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THE THEORETICAL PRINCIPLES
OF THE
METHODS OF ANALYTICAL CHEMISTRY
BASED UPON
CHEMICAL REACTIONS
GENERAL
>6
THE MACMILLAN COMPANY
NEW YORK BOSTON CHICAGO
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THE MACMILLAN CO. OF CANADA, LTD
TORONTO
Theoretical Principles
OF THE
Methods of Analytical Chemistry
Based Upon Chemical Reactions
BY
M. G. CHESNEAU
Ingenieur en Chef des Mines: Professeur d'analyse minerale
a 1'ecole nationale des Mines
Authorized Translation
BY
AZARIAH THOMAS LINCOLN, PH.D.
Assistant Professor of Chemistry, Rensselaer Polytechnic Institute
AND
i
DAVID HOBART CARNAHAN, PH.D.
Associate Professor of Romance Languages, University of Illinois
got*
THE MACMILLAN COMPANY
1910
jlll right* r$rvtd
COPYHGHT, 1910
BY THE MACMILLAN COMPANY
Set up and electrotyped. Published March, igio.
TIM MASON-HENRY PRUSS
SYRACUSE AND NEW YORK
Translators' Note
The translators desire hereby to express their hearty thanks and
acknowledge their indebtedness to Dr. H. E. Patten, Professor Herman
Schlundt and Dr. J. H. Walton for reading the manuscript and for
their suggestions.
Troy, New York. A. T. L.
April, 1909. D. H. C.
205444
TABLE OF CONTENTS
Page
INTRODUCTION i
PLAN OF THE WORK 4.
CHAPTER I
INFLUENCE OF THE PHYSICAL STATE OF PRECIPITATES UPON
THEIR PURIFICATION BY WASHING
1. WASHING OF PRECIPITATES . . .... .. . . 6
Phenomena of Absorption and of Adsorption ... . . 8
2. INCREASE OF SIZE OF GRAINS IN CRYSTALLINE PRECIPITATES 10
Influence of the Fineness of Grains upon Solubility . 10
Influence of Surface Tension upon Increase in Size of the Grains 15
3. COLLOIDAL AMORPHOUS PRECIPITATES AND PSEUDO-SOLUTIONS . . 17
Precipitation of Pseudo-solutions by Salts . ....-./. 22
Rules for Washing Colloidal Precipitates . . . . 25
CHAPTER II
THEORETICAL PRINCIPLES OF THE METHODS OF ANALYSIS
BASED UPON IRREVERSIBLE REACTIONS
1. PRINCIPAL TYPES OF IRREVERSIBLE REACTIONS USED IN ANALYSIS . 27
Methods Based upon the Stability of Bodies at Definite Tem-
peratures . ... . . . . . . .".-. . . .27
Methods Based upon the Use of Oxidizing or Reducing Reagents 28
Formation of Soluble or Insoluble Complexes . . . . . 28
2. THEORETICAL PRINCIPLES INVOLVED IN IRREVERSIBLE REACTIONS . .31
Role of the Speed of Reaction in Analysis 31
Intermediate Reactions Between the Initial and Final States . 35
Thermodynamic Law of Irreversible Reactions ... . 37
3. STUDY OF SOME METHODS BASED UPON IRREVERSIBLE REACTIONS . . 38
Methods of Nitrates of H. Sainte-Claire Deville . . . . 39
Methods Based on the Difference in Stability of Thiosulphates . 42
Volumetric Methods by Oxidation or Reduction . . , . 45
vii
viii CONTENTS
CHAPTER III
STUDY OF DOUBLE DECOMPOSITION OF SALTS BY THE
CALORIMETRIC METHOD
Page
Reversible Reactions in Analysis 52
1. THERMAL CHANGES IN DOUBLE DECOMPOSITION OF SALTS ... 55
Neutralization of Acids by Bases 55
Action of Water upon Salts in Solution; Phenomena of Hy- 56
drolysis 56
Mutual Action of Two Salts 64
2. EXPERIMENTAL RESEARCH UPON THE MATHEMATICAL LAW OF EQUILI-
BRIUM IN DOUBLE DECOMPOSITION OF SALTS 67
History . . 67
Work of Guldberg and Waage 70
Verification of the Guldberg-Waage Law 74
3. ANALOGOUS LAW DRAWN FROM THE PRINCIPLES OF THERMODYNAMICS . 76
Equation of Van't Hoff and of H. Le Chatelier .... 76
