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LIBRARY

OF THE

UNIVERSITY OF CALIFORNIA.
Cla&s






QUANTITATIVE
CHEMICAL ANALYSIS

BT

ELECTROLYSIS



BY

PKOF. ALEXANDER CLASSEN, PH.D.

PRIVY COUNCILLOR

Director of the Laboratory of Electrochemistry and Inorganic Chemistry in the
Royal Institute of Technology at Aachen



AUTHORIZED TRANSLATION

FOURTH ENGLISH FROM THE FOURTH GERMAN EDITION

REVISED AND ENLARGED

BY

BERTRAM B. BOLTWOOD, PH.D.

Formerly Instructor in Physical and Analytical Chemistry in the
Sheffield Scientific School of Yale University



FIRST THOUSAND

IE

- ITY

A\^ '




JOHN WILEY & SONS

LONDON : CHAPMAN & HALL, LIMITED

1903



CG



GENERAL



Copyright, 1903,

BY

BERTRAM B. BOLTWOOD.



ROBERT DRUMMOND, PRINTER, NEW YORK.



AUTHOR'S PREFACE TO FOURTH GERMAN

EDITION.



THE present edition, revised with the assistance of Dr.
Walter Lob, differs from the previous edition in having the
Introduction amplified by the insertion of a section devoted to
theory. This was required the more since recent investiga-
tions in electrochemical analysis have been directed to explain-
ing the reactions taking place in solutions and to determining
the magnitudes of the electrical factors.

The importance of specific directions respecting the elec-
trode potential, current-strength, and decomposition potential
has been demonstrated. The author, with the help of his
assistants, has experimentally determined these electrical
factors, for not only his own methods, but also for a consid-
erable number of other methods, and has incorporated them
in the text. Additional methods by other authors, in which
directions concerning these important electrical factors are
lacking, have been omitted, in view of the fact that these
methods are either uncertain or entirely impractical; refer-
ences to them will be found, however, under the heading of
Literature.

The book has been made the more complete by the
descriptions of a variety of measuring instruments, sources of
current and other apparatus, as well as simple and complete
appliances for carrying out electrolytic experiments. These
have been illustrated by a large number of new cuts, in the
text and in the appended tables.

A. CLASSEN.

AACHEN, January 18, 1897.



117280



TRANSLATOR'S PREFACE.



THE chief object in preparing the present English edition
of this book has been to include a considerable number of new
electroanalytical methods which have been published since
the appearance of the fourth German edition in 1897.

This task has been greatly simplified by the very kind
assistance of Professor Classen, who has generously placed his
new and valuable work on "Ausgewahlte Methoden der
Analytischen Chemie," published in Berlin in 1902, at the
disposal of the translator. This book covers a wide field in
analytical chemistry and embraces a variety of special
subjects. It has been freely used in preparing the present
English edition.

Part First of the German original has been divided into
two sections, and the arrangement of the text has been
altered to permit of a more systematic treatment of the
subject. Much new material has been introduced into this
part, and acknowledgment is especially due to Professors
Hastings and Beach, from whose Text-Book of General Physics
many of the descriptions of electrical apparatus have been
taken.

To Part Second many new methods of analysis have been
added, the source of these being Professor Classen's book
mentioned above and the original papers in the chemical
journals.



vi TRANSLATOR'S PREFACE.

The translator has attempted to retain all of the valuable
material contained in the fourth German edition and is solely
responsible for any errors or mistakes in the new material
which has been inserted. He desires to express here his
thanks to Professor H. A. Bumstead of the Sheffield Scientific
School for his valuable advice and criticism.

B. B. BOLTWOOD.

NEW HAVEN, Conn., April, 1903.



CONTENTS.



PART FIRST.
SECTION I. INTRODUCTORY.

CHAPTER PAGE

I. HISTORICAL ' 1

II. THEORY OF SOLUTION 6

III. ELECTROLYTES 10

IV. CURRENT-STRENGTH, POTENTIAL 16

V. FARADAY'S LAW 19

VI. OHM'S LAW . ., 22

VII. MIGRATION OF IONS 26

VIII. CONDUCTIVITY OF SOLUTIONS 30

IX. ELECTROLYSIS 34

X. ELECTROMOTIVE FORCE 42

XI. POLARISATION . 47

SECTION II. DESCRIPTIVE.

