is a coloured one as, for example, that of copper or of the
peroxides the observation is carried out by adding a little
distilled water to the electrolyte, and thus covering a fresh
portion of the kathode surface with the liquid. This newly
covered surface is examined after an interval of five or ten
minutes ; if no trace of a deposit be visible, the electrolysis
is completed. When the basin electrode has been used,
instead of adding water to the electrolyte, a small strip of
clean platinum foil is hung over the edge of the basin, so
that it dips into the liquid.
When the jacket electrode is used, it is extremely easy
to change the first electrode for a second one free from any
deposit. These methods are, however, not applicable in the
case of those metals the deposits of which differ little from
that of the platinum in colour as, for example, the deposits
of cobalt, nickel, iron, and silver. In these cases the test
is usually made by withdrawing a small portion of the
electrolyte by means of a small pipette, and by applying to
this small test portion some delicate chemical test. If no
reaction for the concerned metal is obtained, one concludes
that the last traces have already been removed from the
electrolyte by the action of the current.
When the electrolysis is completed, the circuit is broken,
the electrodes are freed from the binding screws, and the
liquid is poured out of the basin, or the jacket electrode is
lifted out of the liquid in the beaker. The portion of the
liquid that clings to the deposit is washed away with dis-
tilled water, and the water is then itself removed by rinsing
the deposit with strong alcohol. The final drying is accom-
plished either over the open flame, upon a heated asbestos
or iron plate, in the air-bath at 100 C., or in a desiccator,
according to the nature and character of the deposit. The
alcohol is used to prevent, as far as possible, the oxidation
of the moist metallic coating.
In many electrolytic depositions the breaking of the
circuit leads to a re-solution of the deposited metal by the
electrolyte, and in these cases it is necessary to displace
and wash out the electrolyte before breaking the circuit.
This is accomplished by hanging over the side of the vessel
90 THE ELECTEOLYTIC PROCEDURE
a small siphon filled with water, and by adding distilled
water to the vessel as the original liquid is carried away
by this siphon. A conducting medium thus remains be-
tween the two electrodes, the current continues to pass in
the original direction, and only after suitable dilution of
the original electrolyte is the circuit broken, and the elec-
trode with its deposit removed for further treatment in the
usual way. When the jacket electrode has been employed,
one may avoid using this displacement method of removal
of the electrolyte by a quick withdrawal of the electrode
from the electrolyte. The former method ought to be used
for deposits of copper from nitric acid solutions, of antimony
from sodium sulphide solutions, of lead peroxide from nitric
acid solutions, and of other metals ; but one may even in
these cases substitute for it a rapid emptying of the basin
without introducing any serious error into the results.
The weight of metal which can pass into solution again
during the few seconds required for pouring off the electro-
lyte and diluting the remainder, varies with the metal, the
solution, arid the duration of the operation ; but experiments
have shown that it is, as a rule, between -0004 and '001
grm. The electrode bearing the dry deposit must never
be weighed until it has cooled down to the temperature of
the balance room.
/ Since a repetition of any experiment is only possible
' when data concerning the concentration and temperature
of the electrolyte, the strength and electro-motive force of
the current, and the superficial area of the electrodes have
been kept, it is advisable even from the commencement of
the experiments to record all these details, as well as the
observations made during the progress of the experiment,
and the results obtained, in some such form as the follow-
Experiment. (Example Copper, or separation of
copper and lead.)
Electrolyte. (1 grm. CuS0 4 ; 5 c.cms. HNO 3 ; 150
c.cms. H 2 O.)
Source of Energy. (Two accumulator cells in series.)
Electrodes, (Basin jacket electrode or foil.)
OBSERVATIONS AND MEASUREMENTS
in ohms '
Current density (per 100 Character of the deposit
sq. cms.) Used for analysis
DEPOSITION OF THE METALS FROM
SOLUTIONS OF PURE SALTS
In the following division of this work a number of dif-
ferent methods are given under the headings of the indivi-
dual metals, by which the separation of the concerned metal
can be effected.