Experimental Verification of the Analogous Law of Equilibrium
in Double Isothermic Decompositions 79
Meaning of the Laws of Berthollet 83
CHAPTER IV
ELECTROLYTIC THEORY OF DOUBLE DECOMPOSITION OF
SALTS
1. LAW OF OSMOTIC PRESSURES 87
Experiments of Pfeffer and of de Vries : the Van't Hoff Law . 87
Molecular Lowering of Freezing Point and Vapor Pressures:
Raoult's Laws 91
Identity of Coefficients i Deduced from Osmotic, Cryoscopic and
Boiling Point Measurements 92
2. ELECTROLYTIC DISSOCIATION OF SALT SOLUTIONS 93
Hypothesis of Arrhenius, lonization of Electrolytes 93
Agreement of the Coefficients Obtained from the Electrical Con-
ductivity and Cryoscopic Measurements 95
3. EXPLANATION OF DOUBLE DECOMPOSITION OF SALTS BY THE IONIC
THEORY 97
Additive Chemical Properties 98
Speed of Reactions 08
Equilibrium Between an Electrolyte and Its Ions ... 99
Influence of the Addition of an Ion of an Electrolyte to this
Electrolyte 100
Mutual Action of Two Electrolytes 101
Relative Strengths of Acids and Bases . . . . . .104
Heat of Neutralization and the Law of Thermoneutrality . . 105
Direction of Transformations by Double Decomposition of Salts 106
CONTENTS ix
CHAPTER V
OBJECTIONS TO THE IONIC THEORY
Page
1. OBJECTIONS OF A THEORETICAL CHARACTER 108
Additive Properties, Speed of Reactions, etc 108
Law of Thermoneutrality . . in
Disagreement Between the Phenomena of Hydrolysis and the
Deductions of the Electrolytic Theory 114
2. OBJECTIONS OF AN EXPERIMENTAL CHARACTER 116
Non-aqueous Solutions and Reactions Between Non-aqueous
Solutions Containing Electrolytes 117
Experiments of Kahlenberg upon Aqueous Solutions . . .118
Transfer of Dissolved Electrolytes: Experiments of Chassy . 126
3. POLYMERIZATION OF SOLVENTS AND DISSOLVED SUBSTANCES . . . 127
Experiments of Ramsay and Shields 128
Crompton's Theory of the Coefficient i 131
Hypothesis of Reychler 132
Conclusion . . 133
CHAPTER VI
GENERAL PROCESSES OF ANALYSIS BA'SED UPON DOUBLE
DECOMPOSITION OF SALTS
1. METHODS OF MAKING AS COMPLETE A PRECIPITATION AS POSSIBLE . 135
Influence of an Excess of Reagent 135
Ostwald's Solubility Product 141
Substitution of a Weak Acid for a Free Strong Acid in a Solution 143
Decreasing the Free Weak Acid by the Addition of an Alkali
Salt of the Same Acid , - , ; *. J 45
2. METHODS OF BRINGING INTO SOLUTION A COMPOUND INSOLUBLE IN
WATER OR ACIDS .... ... , ". . 152
Dulong's Wet and Dry Methods . . . ... . 152
Redissolving an Insoluble Substance by a Suitable Reagent . 154
3. PRECIPITATES OF VARIABLE COMPOSITION . . . . . . .156
Hydrates of Variable Composition . 156
Double Ammonium Salts (Determination of Arsenic and Phos-
phorus) 157
Use of the Phase Rule 160
x CONTENTS
CHAPTER VII
STUDY OF SOME METHODS BASED UPON DOUBLE DECOM-
POSITION OF SALTS
Page
1. VOLUMETRIC METHODS 165
Methods by Precipitation . . . 165
Methods by Neutralization: Acidimetry and Alkalimetry . . 167
Polyvalent Bases and Polybasic Acids 171
2. GRAVIMETRIC METHODS BY DOUBLE DECOMPOSITION . . . .173
Detection and Separation of Metals by Hydrogen Sulphide and
Alkali Sulphides 173
3. METHODS BASED ON HYDROLYSIS 177
General Characteristics of These Methods 177
"The Acetate Method" 179
THEORETICAL PRINCIPLES
OF THE
METHODS OF ANALYTICAL CHEMISTRY
BASED UPON
CHEMICAL REACTIONS
INTRODUCTION