XII. ELECTROCHEMICAL ANALYSIS 49

XIII. DETERMINATION OF ELECTRICAL MAGNITUDES -55

XIV. SOURCE OF CURRENT 73

XV. REGULATING CURRENT-STRENGTH AND POTENTIAL 95

XVI. ACCESSORY APPARATUS 110

XVII. THE ANALYTICAL PROCESS 128

XVIII. ARRANGEMENTS FOR ANALYSIS 312

PART SECOND.
SPECIAL.

SECTION

I. QUANTITATIVE DETERMINATION OF METALS. 153

II. DETERMINATION OF NITRIC ACID IN NITRATES 221

III. DETERMINATION OF THE HALOGENS 223

IV. SEPARATION OF METALS 225

V. SEPARATION OF THE HALOGENS 278

PART THIRD.

I. APPLIED EXAMPLES OF ELECTROCHEMICAL ANALYSIS .... 281

II. REAGENTS 293

INDEX '. 297

vii



QUANTITATIVE ANALYSIS BY ELECTKOLYSIS,



PART FIRST.
SECTION I. INTRODUCTORY.



CHAPTER I.
HISTORICAL.

THE development of electrochemical analysis has been
almost wholly empirical. The most suitable conditions for
the quantitative separation of metals by electricity have
been determined from a great number of experiments, con-
ducted with diligence and perseverance, while the nature of
the reactions involved has not always at the time been clearly
understood. The relatively recent development of electro-
chemistry has served to throw much light on the theory of
quantitative electrolysis, and the importance and significance
of the electrical factors and other conditions are now much
more clearly understood.

The first attempts at the electrolytic determination of
the metals were entirely qualitative in character. Shortly
after the discovery, by Nicholson and Carlisle (1800), of the
decomposition of water by the electric current, Cruikshank



2 QUANTITATIVE ANALYSIS BY ELECTROLYSIS.

(1801), having observed the separation of metallic copper,
suggested that the galvanic current might be used for the
qualitative determination of other metals. This suggestion
awakened but little interest. In 1812 Fischer employed
an electrolytic method for identifying arsenic in animal fluids,
and later, in 1840, Cozzi used a similar method for the de-
tection of metals in general in such solutions.

The discovery of galvanoplasty, a most important techni-
cal process closely allied to electrochemical analysis, dates
from 1839 and was made by Jacobi.

Gaultier de Claubry, in 1850, recommended the use of the
electric current for detecting poisonous metals in mixtures
containing organic substances, and in 1860 Bloxam continued
this work and devised numerous methods by which he at-
tempted to make the identification of arsenic and antimony
possible in the presence of other metals. In this work he
was assisted somewhat by the directions for the separation
of metals from mixtures published by Morton in 1851.

Becquerel observed, as early as 1830, that lead and
manganese often separated, not as metals at the negative
pole, but in the form of , oxides on the positive pole, a property
which permitted these metals to be readily separated from
others. Investigations on the qualitative decomposition
of inorganic salts of the metals were also carried out by
Despretz (1857), Nickles (1862), and Wohler (1868). The
work of A. C. and E. Becquerel (1862) on the electrolytic
reduction of the metals was likewise of an entirely qualitative
character.

It can be readily understood that with such abundant
data at hand the development of quantitative electrolysis
was comparatively rapid.

The field of quantitative investigation was first opened
by W. Gibbs (1864), who carried out an investigation on the



HISTORICAL, 3

electrolytic determination of copper and nickel, which in-
cluded a description of the methods for the determination of
silver and bismuth in the form of metals, as well as of lead
and manganese in the form of peroxides. He also published
studies on the separation of zinc, nickel, and cobalt. The
possibility of the quantitative determination of copper was
confirmed by Luckow (1865), who had worked at it for a
number of years. The quantitative electrolytic determina-
tion of metals was entitled by him ' i electro-metal-analysis. ' '
This author published at the same time a series of directions
for the method of using the current for analytical work, and
by these precise instructions laid the foundation for many
later researches.

The attention of investigators was then directed principally
to the chemical reactions which took place when different
sources of current were used and when the other physical
conditions were varied. The salts of the metals and the
solvents suitable for use and the proper substances to be
added to the solutions were investigated and determined.
Wrightson (1876) called attention to the fact that the accu-
racy of copper determinations was influenced by the presence
of other metals and ascertained the limits under which copper
could be accurately determined in the presence of antimony.

