in chloroform, no chlorine reaction is obtained with silver
nitrate, for the ions of monochloracetic acid are Na or H,
and CH 2 C1.COO. The same reasoning applies to the test
for iron in ferric chloride or sulphate of iron, as opposed
to the test for iron in potassium ferrocyanide ; for here
also the splitting up into ions is different, and is as
follows: Fe 2 C1 6 ; Fe|SO 4 ; but K 4 |Fe(CN) G . 1
When such salts as ferric chloride are electrolysed, the
constituents which migrate as ions are found to be those
which are detected by chemical reactions, and these separate
at the electrodes. When, however, potassium ferrocyanide
in which no iron can be detected by the ordinary tests,
and in which therefore iron does riot exist in the ionic
state, is electrolysed, the complex group Fe(CN) G migrates
as a simple ion towards the anode, while no deposition
of iron occurs at the kathode. Salts which contain two
different bases are usually designated double salts.
The term strictly, however, ought to be confined to those
salts which on electrolysis yield both the metallic con-
stituents at the kathode ; and salts of the class repre-
sented by potassium ferrocyanide, where the potassium
alone migrates towards the kathode and the whole of the
remaining complex group with the other metal migrates
towards the anode, should be denoted by the term ' com-
1 The bearing of these new views upon analytical chemistry is
treated of in Ostwald's ' Theoretical Chemistry.'
THE PHENOMENA OF ELECTEOLYSIS 11
plex.' (Further reference will be made to this point in
Chapter VI., upon 'Dissociation of Salts in Solution.')
The double salts of potassium cyanide, the double oxalates
and phosphates, are examples of these complex salts. One
may indeed regard these complex salts, owing to their
method of splitting up, as binary salts, of which the
anion is a complex acid radical. For example, we have
potassium ferrocyanide, K 4 |Fe(CN) 6 , the potassium salt
of hydroferrocyanic acid ; sodium platinum chloride,
Na 2 !PtCl 6 , the sodium salt of hydroplatinic acid ; and,
similarly, the cyanides K 2 Ni(CN) 4 , K Ag(CN) 2 , and the
oxalateK 3 |Cr(C 2 4 ) 3 .
It is noteworthy that, while potassium zinc sulphate,
K 2 Zn(SO 4 ) 2 , is a true double salt, a complex salt potassium
zincate is formed when caustic potash in excess is added to
its solution. This salt is to be regarded as the potassium
salt of zincic acid. Similar salts of bismuth, arsenic, and
antimony are known.
If the complex anion produced by electrolysis of these
salts did not enter into secondary reactions, the separation
of the metallic element would be as little possible by
electrolysis as its detection by chemical reactions. In
some cuses these complex groups do indeed remain practi-
cally unchanged ; in others they decompose by secondary
reactions more or less quickly. In the case of a few, this
decomposition is so rapid as for example in the double
cyanide of nickel and potassium that one is inclined to
regard them as true double salts. By such secondary
decompositions of the anion, the metallic constituent of the
group becomes an ion and migrates as a kation towards
the kathode, where its deposition occurs. It may be
remarked here that this kind of secondary deposition has
become for many metals of practical importance, not only
in chemical analysis, but also in electrotyping and electro-
plating.
From such solutions of complex salts it is possible
to obtain even compact and adherent deposits of certain
12 THEOEY OF ELECTROLYSIS
metals which separate in crystalline or dendritic form, or
which show a tendency towards formation of spongy
deposits, when their simple salts are electrolysed. It is for
this reason that silvering and gilding by ' the wet method '
that is to say, by electrolysis are always undertaken by
means of solutions of the double cyanides.
Hittorf has given the following detailed explanation of
the electrolytic decomposition of the double cyanide of
silver and potassium, when insoluble platinum electrodes
similar to those used in electrolytic analyses are em-
ployed.
The kation potassium migrates towards the kathode ;
the remaining part of the molecule, Ag(CN) 2 , migrates as
anion towards the anode, where it coats the surface of the
platinum with AgCN, and (CN) is set free as gas. The
migrating potassium anion, however, reacts secondarily with
the uiidecomposed original salt, according to the equation
K + KAg(CN) 2 =2KCN + Ag
The liberated silver migrates towards the kathode, and
is there deposited ; the newly formed potassium cyanide
re-dissolves the silver cyanide (AgCN) which covers the
platinum anode, and forms anew the complex salt ; and
this cycle of changes continues until all the silver has been
deposited at the kathode. An anode of silver is used when
silver-plating ; the anion Ag(CN) 2 in this case dissolves an
atom of silver from the anode surface, and re-forms with the
2KCN the complex salt KAg(CX) 2 .