These will be found to include not only electrolytic
methods of a particular class, but also practically all the
methods in actual use, whoever may have been their
Those methods, which are easily carried out, and which,
in some cases by variation in the conditions, can be made
to yield reliable results, have received especial attention ;
since such methods have been adopted in some technical
laboratories as substitutes for the older analytical pro-
In all cases the necessary information concerning
92 THE ELECTROLYTIC PROCEDURE
current density, concentration, temperature, and voltage
has been given. A very full list of references to original
papers will facilitate the consultation of these when neces-
sary, and will enable the reader to obtain a comprehensive
survey of the methods proposed up to the present date. It
will also serve to check the very common rediscovery of old
methods by new workers in this branch of science.
Those separations which are of technical importance
have been indicated, so that the choice of methods for prac-
tice by the novice has been simplified as far as possible.
Copper was the first metal of which proof was given
that it could be deposited quantitatively by electrolysis.
Electrolysis was thus shown to be applicable to analytical
Copper is distinguished from other metals by the ease
with which it can be deposited from acid solutions, and by
the character of the deposit so obtained. This is nearly
always bright red in colour, and of a metallic lustre. The
position of copper below hydrogen in the series of metals
given in Chapter VII (p. 35) signifies that the metal can
be deposited without difficulty, especially from solutions
containing free acid. In such solutions the decomposition
value of the salt of copper is comparatively low, and the
deposition can be effected with a very small expenditure
An E.M.F. of 1*8 volts suffices to separate copper from
solutions containing nitric acid ; from solutions containing
ammonia a voltage rather lower than this is sufficient ;
while from solutions containing ammonium oxalate only
1 -5 to 1 -6 volt is required.
Solutions of copper sulphate or copper nitrate containing
free nitric acid are especially suited for laboratory electro-
lysis ; l but the amount of free acid present must not be
1 Zeitschr.f. anal, Chem. 19, 1.
DEPOSITION FROM PURE S^K^^^^T 93
allowed to exceed 8 to 10 per cent. 1 For the carrying out
of such an electrolysis a weighed amount of copper sul-
phate, generally about 1 grin., is dissolved in water, the
solution is diluted to about 150 c.cms., and from 3 to
5 per cent, by volume of cone, nitric acid (sp. gr. 1*40) is
added to it. The solution is heated to 50 or 60 C. in
the basin or in the beaker in which the electrolysis is
to be performed, and a current of about 1 ampere
is passed through it. The E.M.F. should be from 2 to
The reddish deposit of metallic copper upon the kathode
can be noticed immediately the circuit is completed. The
current is allowed to continue until the blue copper solution
appears to have lost all its colour ; this result should be
obtained in the course of about two hours. Before the
circuit is broken, however, it is necessary to prove that the
whole of the copper has been deposited. The complete
separation of the copper contained in 1 grm. of copper
sulphate, which contains theoretically 25'33 per cent. Cu,
should require, with the strength of current named above,
from two to three hours. In order to test whether in a par-
ticular case traces of copper are still present in the solu-
tion, sufficient water is added to the electrolyte to cause
the immersion of a clean portion of the kathode. As the
reddish colour of the deposited copper is very distinct
against the platinum kathode, a ready means of checking
the completion of the deposition is afforded.
After the addition of water to the electrolyte, the current
is allowed to continue for ten or fifteen minutes, and the
freshly immersed surface of the kathode is then examined
for a thin coating of copper. A more delicate method of de-
termining when the electrolysis is complete consists in the
withdrawal of a very small quantity of the electrolyte by
means of a glass tube which has been drawn out at one end
to a fine jet, and in the testing of this by chemical methods.
The two reactions made use of for this purpose are those
1 Berg- u. Hiitten-Zcit. 21, 220.
94 THE ELECTROLYTIC PROCEDURE
with sulphuretted hydrogen and potassium ferrocyanide ;
the test with ammonia is not sufficiently sensitive for such
small amounts of copper. The liquid that has been with-
drawn is treated with a few drops of sodium sulphide
solution, after addition of acid if it be a neutral or alkaline
solution of copper; the presence of traces of copper is
indicated by a distinct brown colour of the solution. The
use of sulphuretted hydrogen gas is less convenient. In
order to apply the potassium ferrocyanide test, a few drops
of the solution of this salt are placed in each of two test
tubes, the liquid in each is acidified with a little hydrochloric
acid, and the solution withdrawn from the electrolyte is
added to the contents of one of these test tubes. On
viewing the tubes against a white background, the presence
of a very small amount of the reddish ferrocyanide of
copper is easily detected. One can also mix finely ground
potassium ferrocyanide with a drop or two of hydrochloric
acid upon a porcelain tile, and allow the solution which is
to be tested to fall in drops into the middle of the mixture ;
a reddish coloration will appear at the edges of the mixed
solutions if traces of copper be present.