Simultaneous with the announcement of the electrolytic
determination of gallium in alkaline solutions by Lecoq de
Boisbaudran (1877) came the announcement by Parodi and
Mascazzini that zinc could be determined in a solution of its
sulphate to which an excess of ammonium acetate had been
added, and that metallic lead could be quantitatively pre-
cipitated from an alkaline tartaric acid solution containing
an alkali acetate.

We are indebted to Bichert (1878) for the first accurate
directions for the determination of manganese. He ob-



4 QUANTITATIVE ANALYSIS BY ELECTROLYSIS.

served that this element may be completely separated at the
positive pole in the form of an oxide from solutions of the
nitrate. This property permits of the electrolytic separation
of manganese from other metals, e.g., copper, cobalt, nickel,
sine, etc.

Other papers which were published at that time by
Luckow, F. W. Clarke, and J. B. Haunay described the
electrolytic determination of mercury, which was found to
separate readily from solutions of the chloride and sulphate.

A method for the electrolytic determination of cadmium
was found by F. W. Clarke (1878), who succeeded in precipitat-
ing this metal from solutions of its acetate, and Yver (1880)
employed a similar solution for separating cadmium from zinc.

The determination of zinc from solutions of the double
cyanides was carried out by Beilstein and Jawein (1879), and
Fresenius and Bergmann (1880) successfully precipitated
metallic nickel and cobalt from solutions containing an excess
of free ammonia and ammonium sulphate.

Edgar F. Smith showed (1880) that if uranium acetate
solutions were electrolysed the uranium was completely
precipitated as a hydrated protosesquioxide ; and, further,
that molybdenum could be deposited as hydrated sesqui-
oxide from warm solutions of ammonium molybdate in the
presence of free ammonia. We are indebted to the same
author and his students for a large number of valuable con-
tributions to the literature of electrochemical analysis.

Luckow (1880) rendered a special service in the publica-
tion of his observations on the reactions which take place
during electrolysis. He pointed out the reduction from
higher to lower states of oxidation in the case of chromic
acid, iron, and uranium salts, and demonstrated, on the other
hand, that sulphites and thiosulphates are oxidised to sul-
phates. He summed up the results of his observations in a



HISTORICAL. 5

law, that in general the electric current exerts a reducing
action on acid, and an oxidising action on alkaline, solutions.
Recent investigations have shown, however, that other factors
are of importance in these reactions.

In the year 1881 Alexander Classen and his students
began a series of investigations on quantitative analysis by
electrolysis which ultimately included nearly all of the metals.
It was he who first pointed out the value of oxalic acid and
the double oxalates. A large number of electrolytic methods
originated by him will be described in this book.

At about the same time as and quite independently of
Classen, Reinhardt and Ihle proposed the double oxalates
for the electrolytic determination of zinc.

An attempt was made (1880) by Gibbs, who used a mer-
cury cathode, to determine metals by observing the increase
in weight of the mercury due to the formation of an amalgam,
and a similar method was employed by Luckow (1886) for
the determination of zinc.

Since the year 1886 a great number of publications on
electrochemical analysis have appeared, and the most im-
portant of these will be mentioned later.

Especially worthy of mention at this point, however, are
the experiments conducted by Vortmann (1894) on the elec-
trolytic determination of the halogens.

The investigations of Kiliani (1883), on the significance of
the potential-difference in electrolytic determinations, served
to draw attention to this important factor, and the later work
of Le Blanc (1889) on the potential-differences necessary
for the decomposition of solutions of the salts of various
metals added greatly to the available theoretical data. In
1891 Freudenberg successfully separated a number of metals
from solutions containing several by carefully regulating the
potential-difference of the current which he employed.



CHAPTER II.
THEORY OF SOLUTION.

THE modern theory of solution is the foundation of the
science of electrochemistry. It is therefore most essential
that this theory should be clearly understood by all workers
in this branch of chemical science.

Until recent years solutions were considered to be mere
mechanical mixtures of solvent and solute and no general
laws governing such mixtures had been discovered. A
theory assuming chemical interaction between solvent and
solute, the so-called hydrate theory, involving the chemical
combination of the molecules of the solute with the mole-
cules of water, was proposed, but since this theory did not
prove to be a satisfactory working hypothesis it was grad-
ually abandoned.