The decomposition of the double chloride of potassium
and gold follows the same course.
The ions K and AuCl 4 are first formed ; the anion
AuCl 4 splits up into AuCLj and Cl, especially easily as the
dilution increases ; and this gold chloride (AuCl 3 ) then
breaks up into its constituent elements, gold and chlorine.
If a solution of potassium ferrocyanide, K 4 Fe(CN) G ,
slightly acidified with hydrochloric acid, be electrolysed be-
tween insoluble electrodes, Prussian blue, (Fe 2 ) 2 [F e (CN) 6 ] ;3 ,
THE PHENOMENA OF ELECTROLYSIS 13
is formed after some time, when the solution used is very
dilute.
According to Hittorf, the potassium ion migrates to-
wards the kathode and there decomposes water with libera-
tion of hydrogen :
K 4 + 4H 2 0=4KOH + H 4 .
The radical Fe(ClSr) 6 drifts towards the anode, and in
concentrated solutions that is to say, in solutions contain-
ing sufficient potassium ferrocyanide forms potassium
ferricyanide :
3K 4 Fe(CN) 6 + Fe(CN) 6 =4K 3 Fe(CN) 6 .
When, however, the solution is very dilute, the reaction
takes a different course, and Prussian blue is formed
according to the following equation :
Fe(CN) 6 + 2H 2 0=H 4 Fe(CN) 6 + 2
7H 4 Fe(CN) 6 + O 2 = 24HCN + (Fe 2 ) 2 [Fe(CN) 6 ] 3 + 2H 2
When the anions are constituted of many elements,
especially in the case of the radical groups of organic acids,
one can frequently observe that reactions occur between
the similarly constituted anions. These reactions result
generally in the formation of gaseous products, which
either escape or enter again into combination with other
ions simple or complex present in the solution. Such
reactions occur during the electrolysis of nearly all organic
acids and salts.
The decomposition of formic acid by electrolysis takes
place according to the following equations :
HCOOH=H + HCOO
HCOO + HCOO=H 2 + 2C0 2
2HCOO + H 2 0=2HCOOH +
At the anode, carbon dioxide and oxygen are evolved,
while hydrogen is evolved at the kathode. When the
alkali salts of this acid are electrolysed, hydrogen is also
evolved at the kathode in consequence of the reaction
between the liberated alkali metal and the water. When
14 THEOEY OF ELECTROLYSIS
the formic acid salts of the heavy metals are electrolysed,
the metal itself is of course deposited at the kathode.
The electrolysis of acetic acid or its salts follows a
similar course :
PTT PO
i H 2
or one may assume that a direct splitting up into the two
anions CH 3 COO occurs :
2CH 3 COOH=2CH 3 COO + H 2
2CH 3 COO=C 2 H 6 + 2CO 2
The final products at the anode are in either case
ethane and carbon dioxide.
It is possible, however, for other products to occur,
according to the concentration of the solution and the
strength of the current.
Bourgoin found only carbon monoxide and carbon
dioxide ; Bunsen found other products in addition to these ;
while Jahn, when using a low- current density, found
carbon dioxide and ethane.
When solutions of oxalic acid or its salts are decomposed
by the electric current, the acid radical of the anion falls
at once into two molecules of carbon dioxide.
r COOH 9m ITT
lCOOH =2C 2|H2
If the potassium salt of this acid be electrolysed, the
liberated carbon dioxide reacts with the potassium hydrate
formed at the kathode, and potassium bicarbonate is
formed :
2K + 2H 2 O=2KOH + H 2
2KOH + 2C0 2 =2KHCO 3
When the electrolysis is performed with the ammonium
salt, the corresponding ammonium compound is formed ; but
this immediately splits up into ammonium hydrate and
carbon dioxide.
THE PHENOMENA OF ELECTROLYSIS 15
From the heavy-metal salts the metals alone are
deposited.
The salts of tartaric acid yield on electrolysis carbon
dioxide, carbon monoxide, and oxygen as final products at
the anode, with small quantities of formic aldehyd and
formic acid.
The metal double salts of the above-named organic acids
are occasionally made use of in electrolytic methods of
analysis.
It is to be noted that only those organic compounds
which correspond to the inorganic salts in constitution are
to be regarded as true electrolytes.
16 THEOKY OF ELECTEOLYSIS
CHAPTER II
WHEN an electric current is passed through different bodies
the movement of electricity in these bodies can occur in
two different ways.
