The electrodes then exerted a directive influence upon
the polarised molecules of water, so that the oxygen side of
the molecules faced the negative electrode, and the
hydrogen side the positive. During electrolysis, only the
two end molecules of each chain were decomposed, and the
respective oxygen or hydrogen atoms set free ; the remain-
ing hydrogen and oxygen of these molecules united with
the oxygen and hydrogen of the two neighbouring molecules,
so that combination and decomposition alternated continu-
ously in the electrolyte.
A definite electro- motive force was, according to this
theory, necessary in order to start the decomposition,
whereas experiment showed that solutions would conduct
even with the feeblest currents.
Clausius pointed out this contradiction between fact
and theory, and declared the theory to be untrustworthy.
He ought to have been forced by this reasoning into a
recognition of the absolute freedom of the ions in the
electrolyte, but he saw only half the truth, and, as noted in
the last chapter, advanced the view that the current does
THE MIGRATION OF THE IONS
25
not directly cause the breaking up of the molecule, but that
by its action the loosely bound constituent atoms of the
molecule are set in more rapid vibration and movement, and
o
o
o
o
that, as a result of this, some
molecules break up and the con-
stituent parts of these migrate
towards the electrodes. It was not
until the year 1887 that Arrhenius
published his Dissociation Theory
and finally solved the problem.
Hittorf had, indeed, in the
years 1853-1858 been engaged
upon a study of the alterations in
concentration of electrolytes at the
electrodes, and had obtained in the
course of this work a deep insight
into the subject of the migration of
the ions.
If the rate of migration of
the two ions during electrolysis
be the same, then the loss of the
liquid around the two electrodes
will be equal. This is, however,
rarely the case, and Hittorf there-
fore concluded that the ions pos-
sess different velocities. Let one
imagine an electrolyte, which con-
tains an equal number of anions
and kations (represented by the
black and white circles in Fig. 1),
divided in the middle by a porous
partition (the vertical line in Fig.
1) so that equal numbers of anions
and kations are present on each side of it. The passage
of the current will speedily disturb this equilibrium. In
Fig. 1 the row (a) represents the electrolyte before the
action of the electro-motive force ; the row (b) repre-
o
c
) 1
o
c
) J
o
c
)
o
c
)
o5
c
)
1
o
c
)
1
.c
)
I
o
c
)_
'
E
c
)
I
o
c
)
rH
o
c
)
8
o
c
)
o'
^
o
o
26 THEORY OF ELECTROLYSIS
sents the same electrolyte after a migratory movement of
the ions.
In this movement it has been assumed that the anions
(the white circles) have moved twice as fast as the kations
(the black circles), and the horizontal lines (u) and (v)
represent the extent of the movement. Six ions have been
liberated at each electrode ; consequently six equivalents
of the electrolyte must have been decomposed and de-
stroyed.
Four of these have been lost from the left-hand side of
the partition, and the remaining two from the right-hand
side ; in other words, these losses are in the ratio of the
relative migration velocities of the anion and kation.
This system gives approximately a picture of the
migration velocities of the ions of copper sulphate ; the
S0 4 ion migrates nearly twice as fast as the Cu ion, and
covers four units of space in the time that the Cu ion
migrates through two. The quotients 2 (6= -33 and
4 [6= '66 are named by Hittorf the transport ratios for
the concerned ions. This ratio, i.e. the relative velocities
of migration, is quite independent of the working force ;
the absolute velocities are on the contrary directly pro-
portional to it. The temperature and concentration of the
solution do not materially affect the figures.
As a result of these investigations Hittorf concluded,
that in salts of which the double cyanide of silver and
potassium K.Ag(CN) 2 is a type, potassium is the positive
ion and Ag(CN). 2 the negative, and that these are the
migrating ions ; while the separation of the silver from
the anion is the result of a secondary reaction.
When a mixture of two salts is electrolysed, the ions,
if they possess similar migration velocities, as for example
chlorine and iodine, share the current in the proportion in
which they are present in the mixed electrolyte ; the
separation at the electrodes may however occur differ-
ently.