The phenomenon of diffusion was well known. This is
exhibited when solutions of dissolved substances are placed
in contact with the pure solvent. In such cases the dissolved
substance gradually works its way from the stronger solution
through the entire solvent until finally after sufficient time
has elapsed the mixture of solvent and solute is found to be
absolutely uniform and all portions of the solution are of
uniform concentration. This behavior of dissolved sub-
stances suggests the existence of a force tending to drive the
particles out into the adjoining solvent, and in 1877 Pfeffer*

* Osmotische Untersuchungen. Leipzig 1877.



THEORY OF SOLUTION. 7

showed by a series of experiments that when the dissolved
substance was prevented from diffusing into the solvent a
pressure of considerable magnitude was exerted upon the
retarding membranes which he employed. One of the mem-
branes which he used was copper ferrocyanide precipitated
in the walls of a porous earthenware cylindrical jar. This
membrane allows the water used as a solvent, but not the
substance contained in the solution, to pass through it (semi-
permeable membrane), and by placing a solution in the jar
which was surrounded by pure water, he was able to measure
approximately the pressure which was exerted.

This pressure is known as the osmotic pressure of the
substance in solution, and as a result of his experiments
Pfeffer reached the following conclusions:

1. That the pressure is dependent on the nature of the
dissolved substance.

2. That for any given substance the pressure depends
on the concentration of the solution and is in direct propor-
tion to this.

3. That the pressure at a given concentration is depend-
ent on the temperature, and shows a regular increase with
rising temperature.

Pfeffer also concluded that the magnitude of the pressure
was influenced by the nature of the membrane, but this
assumption was later shown to be erroneous.

Pfeffer's investigations attracted but little attention at
the time they were published. It was not until the year
1885 that their important bearing on the theory of solution
was appreciated.

In 1885 Van't Hoff called attention * to the fact that there



* Lois de 1'Equilibre Chimique. Memoire presente .a 1' Academic
des Sciences de Suede le 14 Octobre 1885.



8 QUANTITATIVE ANALYSIS BY ELECTROLYSIS.

seemed to be a very close and .striking relation between the
laws of gas pressure and the laws of osmotic pressure, and
showed that the osmotic pressures of the dilute solutions
measured by Pfeffer could be calculated from the gas laws
alone, the values thus obtained corresponding within the
limits of experimental error with the values measured by
Pfeffer. He thus demonstrated that substances in dilute
solutions have an osmotic pressure which is equal to the
pressure which they would exert if they were in a gaseous
form at the same temperature and occupied under these
conditions a volume equal to the volume of the solution.
Not only does the osmotic pressure vary inversely as the
volume (Boyle's law), but the osmotic pressure is also directly
proportional to the absolute temperature (Gay-Lussac's law).

If it be assumed, therefore, that the laws of gases apply
generally to substances in solution, Avogadro's hypothesis
may be applied in the following form :

Equal volumes of solutions of different substances at the
same temperature and having the same osmotic pressure
contain an equal number of molecules. This is known as
Van't Hoff's law for solutions.

This law furnishes a valuable means for determining
the molecular weight of chemical compounds. It is only
necessary to determine the osmotic pressure and temperature
of a solution containing a known weight of the compound
in a given volume of solution. From the data thus obtained
the molecular weight of the substance in solution can be
readily calculated.

Since the direct measurement of the osmotic pressure
is, for various reasons, a very difficult operation it is seldom
resorted to in practice. Indirect methods which are more
convenient are employed instead. These methods are based
on the determination of other properties of solutions which



THEORY OF SOLUTION. 9

show a direct variation with changes in the osmotic pres-
sure. Chief among these indirect methods are those which
depend on the measurement of the depression of the freezing
point, the elevation of the boiling point, and the lowering of
the vapor pressure of any pure solvent caused by the intro-
duction of a known weight of any soluble substance. These
are all directly proportional to the osmotic pressure of the
substance in solution.



CHAPTER III.

ELECTROLYTES.

THE development of the theory of osmotic pressure
brought to light the fact that a great number of chemical
compounds when dissolved in water exerted osmotic pres-
sures which did not agree with those which would be expected
from Van't HofPs law alone. These compounds, among which
were included most of the substances used as reagents in
analytical chemistry, could be divided into three general
classes, i.e., acids, bases, and salts.