Conductors of the first class metals, alloys, carbon,
and a few other materials exhibit a heating effect which
follows the law of Joule ; but beyond this they exhibit no
change. In the conductors of the second class electro-
lytes however, chemical change is a condition of the
transfer of electricity.
In the previous chapter a large number of examples
have been given of the chemical changes which accompany
the passage of the electric current through molten or dis-
solved salts.
Michael Faraday, who was engaged with experiments
bearing upon the measurement of electrical energy,
discovered, as a result of these, in 1833, the law of 'invari-
able electrolytic action.' When any compound is decom-
posed by an electric current, the weight decomposed is found
to be proportional to the amount of current used ; and the
relative weights of the different elements or groups of
elements separated in the same time are found to be
represented by the equivalent weights of the elements. 1
Helmholtz expressed this law as follows :
The same current liberates in different electrolytes the
1 The equivalent of an element is the atomic weight divided by
the valency.
FAEADAY'S LAW 17
same number oj valency bonds, or engages a like number in
new combinations.
In general terms one may also express it thus : All
movement of electricity in an electrolyte is conditional upon
simultaneous movement of the ions, and the connection
between these is such that, with equal quantities of electricity,
chemically equivalent amounts of the different ions must be
in movement.
The Law of Faraday thus makes no direct reference
to the separation of the ions at the electrodes, but confines
itself to the movement of electricity in the electrolyte.
Faraday had himself already suggested that the conduction
of the current, and separation of the products of the
decomposition at the electrodes, were two distinct pheno-
mena.
Nevertheless, the separation of the ions at the electrodes
is the most convenient means by which to test the accuracy
of Faraday's law.
This law has up to the present survived all the tests to
which it has been submitted. If one connects in series in
the same circuit, cells containing solutions of silver-nitrate,
copper sulphate, and antimony chloride, the same quantity
of electricity must pass through each cell, and by the law
the weights of metals separated must be in the proportion of
their equivalent weights or 107'6 Ag : ^Cu : .^ Sb.
2 3
The relative proportions of the acid radicals simultaneously
o/~v pn
separated at the anodes would be NO 3 : J : 3
For a practical illustration of this law Liipke 1 recom-
mends dilute sulphuric acid, potassium -silver cyanide,
cuprous chloride solution acidified with hydrochloric acid,
copper sulphate solution acidified with nitric acid, and a
tin tetrachloride solution containing oxalate of ammonium.
1 E. Liipke, Grundzilge der Elektrochemie auf experimenteller
Basis, Berlin, Springer. Also English translation of above by
Pattison Muir.
18
THEORY* OF ELECTROLYSIS
Platinum foil is to be used as electrode material, or one
may use for anode a strip of the metal the solution of
which is to be electrolysed. A current obtained from 5
accumulators, which was allowed to pass through the cells
for a period of 30 minutes, gave the following results :
I
II
III
IV
V
H 2 So 4 l:12
KAg(ON) a
Cu 2 01 3
CuSo 4
Sn01 4
- +
- 4-
+
- +
+
Electrode )
material J
Pt Pt
Pt Ag
Pt Cu
Pt Cu
Pt Pt
Weight of ]
deposited }
kations )
67c.c.H= )
6-002 m.g. j
650 m.g. Ag
380 m.g. Cu
190 m.g. Cu
170 m.g. Sn
Ditto per )
1 m.g. H. )
1 mg. H.
108-2 m.g. Ag
63-6 ni.g. Cu
31-8 m.g. Cu
28-3 m.g. Su
Atomic )
weights j
1
107-6
63-3
63-3
117-8
Error per )
cent j
+ 6
+ 4
+ 4
-4-0
These numbers enable one to obtain a very useful
insight into the course of the electrolysis ; in II and III
the metal ions, silver and copper, are univalent ; in' IV the
copper is divalent, and in V the tin is quadrivalent. Abso-
lute accuracy is not to be expected in such experiments
when complex electrolytes are used.
The use of the voltmeter, which will be described later,
rests upon the absolute truth of this law, as confirmed by
extended experiments, of the relative proportions of the
deposited weights of metal or liberated volumes of gas.
In the following Table (p. 19) are given the electro-
chemical equivalents for those elements of chief importance
to the electro-chemist. The weights given represent the
amount separated or deposited by 1 ampere (= unit
current intensity) during an interval of one second in
m.grams, or during one hour in grams.