When it has been proved that the whole of the copper
has been removed from the solution, it is simply necessary,
if a jacket electrode has been used, to remove this from the
holder, to lift it quickly out of the solution, and to wash it
under a running stream of water in order to free it from
the adherent acid liquid. When a basin has been used as
negative electrode, this would demand some little time, and
it is necessary to proceed differently if all danger of the
acid liquid acting upon the deposit of copper is to be
avoided. In this case the copper must be washed before
the current is discontinued. This is effected by using a
siphon to remove the electrolyte from the basin, while
water is allowed to flow in. In this way the acid in the
solution becomes diluted to a point at which action upon
the deposit is impossible. The contents of the basin are
then emptied out, it is rinsed a few times with water, and
DEPOSITION FROM PURE SALT SOLUTIONS 95
lastly with strong alcohol, and quickly dried in an air bath
or by the naked flame.
The solvent action of the remaining electrolyte upon
the deposited metal is, it must be admitted, not very
great, since a portion of the nitric acid present at the
commencement of the electrolysis will have been converted
into ammonia. Experiments made with a copper sulphate
solution to which 8 per cent, of nitric acid had been added,
which was electrolysed for an hour with a current of
1 ampere, showed that '0004 grm. of the deposited copper
was dissolved in each case by the remaining solution in a
quarter of a minute from a surface of 100 sq. centimetres.
An expert analyst would never require such an interval
of time for disconnecting and emptying the basin ; and,
further, the loss of '0004 grm. would be unimportant in its
effects upon the results of technical analyses.
The proposal of Riidorff to add sodium acetate to the
electrolyte, instead of breaking the circuit in order to carry
out the washing, is likewise a useful one.
The deposit of copper obtained from a solution con-
taining free nitric acid possesses a bright red colour, and is
of crystalline structure. If the current be passed for a
considerable time through such a solution, a portion of the
nitric acid will be itself decomposed with formation of
ammonia ; this ammonia neutralises a further portion of
the free acid, so that the amount of free acid present
diminishes as the electrolysis proceeds.
The separation of copper from its solution containing
free nitric acid can also be effected by allowing a weak
current to act for a longer period of time. Thus, the
solution of which details have been given above may be
left overnight with a current of from '2 to '3 ampere
passing through it ; the deposition will be complete by the
next morning, and no heating of the solution prior to the
commencement of the electrolysis will have been necessary.
In this case it is, however, requisite to increase the
amount of free nitric acid added to the solution ; an
96 THE ELECTROLYTIC PROCEDURE
addition of 10 c.cms. for a volume of 150 c.cms. of the
solution suffices. In the presence of insufficient free
acid, and of increase of the ammonia contents of the
solution, the copper is deposited in a brown spongy form,
which adheres so slightly to the electrode that, even on
washing, portions become loosened and are lost. Such bad-
coloured spongy deposits are not adapted for weighing,
and the results obtained with them are inaccurate.
The fact that it is possible to obtain satisfactory
deposits of copper from the solutions containing free nitric
acid, with current densities up to and over 3 amperes, is
worthy of notice.
In the same manner that copper may be separated
from solutions of its sulphate after addition of nitric
acid, it is possible to obtain useful deposits from solutions
containing free sulphuric acid. 1 The amount of acid
present must, however, not exceed 8 to 10 per cent.
In order to carry out this method, 1 grm. of copper sul-
phate is dissolved in about 150 c.cms. of water, 2 to 3 c.cms.
of cone, sulphuric acid (or a corresponding amount of dilute
acid) are added, and the solution is then electrolysed at the
normal temperature with a current density of 1 ampere. In
the course of one and a half or two hours the whole of the
copper will be separated as a red deposit. When the
amount of copper is considerable, this deposit is never so
brilliant in colour as the deposit obtained from nitric acid
solutions, but it nevertheless yields exact results.
