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ELECTROLYTIC METHODS
OF ANALYSIS
ORGANIC CHEMICAL MANIPULATION. By J. T. HEWITT,
M.A., D.Sc., Ph.D., Fellow of the Chemical Societies of London and
Berlin, Professor of Chemistry in the East London Technical College.
With 63 Illustrations. Crown 8vo. 272 pp. 7s. Qd. net.
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'Everything that a steel- works analyst may fairly be called upon to examine
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EXPLOSIVES, The Manufacture Of. A Theoretical and
Practical Treatise on the History, the Physical and Chemical Properties,
and the Manufacture of Explosives. By OSCAR GUTTMANX, Assoc.
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' Mr. Guttmann's important book.' ENGINEER.
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THE ELECTRO -PLATERS' HANDBOOK. A Practical
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Revised and Enlarged, with an Appendix on Electro-typing. 3s.
'An amateur could not wish for a better exposition of the elements of the
subject.' ELECTRICAL REVIEW.
London: WHITTAKEE & CO.
THE THEOEY AND PRACTICE
OF
ELECTEOLTTIC METHODS
OF
ANALYSIS
BY
DR BERNHARD NEUMANN
M
ASSISTANT LECTURER ON METALLURGY AT THE TECHNICAL
SCHOOL AT AACHEN
TRANSLATED BY
JOHN B. C. KEESHAW, F.I.C.
WHITTAKER & CO.
WHITE HART STREET, PATERNOSTER SQUARE, LONDON
AND 66 FIFTH AVENUE, NEW YORK
1898
I'lUNTED BY
SPOTTISWOODE AND CO., NEW-STREET SQUARE
LONDON
TRANSLATOR'S PREFACE
IN the preparation of the English translation of Dr.
Neumann's work on Electrolytic Methods of Analysis a
comparatively small number of alterations or corrections
have been found necessary ; and the Author's consent has
been in every case obtained for the few that have been
made.
The portion of the original work dealing with primary
arid secondary batteries, the dynamo, and the thermo-pile
has been omitted. The electro- chemical equivalents have
been recalculated upon the most reliable figures for the
atomic weights of the elements. The Translator has added
a few notes to the original text where such notes were
considered to be of utility. These notes are in every case
included in square brackets, in order to distinguish them
from the original text. The translation is provided with a
fairly complete subject index, and a name index, which it
is believed will increase the value of the book to those
engaged in original research in this branch of chemistry.
With regard to the question of current supply, it may
be pointed out that city and town laboratories can obtain
the current required from the supply mains of the electric-
lighting companies.
The installation of small transformers for reducing the
current voltage to five volts, and where necessary for
converting alternating currents into direct ones, is a com-
paratively simple matter. When continuous work is con-
templated, the addition of one or two storage batteries
would be requisite.
VI TRANSLATOR'S PEEFACE
Three warnings may be given to those who through the
instrumentality of this book are led to make practical use of
electrolytic methods in their laboratories for the first time :
1. Electrolytic methods should only be used when
decided advantages in time or accuracy will result. The
practical examples given in Part III., D, clearly indicate
the principles upon which electrolysis can be used with
advantage in analytical work. The Author's concluding
paragraph on page 245 wisely states that it will 'always be
found most convenient to combine the chemical and electro-
lytic methods of separation.' The attempt to carry out
complete analyses by means of electrolysis, or to use
electrolytic methods for the determination of metals more
conveniently estimated by gravimetric or volumetric
methods of analysis, will generally result in failure, and
may lead the chemist who has not had any experience of
the advantages to be gained from these new methods, when
rightly applied, to confine himself more rigidly than before
to the older methods of analytical work.
2. The greatest care and attention must be given to
the precautions, mentioned on page 85, relative to the elec-
trodes. Many failures of electrolytic methods in the hands
of students and novices could, no doubt, be traced to neglect
of these very elementary conditions of successful work.
3. The conditions as to current density, temperature,
and E.M.F. mentioned in the detailed descriptions of the
various methods in Part III. must be strictly observed.
Slight variations in these conditions will in many cases
suffice to entirely alter the nature or character of the deposit.
In conclusion, the Translator hopes that the reception
accorded in England and America to the English edition
of Dr. Neumann's work may be as favourable as that
given to the original in Germany one year ago.
LONDON : January 1898.