27
CHAPTER V
THE CONDUCTIVITY OF THE ELECTROLYTE
HITTORP had often given expression, in his papers upon the
migration of the ions, to the opinion that a deeper know-
ledge of the real nature of electrolysis would be obtained
by determinations of the specific conductivities of the
different electrolytes. No reliable method of making such
determinations was however known to him. The con-
ductivity of a body is represented by the reciprocal of its
resistance. For conductors of the first class, the resistance
is dependent upon the form, the nature, and the temperature
of the material used.
The unit of specific resistance is the ohm, or the resist-
ance of a column of mercury 106-3 c.m. in length, and
1 sq. m.m. in sectional area at OC. (The Siemens unit
of resistance, represented by a similar column 100 c.m. in
length, is still occasionally used.) In connection with
liquids, it is customary to speak of the conductivities
rather than of the resistances, and to express these in the
reciprocals of the ohm.
If one dissolves the molecular weight in grams of any
salt in 1 litre of water, and brings this solution between
two parallel electrode surfaces, placed at a fixed distance
apart, the system will be found to possess a definite re-
sistance in ohms, and corresponding to this a definite
conductivity. These constants are named the molecular
resistance, and the molecular conductivity. The molecular
conductivity of an electrolyte increases with the tempera-
28 THEOEY OF ELECTROLYSIS
ture ; metallic conductors show the reverse phenomena.
The molecular conductivity increases also with the dilution
of the electrolyte ; but in this case the number of ions in
the unit of volume, and therefore the specific conductivity
of the electrolyte, is diminished. The maximum con-
ductivity is therefore found at that point of dilution where
the second effect commences to exceed the first.
In 1880 F. Kohlrausch published a useful method for
determining the relative conductivities of electrolytes.
The principle of this method consists in the use of the
Wheatstone bridge with alternating currents in order to
avoid the errors caused by polarisation ; a telephone is also
used to replace the galvanometer.
The conductivities of liquids in comparison with the con-
ductivities of the metals are very small. For example, such
a good electrolyte as 20 per cent, hydrochloric acid solution
at 18C. possesses a conductivity only 71*4 millionths of
that of mercury at 0C. The fraction for 30 per cent, sul-
69*1
phuric acid is - , for 25 per cent, sodium chloride
1,000,000
20
solution " , and for 10 per cent, copper sulphate
1,000,000
solution it is only
The relative conductivities have also been determined
for solutions which contain the equivalent weight of the
salt in grams dissolved in one litre of water ; these are
named the equivalent conductivities. 1
Kohlrausch found that the equivalent conductivities of
the neutral salts were additively composed of two values,
the one depending only on the anion, the other depending
only on the kation.
1 Further information upon conductivity can be obtained from
the following works : Lehrbuch der Allgemeinen Chemie, vol. ii.,
by W. Ostwald ; Theoretische Chemie, by W. Nernst ; Grundziige der
Elektrochemie auf experimenteller Basis, by E. Liipke ; Elektro-
chemie, by M. Le Blanc. The first-named most excellent book is
that recommended for the study of the theoretical side of the subject.
THE CONDUCTIVITY OF THE ELECTROLYTE 29
These values represented in fact the relative migration
velocities of the different ions.
Finally, the absolute values of the migration velocities
of single ions have been determined.
These, with a potential drop of 1 volt per c. metre at
18C., are as follows :
Potassium '00057 c.m. ; sodium -00035 c.m. ; hydrogen
00300 c.m. ; hydroxyl -00157 c.m. ; ammonium -00055
c.m. ; silver -00046 c.m. ; chlorine -00059 c.m. The ions
of hydrogen and hydroxyl are therefore those which move
most quickly.
30 THEOEY OF ELECTROLYSIS
CHAPTER VI
THE DISSOCIATION THEORY
VAN'T HOFF has shown in his theory of solution that
Avogadro's law may be extended to dilute solutions, and
that even the laws of gas volumes formulated by Boyle and
Gay Lussac are still correct when applied to dilute salt
solutions.
In other words, dissolved salts behave as gases. From
the law of osmotic pressure Van't Hoff deduced other laws
concerning the influence exerted by the dissolved salt upon
the vapour pressure and the freezing point of the solvent.