The E.M.F. required for the electrolysis of sulphuric
acid solutions is from 2 '5 to 3 volts.
It is possible by warming the solution bo lessen the
time required for this electrolysis. This diminishes the
resistance of the electrolyte, and if the current con-
ditions remain unaltered in the external circuit, the
current will consequently increase, and with it the amount
of deposit, in a definite time. When using solutions con-
1 Luckow, Dingl. polyt. Jour. 1865, 177 ; Gibbs, Zcitschr. f. anal.
Chem. 3, 334.
DEPOSITION FROM PUEE SALT SOLUTIONS 97
taining free sulphuric acid in the proportion described
above, it is not wise to allow the current density to exceed
1'5 amperes per 100 sq. centimetres, because with higher cur-
rent densities, in spite of the free acid present, the deposit
has a tendency to separate in a less compact and some-
what spongy form. In order to gauge the end of the
electrolysis, the same reactions are made use of as with
nitric acid solutions.
The same precautions with regard to stopping the
current and washing the deposit must also be observed.
The deposition of copper from a sulphuric acid solution
was the first example of the use of electrolysis for technical
Gibbs, in the year 1864, determined the copper in
copper-nickel coinage by this method, and in 1865 Luckow
made use of this same process for the determination of
copper in the metal used for the fireboxes of locomo-
One would expect that copper could also be obtained
as a beautiful deposit from the neutral sulphate solution,
since in this case the electrolyte gradually becomes acid
owing to the formation of sulphuric acid at the anode.
This assumption is found to be correct ; but the resistance
of the neutral electrolyte is so great that an E.M.F. of 6
volts is requisite at the commencement in order to obtain
a current strength of '5 ampere. Heating this electrolyte
does not much increase its conductivity.
For practical work, it is therefore better to lessen the
resistance by the addition of a few cubic centimetres of
nitric or sulphuric acid.
Luckow and Drossbach have made attempts to elec-
trolyse copper sulphate solutions to which excess of ammo-
nia had been added ; but bright deposits could only be ob-
tained when very feeble currents were employed. Oettel 1 and
MacCay 2 found, however, that the conditions were much
1 Chem. Zeitg. 1894, 879 ; Zeitschr.f. EkUtrocliem. 1894, 142.
2 Chem. Zeitg. 1890, 509,
98 THE ELECTROLYTIC PROCEDURE
improved by the addition of definite amounts of ammonium
nitrate to the solution.
In order to carry out this electrolysis 1 grm. of copper sul-
phate and 4 grms. of ammonium nitrate should be dissolved
in water, and ammonium hydrate should be added until it is
present in slight excess. The deep blue solution is diluted
until it occupies about 150c.cms., and it is then electrolysed
at the normal temperature with a current of from -1 to -3
ampere. The E.M.F at the commencement of the electro-
lysis is about 2 volts ; during its course, however, it rises to
about 3 volts. At the end of a period of from six to seven
hours the deposition is complete, the copper being obtained
as a brilliant metallic coat. If the amount of ammonium
nitrate be decreased, or if that of ammonium hydrate be
increased, there is some danger of the copper separating in
the brown and spongy form. This method can be employed
for effecting copper separations during the night.
While copper can be separated in bright metallic form
from solutions of the sulphate and nitrate containing free
sulphuric or nitric acids, experiments with cupric chloride
and hydrochloric acid have shown that the solution of this
salt is not suited for analytical purposes, since the copper
has a tendency to separate from it in the spongy form.
It may be noted here that it is necessary to avoid as
far as possible the use of chlorides for electrolytic work if
metal electrodes are to be employed, as the free chlorine
produced by the electrolysis has always some action upon
Kiidorff ] has nevertheless found that a useful deposit
can be obtained from solutions of the chloride to which
ammonium nitrate and excess of ammonium hydrate have
been added. To obtain a deposit from such a solution, -5
to 1 '0 grm. of cupric chloride and 4 to 5 grms. of ammo-
nium nitrate are dissolved in 100 to 125 c.cms. of water, and
25 to 30 c.cms. of ammonium hydrate are added. This solu-
tion is then electrolysed at the normal temperature with a
1 Berichte, 21, 3050.
DEPOSITION FROM PUEE SALT SOLUTIONS 99
current density of about 1 ampere ; the E.M.F. required is
from 3-3 to 3-6 volts.