AUTHOE'S PEEFACE
UP to the present time, two works have been published
which treat of electrolytic methods of chemical analysis,
One of these was written by E. Smith, and has been
translated into German by Ebeling ; the other was
written by A. Classen.
Both of these works deal principally with the authors'
own methods, although a few others receive mention.
In the meantime electrolytic methods of analysis have
been adopted in many technical laboratories, and have been
accepted as valuable aids, and in some cases as useful
alternative processes, to the ordinary analytical procedure.
The methods now customarily, and even exclusively,
used in these technical laboratories for the determination of
different metals, receive in the above-named works only
subsidiary mention ; the current is given in terms of
detonating gas ; the voltage is not even mentioned. In
the present work these faults and omissions are rectified.
In the consideration of the methods of electrolytic
determination of single metals, the methods of greatest
technical importance receive the most ample treatment.
These are described in detail ; exact data regarding current,
voltage, and temperature are given, so that even the novice
will be in a position to carry them out with some degree of
success. The more important of the remaining methods
are also noticed briefly, and their relative advantages and
disadvantages are discussed.
Vlll AUTHOR'S PEEFACE
Following the next division of the work, which is
devoted to metal -separations, there comes a subdivision
containing a selection of practical examples. In this
it is shown that in the analysis of metals, alloys, and
smelting works' products, electrolytic methods of analysis
have already found acceptance, or could advantageously
be adopted.
Since the newer theories relating to electrical phe-
nomena are steadily meeting with more general acceptance,
it is certainly fitting that works on electrolysis, both
analytical and technical, should be provided with a brief
review of them.
The Author has therefore devoted the opening chapters
of the present work to such a summary. The phenomena
and laws of electrolysis are discussed and explained in the
light of the newer theories.
The most convenient forms of current-measuring and
regulating apparatus are specially described.
It has been throughout the Author's chief aim to
provide both the student and the practical chemist with a
work which should cover the whole of the ground ; one
which, while it treated fully of the theoretical side of the
subject, and gave all the necessary explanation of electro-
chemical phenomena, should still deal in an unusually full
manner with the practical aspects of these new analytical
methods, and should enable both the student and practical
chemist, by its large number of practical examples, and by
its full descriptions of the apparatus and instruments used,
to avoid those errors into which they might otherwise fall.
The numerous references to the original literature of
the subject may be regarded as a useful appendix to the
work.
DR. B. NEUMANN.
STOLBERG : September 1896.
CONTENTS
PAQB
INTRODUCTION 1
PART I
THEORY OF ELECTROLYSIS
CHAP.
I. THE PHENOMENA OF ELECTROLYSIS . . . 6
ii. FARADAY'S LAW 16
III. THE CONSTITUTION OF THE ELECTROLYTE . . 21
IV. THE MIGRATION OF THE IONS 24
V. THE CONDUCTIVITY OF THE ELECTROLYTE . . 27
VI. THE DISSOCIATION THEORY 30
VII. THE CHEMICAL AND MOLECULAR CHANGES DURING
ELECTROLYSIS 33
PART II
MEASURING AND REGULATING THE CURRENT
A. CURRENT MEASUREMENT
VIII. MEASUREMENT OF THE CURRENT STRENGTH . . . 50
IX. MEASUREMENT OF ELECTRO-MOTIVE FORCE (B.M.F) . 61
B. REGULATING THE CURRENT"
X. INCREASING THE CURRENT STRENGTH . . . . 65
XI. REDUCING THE CURRENT STRENGTH . . . .74
x CONTENTS
PART III
THE ELECTROLYTIC PROCEDURE
J'AC.K
A. INTRODUCTORY . . 79
B. DEPOSITION OF THE METALS FROM SOLUTIONS OF PURE
SALTS 91
C. SEPARATION OF THE METALS 165
D. PRACTICAL EXAMPLES 220
APPENDIX
THEORETICAL PERCENTAGE OF THE METALLIC ELEMENTS IN
CERTAIN METALLIC SALTS ....... 246
NAME INDEX 247
SUBJECT INDEX ........ 249
ELECTBOLYTIC METHODS
OF
ANALYSIS
INTRODUCTION
IN the year 1792 Volta commenced to investigate Gal-
vani's discovery, and there resulted from his investigations
1 the voltaic pile.' Since that time a very large number
of arrangements of conductors of the first and second class
have been constructed, by means of which it has been possible
to produce an electric current with ease. It was there-
fore natural that the mode of action of the different
elements, or piles, should have received close study, and
that careful attention should have been given to the
attendant phenomena. One of the earliest observations
was, that water which had been made acid with sulphuric
acid was split up into its components oxygen and
hydrogen by the electric current. The discovery of the
decomposition of metallic salt solutions, and of the easy
separation of the metallic component, generally as a metallic
coating upon one electrode, quickly followed. We find
such depositions already technically employed at the end
of the thirties. Jacobi, who is to be regarded as the
founder of the art of electroplating, had already in 1839
prepared electrotypes of various objects, and these were
a
2 ELECTROLYTIC METHODS OF ANALYSIS
exhibited to the members of the St. Petersburg Academy.