It was found, however, that all the acids, bases, and salts
dissolved in water gave, when the normal molecular
weights were accepted as correct, too high results for the
osmotic pressure, vapour pressure, and freezing point
determinations, or the molecular weights calculated from
these results were too small. In 1887 S. Arrhenius gave
the explanation of this discrepancy between the theoretical
and observed results.
He had in an earlier paper upon the conductivity of
electrolytes given expression to the view that two kinds of
molecules active and inactive are present, and that part
only of the active molecules conduct the current. The
ratio of the active molecules to the total number of mole-
cules present he named the coefficient of activity.
The comparison of the properties of the electrolyte as
regards the depression of its freezing point, and its ability
to conduct the electric current, led him to formulate the
THE DISSOCIATION THEOKY 31
theory so fruitful in its after results of the dissociation
of all bodies dissolved in water. The discrepancy in the
freezing point determination was, in the light of this theory,
seen to be caused by the splitting up of the salt into its two
component parts on solution ; these fragments, or disso-
ciated parts, giving too high a value to the gram molecule.
The degree of dissociation of a salt on solution, i.e., the
number of decomposed molecules, as determined by the
observation of the freezing point, was found to be in very
fair agreement with the number calculated from the elec-
trical conductivity.
Since only the decomposed molecules conduct, Arrhenius
assumed that these sub-molecules or ions carried electrical
charges, even in solutions which formed no part of an
electric circuit.
According to this theory of Arrhenius, the electric
current in passing through an electrolyte does not decom-
pose the molecules ; these are already present charged
with their respective electric charges in the ionic state.
Inversely, the conduction of the current by an electro-
lyte is dependent upon the presence of free ions ; those
molecules which have not undergone dissociation take no
part in the electrolysis. The activity of the sub-molecules
does not depend alone upon their conductivity, but, as
already remarked, upon the chemical affinity of the mole-
cule. When one dilutes an electrolyte with water, the
conductivity increases up to a certain point ; at and
beyond this point of dilution, all the molecules are to be
regarded as in the dissociated state. Anhydrous liquids,
such as 100 per cent, sulphuric acid, and concentrated
hydrochloric acid, &c., &c., do not conduct, because no disso-
ciated molecules are present. Chemically pure water is
also a non-electrolyte ; the specific resistance at 18C.
being 24'75xl0 10 mercury units, or, expressed in another
way, 1^ million litres of water contain only 1 g. hydrogen,
and 17 g. hydroxyl as ions.
It is noteworthy, therefore, that good conducting
32 THEOKY OF ELECTROLYSIS
liquids are formed by the solution of acids, bases, and salts
in water. One is obliged to assume that water possesses
in a peculiar degree the property of producing dissociation
effects ; for the water molecules remain in these salt solu-
tions practically unchanged, and do not share in the con-
duction of the current. Molten bodies act as electrolytes ;
in this case dissociation would appear to be an effect of the
increase of heat.
The fact that the ions possess charges of energy, differ-
ing from those of the corresponding atoms, and as a result
of this possess different chemical properties, has already
received mention in Chapter III (p. 23).
Whence comes then this property of water to effect
dissociation ? One may perhaps assume that since solution
is generally attended by a depression in temperature, there
is an absorption of energy from without, which has some
connection with the dissociation of the salt. Up to the
present, however, no satisfactory explanation has been
given as regards the origin of the electric charges carried
by the separate ions. lonisation is certain to cause some
conversion into other forms, of the original energy of the
atoms.
33
CHAPTER VII
THE CHEMICAL AND MOLECULAR CHANGES DURING
ELECTROLYSIS
ELECTROLYSIS is conditional upon the passage of measur-
able quantities of electricity into the electrolyte by its
boundary surfaces.
This occasions a movement of electricity through the
electrolyte, which is intimately connected with the move-
ment of the ions. The current causes the anions to drift
towards the anode, and the kations towards the kathode.
An accumulation of negative electricity at the anode
and of positive electricity at the kathode results, which
would speedily lead to a cessation of the current if this
excess of electricity and accumulation of ions at the
two electrodes were not destroyed. At the kathode posi-
tive electricity is drawn from the kations ; at the anode
negative electricity is abstracted from the anions : and this
withdrawal of the charges from the ions is followed by
their change into neutral bodies. Electrolysis, strictly con-
sidered, therefore occurs in the voltaic cell. Of the ions
which have delivered up their electric charges, only the
metals can exist as such, and these are to be regarded there-
fore as primary products of the electrolysis. All non-
metallic ions have but a short existence, and the sub-
stances which form at the electrodes are transformation
products of ions which have lost their electric charges, i.e.
secondary products. Examples of these are the molecular
gases chlorine, C1 2 , hydrogen, H 2 , &c. &c.
