 



74. 



SUGAR. 



313 



Where 
a number of 1 c.c. Fehling, reduced by 1 gm. grape sugar. 




Saclisse 



invert sugar. 



,, grape sugar. 

d ,, ,, ,, invert sugar. 

F ,, Fehling, used for 1 vol. sugar solution. 

S = Saclisse 

x amount of grape sugar in gms. in 1 vol. of the solution. 
y= invert sugar 

It need hardly be mentioned that the above, like all other indirect 
methods, leaves room for increased accuracy ; but nevertheless the 
combination of a mercury method with a copper method in the 
determination of a sugar whose nature is not exactly known, gives 
a more serviceable result than the hitherto adopted plan, by which 
a solution that reduced 10 c.c. .Fehling was said to contain 
0-05 gm. of sugar (J. C. S. Abstracts, 1880, 758). 

Taking the reducing power of grape- sugar =100, the reducing 
powers of the other sugars are : 

Fehling (undiluted). Knapp. Sachsse. 

Grape sugar ............. , ..... 100 100 100 

Invert sugar .................. 96'2 99'0 124-5 

Levulose (calculated) ......... 92*4 102*2 148-6 

Milk sugar ..................... 70-3 64-9 70'9 

Galactose ..................... 93'2 83*0 74'8 

Inverted milk sugar ... ...... 96'2 90'0 85*5 

Maltose .. 61 -0 63'8 65-0 



4. Sidersky's Method. 



This process has found great favour among French sugar experts, 
and is based on the use of Soldaini's cupric solution, which was 
devised to remedy the faults common to Fehling and other 
copper solutions containing tartrated and caustic or carbonated 
alkalies. 

This liquid is prepared, according to Degener, in the following 
manner : 40 gm. of cupric sulphate are dissolved in water, and, in 
another vessel, 40 gm. of sodic carbonate are also dissolved in water. 
The two solutions are mixed, and the copper precipitated in the 
state of hydrobasic carbonate. The precipitate is washed with 
cold water and dried. This precipitate is added to a very con- 
centrated and boiling solution of bicarbonate of potash (about 
415 gm.) and agitated until the whole is completely or nearly 
dissolved, water is added to form a volume of 1400 c.c., and the 
whole mass heated for two hours upon a water-bath. The insoluble 
matter is filtered, and the filtrate, after cooling, is of a deep blue 
colour. The sensibility of this liquid is so great that it gives 




3J4 VOLUMETRIC ANALYSIS. 74. 

a decided reaction with 0*0014 gm. of invert sugar. The presence 
of sucrose in the solution increases this sensibility still more. 

Sidersky has recently offered a new volumetric method, based 
upon the use of Soldaini's solution. With .sugars the same 
method as is now in use with Fehling's solution can easily be 
followed, watching the disappearance of the blue colour, and 
testing the end with ferrocyanide and acetic acid. This process 
offers no serious objections common to Fehling's solution, but is 
inapplicable to coloured sugar solutions, such as molasses, etc. For 
the last the following is recommended : 25 gm. of molasses are 
dissolved in 100 c.c. of water and sub-acetate of lead added in 
sufficient quantities to precipitate the impurities, and the volume 
raised to 200 c.c. and filtered. To 100 c.c. of the filtrate are 
added 25 c.c. of concentrated solution of carbonate of soda, 
agitated, and filtered again. 100 c.c. of the second filtrate with 
excess of lead removed are taken for analysis. On the other hand, 
100 c.c. of Soldaini's solution are placed in a flask and heated five 
minutes over an open flame. The sugar solution is now added 
little by little, and the heating continued for five minutes. Finally, 
the heat is withdrawn and cooled by turning in 100 c.c. of cold 
water, and filtered through a Swedish filter, washed with hot 
water, letting each washing run off before another addition. Three 
or four washings will generally remove completely the alkaline 
reaction. The precipitate is then washed through a hole in the 
filter into a flask, removing the last trace of copper. 25 c.c. of 
normal sulphuric acid are added with two or three crystals of 
chlorate of potash, and the whole gently heated to dissolve com- 
pletely the oxide of copper, which is transformed into copper 
sulphate. The excess of sulphuric acid is determined by a 
standard ammonia solution (semi-normal), of which the best 
indicator is the sulphate of copper itself. When the deep blue 
colour gives place to a greenish tinge the titration is completed. 
The method of titration is performed as follows : Having cooled 
the contents of the flask, a quantity of ammonia equivalent to 
25 c.c. of normal sulphuric acid is added. From a burette 
graduated into one-tenth c.c. standard sulphuric acid is dropped 
in drop by drop, agitating after each addition. The blue colour 
disappears with each addition to reappear after shaking. When 
the last trace of ammonia is saturated the titration is complete, 
which is known by a very feeble greenish tinge. The number of 
.c. is read from the burette, which is equivalent to the copper 
precipitated. The equivalent of copper being taken at 31 '7, the 
normal acid equivalent is 0'0317 of copper. Multiplying the 
topper found by 3546 the invert sugar is found. A blank titration 
is needed to accurately determine the slight excess which gives 
the pale green tinge.'" 

* Report of Proceedings of Fifth Animal Co-.ivcn'iou of the Ameiicui Association 
of Official Agricultural Che nists (188S). 



74 SUGAR. 315 

5. Pavy's modified Fehling; Process. 

This method consists in adding ammonia to the ordinary 
Fehling solution, by which means the precipitation of cuprous 
oxide is entirely prevented, the end of the reaction being shown by 
the disappearance of the blue colour in a perfectly clear solution 
(C. N. xL 77). 

The solution recommended by Pavy is made by mixing 120 c.c. 
ordinary Fehling solution* (see p. 309) with 300 c.c. of strong 
ammonia (sp. gr. 0*880), adding 100 c.c. of a 10 per cent, caustic 
soda solution or of a 14 per cent, solution of potash, and diluting 
to a liter. If Fehling's solution is not available, Pavy's solution 
may be made directly by adding a cooled solution of 21 '6 gm. 
Eochelle salt and 1S ! 4 gm. of soda (or 25 '8 gm. of potash) to 
a solution of 4'157 gm. pure cupric sulphate, adding 300 c.c. 
of strong ammonia, and making up to a liter. 100 c.c. 
Pavy's solution =10 c.c. Fehling's solution = 0*05 gm. of 
glucose. 

As ammoniacal cuprous solutions are readily oxidized, it is 
important to exclude air from the liquid during titration. The 
titratioii should be made in a small boiling flask, through the 
cork of which the elongated end of the burette is passed. A small 
escape tube, preferably with a valve, also passes through the same 
cork, and leads into a vessel containing water or weak acid, to 
condense the ammonia. Allen has found a layer of paraffin over 
the liquid an effective means of excluding air. 

In carrying out the titration (100 c.c. of the Pavy's solution is 
a convenient quantity to take) a few pieces of pumice or pipe- 
stem are added, the liquid brought to boiling, and kept boiling 
whilst the sugar solution is gradually run in. The end-point is 
very sharp. Whilst rapid manipulation is desirable, the solution 
must not be run in too quickly, because reduction takes place 
more slowly than with Fehling's solution. 

The method is well adapted for the examination of diabetic 
urine and milk, also mixtures of milk and cane sugars, and 
certainly has the advantage over the ordinary Fehling method 
by its definite end-point. 

Z. Peska gives the following method for the volumetric 
estimation of sugar by means of ammoniacal copper solution 
(Chem. Zeit. Rep. 1895, 257). In order to avoid the oxidation of 
the copper oxide in solution, a layer of vaseline is used instead of 
the usual current of hydrogen. Two solutions are prepared : 
6 -927 gin. of the purest crystallized copper sulphate are dissolved 
in water, 160 c.c. of 25 per cent, ammonia added, and the whole 
made up to 500 c.c. ; 3 4 '5 gm. of Rochelle salt and 10 gm. of 
caustic soda are also dissolved and diluted to 500 c.c. 

* In ammoniacal solution only 5 molecules CuO are reduced by 1 molecule glucose 
instead of 6 CuO, as in Fehling's solution, hence 120 c.c. of the latter are used in 
making Pavy's solution, and not 100 c.c. 



316 



VOLUMETRIC ANALYSIS. 



74 



Process : A mixture of 50 c.c. of each liquid is heated in a beaker under 
a layer of vaseline oil 5 m.m. thick, to a temperature of 80 C. ' The sugar 
solution is run in 1 c.c. at a time for the first test, but on a repetition the 
whole amount may be added at once. Towards the end of the titration, the 
temperature must be raised to 85, and the heating continued for two minutes 
when working on either glucose or invert sugar, four minutes for maltose, 
and six minutes for milk sugar. Dextrine increases the reducing power of 
the sugar in this solution less than in the one prepared with potash, and as 
the ammonia has no injurious action, the whole process is both exact and 
convenient. When saccharose is present, 1 gm. of it has a reducing action 
equivalent to 0'026 gm. of invert sugar. In the determination of lactose in 
milk the albuminoids should be precipitated with lead acetate and the excess, 
of lead removed by sodium sulphate. The following table gives directly the 
number of milligrams of each sugar in 100 c.c. of solution. 



c.c.'s Glucose. 


Invert 


Milk 


Maltose. c.c. 


's Glucose. 


Invert 


Milk 


Maltose. 


used. 




sugar. 


sugar. 


used. 


sugar. 


sugar. 


. 


8 


997-8 


1049-2 




50 


163-0 


173-2 


318-1 


360-0 


9 


889-4 


935-1 








51 


159-8 


169-8 


311-9 


353-0 


10 


802'3 


844-6 








52 


156-8 


166-5 


306-0 


346-3 


11 


730-7 


770-0 








53 


153-9 


163-4 


300-3 


339-9 


12 


670-8 


707-6 





_ 


54 


151-1 


160-4 


294-8 


333-8 


13 


620*0 


654-5 








55 


148'4 


157-5 


289-4 


327-9 


14 


576-3 


608-7 


. . 





56 


145-7 


154-7 


284-2 


322-2 


15 


538-4 


568-9 


1033*9 





57 


143-1 


152-0 


279-3 


316-7 


16 


505-2 


534-2 


971-4 


58 


140-6 


149-4 


274-5 


311-4 


17 


475-8 


503-3 


916*0 


1023-0 59 


138-2 


146-9 


269-9 


306-3 


18 


449-7 


475-7 


866-5 


968-8 60 


135-9 


144-5 


265-4 


301-3 


19 


426-3 


451-2 


822-3 


920-3 61 


133-7 


142-2 


261-1 


296-4 


20 


405-2 


429-0 


782-4 


876-3 62 


131-5 


139-9 


256-9 


291-6 


21 


386-0 


408-8 


746-0 


836-4 63 


129-4 


137-7 


252-9 


287-0 


22 


368-7 


390-6 


713-0 


800-0 64 


127-4 


135-5 


249-0 


282-6 


23 


352-8 


373-8 


682-7 


766-5 65 


125-4 


133-4 


245-2 


278-3 


24 


338-2 


358-4 


654-8 


735-8 66 


123-5 


131-4 


241-5 


274-1 


25 


324-8 


344-3 


629-2 


707-5 67 


121-7 


129-5 


237'9 


270'0 


26 


312-4 


331-2 


605-5 


681-3 68 


119-9 


127-6 


234-4 


266-1 


27 


300-9 


319-3 


583-5 


656-8 69 


118-2 


125-7 


231-0 


262-3 


28 


290-3 


307-8 


563-1 


634-1 70 


116-5 


123-9 


227-7 


258-6 


29 


280-3 


297-3 


544-1 


613-0 71 


114-9 


122-2 


224-6 


255-0 


30 


271-1 


287*5 


526-2 


593-2 i 72 


113-3 


120-5 


221-5 


251-5 


31 


262-4 


278-2 


509-5 


574-5 73 


111-8 


118-9 


218-5 


248-1 


32 


254-2 


269-6 


493-8 


557'1 


74 


110*3 


117-3 


215-6 


244-8 


33 


246-6 


261-6 


479-1 


540-8 


75 


108-8 


115-8 


212-8 


241-6 


34 


239-3 


253-9 


465-3 


525-3 


76 


107-4 


114-3 


210-0 


238-4 


35 


232-6 


246-7 


452-2 


510-7 


77 


106-0 


112-8 


207-3 


235-3 


36 


226-1 


240-0 


439-8 


496-8 


78 


104-6 


111-4 


204-7 


232-3 


37 


220-0 


233-5 


428-1 


483-7 


79 


103-3 


110-0 


202-1 


229-4 


38 


214-3 


227-4 


417*0 


471-3 


80 


102-0 


108-6 


199-6 


226-6 


39 


208-8 


221-7 


406-5 


459*5 


81 


100-8 


107-2 





223-9 


40 


203-6 


216-2 


396-5 


448*3 


82 


99-6 


105-9 





221-2 


41 


198-7 


211-0 


387-0 


437-6 


83 





104-6 





218-6 


42 


194-1 


206-0 


377-8 


427-4 


84 


, 


103-4 





216-0' 