ANALYTICAL chemistry, freed from the traditional empiricism of
the metallurgists in the eighteenth century through the genius of
Bergman and Lavoisier, was placed upon a definite basis in the nine-
teenth century by Gay-Lussac and Berzelius. It then became the
indispensable tool which permitted the discovery of the chemical
laws of mass and of volume governing the proportions in which
simple bodies combine in order to form definite compounds. The
history of the beginning of analytical chemistry is thus intimately
connected with the early development of general chemistry.* After
Berzelius, however, had brought a definite system into the operations
of analytical chemistry, this subject seems to have been considered
a definitely closed chapter of general chemistry. The connection
between the two has diminished little by little until analytical chem-
istry is regarded in general as an art (essentially practical) rather
than as a science in which the speculations of theory can find a place.
Rivot, Fresenius, H. Rose, the immediate followers in the work
of Berzelius, seem to abstain intentionally in their treatises from
applying the methods which they advocate to the conceptions of
general chemistry. They all limit themselves to the exact presenta-
tion of the details of manipulation, which are only essential in obtain-
ing accurate results through their value as directions for carrying
out the operation and have no visible connection with the general
laws of chemistry.
H. Sainte-Claire Devillef tried, it is true, as early as 1853, to
counteract the prejudice of analysts against any purely scientific
*G. Chesnau, I'Evolution de V Analyse minerale (Revue scientifigue
Series (5), Hi, 321 and 357 (1905) Paris).
t Saint-Claire Deville, Am. Chim. Phys. (3) xxxviii, 5 (1853).
i I
2 METHODS OF ANALYTICAL CHEMISTRY
discussion by showing in an article, highly important in the history
of analytical chemistry, in what manner the systematic study of
the stability of a class of salts, the nitrates, subjected to the action
of heat, permits the formulation of an irreproachable method of
separation of the alkaline earth elements from the sesquioxide of
iron and aluminium.
This example was practically the only one of its kind for a long
time, and it is only in recent years that, for the first time, one of the
masters of modern theoretical chemistry, W. Ostwald, has under-
taken, in a work which has quickly -become classic in Germany,* to
subject the methods of analytical chemistry to the recently estab-
lished laws of equilibrium.
Most of these methods are, in reality, based upon the fact that
the decomposition of salts gives rise to the phenomenon of equilib-
rium between two opposite states (the initial substances and the
products of the reaction), as Berthollet had so accurately foreseen
a century ago and which has been definitely established by the
experiments of the Norwegian chemists, Guldberg and Waage, those
of W. Ostwald and of several others, as well as by the thermo-
chemical investigations of Thomsen and M. Berthelot. Van't Hoff
and H. Le Chatelier, guided by the theoretical conceptions of H.
Sainte-Claire Deville, have shown that the law which governs the
double decomposition of salts may be deduced from the principle of
thermodynamics applied to chemical equilibrium.