Since, as already pointed out, a known quantity of
electricity occasions the movement or migration of equiva-
lent weights of the different ions present in the electrolyte,
FAEADAY'S LAW
19
Element
Symbol
Valenci
Atomic
weight
Weight separated
per ampere
m.g. per second
gr. per hour
Aluminium
Al
III
26-90
093583
3369
Antimony
Sb
III
119-40
415387
1-4953
Arsenic .
As
III
74-40
258834
9318
Bismuth .
Bi
III
206-40
718055
2-5849
Cadmium
Cd
II
111-30
580811
2-0909
Chlorine .
Cl
I
35-19
367273
1-3221
Cobalt .
Co
II
58-60
305800
1-1008
Cobalt .
Co
III
58-60
203866
7339
Copper .
Cu
I
62-80
655434
2-3595
Copper .
Cu
II
62-80
327717
1-1797
Gold
Au
III
195-70*
680830
2-4509
Hydrogen
H
I
1-000
0104368
03757
Iron
Fe
II
55-60
290144
1-0445
Iron
Fe
III
55-60
193429
6963
Lead
Pb
II
205-40
1-082300
3-8962
Magnesium
Mg
II
24-20
126286
4546
Manganese
Mn
II
54-60
284926
1-0257
Manganese
Mn
III
54-60
189950
6838
Mercury .
Hg
I
198-90
2-075890
7-4732
Mercury .
Hg
II
198-90
1-037945
3-7366
Nickel .
Ni
II
58-60
305800
1-1008
Nickel .
Ni
III
58-60
203866
7339
Oxygen .
II
15-88
082868
2983
Palladium
Pd
II
104-70
546369
1-9669
Platinum
Pt
IV
193-30
504361
1-8156
Potassium
K
I
38-85
405472
1-4596
Silver .
Ag
I
107-13
1-118100
4-0251
Sodium .
Na
I
22-87
238691
8592
Thallium j Tl
II
202-64
1-057370
3-8065
Tin . . Sn
II
117-20
611599
2-2017
Zinc
Zn
II
65-00
339197
1-2211
one may infer that a definite quantity of electricity moves
with each equivalent of weight. Equivalent weights of
the different ions have, that is to say, like capacities for
electrical energy, and resemble in this the atomic masses of
the elements, which, according to the law of Dulong and
Petit, have like capacities for heat. Weber and Kohlrausch
were the first who attempted to answer the question How
great is this quantity of electricity, and to express in abso-
lute units the electricity which is carried by 1 gram of
hydrogen, or the equivalent weight of any element. The
c2
20 THEOKY OF ELECTEOLYSIS
researches of F. and W. Kohlrausch and of Lord Rayleigh
have proved that this quantity is 96537 coulombs, the
coulomb being the unit of electrical quantity. One coulomb
therefore demands for its transport a mass of any ion repre-
sented by its equivalent weight expressed in grams, and
multiplied by -000010359.
Faraday's law must not be interpreted to indicate that
like quantities of electricity demand the expenditure of
like amounts of work upon the different equivalents of
matter. The law does not touch upon the work or
energy ratios, but relates only to the one factor of elec-
trical energy measurements the quantity of current ; the
second factor the pressure or potential is unnoticed in
this law.
In concluding this chapter it will be well to note briefly
the units of electrical measurement. The unit of electrical
energy is equal to 10 7 absolute units, and is called the
Joule. The unit of potential, pressure, or electro-motive
force is the Volt. The Latimer -Clark cell at 15C. has
an E.M.F. of 1*437 volts [the temperature correction is
obtained by use of the formula -0010 (t-15], while the
Daniell cell has an E.M.F. of about 1-1 volts.
The unit of electrical quantity is the Coulomb, which
represents the quantity of electricity that by a fall of
potential = 1 volt liberates 10 7 absolute units of energy.
If one coulomb pass any cross-section of the circuit in
one second of time, the current is said to have a strength
or intensity of I Ampere. The ampere is then the unit
of current strength. If in any conductor a current of 1
ampere is produced by a fall of potential = 1 volt, the con-
ductor possesses a resistance of 1 Ohm.
The standard resistance of 1 ohm is obtained by use at
0C. of a column of mercury, 106'3 c.m. in length, and 1 sq.
m.m. in sectional area.
21
CHAPTER III
THE CONSTITUTION OF THE ELECTROLYTE
THE conductors of the second class, the electrolytes, must
necessarily be chemical compounds, since decomposition is
a condition of current conduction.
The converse of this is not however equally true ;
many compounds are known which do not conduct. This
ability to act as conductors for the electric current is
possessed generally by all substances in the molten state
or in aqueous solution. No pure substance is however
known, fluid at the ordinary temperature, that is a con-
ductor to any marked degree. The pure acids sulphuric
acid, hydrochloric acid, &c. which in aqueous solutions
form some of the best conductors, are non-conductors.