43 


189-7 


201-3 


369-2 


417-7 


85 





102-2 





213-5 


44 


185-4 


198-7 


360-9 


408*4 


86 





101-1 





211-1 


45 


181-2 


192-3 


353-0 


399-5 


87 











208-7 


46 


177-3 


188-1 


345-4 


391-0 


88 











206-4 


47 


173-5 


184-1 


338-1 


382-8 


89 











204-1 


43 


169-9 


180-3 


331-2 


374' 9 90 











201-9 


49 


166-4 


176-7 


324-5 


367-3 ; 91 











199-7 



74. SUGAB. 317 

6. Gerrard's Cyano-cupric Process. 

This process (Year Book Pharm. 1892, 400), as improved by 
Gerrard and A. H. Allen, promises to prove a valuable addition 
to the processes of titration based on the reducing power of glucose. . 
It has the advantage over Pavy's method in causing no evolution 
of ammonia ; moreover, the reduced solution is reoxidized so slowly 
that titration may even be conducted in an open dish with reason- 
able expedition. The process is based on the following facts : - 
When a solution of potassium cyanide is added to a solution of 
copper sulphate a colourless stable double cyanide of copper and 
potassium is formed, thus : 

CuSO* + 4KCy = CuCy 2 ,2KCy + K 2 SO*. 

This salt is not decomposed by alkalies, hydrogen sulphide, or 
ammonium sulphide. If potassium cyanide be added to Feh ling's 
solution the latter is decolourized, the above double salt being 
formed at the same time, and if the colourless solution be boiled 
with glucose no cuprous oxide is precipitated. If there be present 
-excess of Feh ling's solution over the amount capable of being 
decolourized by the potassium cyanide, the mixture is blue, and when 
it is boiled with a reducing sugar the extra portion is reduced, but 
no cuprous oxide is precipitated, the progress of the reduction 
being marked by the gradual and final disappearance of the colour 
of the solution, just as in Pavy's process. 

Process of Titration. 10 c.c. of fresh Pehling's solution, or 5 c.c. of 
each of the constituent solutions are diluted with 40 c.c. of water in 
a porcelain dish and heated to boiling. An approximately 5 per cent, 
solution of potassium cyanide is added very cautiously from a "burette or 
pipette to the still boiling and "well agitated blue liquid, till the colour is 
just about to disappear. Excess of cyanide must be carefully avoided.* 

10 c.c. of Fehling solution are now accurately measured into the dish, 
and the sugar solution (of about \ per cent, strength glucose) run in slowly 
from a burette with constant stirring and ebullition, till the blue colour 
disappears. Only the second measure of Fehling's solution suffers 
reduction. The volume of sugar solution run in contains 0'05 gin. of 
glucose. 

Some technical applications of these Solutions to mixtures of 
various Sugars. 

It cannot be claimed for these estimations that they are 
absolutely exact ; but with care and practice, accompanied with 
uniform conditions, they are probably capable of the best possible 
results whatever methods may be used. 

Cane Sugar, Grape Sugar, and Dextrine (Biard and Pellet, 
Z. a. C. xxiv. 275). The solution containing these three forms is first 
titrated with the usual Fehling solution for grape sugar. A second portion 

* As the double cyanide solution keeps for some time, a stock may be made up, so 
that 59 c.c. contain 10 c.c. of Fehling' s solution, and that volume taken for each 
titration, instead of going through the process of exact decolonization every time. 



318 VOLUMET1UC ANALYSIS. 75. 

is boiled with acetic acid (which only inverts cane sugar) and titrated. 
Finally, a third portion is completely inverted with sulphuric acid and 
titrated. The difference of the first and second titrations gives the cane 
sugar, and that of the second and third the dextrine. 

Milk and Cane Sug-ar. If the estimation of milk sugar is alone re- 
quired, and by the usual Fehling solution, the casein and albumen must 
be first removed. Acidify the liquid with a few drops of acetic acid, warm 
until coagulation is effected, and filter. Boil the filtrate to coagulate the 
albumen. Filter again, and neutralize with soda previous to treatment for 
sugar by the copper test. The number of c.c. of Fehling's solution 
required, multiplied by 0'0067S6, will give the weight of milk sugar in 
grams. Direct estimation by Pa vy-F eh ling is preferable to this method. 
Cane sugar in presence of milk sugar may be estimated as follows : Dilute 
the milk to ten times its bulk, having previously coagulated it with a little 
citric acid, filter, and make up to a definite volume, titrate a portion with 
Pavy-Fehling solution, and note the result. Then take 100 c.c. of the 
filtrate, add 2 gm. of citric acid, and boil for 10 minutes, cool, neutralize, 
make up to 200 c.c., and titrate with copper solution as before. The difference 
between the reducing powers of the solutions before and after conversion is 
due to the cane sugar, the milk sugar not being affected ~by citric acid. 

Stokes and Bodmer (Analyst x. 62) have experimented largely on this 
method, and with satisfactory results. The plan adopted by them is to use 
40 c.c. of Pavy-Fehling liquid ( = 0'02 gm. glucose), and to dilute the 
sugar solution (without previous coagulation), so that from 6 to 12 c.c. are 
required for reduction. By using a screw-clamp on the rubber burette tube, 
the sugar solution is allowed to drop into the boiling liquid at a moderate 
rate. If Cu 2 O should be precipitated before the colour disappears, a fresh 
trial must be made, adding the bulk of the sugar at once, then finishing by 
drops. If, on the other hand, the sugar has been run in to excess, which 
owing to the rather slow reaction is easily done, fresh trial must be again 
made until the proper point is reached : this gives the milk sugar. Mean- 
while a portion of the mixed sugar solution is boiled with 2 per cent, of 
citric acid, neutralized with NH 3 , made up to double its original volume, 
and titrated as before. 

These operators have determined the reducing action of milk, 
cane, and grape sugar on the Pavy-Fehling liquid, the result 
being that 100 lactose represents respectively 52 glucose, or 49 '4 
sucrose. 

The Pavy-Fehling liquid is admirably adapted for the esti- 
mation of lactose in milk direct after dilution, no coagulation being 
necessary. 

SULPHUR. 



Estimation in Pyrites, Ores, Residues, etc. 
1. Alkalimetric Method (Pelouze). 

75. THIS process, designed for the rapid estimation of sulphur 
in iron and copper pyrites, has hitherto been thought tolerably 
accurate, but experience lias shown that it cannot be relied upon 
except for rough, technical purposes. 



75. SULPHUR, 319 

The process is based on tlie fact, that when a sulphide is ignited 
with potassic chlorate and sodic carbonate, the sulphur is converted 
entirely into sulphuric acid, which expels its equivalent proportion 
of carbonic acid from the soda, forming neutral sodic sulphate ; if 
therefore, an accurately weighed quantity of the substance be 
fused with a known weight of pure sodic carbonate in excess, and 
the resulting mass titrated with normal acid, to find the quantity 
of unaltered carbonate, the proportion of sulphur is readily 
calculated from the difference between the volume of normal acid 
required to saturate the original carbonate, and that actually 
required after the ignition. 

It is advisable to take 1 gm. of the finely levigated pyrites, and 
5 '3 gm. of pure sodic carbonate for each assay; and as 5*3 gm. of 
sodic carbonate represent 100 c.c. of normal sulphuric acid, it is 
only necessary to subtract the number of c.c. used after the ignition 
from 100, and multiply the remainder by 0'016, in order to arrive 
at the weight of sulphur in the 1 gm. of pyrites, and by moving 
the decimal point two places to the right, the percentage is obtained. 

Example : 1 gm. of finely ground FeS 3 was mixed intimately with 5'3 gm. 
sodic carbonate, and about 7 gm. each of potassic chlorate, and decrepitated 
sodic chloride, in powder ; then introduced into a platinum crucible, and 
gradually exposed to a dull red heat for ten minutes ; the crucible suffered 
to cool, and warm water added ; the solution so obtained was brought on 
a moistened filter, the residue emptied into a beaker and boiled with a large 
quantity of water, brought on the filter, and washed with boiling water till 
all soluble matter was removed ; the filtrate coloured with methyl orange^ 
and titrated. 67 c.c. of normal acid were required, which deducted from 100, 
left 33 c.c. ; this multiplied by 0*016 gave 0'528 gm. or 52*8 per cent. S. 

Burnt Pyrites. The only satisfactory volumetric method of 
estimating the sulphur in the residual ores of pyrites, is that 
described by Watson (J. S. C. I. yii. 305), and which is in daily 
use in large alkali works. In order to avoid calculation, Watson 
adepts the following method : 

Standard Hydrochloric Acid. 1 c.c. =0*02 gm. ^Na 2 O. 

Sodic bicarbonate. This may be the ordinary commercial salt, 
but its exact alkalinity must be ascertained by the standard acid. 
Where a number of analyses are being made, a good quantity of 
the salt should be well mixed, and kept in a stoppered bottle. Its 
exact alkalinity having been once determined it will not alter, 
though daily opened. 

Process: 2 gm. of bicarbonate is placed in a crucible which may be 
either of platinum, porcelain, or nickel, and to it is added 5'16 gm. of the 
finely powdered ore, then intimately mixed with a flattened glass rod. 
Heat gently over a Bunsen burner for 5 or 10 minutes, and break up the 
mass with a stout copper wire. After stirring, the heat is increased and 
continued for 10 or 15 minutes. The crucible is then washed out with hot 
water into a beaker. The mixture is boiled for 15 minutes, filtered into 
a flask, the residue washed repeatedly with hot water, then cooled and 
titrated with the standard acid, using methyl orange as indicator. 



320 VOLUMETRIC ANALYSIS. 75. 

Example : 2 gm. of the bicarbonate originally required 37' 5 c.c. of acid. 
After ignition with the ore, 28 c.c. were required = 9'5 c.c., this divided by 
5 will give 1'9, which is the percentage of total sulphur in the ore. 

This total sulphur includes that which exists as soluble sulphide, 
and which is not available for acid making. In order to find 
the amount of this soluble sulphur, Watson boils 5*16 gm. of the 
ore with 5 c.c. of standard sodic carbonate (1 c.c. = 0*05 gm. ]N"a 2 0) 
diluted with water, for 15 minutes. After filtering and washing, 
the filtrate is titrated with the standard hydrochloric acid, and the 
difference between the volume used and that which was originally 
required for 5 c.c. of the soda solution is divided by 5, as in the 
ase of the former process, which gives at once the percentage of 
sulphur existing in the ore in a soluble form. The results are not 
absolutely exact, but quite near enough to guide a manufacturer in 
the working of the furnaces. 

This method is not available for unburnt pyrites. 