Other investigators must have also devoted themselves
with zeal about this time to the study of the subject, for
in 1840 and the following years a large number of
methods were published relating to the preparation of
solutions from which one could obtain, without fail, ex-
ceptionally beautiful deposits of certain metals. For
example, in 1840 Ruolz, Elkington, and de la Rive
proposed to use potassium cyanide solutions for obtaining
deposits of gold and silver ; in 1841 the same solution
was proposed for copper and nickel, and a sodium hy-
drate solution for tin. Originally these methods were
only designed to be used for electroplating purposes ; but
since very small amounts of metals can be deposited and
detected in this way, similar methods were sought which
should render feasible the quantitative estimation of
metals, especially of poisonous ones in foods &c. (Bloxam,
Morton). These methods are, for particular determina-
tions, still in use. In 1805 Davy had pointed out in-
cidentally in his working notes that the electric current
might be used for chemical analysis, but it is Antoine
Becquerel (the elder) who must be honoured as the real
founder of analytical electro-chemistry. Becquerel pub-
lished, as far back as 1830, a practical method for
separating small amounts of lead and manganese from
other metals, the manganese being obtained as peroxide
at the anode. He showed, further, that it was possible in
a definite time to separate as peroxide all the manganese
contained in a known weight of manganese acetate. It
was not, however, until the commencement of the sixties
that electrolytic separations began to be used as aids
to, and in some cases as substitutes for, the ordinary
methods of analysis. In 1864 W. Gibbs separated electro-
lytically nickel and copper from nickel coins. In 1865
Luckow published a large number of experiments, and
showed out of what solutions it was possible to obtain
quantitative deposits of metals. In 1867 he received a
INTRODUCTION 3
prize from the ' Mansfeld'schen Ober-Berg- und Hiitten-
Direktion ' for his electrolytic method of estimating the
copper contained in the Mansfeld schists.
Since this date the employment of electrolysis both
for analytical and technical purposes has extended greatly.
While the most important facts e.g. the more or less
good or rapid deposition of metals from different solutions,
the variations in the current required, the separation of
different elements or groups of elements from the same
solution according to the current intensity or voltage, the
influence of temperature, &c. were long known, there
was lacking, until a few years ago, a theory of electrolysis
which explained clearly these phenomena.
Similarly -there was no theory by means of which it
was possible to explain clearly the changes in the energy-
creating couple. It is true that at the commencement of
this century a violent strife arose between the supporters
of the ' contact theory ' and the supporters of the ' chemical
theory' of the origin of the energy in the voltaic cell.
This strife continued into the third quarter of the century
without the supporters of either the one theory or the
other being enabled to give a clear explanation of the facts.
Such an explanation has only become possible within the
last few years, by aid of the theory of osmotic pressure, or
' osmosis.'
In the same way that this modern theory has satis-
factorily explained the origin of the energy in the voltaic
cell, the newer theories of solution and of electrolytic
dissociation, which are the result of the researches carried
on in the domain of physical chemistry during the past
twenty years, have resulted in a deeper insight into the
changes involved in the conduction of the current by
electrolytes, and have elucidated many hitherto puzzling
phenomena connected with this subject.
It will not therefore be superfluous if the opening
chapters of this work be devoted to a brief summary of
those currently accepted theories which are necessary for an
4 ELECTEOLYTIC METHODS OF ANALYSIS
understanding of the phenomena to be observed in the
conduct of electrolytic methods of analysis. 1
1 Those interested in the study of the historical development of
Electro-chemistry are referred to ' Elektrochemie, ihre Geschichte
und Lehre,' by W. Ostwald.