To effect the separation of the copper contained in 1 grm.
of cupric chloride, from two and a half to three hours are
necessary. During the electrolysis the temperature of the
solution is found to increase slightly, and it is therefore
better to carry out the deposition at a temperature of
50 C. by means of a feebler current. The deposit, which is
at first a brilliant red, becomes later dull. In spite of this
change in its character, it is, however, not spongy, and
it is well suited to analytical requirements.
The employment of copper salt solutions to which an
excess of potassium cyanide had been added dates back
very many years, since in 1840 Ruolz had suggested the
use of this form of solution for electrotyping purposes.
Later it was suggested by Luckow l and Moore 2 for ana-
The solution is prepared by dissolving 1 grm. of copper
sulphate in a small amount of water, and by adding suffi-
cient potassium cyanide solution 3 to this, to cause the
re-solution of the precipitate of cupric cyanide which is at
The solution is then made up to 125 to 150 c.cms. by the
addition of water. The double cyanide of potassium and
copper which is present in this solution requires at least
2-2 volts for its decomposition. If a current of 1 ampere
be passed through it at the normal temperature, an E.M.F.
of from 5-2 to 5 -8 volts will be found at the terminals ;
if, however, the electrolyte be heated to 60 C., the same
current density at the kathode can be attained with an
E.M.F. of 4-2 volts. The copper contained in 1 grm of
copper sulphate will be completely deposited in an hour and
a half under these conditions. The deposit obtained from
1 Zeitschr. f. anal Chem. 19, 1.
2 Chem. News, 1886, 53, 209.
3 The purer the potassium cyanide the better the results. The
commercial article is too impure to be used for this purpose.
100 THE ELECTROLYTIC PROCEDURE
cyanide solutions of copper does not possess the crystalline
structure characteristic of the deposits obtained from nitric
acid solutions, but is a pale rose-coloured homogeneous
The whole of the copper will be deposited from a solution
heated to 60 C. in two hours, if a current of \ ampere
should be used in place of the 1 ampere named above.
This method of deposition from cyanide solutions is also
adapted for use with still feebler currents ; and it is also
useful when circumstances prevent close and constant
attention to the course of the electrolysis.
The use of the double oxalate salt of copper and
ammonia for electrolytic separations of the copper has been
discussed by Classen and von Reiss l and by Classen and
Bongartz. 2 It has been found that better results are
obtained by use of this salt in solutions made acid with
oxalic acid than in the originally used neutral solutions.
In order to carry out this electrolysis, 1 grm. of copper
sulphate and 4 grms. of ammonium oxalate are dissolved
separately in water. On mixing these two solutions, the
precipitate which first forms is seen to redissolve. The
solution is heated to between 50 and 60 C., and a current of
l to 1 ampere intensity is passed through it, after first adding
some few cubic centimetres of a saturated solution of oxalic
acid, sufficient to cause the electrolyte to give a distinctly
acid reaction with litmus paper. In the absence of this
addition of oxalic acid, the deposit obtained quickly changes
to a brown and spongy form ; whilst if too great an excess of
oxalic acid be used, an insoluble form of copper oxalate
separates at the kathode. It is on this account advisable
to add the oxalic acid in small amounts only, from time to
time, during the electrolysis. The oxalic acid is decomposed
by the current with formation of carbon dioxide. A current
of 1 ampere will deposit the copper contained in 1 grm. of
copper sulphate from this electrolyte at a temperature of
50 to 60 C. in two hours, the E.M.F. necessary being from
1 Berichte, 14, 1627. * Ibid. 21, 2898.
DEPOSITION FROM PUEE SALT SOLUTIONS 101
2-8 to 3'2 volts ; while with a current of half this intensity
the deposition would occupy two and a half hours, and the
E.M.F. required would fall to between 2'5 and 2-8 volts. This
method gives bright metallic deposits (the contrary assertions
on this point are incorrect) when exact attention is paid to
the necessary precautions, especially to those concerning the
addition of the acid ; that is to say, continuous and close