34 THEOKY OF ELECTEOLYSIS
The expenditure of work in effecting electrolysis is not,
as already explained, required to split up into ions the
molecules in the electrolyte, but is needed in order to effect
the liberation of the charges of electricity from the ions
at the electrodes. The amount of work demanded for this
cannot be calculated from the heats of combination of the
individual ions, but it is as a general rule proportional to
the sum of these.
It has been customary to distinguish between the
primary and secondary phenomena of electrolysis. Ostwald
rightly points out, that it is neither advantageous nor
logical to maintain this distinction longer. The electro-
lysis of potassium sulphate (see Chapter I) yields hydro-
gen and potassium hydrate at the kathode, oxygen and
sulphuric acid at the anode. If now the separation of the
oxygen and hydrogen at the two electrodes be regarded as
a secondary phenomena, and if one assumes that, first of all,
the ions of the salt K and SO 4 have actually separated
but have immediately reacted with the surrounding water,
one is met by the difficulty that a correspondingly higher
electro-motive force would be required to effect the de-
composition of this compound. Hydrogen and oxygen
could not, if this assumption be correct, be liberated at a
lower E.M.F. Observation has, however, shown that
these gases are liberated with a much lower E.M.F. than
that postulated. The expenditure of electro-motive force
corresponds, then, not to those reactions or products which
we have been in the habit of calling primary, but to the
final products of the electrolysis.
It is, however, necessary to distinguish between those
products which conduct the current and those which
separate at the electrodes. Only in few cases are these one
and the same, as for example in the fused chlorides lead
chloride and magnesium chloride.
In most cases the ions are unable to pass into the
neutral state as regards electrical charge without under-
going an alteration in their chemical constitution. When
CHEMICAL AND MOLECULAE CHANGES 35
different substances are present, the separation is deter-
mined by the electro-motive force necessary to effect it ;
those compounds which demand the least E.M.F. for
their separation will be first obtained, this result being
entirely independent of their classification, according to
the older views, as primary or secondary products.
When the solution to be electrolysed is a mixture of
various electrolytes, the proportion in which the different
ions share in the conduction of the current is governed by
two factors, the relative numbers of the different ions and
their migration velocities. This compound expression is
altered, however, when one comes to consider the separation
of the single ions, since the different ions do not give up
their charges of electricity with equal readiness.
With a slowly increasing electro-motive force, those ions
which relinquish their charges most easily are the first to
experience the change. For example, in a solution con-
taining chloride and iodide of potassium, chlorine and
iodine as ions arrive at the anode in exactly equal propor-
tions, on account of the equality of their migration velocities
(the anode must be of a material that is not attacked by
these gases) ; but only iodine is separated at first. If one
increases the E.M.F. a point is reached at which chlorine
is also liberated ; this is about '35 volt higher than the
first. Similar considerations regulate the deposition of
metals from mixed solutions. The order in which they
are deposited is as follows : Gold, platinum, palladium, silver,
mercury, copper, hydrogen, lead, nickel, cobalt, iron,
thallium, cadmium, zinc, manganese, aluminium, mag-
nesium. Gold and platinum are most easily deposited,
while zinc and aluminium are the most difficult to deposit
of the better-known metals. The metals of the alkaline
earths and the alkali metals can only be obtained as metals
under especial conditions, and by use of a mercury kathode.
The order of the above list corresponds to that of Yolta's
series, but it is dependent upon the electrolyte.
A clear view of this subject of the progressive deposition
D 2
36 THEORY OF ELECTROLYSIS
of the metals is most easily obtained, by assuming that
every ion possesses a definite and fixed force which tends
to keep it in the ionic state ; and that this force can be
measured and expressed in terms of the E.M.F., which is
necessary to effect a separation of the ion, i.e. its transfor-
mation into the neutral condition.