The reactions, utilized in an analysis, ought not then, in many
cases, to be considered as absolutely complete, even when an insolu-
ble precipitate is produced and the reaction may appear to run to an
end. These new laws teach us that there may escape the notice of
the analyst, deceived by the insolubility, appreciable quantities of
the element which he is to determine from the weight of the pre-
cipitate obtained. Without doubt the work of Berzelius and of his
successors has already brought about the empirical solution, in
numerous cases, of the problem of causing the reaction to run to
* W. Ostwald, Die wissenschaftlichen Grundlagen der analytischen Chemie,
Leipzig, 1894. A translation into French, by A. Hollard of the 3d German
edition, has been published under the title of: Les Principes scientifiques de
la Chimie Analytique (Paris, C. Naud, ed. 1903).
An English translation of the various German editions has been prepared
by McGowen and published as The Foundations of Analytical Chemistry by
The Macmillan Company, 3d edition, 1908. Editors' Note.
BASED UPON CHEMICAL REACTION'S 3
practical completion in the desired direction. There is no doubt,
however, but that the recent theories upon equilibrium, which alone
allow one to understand the reason for the processes, will succeed in
determining more definitely the conditions of their application.
The object which analytical chemistry is to pursue, then, is not
only to increase the number of methods by extending the list of
insoluble precipitates, but also, and especially, to increase the exact-
ness of the methods already known, by investigating in the light of
the recent physico-chemical theories the conditions which render the
reactions as complete as possible. The moment seems to have
arrived for analytical chemistry to enter this path under the stimulus
of industrial demands which are endeavoring more and more to
vary the qualities of the ordinary metals by the introduction of
accurately proportioned infinitesimal quantities of foreign elements.
The bond which unites analytical chemistry to general chemistry thus
finds itself strengthened.
In my instruction in the School of Mines, I endeavored to apply
these new theories to the critical examination of the processes of
analytical chemistry, thus following the example of the illustrious
leader of the German School of Physical Chemistry, but in advanc-
ing in this study, I have been led to adopt a method different from
that which is advocated by that school. The fundamental principle
of the theory of Ostwald, based upon the phenomena of Electrolysis,
consists in attributing to salt solutions a hypothetical composition of
elements, or groups of elements, termed "ions," which are the sole
active agents in the double decomposition of salts. This purely
electrolytic theory appeared to me in contradiction to a number of
continually increasing facts, firmly established by experiment, and it
seemed to me preferable in the discussion of the processes of ana-
lytical chemistry to employ a method, which I will call "Calori-
metric," depending only on the calorific phenomena involved in the
occurring reactions, and on the laws deduced from the principles of
Thermodynamics.
My eminent colleague, Professor H. Le Chatelier, kindly per-
mitted me to present these principles before the College de France in
his chair of analytical chemistry, as substitute professor during the
first semester 1904-1905. The work which I present here reproduces
the lectures which I devoted to the methods based upon chemical
reactions, properly speaking. I have omitted the parts of this course
treating of methods based upon Electrolysis, Spectroscopy, Micro-
4 METHODS OF ANALYTICAL CHEMISTRY
graphy, etc., the theories of which enter more especially into the
domain of Physics and have been presented already in a number of
excellent treatises.
Plan of the Work
The processes of analytical chemistry consist, in general, in
bringing each element successively to the state of a definite compound
in a final system, formed of distinct phases, whose nature lends itself
easily to a separation by purely mechanical processes. Such systems
can be obtained chemically from irreversible reactions such as
phenomena of oxidation by dry or wet processes, of decomposition
by heat, etc., or from reversible reactions by double decomposition,
chosen from among those whose equilibria can be obtained in con-
formity with a practically complete reaction.
Since these two classes of reactions depend upon distinct prin-
ciples, it is best to study successively the methods based upon each
class. However, in the case, by far the most frequent, in which the
final system is a precipitate formed in the presence of a liquid, the
two kinds of methods necessitate the same discussion from the point
of view of the conditions necessary to obtain the precipitate in the
pure state. The plan of our study will be then the following :
In Chapter I, we shall examine the influence of the physical state
of the precipitates (size of grains, crystalline state, colloidal state)
upon their purification by washing.