PABT I
THEORY OF ELECTROLYSIS
THE term * electrolysis ' is used to denote the chemical
phenomena and changes, accompanied by movement of the
particles of matter, which are produced when an electric
current is passed through a fluid conductor, i.e. a conductor
of the second class.
- Faraday, with whom originated the terms still in use,
named those bodies which, when in solution or in the
molten state, conduct the current in this manner
* electrolytes.'
The term ' electrode ' is used to denote those parts of
the conductors of the first class, carrying the current to
the electrolyte, which are in contact with it. A difference
of potential is produced by the electric current at the
electrodes, and as a result of this a movement of the ultimate
particles of matter present in the electrolyte the ' ions '
follows.
The particles, which differ fundamentally, move in
different directions ; those which move with the positive
current are called ' kations,' whilst those moving in the
contrary direction are called 'anions.' The electrode
towards which the kations drift is called the * kathode ' ;
that towards which the anions drift is called the l anode.'
The liberation of the ions at the electrodes causes
changes at the latter, which vary greatly in character.
THEOEY OF ELECTKOLYSIS
CHAPTER I
THE PHENOMENA OF ELECTROLYSIS
WHEN an electric current is passed through an electrolyte,
movements of the ions of different kinds towards the
opposite sides occur, and the products liberated at the
electrodes are consequently different.
The most simple case for consideration is that in which
the electrolyte, either in the dissolved or molten state, is
made up of only two component parts, or contains only
a base and an acid radical. For instance, if zinc or copper
chloride be electrolysed with a sufficiently strong current,
zinc or copper will be deposited at the kathode ; while the
chlorine will drift towards the anode, and, when this is of
a non-porous and to some extent chlorine-proof material
(chlorine attacks and destroys in time all electrodes), will
be liberated there as a gas. The same results are obtained
if zinc chloride or lead chloride be melted in a crucible, or
pipe-head of red clay previously warmed, and an electric
current be passed through the mass,, by means of a needle
passing down the straight stem as kathode, and of a carbon
pencil in the bowl as anode. Small spheres of molten zinc
or lead collect at the bottom of the bowl, while a portion
of the chlorine is liberated as gas at the anode. If hydro-
chloric acid be electrolysed with platinum electrodes, the
ions are hydrogen and chlorine. The hydrogen drifts as
the metals generally, towards the kathode ; and, being a gas,
can be partly retained (occluded), according to the character
of the electrode.
OF THE
UNIVERSITY
THE PHENOMENA OF
In the instances given above where the compounds
have been made up of only two components, the products
of the decomposition have been liberated directly at the
electrodes. How will the results be affected when more
complex compounds are electrolysed ?
As a general rule the passage of the current is accom-
panied by a similar division into two sets of drifting ions,
whether the constitution of the molecule be simple or
complex.
Berzelius, who supposed that salts were made up of two
parts the base (an oxide) and the acid (an anhydride) and
consequently wrote the formula of potassium sulphate, for
example, K 2 O.S0 3 , represented the school who thought that
these two component parts not only acted as such in
chemical reactions, but also drifted and separated as such
in electrolysis. In order to bring this theory into harmony
with the observed facts, he was obliged to assume, in
addition, that the electric current decomposed water and
liberated its constituent parts. The deposition of metals
from neutral salt solutions was then explained by him as
follows : The oxide of the metal drifted to the kathode,
and was there reduced by the nascent hydrogen resulting
from the decomposition of the water, and deposited as
metal.
The halogen salts would not fit into this system, and it
was partly on this account that he came to regard them as
oxygen-holding compounds.
Daniell brushed away these contradictions, and proved
that a salt is in every case to be regarded as a combination
of a metal and an acid radical. The latter may be either
a single element, as in the halogens, or a complex group of
different elements.
Hydrogen, on account of its behaviour, is to be regarded
as a metal ; the hydroxyl group of the different bases, on
the contrary, is to be regarded as an acid radical. The acids
are therefore hydrogen salts ; the bases are salts, of which
the acid radicals are the hydroxyl groups. Daniell further
8 THEOEY OF ELECTEOLYSIS
showed that potassium sulphate, for example, on electrolysis
between platinum electrodes, decomposes as other salts, into
the metal ion K and the acid ion SO 4 . These component
parts then lose their charges of electricity at the electrodes,
and secondary action upon the water follows. 1
The potassium decomposes the water :
K 2 + 2H 2 O=2KOHf H 2 ;
likewise the SO 4 ion at the anode :
SO 4 + H 2 0=H 2 SO 4 +
The final products are therefore caustic potash and
hydrogen at the kathode, an acid and oxygen at the anode.