2. Estimation of Sulphur in Coal Gas. 

A most convenient and accurate process for this estimation is 
that of Wildenstein ( 76.2). The liquid produced by burning 
the measured gas in a Lethe by or Tern on Ha re our t apparatus 
is well mixed, and brought to a definite volume ; a portion repre- 
senting a known number of cubic feet of gas is then poured into 
a glass, porcelain, or platinum basin, acidified slightly with HC1, 
heated to boiling, and a measured excess of standard baric chloride 
added ; the excess of acid is then cautiously neutralized with 
rammonia (free from carbonate), and the excess of barium ascer- 
tained by standard potassic chromate exactly as described in 
76.2. 

The usual method of stating results is in grains of sulphur per 
100 cubic feet of gas. This may be done very readily by using 
semi-normal solutions of baric chloride and potassic chromate on 
the metric system, and multiplying the number of c.c. of baric 
solution required with the factor 0*1234, which at once gives the 
.amount of sulphur in grains. 



3. Estimation of Sulphur in Sulphides decomposable by 
Hydrochloric or Sulphuric Acids (Weil). 

This process, communicated to me by M. Weil, is based on the 
fact that, in the case of sulphides where the whole of the sulphur 
is given off as H 2 S by heating with HC1 or H 2 S0 4 , the IPS may 
"be evolved into an excess of a standard alkaline copper solution. 
After the action is complete, the amount of Cu left unreduced is 
estimated by standard .stannous chloride. The method is available 
for the sulphides of lead, antimony, zinc, iron, etc. Operators 



75. SULPHUR. 321 

should consult and practise the methods described in 58.6, in 
order to become accustomed to the special reaction involved. 

Process : Prom 1 to 10 gm. of material (according to its richness in 
sulphur) in the finest state of division, are put into a long-necked flask of 
about 200 c.c. capacity, to which is fitted a bent delivery tube, so arranged as 
to dip to the bottom of a tall cylinder, containing 50 or 100 c.c. of standard 
copper solution made by dissolving 39'523 gm. of cupric sulphate, 200 gm. 
of Eochelle salt and 125 gm. of pure caustic soda in water, and diluting to 
1 liter (10 c.c. = O'l gm. Cu). When this is ready, a few r pieces of granulated 
zinc are added to the sulphide. 75 c.c. of strong HC1 are then poured over 
them, the cork with delivery tube immediately inserted, connected with the 
copper solution, and the flask heated on a sand-bath until all evolution of 
H 2 S is ended. The blue solution and black precipitate are then brought on 
a filter, filtrate and washings collected in a 200 or 250 c.c. flask, and diluted 
to the mark ; 20 c.c. of the clear blue liquid are then measured into a boiling 
flask, and evaporated to 10 or 15 c.c. 25 to 50 c.c. of strong HC1 are then 
added, and the standard tin solution dropped in while boiling, until the blue 
gives place to a clear pure yellow. 

Each c.c. of standard copper solution represents 0*50393 gm. 
of sulphur. The addition of the granulated zinc facilitates the 
liberation of the H 2 S, and sweeps it out of the flask ; moreover, 
in the case of dealing with lead sulphide, which forms insoluble 
lead chloride, it materially assists the decomposition. Alkaline 
tartrate solution of copper may be used in place of ammoniacal 
solution if so desired. 

Examples (Weil) : 1 gm. of galena was taken, and the gas delivered into 
50 c.c. of standard copper solution (=0'5 gm. Cu). After complete pre- 
cipitation the blue liquid was diluted to 200 c.c. 20 c.c. of this required 
12'5 c.c. of stannous chloride, the titre of which was 16'5 c.c. for 0'04 gm. 
Cu. Therefore lf/5 : 0'04 : : 12'5 : 0'0303. Thus 200 c.c. (= 1 gm. galena) 
represent 0'303 gm. Cu. Then 0'5 gm. Cu, less 0'303 = 0'197 gm. for 1 gm. 
galena or 197 for 100 gm. Consequently 197 x 0'50393 = 9'92 per cent. S. 
Estimation by weight gave 9'85 per cent. Again, 1 gm. zinc sulphide was 
taken with 100 c.c. copper solution and made up to 250 c.c., 25 c.c. of which 
required 14'3 c.c. of same stannous chloride, or 143 c.c. for the 1 gm. 
sulphide. This represents 0'347 gm. Cu. Thus 1 0'347 0'653 gm. Cu 
(precipitated as CuS) or 65'3 per 100. Consequently 65'3 x 0'50393 = 32'9 
per cent. S. Control estimation by weight gave 33 per cent. 

The process has given me good technical results with Sb 2 S 3 , but 
the proportion of sulphur to copper is too great to expect strict 
accuracy. 



4. Estimation of Alkaline Sulphides by Standard Zinc Solution. 

This method, which is simply a counterpart of 82.3, is 
-especially applicable for the technical determination of alkaline 
rsulphides in impure alkalies, mother-liquors, etc. 

If the zinc solution be made by dissolving 3 '253 gm. of pure 
metallic zinc in hydrochloric acid, supersaturating with ammonia, 
.and diluting to 1 liter, 1 c.c. will respectively indicate 



322 VOLUMETRIC ANALYSIS. 75. 

0-0016 gin. Sulphur 
0-0039 Sodic sulphide 
0-00551 Potassic sulphide 
0*0034 Ammonic sulphide. 

The zinc solution is added from a burette until no dark colour is 
shown when a drop is brought in contact with solution of nickel 
sulphate spread in drops on a white porcelain tile. 



5. Sulphurous Acid and Sulphites: 

The difficulties formerly presented in the iodometric analyses of 
these substances are now fortunately quite overcome by the 
modification devised by Giles and Shearer (J. S. C. I. iii. 197 
and iv. 303). A valuable series of experiments on the estimation 
of SO 2 , either free or combined, are detailed in these papers. The 
modification is both simple and exact, and consists in adding the 
weighed SO 2 or the sulphite in powder to a measured excess 
of J^. iodine without dilution with water, and when the decomposi- 
tion is complete, titrating back with ^ thiosulphate. Yery con- 
centrated solutions of SO 2 are cooled by a freezing mixture, and 
enclosed in thin bulbs, which can be broken under the iodine- 
solution : this is, however, not required with the ordinary pre- 
parations. Sulphites and bisulphites of the alkalies and alkaline 
earths, also zinc and aluminium, may all be titrated in this way 
with accuracy ; the less soluble salts, of course, requiring more 
time and agitation to ensure their decomposition. A preliminary 
titration is first made with a considerable excess of iodine, and 
a second with a more moderate excess as indicated by the first 
trial. 1 c.c. T ^ iodine = 0-0032 gm. SO 2 . 

The authors found that when perfectly pure iodine and neutral 
potassic iodide were used for the standard solution, its strength 
remained intact for a long period ; and the same with the 
thiosulphate, if the addition of about 2 gm. of potassic bicarbonate 
to the liter was made, and the stock solution kept in the dark. 

From a large number of experiments, they also deduced the 
simple law of the ratio between any given percentage of SO 2 
in aqueous solution at 15-4 and 760 m.m., and its specific gravity ;. 
namely, the percentage found by titration multiplied by OO05' 
and added to unity gives the sp. gr. 

In cases where the iodine method may not be suitable, W. B. 
Giles recommends the use of a standard ammoniacal silver nitrate. 
This process is applicable alike to SO 2 , sulphites and bisulphites. 
The silver solution may conveniently be of ~ strength, but before 
use ammonia is added in sufficient quantity, first to produce 
a precipitate of silver oxide, then to dissolve it to a clear solution. 
A known excess of this solution is digested in a closed bottle. 
with the substance, in a water-bath for some hours, the result of 



75. SULPHUR. 323 

which is the reduction of the silver as a bright mirror on the 
sides of the vessel. The filtered liquid and washings may then 
be titrated by thiocyanate for the excess of silver, or the mirror 
together with any collected on the filter after washing and burning 
to ash may be dissolved in nitric acid and estimated by the same 
process ( 43). 1 c.c. T ^ silver=0'0032 gm. of SO 2 . 

Example : 0'1974 gm. of chemically pure potassium metasulphite was 
weighed out and treated as above described, the mirror of silver and a little 
on the filter estimated gave 0'1918 gm. of metallic silver, which multiplied 
by the factor T028 gives 0'19717 of metasulphite or 99'9 %. 

This method is very useful in determining the percentage of the 
SO 2 in liquefied sulphurous acid, which is now found in large 
quantities in commerce. By cooling down this substance to 
a point where it has no tension, small bulbs can be filled with 
facility and sealed up. After weighing they are introduced into 
a -we/Z-stoppered bottle containing an excess of the ammoniacal 
silver, and the stopper firmly secured by a clamp. By shaking the 
bottle vigorously the bulb is broken, and the estimation is then 
conducted as above described. 

A 2 ON 2 5 + SO 2 + xNH 3 = As 2 + SO 3 + WO 5 + xNH 3 . 



6. Estimation of Mixtures of Alkaline Sulphides, Sulphites, 
Thiosulphates, and Sulphates. 

No method up to the present has apparently been successfully 
devised for the estimation of the above-mentioned substances 
when existing together in any given solution. Richardson and 
Aykroyd (J. S. C. 1. xv. 171) have, however, now published 
a method which seems to give fairly accurate results. 

The estimation of the SO 3 in such a mixture cannot be done 
volumetrically, but by the addition of about 5 gm. of tartaric acid to 
such a quantity of solution of mixed thiosulphate, sulphate, and 
sulphite as would be usually taken for analysis, the SO 3 may 
readily be precipitated with baric chloride in the cold. The 
precipitate of BaSO 4 contains some baric sulphite, but this is 
easily removed by hot dilute HC1 and boiling water. The 
thiosulphate produces no SO 3 whatever under these circumstances, 
whereas in the presence of a mineral acid sulphate is always 
produced. 

The sulphides are estimated by standard ammoniacal zinc 
solution, which may conveniently be of such strength that 
1 c.c. = 0'0016 of S, using nickel sulphate solution as an external 
indicator. 

The zinc solution is easily made from pure metallic zinc 
dissolved in HC1, and the precipitate which is formed by adding 
ammonia, is brought into clear solution by a moderate excess of 
the same re-agent. 

Y 2 



324 VOLUMETRIC ANALYSIS. 75. 

This zinc solution is also used for removing sulphides from 
a mixture of these with thiosulphates, sulphites, and sulphates 
prior to the estimation of the latter bodies. In this case it is 
only necessary to add a slight excess of the zinc solution, and 
filter off the precipitated sulphide. 

The authors of this method after pointing out the value of 
Giles and Shearer's method of estimating sulphites by iodine, 
described in this section (par. 5), mention a method devised by 
themselves, which they believe enables them to estimate not only 
sulphites but free SO 2 , not only in a pure state but in mixtures 
with sulphates, thiosulphates, and sulphides. They avail them- 
selves of the well-known reaction, that when iodine is added to 
a neutral sulphite, neutral sulphate and an equivalent amount of 
hydriodic acid are formed 



H 2 - Na 2 S0 4 + 2HI, 

and the acidity of the solution may be accurately measured by 
standard alkali and methyl orange. 

The authors proceed to state that the best plan is to convert all 
sulphites to bisulphites, i.e., to the hydrogen sulphite of the base : 
this is necessary because a sulphite may be alkaline, or it may be 
exclusively acid. Sodic bisulphite is quite neutral to methyl 
orange, and by titrating the solution of a neutral sulphite with 
decinormal sulphuric acid, using methyl orange, we arrive exactly 
at a point when all the sulphite is converted into the acid sulphite. 
The reason for this is patent when the reaction which takes place 
when an acid sulphite acts upon iodine is considered 

KaH.S0 8 + OH 2 + 1 2 - NaH.SO* + 2HL 

Here is a new factor, inasmuch as the titration with alkali and 
with methyl orange as indicator is concerned ; although the acid 
sulphite of soda is neutral to methyl orange, the acid sulphate of 
soda is acid to the full and exact extent of its combining power. 