In Chapter II, the theoretical principles involved in the methods
based upon irreversible reactions will be studied and then these
principles will be applied to some of the special methods chosen as
types from among the most important of this class.
The study of the methods based upon reversible reactions by
double decomposition of salts will be made afterwards, according to
the same plan, but will be much more extensive on account of their
great importance in analytical chemistry. Besides, it has appeared
necessary to present the electrolytic theory, together with our
calorimetric method, and to indicate the experimental evidence ap-
pearing to us in favor of the latter. Under these conditions, then,
Chapter III will be devoted to the study of the principles involved
according to the calorimetric theory, in reversible reactions, Chap-
ter IV to the presentation of the electrolytic theory of these reactions,
and Chapter V to a comparison of the two methods.
BASED UPON CHEMICAL REACTIONS 5
In Chapter VI we will formulate the theory of the general
processes which produce complete precipitation, as far as possible,
in double decomposition and, inversely, which cause precipitates in-
soluble in water to become soluble.
Finally in the last chapter, we shall apply the principles estab-
lished in the preceding chapters to some particular methods based
upon double decomposition.
We shall omit, in this study, all treatment of the deduction of
the laws of chemical energy based upon the principles of Thermo-
dynamics, merely citing special works or articles published upon
this subject, and we will content ourselves with applying these laws
to reactions employed in analytical chemistry.
CHAPTER I
INFLUENCE OF THE PHYSICAL STATE OF PRE-
CIPITATES UPON THEIR PURIFICATION
BY WASHING
i. Washing the Precipitates
THE most frequent operation in analytical chemistry consists in
producing in a homogeneous liquid an insoluble precipitate, con-
taining in the form of a definite compound one of the elements to
be determined and to separate it from the mother liquor by filtration
through a porous membrane whose pores are large enough to let the
liquid pass and small enough to retain all the solid particles. In
analytical chemistry we utilize, as filtering media, the Berzelius
paper or asbestos and at times diminish the size of the pores by
different artifices (such as covering with a gelatinous substance
which plays the part of a material with pores still finer than those
of the filter). The speed of the flow of the liquid may be acceler-
ated by increasing the temperature, which diminishes the viscosity,
and by diminishing the pressure under the filter.
The simple filtration of a mixture of a solid and a liquid does
not give an exact separation because the solid remains moistened by
the liquid and thus retains a certain amount of it by reason of the
viscosity of the liquid and the attraction (capillarity) between the
solid and the liquid. We assume that the quantity of the liquid thus
retained by wetting is approximately proportional to the total surface
of the grains, and it is easy then to calculate that for the same
weight of solid substance, this quantity is roughly directly propor-
tional to the fineness of the grains. This explains the fact that in
the case of gelatinous bodies which represent the maximum of fine-
ness of precipitates (grains invisible under the most powerful
microscopes) the wetting liquid is practically continuous.*
* There have been issued very recently (1908) by the U. S. Department of
Agriculture, Bureau of Soils, two Bulletins: No. 51 on the Absorption of(
Vapors and Gases by Soils by H. E. Patten and F. E. Gallagher, and No. 52
on Absorption by Soils by H. E. Patten and W. H. Waggaman. In the
6
BASED UPON CHEMICAL REACTIONS 7
The separation of the liquid and the solid forming the initial
heterogeneous mixture can be complete only upon the condition of
displacing the initial liquid by another liquid in which the solid par-
ticles are insoluble, while the original liquid is soluble in it ; this result
is accomplished by the operation of washing.
It should be easy to expel completely the liquid employed in
washing by simple evaporation ; the latter ought to be done in gen-
eral at a temperature higher than the boiling-point of this liquid, for
it is known that the tension of vapor moistening the surface of the
solid is, on account of the capillarity, very much lower than that of
the same liquid taken in mass.