These apparent exceptions to the law were raised as
objections to its adoption, just as the abnormal gas density
of ammonium chloride, before this was fully understood,
was considered a proof of the fallacy of Avogadro's hy-
pothesis.

Arrhenius * was the first to offer a satisfactory explanation
of the cause of these abnormal osmotic pressures.

The theory proposed by him in 1887 may be stated as
follows:

When a solid compound soluble in water is introduced
into this liquid it passes into solution in the form of mole-
cules. If the behavior of the compound is perfectly normal,
e.g. if it gives an osmotic pressure which agrees with Van't
Hoff's law, the molecules undergo no further alteration, but
exist as such in the solution. If, however, the substance

*Zeit. f. phys. Chem., 1, 631 (1887).



ELECTROLYTES. 11

belongs to that class of bodies which give abnormal osmotic
pressures, then, immediately on passing into solution, some
of the molecules dissociate into other particles which .are
called ions. These ions, which may be either single atoms
or groups of atoms, have the same effect on the osmotic
pressure as undissociated molecules. As a result of this
increase in the number of particles in the solution the osmotic
pressure is greater than if no dissociation had taken place.

The ratio of the number of dissociated molecules to the
total number of molecules introduced into the solution is
called the degree of dissociation.

If one gram molecule of a substance the composition of
which is represented by AB is dissolved in a definite volume
of solvent, and if this substance dissociates into two ions, A
and B, the degree of dissociation being equal to x, the state
of the substance in solution will be represented by the fol-
lowing expression:



where 1 represents the gram molecule taken.

For a given solution having a known osmotic pressure
the degree of dissociation can be calculated from the equation



?(*-!)'

in which P stands for the osmotic pressure measured, p the
theoretical osmotic pressure calculated from the gas laws, and
k the number of ions into which each molecule dissociates.

The maximum value which x can attain is unity. This
is its value when all of the substance contained in the solution
is in the form of ions.

Several very important points with respect to the values
of x have been brought out by experiment.



12 QUANTITATIVE ANALYSIS BY ELECTROLYSIS.

1. The degree of dissociation for a given substance dis-
solved in water is the same for all solutions of the same sub-
stance at the same concentration and temperature, i.e., the
degree of dissociation for a given solution at constant tem-
perature is a constant.

2. On diluting a solution the degree of dissociation in-
creases until the maximum value is attained. Beyond this
point further dilution produces no change in the state of the
dissolved substance.

3. Strong acids, strong bases and their salts even in
fairly concentrated solutions are almost completely disso-
ciated into their ions.

4. The degree of dissociation determined by measure-
ments of the osmotic pressure or by any of the indirect
methods already mentioned is found to agree exactly with
the degree of dissociation as determined by an entirely sepa-
rate and independent method depending upon the electrical
conductivity of the solution (see p. 32).

A chemical compound which in a dissolved or melted
condition conducts the electric current is called an electrolyte.
If an electric current is passed through the aqueous solution
of an electrolyte, certain chemical changes are produced.
The process is called electrolysis. The points at which the
current enters and leaves the solution are called the
electrodes.

Arrhenius called attention to the fact that all solutions
which contain dissociated substances have the property of
conducting the electric current, indeed the greater the degree
of dissociation the better the conductivity of the solution,
while this property is not possessed to an appreciable extent
by solutions of substances which correspond to Van't Hoff's
law.

He therefore assumed that the undissociated molecules



ELECTROLYTES. 13

in a solution take no part in conducting the current and that
the conductivity of the solution is due to the ions alone.

This view is borne out particularly by the fact that the
conductivity of a solution per molecule of dissolved electro-
lyte increases with the dilution; namely, with increased
dissociation.

In order to explain the property of conductivity, as
well as other properties, the following conditions have been
assumed :

1. That the separate ions are charged with electricity.

2. That a molecule is dissociated into two different kinds
of ions, one kind being charged positively, the other nega-
tively.

3. That the sum of the negative charges borne by the
negative ions is exactly equal to the sum of the positive
charges borne by the positive ions formed from the same
molecule.

4. That the charges are inseparable from the ions as such
and appear at the very instant of dissociation.

5. That the composition of the ions is similar to that of
the substances which primarily appear at the electrodes
when the solution is submitted to electrolysis.

Since bodies charged with electricity of unlike sign are


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