Thus one molecule of sodic bisulphite, on titration with - iodine, 
liberates acid equivalent to three molecules of sodic or potassic 
hydrate. 

A solution containing 1'62 per cent, of Na 2 SO 3 .7Aq was titrated. Iodine 
solution equivalent to 9'5 c.c. T \ I ; 29'9 c.c. were required; the mixture 
required 14'6 c.c. of ^ NaHO. Now 9'5 c.c. T \ I and 14'6 c.c. T ^ NaHO 
are in the ratio of 2 : 3 almost exactly; by using 0'0126 as the factor for 
the c.c. of T \ I and 0'084 for the ^ NaHO, both results give T64 per cent. 
of Na 2 SO 3 .7Aq. (Of course the sulphite solution had been previously 
titrated with ^ T H 2 SO 4 in the presence of methyl orange.) 

As the details of calculation may be somewhat obscure to those who have 
not experimented in this direction, the working out of an actual analysis 
may be of interest. A solution containing 1 per cent, of pure sodic 
tine-sulphate, and 0'78 per cent, of sodic sulphite, was titrated upon 20 c.c. 
of iodine ; 19'3 c c. were required to decolorize ; to neutralize with methyl 
orange as indicator 17*9 c.c. of 5- soda were required ; therefore ICO c.c. of 



76. SULPHURIC ACID. 325 

the mixture required 103'6 c.c. iodine and 92 - 7 c.c. of T \ soda respectively ; 
the c.c. of soda x 0'0084 give 0'7787 as the percentage of Na 2 SO 3 .7Aq, and 
this figure-:- 0*0126 (the factor for 1 c.c. iodine in Na 2 SO 3 .7Aq) gives 61'8 c.c., 
and this subtracted from 103'6 c.c. of total iodine required gives 41'8 c.c., 
and this x 0'0248 gives T036 instead of 1 per cent, of Na 2 S 2 O 3 .5Aq. 

The immense advantage of this method is better seen in the 
case of a complex mixture, where one must remove sulphides or 
other bodies by the addition of an alkaline solution of zinc or 
other precipitating agent. The alkaline filtrate is speedily brought 
into a suitable condition for iodimetric and alkalimetric titration 
'by the method proposed. 

Example : A solution of known amounts of sodic thiosulphate and 
sulphite was treated with 10 c.c. of a strongly ammoniacal zinc-chloride 
solution, and the mixture was titrated with it until it gave a neutral 
reaction with methyl orange; it was now made to 1000 c.c., and was titrated 
upon a known volume of f$ iodine, using starch to find the end-reaction 
(which is otherwise somewhat obscured by the methyl orange). The 
disappearance of the blue colour and the appearance of the pinkish-purple 
of the acidified methyl orange is both interesting and striking. Titration 
with /TJ- NaHO was now easily accomplished. The results were exact in the 
case of thiosulphate, and very slightly in excess in the case of sulphite. 

After the sulphite and thiosulphate solution has been titrated 
upon a known volume of y^- iodine, the sulphate formed is 
estimated by barium at a boiling heat in the presence of a little 
dilute HC1. Any sulphate in the original solution is, of course, 
estimated by the tartaric acid method and deducted from the 
result. Ammonic tartrate must be avoided in the process, owing 
to its solvent action on barium sulphate. 



SULPHURIC ACID AND SULPHATES. 
Monohydrated Sulphuric Acid. 

H 2 S0 4 = 98. 

Sulphuric Anhydride. 

SO 3 = 80. 

I. Mohr's Method. 

76. THE indirect process devised by C. Mohr (Ann. der 
Chem. u. Pharm. xc. 165) consists in adding a known volume of 
baric solution to the compound, more than sufficient to precipitate 
the SO 3 . The excess of barium is converted into carbonate, and 
titrated with normal acid and alkali. 

formal Baric chloride is made by dissolving 12177 gm. of 
pure crystals of baric chloride in the liter ; this solution likewise 
suffices for the determination of SO 3 by the direct method. 

Process : If the substance contains a considerable quantity of free acid, 
it must be brought near to neutrality by pure sodic carbonate ; if alkaline, 



326 VOLUMETRIC ANALYSIS. 76. 

slightly acidified with hydrochloric acid ; a round number of c.c. of baric 
solution in excess is then added, and the whole digested in a warm place for 
some minutes; the excess of barium is precipitated by a mixture of 
carbonate and caustic ammonia in slight excess ; if a piece of litmus paper 
be thrown into the mixture, a great excess may readily be avoided. The 
precipitate containing both sulphate and carbonate is now to be collected on 
a filter, thoroughly washed with boiling water, and titrated. 

The difference between the number of c.c. of baric solution 
added, and that of normal acid required for the carbonate, will be 
the measure of the sulphuric acid present ; each c.c. of baric 
solution is equal to 0*040 gm. SO 3 . 

Example : 2 gm. of pure and dry baric nitrate, and 1 gm. of pure potassic 
sulphate were dissolved, mixed, and precipitated hot with carbonate and 
caustic ammonia ; the precipitate, after being thoroughly washed, gave 
T002 gm. potassic sulphate, instead of 1 gm. 

For technical purposes this process may be considerably shortened 
by the following modification, which dispenses with the washing of 
the precipitate. 

The solution containing the sulphates or sulphuric acid is first rendered 
neutral ; normal baric chloride is then added in excess, then normal sodic 
carbonate in excess of the baric chloride, and the volume of both solutions 
noted ; the liquid is then made up to 200 or 300 c.c. in a flask, and an aliquot 
portion filtered off and titrated with normal acid. The difference between 
the baric chloride and sodic carbonate gives the sulphuric acid. 

The solution must of course contain no substance precipitable by 
sodic carbonate except barium (or if so, it must be previously 
removed) ; nor must it contain any substance precipitable by 
barium, such as phosphoric or oxalic acid, etc. 



2. Titration by Baric Chloride and Potassic Chroxnate 
(Wildenstein). 

To the hot solution containing the SO 3 to be estimated (which 
must be neutral, or if acid, neutralized with caustic ammonia, free 
from carbonate), a standard solution of baric chloride is added in 
slight excess, then a solution of potassic chromate of known 
strength is cautiously added to precipitate the excess of barium. 
So long as any barium remains in excess, the supernatant liquid is 
colourless ; when it is all precipitated the liquid is yellow, from the 
free chromate ; a few drops only of the chromate solution are 
necessary to produce a distinct colour. 

Wildenstein uses a baric solution, of which 1 c.c. = 0*015 
gm. of SO 3 , and chromate 1 c.c. = 0*010 gm. of SO 3 . I prefer 
to use I- solutions, so that 1 c.c. of each is equal to 0*02 gm. 
of SO 3 . If the chromate solution is made equal to the baric 
chloride, the operator has simply to deduct the one from the other, 
in order to obtain the quantity of baric solution really required to 
precipitate all the SO 3 . 



76. SULPHURIC ACID. 327 

Process : The substance or solution containing SO 3 is brought into a small 
flask, diluted to about 50 c.c., acidified if necessary with HC1, heated to 
boiling, and precipitated with a slight excess of standard baric chloride 
delivered from the burette. As the precipitate rapidly settles from a boiling 
solution, it is easy to avoid any great excess of barium, which would prevent 
the liquid from clearing so speedily. The mixture is then cautiously 
neutralized with ammonia free from carbonic acid (to be certain of this, it is 
well to add to it two or three drops of calcic chloride or acetate solution). 

The flask is then heated to boiling, and the chromate solution added 
in i c.c. or so, each time removing the flask from the heat and allowing to 
settle, until the liquid is of a light yellow colour ; the quantity of chromate 
is then deducted from the barium solution, and the remainder calculated 
for SO 3 . 

Or the mixture with barium in excess may be diluted to 100 or 150 c.c. 
the precipitate allowed to settle thoroughly, and 25 or 50 c.c. of the clear 
liquid heated to boiling, after neutralizing, and precipitated with chromate 
until all the barium is carried down as baric chromate, leaving the liquid of 
a light yellow colour; the analysis should be checked by a second titration. 
The process has yielded me very satisfactor} 7 results in comparison with the 
barium method by weight ; it is peculiarly adapted for estimating sulphur in 
gas when burnt hi the Letheby sulphur apparatus, details of which will be 
found on page 320. 

The presence of alkaline and earthy salts is of no consequence 
Zn and Cd do not interfere Xi, Co, and Cu give coloured 
solutions which prevent the yellow chromate being seen, but this 
difficulty can be overcome by the use of an external indicator for 
the excess of chromate. This indicator is an ammoniacal lead 
solution, made b^ mixing together, at the time required, one 
volume of pure ammonia and four volumes of lead acetate solution 
(1 : 20). The liquid has an opalescent appearance. To use the 
indicator, a large drop is spread upon a white porcelain plate, and 
one or two drops of the liquid under titration added; if the 
reddish-yellow colour of lead chromate is produced, there is an 
excess of chromate, which can be cautiously reduced by adding 
more barium until the exact balance occurs. 



3. Direct Precipitation -with. Normal Baric Chloride. 

Yery good results may be obtained by this method when 
carefully performed. 

Process : The substance in solution is to be acidified with hydrochloric 
acid, heated to boiling, and the baric solution allowed to flow cautiously in 
from the burette until no further precipitation occurs. The end of the 
process can only be determined by filtering a portion of the liquid, and 
testing with a drop of the baric solution. Beale's filter (shown in fig. 23) 
is a good aid in this case. A few drops of clear liquid are poured into a test 
tube and a drop of baric solution added from the burette ; if a cloudiness 
occurs, the contents of the tubes must be emptied back again, washed out 
into the liquid, and more baric solution added until all the SO 3 is precipitated. 
It is advisable to use r \ solution towards the end of the process. 

Instead of the test tube for finding whether barium or sulphuric 
acid is in excess, a plate of black glass may be used, on which a drop 




328 VOLUMETRIC ANALYSIS. 76. 

of the clear solution is placed and tested by either a drop of baric 
chloride or sodic sulphate, these testing solutions are preferably 
kept in two small bottles with elongated stoppers. A still better 
plan is to spot the liquids on a small mirror, as suggested by 
Haddock (C. N. xxxix. 156); the faintest reaction can then be 
seen, although the liquid may be highly coloured. 

Wildenstein "has arranged another method for 
direct precipitation, especially useful where a con- 
stant series of estimations have to be made. The 
apparatus is shown in fig. 51. A is a bottle of 
900 or 1000 c.c. capacity, with the bottom removed, 
and made of well-annealed glass so as to stand 
heating ; B a thistle funnel bent round, as in the 
figure, and this syphon filter is put into action by 
opening the pinch-cock below the cork. The mouth 
of the funnel is first tied over with a piece of fine 
cotton cloth, then two thicknesses of Swedish filter 
_, paper, and again with a piece of cotton cloth, the 

whole being securely tied with waxed thread. 
In precipitating SO 3 by baric chloride, there occurs a point 
similar to the so-called neutral point in silver assay, when in one 
and the same solution both barium and sulphuric acid after a 
minute or two produce a cloudiness. Owing to this circumstance, 
the barium solution must not be reckoned exactly by its amount 
of Bad 2 , but by its working effect; that is to say, the process 
must be considered ended when the addition of a drop or two of 
barium solution gives no cloudiness after the lapse of two minutes. 