Theoretically, if a precipitate had no physical action upon the
soluble matter in solution in the mother liquor, a very limited num-
ber of washings would be sufficient to purify it completely. In this
case, in fact, the volume v of the moistening liquid impregnating
the filter and the precipitate is constant, and at each addition of the
washing liquid, there is a uniform dilution in the volume V of the
whole of the liquid ; the substances in the moistening liquid are then
brought at each addition to the fraction of the weight which they
had after the draining of the preceding liquid, so that, after n addi-
tions of the washing liquid, the impurities are reduced to the frac-
tionf -?- i of their initial value. We thus see that the residue of
impurities will be as much less as n is greater, and V greater in com-
parison with v. In order to diminish v, we start with a weight of
precipitate as small as is allowed by the degree of precision of the
balance compared to the degree of accuracy which we wish to
obtain, and we take filters of small surface. By diminishing the
latter, we reduce necessarily the volume of the wash liquid in a still
greater proportion, which opposes the attainment of the desired
result. Practically we proportion as much as possible the size of
the filter to that of the volume of the precipitate in such a manner
that v does not exceed about the tenth part of the total volume V
of the liquid which the filter can contain. Starting with this, if we
put = jXjp we see that four washings will suffice in order to reduce
latter there is presented an excellent resume of the work which has been
done on the subject of absorption and they both contain a large amount of
valuable experimental data. -Editors' Note.
8 METHODS OF ANALYTICAL CHEMISTRY
the impurities retained by the filter and the precipitate to 10 Q OU of
their initial value, which is quite sufficient in practical work.
Phenomena of Absorption and Adsorption. Experiments prove,
unfortunately, that, in most cases, the purification is much less than
the preceding reasoning indicates, and that a much greater number
of washings are necessary in order to obtain an acceptable degree
of purity. By reason of an attraction which exists between the
solid bodies and the bodies dissolved at the surface of contact, the
liquid impregnating the precipitate and the filter retains a proportion
of impurities greater than the liquid which is filtered, instead of
having the same concentration in impurities, as we assume above.
There is an absorption or adsorption of the dissolved impurities by
the precipitate, so that the quantity of impurities remaining after
each washing is greater than that which we have assumed, and the
quantity carried away during each washing is smaller.
The phenomena of absorption were very accurately observed by
H. Sainte-Claire Deville, in the paper previously mentioned (loc.
cit.) ; "The wet analytical processes," he says, "are often attended by
a source of error upon which the attention of chemists has not been
sufficiently fixed until now, the phenomena of the absorption of
the soluble substances by precipitates at the moment when they are
forming. Each time that a solid body, in becoming separated in
the midst of its solution, takes the form of a voluminous precipitate,
it draws with it, in a proportion often quite large, a part of the
various materials which are contained in the solution. This fact
is well known in the case of ferric and aluminium hydroxides, when
one wishes to separate them from calcium, magnesium, manganese,
etc., and even from potassium. It is difficult to say in what state
the precipitate and the absorbed material are found in relation to
each other; but we can compare them to animal charcoal and to a
coloring matter which, certainly, do not combine in the manner of
very definite compounds. These are the facts which Chevreul
attributes to capillary attraction."
We will see later in the chapter dealing with colloidal bodies,
that many of the more recent works* have added little to the very
exact views of H. Sainte-Claire Deville upon the absorption of dis-
solved substances by precipitates. The absorption is particularly
marked in alkaline solutions, and to a less degree in acids and neutral
*A notable exception is the work of the Bureau of Soils, Bulletins 51 and
52, loc. cit. Editors' Note.
BASED UPON CHEMICAL REACTIONS 9
salts ; it is more manifest in proportion as the precipitates are finer,
and it reaches its maximum in bodies of a gelatinous consistency.
Filter paper itself has a very great absorbing power for alkaline
solutions; the following experiment cited by Ostwald (he. cit.)
is a striking example of this fact. A drop of a solution of barium