It is for this reason that iodine can be separated more
easily than bromine, and the latter more easily than chlorine ;
and the same reasoning explains the ordering of the metals
in the series already given. It is seen from this series
that the noble metals and those allied to them have a
distinct tendency to pass out of the ionic state ; whilst
those at the other end of the series have the opposite
tendency, and are always striving to enter into it. Hydro-
gen occupies a position midway between these two extremes.
The order in which the metals are deposited with a
slowly increasing E.M.F. varies with the nature of the
salt used for the electrolysis. For example, if an excess of
potassium hydrate be added to a solution of the neutral
salts of zinc or tin, an increase of from -5 to '7 volt is
necessary in the E.M.F. required to effect deposition as
compared with that required for the neutral solutions.
The explanation of this lies in the fact that these metals
form respectively zincate and stannate when excess of
potassium hydrate is added to solutions of their neutral
salts, and that these complex salts yield the zinc or tin
as anion when electrolysed ; 1 while only very small
amounts of zinc and tin are present as metals in the ionic
state. Other examples of this kind are the solutions of
the double oxalates and the double cyanides, which are so
frequently used for electrolysis.
If then one has a liquid containing two or more metallic
salts in solution, it ought to be possible, in view of the
above facts, to deposit the single metals one after the other
by the use of an extremely feeble current, which is
gradually increased by means of a higher E.M.F. ; that
1 Cf. Chap. I. p. 12.
CHEMICAL AND MOLECULAE>SM2^^ 37
is to say, an analytical separation of the metals by electro-
lytic methods should be practicable.
Freudenberg 1 has proved that this is possible for a
considerable number of the metals.
If the current be increased before the whole of the
more easily separated metal has been deposited, the second
metal will take part in the deposition, and the limit will
ultimately be attained at which the two metals arrive at
the electrode, and are separated, in the ratio expressed by
their relative migration velocities. A practical application
of this phenomena occurs in the electroplating industry,
when articles are coated with brass (copper and zinc).
With feeble currents, copper alone is deposited. In order
to obtain mixed metallic deposits of the required composi-
tion, it is necessary to pay careful attention to the nature
and quantity of the salts used, and to the current density
employed. An experiment illustrative of this deposition
of alloys of the metals may be easily performed as follows :
A mixed solution of the sulphates of copper and iron
(ferro-salt) containing a little sulphuric acid is electrolysed.
With the electrodes a certain distance apart, copper alone is
deposited at the kathode ; but if they be gradually moved
nearer to each other the resistance of the electrolyte is
reduced, the current density is increased, and a white alloy
of copper and iron is deposited if the conditions be exactly
right, or a black spongy deposit may be obtained.
The term ' current density ' is used to denote the current
strength or intensity divided by the area of the immersed
part of the electrode.
The unit of area generally used for electrolytic separa-
tions in the chemical laboratory is the square decimetre
( = 100 sq. centimetres = -107642 sq. foot). Current
densities expressed in terms of this unit are denoted
as 'normal densities,' and are generally written in this
form : ' KD. 100.'
The expression ' N.D. 100 = 1*5 A ' thus signifies that
1 Zeitsch. f. phys. Chemie, 1893, 12, 197.
38 THEORY OF ELECTROLYSIS
1'5 amperes of current is used for each 100 sq. centi-
metres of the electrode surface. For technical purposes
the square metre (= 10 '76 sq. feet) is used as unit of area.
[In the calculations of current density, for technical
purposes it is necessary to note carefully whether one or
both surfaces of the electrodes will take part in the electro-
lysis. As a rule, more than two electrodes are used in
each vat, and thus both surfaces of anode or kathode come
into play. Translator's noteJ\
It is evident from the foregoing consideration that the
maintenance of a fixed current density is of great import-
ance. The influence which the current density exerts is
manifested in two directions, both the nature of the pro-
duct and the quality of the deposit being dependent upon it.
The latter influence is especially noticeable in the electro-
lysis of metallic salt solutions. As an example of the
former, the statement that, according to the current density
employed, either copper or cuprous chloride may be
obtained on electrolysing a cupric chloride solution is to be
noted. Palladium and molybdenum are obtained either as
metals, as oxides, or there may be no deposit at all, accord-
ing to the current density used for the electrolysis of their