Process : The solution containing the SO 3 being prepared, and preferably 
in HC1, the vessel A is filled with warm distilled water, and the pinch-cock 
opened so as to fill the filter to the bend C ; the cock is then opened and 
shut a few times so as to bring the water further down into the tube, but 
not to fill it entirely ; the water is then emptied out of A, and about 400 c.c. 
of boiled distilled water poured in together with the SO 3 solution, then, if 
necessary, a small quantity of HC1 added, and the baric chloride added in 
moderate quantity from a burette. After mixing well, and waiting a few 
minutes, a portion is drawn off into a small beaker, and poured back without 
loss into A ; a small quantit} 7 is then drawn off into a test tube, and two 
drops of baric chloride added. So long as a precipitate occurs, the liquid is 
returned to A, and more barium added until a test is taken which shows no 
distinct cloudiness; the few drops added to produce this effect are deducted. 
If a distinct excess has been used, the analysis must be corrected with 
a solution of SO 3 corresponding in strength to the barium solution. 

A simpler and even more serviceable arrangement of apparatus 
on the above plan may be made, by using as the boiling and 
precipitating vessel an ordinary beaker standing on wire gauze or 
a hot plate. The filter is made by taking a small thistle funnel, tied 
over as described, with about two inches of its tube, over which is 
tightly slipped about four or five inches of elastic tubing, terminating 
with a short piece of glass tube drawn out to a small orifice like 



77. SULPHURETTED HYDROGEN. 329 

a pipette ; a small pinch-cock is placed across the elastic tube just 
above the pipette end, so that when hung over the edge of the 
beaker with the funnel below the surface of the liquid, the 
apparatus will act as a syphon. It may readily be filled with warm 
distilled water by gentle suction, then transferred to the liquid 
under titration. By its means much smaller and more concentrated 
liquids may be used for the analysis, and consequently a more 
distinct evidence of the reaction obtained. 



SULPHURETTED HYDROGEN. 
IPS = 34. 

1 c.c. -fjj arsenious solution = O00255 gm. IPS. 
1. By Arsenious Acid (Mohr). 

77. THIS residual process is far preferable to the direct titration 
of sulphuretted hydrogen by iodine. The principle is based on the 
fact, that when H 2 S is brought into contact with an excess of 
arsenious acid in hydrochloric acid solution, arsenic sulphide is- 
formed ; 1 eq. of arsenious acid and 3 eq. of sulphuretted hydrogen 
produce 1 cq. of arsenic sulphide and 3 eq. of water, 

As 2 3 + 3H 2 S = As 2 S 3 + 3H 2 0. 

The excess of arsenious acid used is found by iodine and starch y 
as in 40. In estimating the strength of sulphuretted hydrogen 
water, the following plan may be pursued. 

Process : A measured quantity, say 10 c.c. of ^ arsenious solution, is put 
into a 300 c.c. flask, and 20 c.c. of sulphuretted hydrogen water added, well 
mixed, and sufficient HC1 added to produce a distinct acid reaction ; this- 
produces a precipitate of arsenic sulphide, and the liquid itself is colourless. 
The whole is then diluted to 300 c c., filtered through a dry filter into a dry 
vessel, 100 c.c. of the filtrate taken out and neutralized with sodic- 
bicarbonate, then titrated with T ^ iodine and starch. The quantity of 
arsenious acid so found is deducted from the original 10 c.c., and the 
remainder multiplied by the requisite factor for H 2 S. 

The estimation of IPS contained in coal gas, may by this 
method be made very accurately by leading the gas very slowly 
through the arsenious solution, or still better, through a dilute 
solution of caustic alkali, then adding arsenious solution, and 
titrating as before described. The apparatus devised by Mohr for 
this purpose is arranged as follows : 

The gas from a common burner is led by means of a vulcanized tube into- 
two successive small wash-bottles, containing the Alkaline solution; from the 
last of these it is led into a large Woulf f's bottle filled with water. The 
bottle has two necks, and a tap at the bottom ; one of the necks contains 
the cork through which the tube carrying the gas is passed; the other, 
a cork through which a good-sized funnel with a tube reaching to the bottom 



"330 VOLUMETRIC ANALYSIS. 77. 

of the bottle is passed. When the gas begins to bubble through the flask, 
the tap is opened so as to allow the water to drop rapidly ; if the pressure of 
gas is strong, the funnel tube acts as a safety valve, and allows the water to 
rise up into the cup of the funnel. "When a sufficient quantit}' of gas has 
passed into the bottle, say six or eight pints, the water which has issued from 
the tap into some convenient vessel is measured into cubic inches or liters, 
and gives the quantity of gas which has displaced it. In order to insure 
accurate measurement, all parts of the apparatus must be tight. 

The flasks are then separated, and into the second 5 c.c. of arsenious 
solution placed, and. acidified slightly with HC1. If any traces of a 
precipitate occur it is set aside for titration with the contents of the first 
flask, into which 10 c.c. or so of arsenious solution are put, acidified as 
before, both mixed together, diluted to a given measure, filtered, and a 
measured quantity titrated as before described. 

This method does not answer for very crude gas containing large 
quantities of H 2 S unless the absorbing surface is largely increased. 



2. By Permang-anate (Moh.r). 

If a solution of H 2 S is added to a dilute solution of ferric 
.sulphate, the ferric salt is reduced to the ferrous state, and free 
sulphur separates. The ferrous salt so produced may be measured 
accurately by permanganate without removing the separated 
.sulphur. Ferric sulphate, free from ferrous compounds, in 
sulphuric acid solution, is placed in a stoppered flask, and the 
solution of H 2 S added to it with a pipette ; the mixture is allowed 
to stand half an hour or so, then diluted considerably, and per- 
manganate added until the rose colour appears. 

56 Fe=17 H 2 S 

or each c.c. of -^ permanganate represents O0017 gm. of IPS. 
The process is considerably hastened by placing the stoppered llask 
containing the acid ferric liquid into hot water previous to the 
addition of H 2 S, and excluding air as much as possible. 



3. By Iodine. 

Sulphuretted hydrogen in mineral waters may be accurately 
estimated by iodine in the following manner : 

Process : 10 c.c or any other necessary volume of T V iodine solution are 
measured into a 500 c.c. flask, and the water to be examined added until the 
colour disappears. 5 c.c. of starch indicator are then added, and T y iodine 
until the blue colour appears ; the flask is then filled to the mark with pure 
distilled water. The respective volumes of iodine and starch solution, 
together with the added water, deducted from the 500 c.c., will show the 
volume of water actually titrated by the iodine. A correction should be 
made for the excess of iodine necessary to produce the blue colour. 

Fresenius examined the sulphur water of the Grindbrunnen, 
in Frankfurt a. M. (Z. a. C. xiv. 321), both volume trically and 



78. TANNIC ACID. 331 

by weight for H 2 S with very concordant results. 361*44 gm. of 
water (correction for blue colour being allowed) required 20*14 c.c. 
of iodine, 20*52 c.c. of which contained '02 527 of free iodine 
= H 2 S 0*009194 gm. per million. 444'65 gm. of the same water 
required, under the same conditions, 25 '05 c.c. of the same iodine 
solution = H 2 S 0-009244 gm. per million. By weight the H 2 S 
was found to be 0*009377 gm. per million. 

TANNIC ACID. 

78. THE estimation of tannin in the materials used for 
tanning is by no means of the most satisfactory character. Many 
methods have been proposed, and given up as practically useless. 
In the previous editions of this book LowenthaPs method 
as then perfected was given but it is still somewhat deficient 
in accuracy or constancy of results, although much ingenuity and 
intelligence have been expended on it. 

One difficulty is still urisurmounted, and i^hat is, the preparation 
of a pure tannic acid to serve as standard. The various tannins in 
existence are still very imperfectly understood,* but so far as the 
comparative analysis of tanning materials among themselves is 
concerned, the method in question is theoretically the best. 

The principle of the method depends on the oxidation of the 
tannic acid, together with other glucosides and easily oxidizable 
substances by permanganate, regulated by the presence of soluble 
indigo-carmine, which also acts as an indicator to the end of the 
reaction. The total amount of such substances being found and 
expressed by a known volume of permanganate, the actual available 
tannin is then removed by gelatine, arid the second titration is 
made upon the solution so obtained in order to find the amount of 
oxidizable matters other than tannin. 

The volume of permanganate so used, deducted from the volume 
used originally, shows the amount of tannin actually available for 
tanning purposes expressed in terms of permanganate. 

It will be at once seen that this method is essentially a practical 
one, because it is only the particular tannin capable of combining 
with organic tissue which is estimated. It has been critically 
examined with approbation by good authorities, among whom may 
be mentioned, Procter (C. N. xxxvi. 59 ; ibid, xxxvii. 256), 
Kathreiner (Z. a. C. xviii. 112), (Diiigler's Polyt. Jour. 
cxxvii. 481), and Hewitt (Tanner's Jour., May, 1877, 93). My 

*Von Schroder, whose suggestions have been adopted by the German Association 
of Tanners, selects a commercial pure tannic acid for use as a standard by dissolving 
2 gm. in a liter of water. 10 c.c. of this is titrated with permanganate as described. 
50 c.c. are then digested twenty hours with 3 gm. moistened hide powder. 10 c.c. of 
the filtrate from this is then titrated, and if the permanganate consumed amounts to 
less than 10 per cent, of the total consumed by the tannin, it is suitable for a standard. 
1000 parts being considered equivalent in reducing power to 1048 parts of tannin pre- 
cipitable by hide, according to Hammer's experiments, therefore Von Schroder, 
after titrating as described, calculates the dry matter, and multiplies by the round 
number 1 '05 to obtain the value in actual tannin precipitable by hide. 



332 VOLUMETRIC ANALYSIS. 78. 

own experiments have shown that for all materials containing, 
tannin, even catechu, it is the best process yet discovered, but 
requires patient practice to ensure concordant results. Lowenthal's- 
description of the method is given in Z. a. C. xvi. 33. 

The extraction of the tannic acid from the raw material is best 
performed by boiling it in a large flask with about a liter of 
distilled water for half an hour, then straining, and diluting when 
cold to 1 liter. Portions are filtered if necessary. Concentrated 
extracts are dissolved before titration by adding them to boiling 
water, then cooling and diluting to the measure. In the case of 
strong materials such as sumach or valonia 10 gm., or oak-bark 
20 gm., are used. 

The quantity of these extracts to be used for titration must be 
regulated to some extent by the amount of permanganate required 
to oxidize the tannic and gallic acids present. Practice and 
experience will enable the operator to judge of the proper propor- 
tions to use in dealing with the various materials, bearing in mind 
that volumetric processes are largely dependent upon identity of 
conditions for securing concordant results. 

Procter, who is probably one of the best authorities on this 
.subject, has modified to some extent the details of this process 
(/. S. C. I. iii. 82, and ibid. v. 79), and these modifications are 
embodied here. 



Standard Solutions and Re-agents. 

Standard Potassic permanganate. Kathreiner recommends 
that this solution should contain not more than 1*333 gm. of 
the pure salt per liter (better only about 1 gm.) ; therefore, if 
the operator is accustomed to use the decinormal solution, a very 
convenient strength is made by diluting one volume of it with 
two of water, thus obtaining a solution of -$ strength ( = 1 '052 gm. 
per liter). 

This standard is the more advisable because it enables the 
operator to calculate its value into oxalic acid, and so arrive at the 
theoretical standards adopted by Neubauer and Oser; namely, 
that 0*063 gm. of oxalic acid represents 0'04157 gm. of gallo-tannic 
acid (gall-nut tannin), or 0*062355 gm. of querci-tannic acid (oak 
bark tannin). These coefficients for calculation are now largely 
adopted, and are certainly preferable to standardizing the perman- 
ganate upon any specimen of so-called pure tannin. 

30 c.c. of -jf^j- permanganate will therefore represent 0'063 gm. 
of oxalic acid or the weights of tannin above mentioned. 

Solution of Indigo Carmine. This should be a clear solution of 
about 5 gm. to the liter with about 50 c.c. of pure H 2 S0 4 . 

Solution of Gelatine. This solution is used to precipitate the 
available tannin in any given solution after its total oxidizable 
matters have been determined by the indigo and permanganate. It 



78. TANNIC ACID. 333 

should be made fresh for each series of titrations, by dissolving 
2 gm. of Nelson's gelatine in 100 c.c. of water and filtering. 
Dilute Sulphuric Acid. MO. 