Organic compounds conduct only in the degree in which
they possess the constitution and characteristics of true
salts. The possibility of functioning as a conductor thus
depends upon the ability to form from the molecules of
the dissolved substance, particles of matter, which charged
with positive or negative electricity are free to move
in opposite directions. Since no substance in the mole-
cular state can become charged with positive or negative
electricity, one is obliged to assume that this property
belongs exclusively to the parts of the molecule, the ions.
The view that the electric current first causes a splitting
up of the molecule, and then makes use of the sub-molecules
for its transport, does not, however, correspond to the facts.
Such a splitting up of the molecule would demand the expen-
diture of a definite amount of work. Clausius, therefore, in
22 THEORY OF ELECTROLYSIS
1857 formulated the theory, that the current caused an ac-
celeration of the molecular movements, and that the splitting
up was a result of the collisions that ensued. The relative
proportions of the numbers of molecules and sub-molecules
remained at that time undetermined. It was not until
1887 that Arrhenius proved, from other characteristics of
the electrolyte, that in solutions of salts, strong bases, and
acids, these bodies are contained only in small part as such,
and that they are for the most part split up into their
respective ions. If the electric current were the cause of
this ionisation of the substance, those chemical substances,
the elements of which possessed the weakest chemical
affinities, would be found to be the best conductors. Ex-
periments, however, prove exactly the reverse. It may
indeed be considered somewhat strange that it should be
salts like potassium sulphate and sodium chloride, the
elements of which have the strongest affinity for each
other, that show the greatest ionisation when dissolved.
This strangeness is, however, merely the result of a con-
fusion of thought in regard to the stability, and the
chemical activity of a substance.
The metals displace hydrogen with the greatest ease
from its combinations in the mineral acids ; while in the
hydro-carbons the hydrogen is unacted on by metals. The
hydroxyl group in the alkaline hydrates is easily displaced
by an acid radical ; in the alcohols the same group remains
unattacked by acids. Thus it is the chemically inert
bodies which show the strongest chemical affinities
among their component elements. The chemically
active bodies form on the other hand the best electro-
lytes, and the relationship between these two properties
of compounds is so close that one can determine the con-
ductivity by the chemical activity, and vice versa.
The theory that free ions exist in solutions may seem
strange to those to whom it is new, for we are accustomed
to associate with the free elements other properties than
those noted in solutions.
THE CONSTITUTION OF THE
For example, in a potassium chloride solution, which
in the light of this theory contains chiefly potassium and
chlorine as ions, one can observe neither the water-de-
composing properties of the former, nor the characteristic
smell of the latter.
The explanation of this is to be found in the fact that,
although the free atoms of potassium and chlorine are
present as ions in the solution, they carry extremely
large charges of electricity, and on this account possess
chemical properties differing widely from the normal ones.
The energy charge of an ion is different from that of a free
atom, and it is this that determines the different properties
of the two. Let such a charged ion, as for example a
potassium ion, deliver up its electrical charge at the
electrode ; the properties of the normal potassium atom
at once reappear. The same is true of gaseous as well as
of metallic ions, of anions as well as kations. From the
above it follows, that when a metal salt is electrolysed and
a deposition of the metal obtained at the kathode, the
latter has occurred owing to the delivery of the electrical
charges brought by the metal ions to the electrode ; the
now electrically neutral sub-molecules of the metal being
thus able to manifest the usual metallic properties.
The acceptance of the theory, that the properties of an
atom of an element may be entirely altered by the presence
of an electric charge upon it, also explains the fact that
isomeric ions of different valency possess different properties.
The ferro- ion in divalent iron compounds, for example,
exhibits different properties as regards colour and behaviour
towards reagents, from those of the ferri- ion in ferric-salts.
A similar contrast is exhibited by the two groups
Fe(CJST) 6 in yellow and red prussiate of potash ; and the
distinctive properties of the two MnO 4 ions in manganic
and permanganic acids is another instance of the same
kind. The differences in all these cases spring from differ-
ences in the charges of energy ; the ions in each case
carrying electric charges which vary as their valency.
24 THEORY OF ELECTROLYSIS
CHAPTER IV
THE MIGRATION OF THE IONS
IN order to explain the fact that the passage of the electric
current through acidified water caused a liberation of
hydrogen and oxygen at the electrodes, different theories
were put forward, even in the early days of the science.
According to that advanced by Grotthiiss (1805) the current
made the one electrode positive, the other negative.