The amounts of alkali and acid formed at the electrodes are
equivalent to the amounts of the respective gases.
That there is an actual formation of caustic potash
solution at the one electrode, and of sulphuric acid at the
other, is most simply shown by performing the electrolysis
in a V-tube containing a litmus-coloured solution of the
salt. The colour of the acid solution then changes to red,
and of the alkali solution to blue. A similar proof that K
and SO 4 are the products which drift towards the electrodes
is obtained by covering some mercury contained in a vessel
with potassium sulphate solution and by use of the mercury
as kathode with a strong current. Potassium amalgam is
formed, and this, separated from the electrolyte and treated
with water, gives visible proof of the presence of potassium,
Water shares in the carrying of the current only to a
very small degree.
The view held by Berzelius, that when potassium
sulphate was electrolysed the water also suffered con-
siderable decomposition, must be regarded as incorrect.
Daniell himself proved this, by placing in the circuit a
voltmeter containing dilute sulphuric acid, and noting that
the volumes of gases liberated in both electrolytic cells
1 The modern theory, as will be shown later, gives a simpler
explanation of the still customary conception of secondary decom-
position.
THE PHENOMENA OF ELECTEOLYSIS 9
were the same. The electrolysis of dilute sulphuric acid,
it is evident, must yield the same gases as that of potassium
sulphate namely, hydrogen and oxygen.
The electricity moves in such a way in conductors of the
second class i.e. electrolytes that the metals, the metal-
loid radicals of salts and bases, and the hydrogen of the
acids, all drift from the positive side of the cell-circuit
towards the negative ; while the acid radicals, the halogens,
and the hydroxyl groups of the basic compounds drift in
the contrary direction. No element or ion is known
which can appear both as kation and anion. The following
are to be classed as kations : The metals ; hydrogen and
the radical NH 4 ; organic substitution products of NH 4 ,
PH 4 , AsH 4 ; further, SR 3 , SeR 3 , TeR 3 , and other similar
series in which R represents hydrocarbon radicals. The
anions may be regarded as the remaining radicals of con-
ducting bodies ; as, for example, OH, Cl, Br, I, N0 2 , NO 3 ,
C10 3 , C10 4 , S0 4 , Se0 4 , P0 4 , As0 4 .
In general, one may use the following definitions : The
anion is all that which, combined with hydrogen or a
metal, forms an electrolyte ; the kation is all that which,
combined with a halogen or an acid radical, forms an
electrolyte. Oxygen, sulphur, selenium, and tellurium are
anions, but they occur chiefly in the forms OH, SH, SeH,
and TeH. The ions do not all possess the same valency ;
for instance, zinc is a dyad (Zn 11 ), bismuth a triad (Bi 111 ), while
manganese and antimony are Mn 11 and Sb m respectively.
The anions S0 4 and PO 4 are the first a dyad and the second
a triad, and are written S0 4 " and PO 4 m . Metal ions and
acid ions with variable valency are also known. The
following list of metals, in which the valency is signified
by the Roman numerals, shows examples of this : Fe 11 ,
Fe m ; Ou 1 , Cu 11 ; Hg 1 , Kg" ; Au 1 , Au m ; Sn", Sn iv . Simi-
larly the following variable anions are known : Fe m (CN) 6 ,
Fe IV (CN) 6 ; Mn'0 4 , Mn"O 4 .
It has already been shown that one may regard both
acids and bases as salts, and Hittorf has founded upon this
10 THEOEY OF ELECTEOLYSIS
view the following general definition : Electrolytes are
salts ; they break up on electrolysis into the same atoms
or atom groups which they exchange in chemical reactions.
Later it was found that one could go further and say that
all chemical reactions are exclusively reactions between
ions that is to say, elements or groups of elements can
only be detected by the customary reagents when they are
present in the ionic state. As an example of this we have
the detection of Cl in common salt or in hydrochloric acid
by means of silver nitrate.
In the chlorine substitution "products of acetic acid, or