Processes of Tit-ration : The first thing to be done is to ascertain the 
relationship between the permanganate and indigo solutions (it is assumed 
that the permanganate is correct as regards its relation to oxalic acid), and 
therefore 10 or 20 c.c. of the indigo are measured into a white porcelain basin, 
and diluted to f of a liter with distilled w r ater, or good ordinary water free 
from organic matter or other substances capable of reducing permanganate. 
10 c.c. of the dilute acid are measured in, and the permanganate delivered 
in with a hand-pipette in drops, with constant stirring, until the colour is 
just discharged, leaving a clear faint yellow tint, with just a shade of pink 
at the rim. 

This experiment will act as a guide to the final adjustment of the indigo 
with an accurate 30 c.c. burette in $, which should be of such dilution that 
about 20 c.c. correspond to about 15 c.c. of permanganate. 

Titration of the Tanning Material : It is very important, in order to 
avoid uncertainty in the end-point of the reaction,' that only so much 
material shall be used as shall consume about 7 or 8 c.c. of permanganate of 
-5^5- strength above that point which is required for the indigo. 

Procter and Kathreiner both insist upon these proportions, and the 
general method adopted by them is to add 20 c.c. of indigo with 10 c.c. of 
dilute acid to about i of a liter of water, in a porcelain dish, followed by 5 c.c. 
of tannin solution. The permanganate is then delivered in very slowly, with 
constant stirring, until a faint rose colour appears round the edges of the 
liquid. The time allowed for the titration is also very important. " 

Von Schroder, representing the Association of German Tanners, 
prefers to add the permanganate 1 c.c. at a time with vigorous 
stirring, until the colour of the liquid indicates that a few drops 
only are required to end the titration. Procter, on the other 
hand, prefers the rapid drop method for the commencement, and 
until near the end. He also finds that the method of stirring 
influences the result in no very slight degree. Whatever plan the 
operator adopts, it is advisable to keep consistently to it in order 
that the results may be comparatively the same. 

It must be remembered that neither by this nor any other 
method is it possible to accurately estimate the tannin, but only as 
a means of comparing two samples of the same material. 

Precipitation of the Tannin, and subsequent Titration of Substances 
other than Tannin. Procter's procedure is to take 50 c.c. of the tannin 
infusion (5 c.c. of which has been titrated), and add to it 28'6 c.c. of gelatine 
solution in a flask holding about 150 c c. The mixture is well shaken, then 
saturated with clean table salt, and 10 c.c. of the dilute acid added, together 
with a teaspoonful of kaolin : the whole is vigorously shaken, then filtered, 
and made up to exactly 100 c.c. 10 c.c. of this liquid, representing 5 c.c. of 
the tannin decoction, are then titrated in precisely the same manner as before. 
The calculation of percentage is then made as follows : Let the first titration 
(two of which should be made for security) be called a ; the second, also in 
duplicate, b. If further, c be the quantity of permanganate required to 
oxidize 10 c.c. of ^V oxalic acid, and 10 gm. of substance have been employed 
for 1 liter of decoction, then c : (a b) : : 6'3 : x, where x is the percentage 
of tannin expressed in terms of oxalic acid. 



334 VOLUMETRIC ANALYSIS. 78. 

Hunt, who is also an undoubted authority on tannin estimation, 
differs from Procter on the question of saturating the liquid for 
final titration with salt (J. C. S. I. iv. 263), on the ground that, in 
the case of material containing much gallic acid, some of it is 
precipitated with the tannin, thus leading to higher results. This 
he has proved by experiment, and therefore prefers to act as- 
follows : 

50 c.c. of the tannin solution are run into a small dry flask, to this 25 c.c. 
of the fresh filtered gelatine solution are added, and the flask shaken. 25 c.c. 
of a saturated solution of salt, containing 50 c.c. of strong H' 2 SO 4 per liter, 
are now added, and about a teaspoonful of kaolin, or baric sulphate. The 
flask is thoroughly shaken for a feAV minutes, after which a clear bright 
filtrate may be obtained. 

For materials containing over 45 per cent, tannin, it is advisable 
to take 25 c.c. instead of 50, and to use 50 c.c. of salt, the amount 
of gelatine solution. being the same. The same authority also states 
that, for gambler and its allies, the method of titration as above 
described does not give accurate results, inasmuch as the gelatine 
and salt do not remove all the substances of tanning value from the 
liquid. In such case it is necessary to digest the liquid for at least 
twelve hours with pure hide powder. The mixture is then filtered 
and titrated in the usual way. 

It is impossible to give here the opinions held by various 
authorities on this subject, therefore the reader who desires fuller 
information should consult the papers to which reference has been 
made. 

The table on next page by Hunt is appended, as the result of 
careful working, and as a guide to the nature of various tanning 
materials : 

The " total extract " in the table was determined by evaporating 
a portion of the tannin solution to dryness in a small porcelain 
basin and drying the residue at 110 C. The "insoluble matter" 
was also dried at 110 C. 

The hide powder process for tannin not being a volumetric one 
is not described here. 

Tannin in Tea. The extract in this substance is made upon 
10 gm. of the tea, by boiling it with repeated quantities of distilled 
water, filtering and diluting the liquid when cool to a liter. The 
percentage varies from about 12 in black tea to 18 or 20 in green. 



78. 



TANNIC ACID. 



335> 



Total 
matters 

NAME or MATERIAL. ^g^. 
g-auate, as 
i Oxalic Ac. 


Tannin, as 
Oxalic Ac, 
(Procter) 


Tannin, as 
Oxalic Ac. 
(H u n t) 


Total 
Extract. 


Insoluble. 




per cent. 


per cent. 


per cent. 


per cent. 


per cent. 


English Oak Bark ... 1570 


13-54 


11-97 


18-38 


66-15 


CanadianHemlockBark 9'03 


7-46 


7-08 


13-96 


75-25 


Larch Bark 


8-20 


7-17 


6-15 


20-64 


60'80 


Mangrove Bark 


31-35 


29-71 


28-48 


26-60 


49-70 


Alder Bark 


8'27 


6-15 


5-73 


19-36 


68-00 


Blue Gum Bark 


10-18 


8-91 


8-91 


11-76 


74-65 


Valonia 


37-41 


35-24 


30-50 


38-50 


46-05 


Myrabolans 


48-23 


38-43 


38-00 


42-80 





Sumach 


42-53 


34-30 


31-46 


44-10 


47-77 


BetelNut 


15-91 


13-87 


13-79 


17-94 


67-00 


Turkish Blue Galls ... 


73-38 


65-83 


59-96 


48*40 


36-35 


Aleppo Galls 


98-85 


87-82 


83-05 


68-80 


1432 


Wild Galls 


26-21 


18-75 


16-56 


31-70 


54-17 


Divi-Divi 


66-98 


62-62 


61-22 


54-38 


29-90 


Balsamocarpon (poor 












and old sample) ... 


50-49 


37-76 


32-88 


57-14 


28-25 


Pomegranate Rind . . . 


27-58 


21-18 


23-12 


41-00 


49-50 


Tormentil Root 


22-27 


20-98 


20-68 


1970 


67-95 


Rhatany Root 


22-27 


20-15 


19-30 


18-80 


66-00 


Pure Indian Tea 


23-06 


18-65 


17-40 


34-46 


53-40 


Pure China Tea 


1 8-03 


14-21 


14-09 


24-50 


62'6'0 


Cutch 


57"65 


51-95 


44-24 


61-60 


4-75 


Gum Kino 


66-39 


59-55 


51-55 


7930 


i-oo 


Hemlock Extract ... 


35-16 


33-17 


30*98 


48-78 


i 


Oak wood Extract ... 


33-49 


26-90 


23*86 


37-78 





Chestnut Extract ... 


39-77 


32-63 


28'88 


50-28 





Quebracho Extract ... 


48-22 


44-45 


40-84 


49-00 





"Pure Tannin" 


135-76 


122-44 


121-93 





. 


TanLiquor,sp. gr.1'030 


4-84 


3-14 


2-10 


6-01 





Spent Tan Liquor, sp. 












gr. 1'0165 


1-40 


0-37 


0-25 


3-10 











Absorbed 












by Dry 












Pure Skin. 






Gambier, Cube 


70-12 


. 


51-07 


74-40 


5-31 


Sarawak . . . 


63-13 





47-09 


70-70 


3-67 


Bale 


56-00 





43-70 


63'54 


1'40 



Tannin in Wine, Cider, etc. The method now generally adopted 
for this estimation is that of treating a known volume of the 
wine, etc., with catgut (violin strings which have not been oiled, 
and which have been purified by washing in dilute alcohol acid 
and water, until they have no reducing action on permanganate in 
the cold). The digestion is carried on at ordinary temperature for 
a week, in a closely stoppered bottle. The original substance, and 
that from which the tannin has been removed, are then titrated 
with permanganate, and the difference calculated to tannin. 

Another method consists in mixing equal parts of an eight per 



.336 VOLUMETRIC ANALYSIS. 78. 

cent, solution of alum and the wine, collecting the precipitate on 
.a filter, washing slightly with cold water, transferring the precipitate 
*by a stream of water from a wash-bottle to a beaker, then acidifying 
with H 2 S0 4 and titrating with indigo and permanganate as usual. 

Dreaper's Copper Process for Tannic and Gallic Acids. This 
as described in a paper contributed to /. C. S. I. xii. 412, from 
which the following abstract is taken. 

The methods hitherto proposed for the estimation of tannin may 
IDC divided into two classes, viz. : 

(1) Those which act by precipitating the tannic acid as an 
insoluble compound. 

(2) Those which act by oxidation. 

To the former class belongs the well-known hide powder process, 
.and to the latter Lowenthal's permanganate method, which has 
been modified by Procter and others. These fairly represent the 
two classes, and are the only ones in general use at the present 
-day. 

Dreaper, however, has adopted a modified form of Darton's 
method, the novelty of which consists in precipitating the tannic 
.acid by means of an ammonio-copper sulphate solution, after 
a preliminary treatment with sulphuric acid to remove the ellagic 
acid, and then a treatment with ammonia, filtering after each 
treatment. Procter states that this preliminary treatment is 
unnecessary in the case of some extracts, but Dreaper has never 
found any precipitation to take place in the case of the so-called 
pure tannic acids, probably owing to the removal of the impurities 
during the process of purification. The original solution and the 
(filtrate are titrated with permanganate as in Lowenthal's method, 
the difference in the two results being due to the tannic acid 
present. The copper compound may be dried at 110 C. and 
weighed, or else ignited and weighed as copper oxide. Fleck 
states that the tannic acid can be calculated from this by multi- 
plying by the factor 1 "034. 

The standard copper solution used by the author contained 
."30 gm. of pure crystallized copper sulphate in a liter of water. 
Baric carbonate is also required, which should be free from calcic 
.-salts. 

The process is based on the direct precipitation of the gallic and tannic 
acids by means of a copper salt, using as outside indicator potassic ferro- 
cyanide. If a standard solution of copper sulphate be run into a solution 
of the mixed acids, a certain amount of copper tannate and gallate will be 
precipitated, depending on the dilution of the solution and the amount of 
acid set free from the copper sulphate. The precipitate is, under these 
circumstances, of a bulky nature and ill adapted to any separation by quick 
filtration, so necessary in a process of this description. It was found that 
when a solution of copper sulphate was added to a solution of the mixed 
acids in the presence of baric carbonate, the precipitation proceeds with the 
utmost regularity. The carbonate immediately forms insoluble sulphate 
-with the free acid, and also helps to consolidate the precipitated copper salts, 



78. 



TANNIC ACID. 



so that towards the end of the reaction they fall rapidly to the bottom of 
the vessel, leaving the supernatant liquid clear. This separation is a good 
indication that the end of the titration is near, and is supplemented hy the 
ferrocyanide test. 

A modified method of testing for the excess of copper in the solution 
is as follows : Pieces of stout Swedish filter-paper one inch square are folded 
across the middle, and a drop of the liquid to be tested taken up on a glass 
rod and gently dropped on to the top surface. The liquid will percolate 
through to the under fold, leaving the precipitate on the upper one. It 
is then only necessary to unfold the sheet and apply a drop of ferrocyanide 
to the under surface. If the reaction is complete a faint pink colouration 
will take place, which is perhaps more easily recognized by transmitted light. 

The results obtained by duplicate experiments tend to show that the copper 
salts are perfectly constant in composition when precipitated in this manner, 
and the results equal in accuracy any obtained with other processes. 

About 1 gin. of baric carbonate was added in each case and the solution 
heated up to 90 C. before titration. The temperature at the end of the 
titration should not be less than 30 C. 

The precipitation by copper is done say on 25 c.c. of the solution of the 
sample, and the results noted. 50 c.c. of the same sample are then mixed 
with the usual proportions of gelatine, salt, acid, and baric sulphate ; diluted 
to 100 c.c., then filtered through a dry filter and 50 c.c. ( = 25 c.c. of the 
original liquid) titrated with copper solution as before, the difference being 
calculated to available tannin. 

The experiments show that the separation of the tannic acid by means of 
an acid solution of gelatine and salt will not affect the general results 
obtained, and this method for want of a better was used in the experiments, 
Procter's modification being considered the most accurate, and therefore 
adopted. 

The following table was prepared from experiments, showing the error 
due to the indicator in c.c. of standard solution added to different quantities 
of water: 



c.c. of Water. 


c.c. of Standard Solution 
required. 


20 


0-3 


30 


0-4 


60 


07 


100 


1-0 


150 


1-5 



The above correction should be made in all cases. 

A sample of so-called pure tannic acid gave the following results 



Weight taken. 


c.c. required. 


Gni. 




O'o 


25-0 


0-5 


25-2 


0-5 


25-2 



Slightly lower results were obtained when the operation was conducted in 
the cold, probably owing to the slower action of the carbonate on the free 

z 



338 



VOLUMETEIC ANALYSIS. 



78. 



acid ; but the rate of running in of the solution had no appreciable effect 
on the quantity required. 

A sample of the purest gallic acid that could be obtained gave the 
following figures : 



Weight taken. 


c.c. required. 


GUI. 




0-5 


45'0 


0-5 


448 



Allowing that the acid was of 90 per cent, purity, these results would give 
a value for each c.c. of O'Olll gm. This figure must of course only be 
taken as approximate. It will be seen that more solution is required to 
precipitate the gallic than the tannic acid. This is also noticed in 
Lowenthal's method. 

The chief advantages claimed by the author of this method over 
Lowenthal's are as follows : 

(1) Both the tannic and gallic acids are estimated. 

(2) Rapidity of estimation where a simple assay is sufficient. 

(3) The results are expressed in terms of the copper oxide 
precipitated. 

(4) The standard solution keeps well, and there is no correction 
necessary for indigo-carmine solution or gelatine. 

(5) Larger quantities of the solution can be titrated, thus 
reducing the working error. 

It seems to be possible to use this method for substances other 
than tannic or gallic acids, e.g. Fustic. 

The following results were obtained with a sample of pure 
Fustic extract 51 Tw. 

0*5 gin. taken required 11 '5 c.c. of standard solution. 

0'5 gm. taken required 11*6 c.c. of standard solution. 

The end of the reaction was sharp when the titration was 
carried on at the boiling-point and the precipitate settled well. 



Other Methods of Estimating- Tannin. 

Direct Precipitation by Gelatine. The difficulty existing with 
this method is that of getting the precipitate to settle, so that it 
may be clearly seen when enough gelatine has been added. 

Tolerably good results may sometimes be obtained by using 
a strong solution of sal ammoniac or chrome alum as an adjunct. 
The best aid is probably barium sulphate, 2 or 3 gm. of which 
should be added to each portion of liquid used for titration. 

The Standard Solution of Gelatine should contain 1*33 gm. of 
dry gelatine per liter, in which is also mixed a few drops of 
chloroform or a small quantity of thymol to preserve it. 45 c.c. 
= 0'05 gm. tannin (Carles). This method is adapted only for 
rough technical purposes, as also the following. 




' 79. TIN. 

Direct Precipitation by Antimony. This method is still in 
favour with some operators ; but, like the gelatine process, is beset 
with the difficulty of getting the precipitate to settle. 

The Standard Antimony solution is made by dissolving 2*611 
.gm. of crystals of emetic tartar dried at 100 C. in a liter. I c.c. = 
'0*005 gm. tannin. This liquid may also be kept from decomposition 
by a few grains of thymol. 50 c.c. of the tannin solution may be 
taken for titration, to which is added 1 or 2 gm. of sal ammoniac, 
:and the antimonial solution run in until no further cloudiness is 
produced. 

In both the above methods the final tests must either be made 
by repeatedly filtering small portions to ascertain whether the 
precipitation is complete, or by bringing drops of each liquid 
together on black glass or a small mirror. 

TIN. 

Sn = 118. 

Metallic iron 1*0536 =Tin. 

Double iron salt 0*1505= 

Factor for T ^- iodine 

or permanganate 

solution 0*0059 

79. THE method, originally devised by Streng, for the 
direct estimation of tin by potassic bichromate, or other oxidizing 
agents in acid solution, has been found most unsatisfactory, from 
the fact that variable quantities of water or acid seriously interfere 
with the accuracy of the results. The cause is not fully under- 
stood, but that it is owing partly to the oxygen mechanically 
contained in the water reacting on the very sensitive stannous 
chloride there can be very little doubt, as the variations are 
considerably lessened by the use of water recently boiled and 
cooled in closed vessels. These difficulties are set aside by the 
processes of Lenssen, Lowenthal, Stromeyer, and others, now 
to be described, and which are found fairly satisfactory. 

1. Direct Titration by Iodine in Alkaline Solution (Lenssen). 

Metallic tin or its protosalt, if not already in solution, is 
dissolved in hydrochloric acid, and a tolerable quantity of Rochelle 
salt added, together with sodic bicarbonate in excess. If 
enough tartrate be present, the solution will be clear ; starch is 
then added, and the mixture titrated with ~ iodine. Metallic tin 
is best dissolved in HC1 by placing a platinum crucible or cover in 
contact with it, so as to form a galvanic circuit. 

Benas (Cliem. Gentr-blatt. li. 957) points out that the chief 
error in the estimation as above arises from oxygen dissolved in 

z 2 



340 VOLUMETRIC ANALYSIS. 79. 

the liquid, or absorbed during the operation. In order to obtain 
constant results, it is necessary to dissolve the tin compound 
in HC1, dilute with oxygen-free water, and add at once excess of 
standard iodine, which excess is found by residual titration with 
standard thiosulphate. 

2. Indirect Titration by Ferric Chloride and Permanganate 
(Lbwenthal, Stromeyer, etc.). 

This method owes its value to the fact, that when stannous 
chloride is brought into contact with ferric or cupric chloride, it 
acts as a reducing agent, in the most exact manner, upon these 
compounds, stannic chloride being formed, together with a pro- 
portionate quantity of ferrous or cuprous salt, as the case may be. 
If either of the latter be then titrated with permanganate, the 
original quantity of tin may be found, the reaction being, in the 
case of iron, 

SnCl 2 + Fe 2 Cl 6 =SnCl 4 + 2FeCl 2 . 

56 iron=59 tin. If decinormal permanganate, or the factor 
necessary to convert it to that strength, be used, the calculation by 
means of iron is not necessary. 

Process: The solution of stannous chloride, or other protosalt of tin in 
HC1, or the granulated metal, is mixed with pure ferric chloride, which, 
if tolerably concentrated, dissolves metallic tin readily, and Avithout evolution 
of hydrogen, then diluted with distilled water, and titrated with perman- 
ganate as usual. To obtain the most exact results, it is necessary to make an 
experiment with the same permanganate upon a like quantity of water, 
to which ferric chloride is added; the quantity required to produce the 
same rose colour is deducted from the total permanganate, and the remainder 
calculated as tin. 

Stannic salts, also tin compounds 'containing iron, are dissolved in water, 
HC1 added, and a plate of clean zinc introduced for ten or twelve hours ; 
the tin so precipitated is carefully collected and washed, then dissolved 
in HC1, and titrated as ahove; or the finely divided metal may at once 
be mixed with an excess of ferric chloride, a little HC1 added, and when 
solution is complete, titrated with permanganate. 4 eq. of Iron ( = 224) 
occurring in the form of ferrous chloride represent 1 eq. ( = 118) of tin. 

Tin may also be precipitated from slightly acid peroxide solution 
as sulphide by H 2 S, the sulphide well washed, and mixed with 
ferric chloride, the mixture gently warmed, the sulphur filtered 
off, and the filtrate then titrated with permanganate as above. 
4 eq. of iron=l eq. of tin. 

Tin Ore. In the case of analysis of cassiterite, Arnold (C. N. 
xxxvi. 238) recommends that 1 gm. of the very finely powdered 
mineral be heated to low redness for two hours in a porcelain boat 
in a glass tube with a brisk current of dry and pure hydrogen gas, 
by which means the metal is reduced to the metallic state. It is 
then dissolved in acid ferric chloride, and titrated with perman- 
ganate or bichromate in the usual way. 



80. VANADIUM. 341 

URANIUM. 

Ur = 240. 

80. THE estimation of uranium may be conducted with great 
accuracy by permanganate, in precisely the same way as ferrous 
salts ( 63). The metal must be in solution either as acetate, 
sulphate, or chloride, but not nitrate. In the latter case it is 
necessary to evaporate to dryness with excess of sulphuric or 
hydrochloric acid, or to precipitate with alkali, wash and redissolve 
in acetic acid. 

The reduction to the ura nous state is made with zinc, but as the 
end of reduction cannot, like iron, be known by the colour, it is 
necessary to continue the action for a certain time ; in the case of 
small quantities a quarter, larger half an hour, at a temperature of 
50 to 60 C., and in the presence of excess of sulphuric acid; 
all the zinc must be dissolved before titration. The solution is 
then freely diluted with boiled water, sulphuric acid added if 
necessary, and then permanganate until the rose colour is faintly 
permanent. The ending is distinct if the solution be well diluted, 
and the reaction is precisely the same as in the case of ferrous 
salts ; namely, 2 eq. of uranium existing in the uranous state 
require 1 eq. of oxygen to convert them to the uranic state ; hence 
56 Fe = 120 Ur, consequently the strength of any permanganate 
solution in relation to iron being known, it is easy to find the 
amount of uranium. 

VANADIUM. 



' 81. VANADIUM salts, or the oxides of this element, may be 
very satisfactorily titrated by reduction with a standard ferrous 
solution ; thus 

2FeO + VO 3 = Fe 2 3 + VO. 

1 gm. of Fe represents 1 "630357 gm. of vanadic pentoxide. 

Lindemann (Z. a. C. xviii. 99) recommends the use of a 
solution of ferrous ammonio-sulphate (double iron salt) standardized 
by y 1 ^ potassic bichromate. 

Of course it is necessary that the vanadium compound should be 
in the highest state of oxidation, preferably in pure sulphuric acid 
solution. The blue colour of the tetroxide in the dilute liquid has 
no misleading effect in testing with ferridcyanide. 

With hydrochloric acid great care must be taken to insure 
absence of free Cl or other impurities. The end-point in the case 
-of this acid is different from that with sulphuric acid, owing to the 
colour of the ferric chloride, the mixture becoming clear green. 

The accuracy of the reaction is not interfered with by ferric or 
chromic salts, alumina, fixed alkalies, or salts of ammonia. 



342 VOLUMETRIC ANALYSIS. 82. 

Vanadic solutions being exceedingly sensitive to the action of 

reducing agents, great care must be exercised to exclude dust 
or other carbonaceous matters, alcohol, etc. 



ZINC. 
Zn = 65. 

1 c.c. T ^ solution =0-003 2 5 gm. Zinc. 
Metallic iron x 0*5809 = Zinc. 

,, x 0-724 = Zinc oxide. 

Double iron salt x 0-08298 = Zinc. 

x 0-1034 = Zinc oxide. 

1. Indirect Method (Mann). 

82. THIS process gives exceedingly good results, and consists 
in precipitating the zinc as hydrated sulphide, decomposing the 
sulphide with moist silver chloride, then estimating the zinc 
chloride so formed with ammonic thiocyanate as in Volhard's 
method ( 43). 

The requisite materials are 

Silver chloride. Well washed and preserved from the light- 
under water. 

Standard Silver nitrate. 33'18 gm. of pure silver, dissolved in 
nitric acid and made up to 1 liter, or 52'3 gm. silver nitrate per 
liter. If made direct from silver, the solution must be well boiled 
to dissipate nitrous acid. 1 c.c. = 0*01 gm. of zinc. 

Ammonic thiocyanate. Of such strength that exactly 3 c.c.. 
suffice to precipitate 1 c.c. of the silver solution. 

Ferric Indicator and Pure Citric Acid (see 43.3 and 4). 

Process: 0'5 to 1 gm. of the zinc ore is dissolved in nitric acid. Heavy 
metals are removed by H 2 S, iron and alumina by double precipitation 
with ammonia. The united filtrates are acidified with acetic acid, and H 2 S 
passed into the liquid until all zinc is precipitated as sulphide. Excess of 
H'-S is removed by rapid boiling, so that a drop or two of the filtered liquid 
gives no further stain on lead paper. The precipitate is then allowed to settle, 
decanted while hot, the precipitate brought on a filter with a little hot water, 
and without further washing, the filter with its contents is transferred to 
a small beaker, 30 50 c.c. of hot water added, well stirred, and so much 
moist silver chloride added as is judged necessary to decompose the sulphide, 
leaving an excess of silver. The mixture is now boiled till it shows signs of 
settling clear ; 5 or 6 drops of dilute sulphuric acid (1 : 5) are added to the 
hot mixture, and in a few minutes the whole of the zinc sulphide will be 
converted into zinc chloride. The free sulphur and excess of silver chloride 
are now filtered off, washed, and the chloride in the mixed filtrate and 
washings estimated as follows : 

To the cool liquid, measuring 200 or 300 c.c., are added o c.c. of ferric 
indicator, and so much pure nitric acid as is necessary to remove the yellow 
colour of the iron. A measured excess of the standard silver solution is then 



82. ZINC. 343 

delivered in with the pipette, and without filtering off the silver chloride, or 
much agitation, so as to clot the precipitate, the thiocyanate is cautiously 
added, with a gentle movement after each addition, until a permanent light 
brown colour appears. 

The volume of silver solution represented by the thiocyanate 
being deducted from that originally used, will give the volume to 
be calculated to zinc, each c.c. being equal to 0*01 gm. Zn. 



2. Precipitation as Sulphide and subsequent titration with. Ferric 
Salts and Permang-anate (Schwarz). 

The principle of this method is based on the fact, that when zinc 
sulphide is mixed with ferric chloride and hydrochloric acid, or 
better still, with ferric sulphate and sulphuric acid, ferrous or zinc 
chloride, or sulphates respectively, and free sulphur are produced. 
If the ferrous salt so produced is estimated with permanganate or 
bichromate, the proportional quantity of zinc present is ascertained. 
2 eq. Fe represent 1 eq. Zn. 

Preparation of the Ammpniacal Zinc Solution. In the case of rich 
ores 1 gm., and poorer qualities 2 gm., of the finely powdered material are 
placed into a small wide-mouthed flask, and treated with HC1, to which 
a little nitric acid is added, the mixture is warmed to promote solution, and 
when this has occurred the excess of acid is evaporated by continued heat. 
If lead is present, a few drops of concentrated sulphuric acid are added 
previous to complete dryness, in order to render the lead insoluble ; the 
residue is then extracted 'with water and filtered. Should metals of the fifth 
or sixth group be present, they must be removed by H 2 S previous to the 
following treatment. The solution will contain iron, and in some cases 
manganese. If the iron is not already fully oxidized, the solution must be 
boiled with nitric acid ; if only traces of manganese are present, a few drops 
of bromized HC1 should be added. When cold, the solution may be further 
diluted if necessary, and then super-saturated with ammonia to precipitate 
the iron ; if the proportion of this metal is small, it will suffice to filter off 
and wash the oxide with ammoniacal warm water, till the washings give no 
precipitate of zinc on adding ammonic sulphide. Owing to the fact that 
this iron precipitate tenaciously holds about a fifth of its weight of zinc, it 
will be necessary when the proportion is large to redissolve the partly washed 
precipitate in HC1, and reprecipitate (best as basic acetate) ; the filtrate from 
this second precipitate is added to the original zinc filtrate, and the whole 
made up to a liter. 

Process : The ammouiacal zinc solution (prepared as described above) is 
heated, and the zinc precipitated in a tall beaker, with a slight excess of 
sodic or ammonic sulphide, then covered closely with a glass plate, and set 
aside in a warm place for a few hours. The clear liquid is removed by 
a syphon, and hot water containing some ammonia again poured over the 
precipitate, allowed to settle, and again removed, and the washing by 
decantation repeated three or four times ; finally, the precipitate is brought 
upon a tolerably large and porous filter, and well washed with warm water 
containing ammonia, till the washings no longer discolour an alkaline lead 
solution. The filter pump may be used here with great advantage. 

The filter with its contents is then pushed through the funnel into a large 
flask containing a sufficient quantity of ferric sulphate mixed with sulphuric 
acid, immediately well stopped or corked, gently shaken, and put into a warm 
place ; after some time it should be again well shaken, and set aside quietly 



344 VOLUMETRIC ANALYSIS. 82. 

for about ten minutes. After the action is all over the mixture should 
possess a yellow colour from the presence of undecomposed ferric salt ; when 
the cork or stopper is lifted there should be no odour of H 2 S. The flask is 
then nearly filled with cold distilled water, if necessarj- some dilute 
sulphuric acid added, and the contents of the flask titrated with permanganate 
or bichromate as usual. 

The free sulphur and filter will have no reducing effect 
upon the permanganate if the solution be cool and very dilute. 

3. Precipitation by Standard Sodic Sulphide, with Alkaline Lead 
Solution as Indicator (applicable to most Zinc Ores and Products). 

The Ammoniacal Solution of Zinc is prepared just as previously 
described in Schwarz's method. 

Standard Sodic sulphide. A portion of caustic soda solution is 
saturated with H 2 S, sufficient soda added to remove the odour of 
the free gas, and the whole diluted to a convenient strength for 
titrating. 

Standard Zinc Solution. 44*12 gin. of pure zinc sulphate are 
dissolved to the liter. 1 c.c. will then contain 0*01 gm. of 
metallic zinc, and upon this solution, or one prepared from pure 
metallic zinc of the same strength, the sulphide solution must be 
titrated. 

Alkaline Lead Indicator. Is made by heating together lead 
acetate, tartaric acid, and caustic soda solution in excess, until 
a clear solution is produced. It is preferable to mix the tartaric 
acid and soda solution first, so as to produce sodic tartrate ; or if 
the latter salt is at hand, it may be used instead of tartaric acid. 
Some operators use sodic nitroprusside instead of lead. 

Process: 50 c.c. of zinc solution (=0*5 gm. Zn) are put into a beaker, 
a mixture of solutions of ammonia and ammonic carbonate (3 of the former 
to about 1 of the latter) added in sufficient quantity to redissolve the 
precipitate which first forms. A few drops of the lead solution are then, by 
means of a glass rod, placed at some distance from each other, on filtering 
paper, laid upon a slab or plate. 

The solution of sodic sulphide contained in an ordinary Mohr' s burette 
is then suffered to flow into the zinc solution until, on bringing a drop from 
the mixture and placing it upon the filtering paper, so that it may expand 
and run into the drop of lead solution, a black line occurs at the point of 
contact ; the reaction is very delicate. At first it will be difficult, probably, 
to hit the exact point, but a second trial with 25 or 50 c.c. of zinc solution 
will enable the operator to be certain of the corresponding strength of the 
sulphide solution. As this latter is always undergoing a slight change, it is 
necessary to titrate occasionally. 

Direct titration with pure zinc solution gave 99'6 and 100'2, instead of 100. 

Groll recommends the use of nickel protochloride as indicator, 
instead of sodic nitroprusside or lead. The drops are allowed to 
now together on a porcelain plate ; while the point of contact shows 
a blue or green colour the zinc is not all precipitated by the sodic 
sulphide, therefore the latter must be added until a greyish black 
colour appears at contact. 



82. ZINC. 345 

4. Precipitation as Sulphide with Ferric Indicator (Schaffner) . 

Schaffner's modification of this process, and which is used 
constantly at the laboratory of the Vieille Montagne and the 
Rhenish Zinc Works, is conducted as follows : For ores containing 
over 35 per cent, zinc, 0*5 gm. is taken ; for poorer ones, 1 gm. to 
2 gm. Silicates, carbonates, or oxides, are treated with hydro- 
chloric acid, adding a small proportion of nitric acid at boiling heat 
to peroxidize the iron. Sulphur ores are treated with aqua regia, 
evaporated to dryness, and the zinc afterwards extracted by hydro- 
chloric acid ; the final ammoniacal solution is then prepared as 
described on page 343. 

Process : The titration is made with a solution of sodio sulphide, 1 c.c. of 
which should equal about O'Ol gm. Zn. The Vieille Montagne laboratory 
uses ferric chloride as an indicator, according to Schaffner's method. 
For this purpose a single drop or some few drops of this chloride are let fall 
into the ammoniacal solution of zinc. The iron which has been added is at 
once converted into red flakes of hydrated ferric oxide, which float at the 
bottom of the flask. If sodic sulphide be dropped from a burette into the 
solution of zinc, a white precipitate of zinc sulphide is at once thrown down, 
and the change in the colour of the flakes of iron from red to black shows 
the moment when all the zinc is sulphuretted, and the titration is ended. It 
is advisable to keep the solution for titration at from 40 to 60 C. Titration 
carried out under exactly equal conditions, with a known and carefully 
weighed proportion of zinc, gives comparative data for calculation, and thus 
for the determination of the contents of any zinc solution by means of 
a simple equation. If, for example, 30'45 c.c. of sodic sulphide have been 
used to precipitate 0'25 gm. of zinc, 1 c.c. of it will precipitate 8'21 m.gm. 
of zinc (30'45 : 0'25 -1 : x, and therefore #=0-00821). 

The following method is adopted in the laboratory of a well- 
known copper works in Wales : 

Reduce the sample to fine powder, and dry at a temperature of about 
105 C. Dissolve 0'5 gm. of the sample thus prepared in aqua regia, 
evaporate nearly to dryness, take up with hot water, add 20 c.c. of 
ammonia and 10 c.c. of a solution of ammonic carbonate (1 to 10), then 
a few drops of solution of permanganate to precipitate lead and manganese. 
Now heat nearly to boiling-point and filter into a larger flask, wash the 
precipitate well with hot water containing ammonia until a drop of the 
washings shows no reaction with sodic sulphide. The volume of the filtrate 
and washings should be about 250 c.c., and the temperature about 50 C. Now 
titrate with a standard solution of sodic sulphide. The most convenient 
strength is 70 c.c. = 0'5 gm. of pure zinc, heat the sample liquid almost to 
boiling-point, and add no