separate these metals by means of a current of moderate
density in an acid solution, it will be found that both metals
are deposited simultaneously at the kathode. Copper is,
however, deposited alone when the E.M.F. does not exceed
1*8 volts ; and it is therefore possible to obtain a deposit of
copper free from antimony, especially when the amount of
the latter metal present is small, if the electrolysis be not
allowed to continue for too long a period that is, if it be
stopped as soon as the whole of the copper is removed from
the solution. Such a method of separation cannot, however,
SEPARATION OF METALS 187
be recommended. Antimony is a metal that resembles
arsenic in its behaviour in alkaline solutions. One can
therefore separate copper and antimony in an ammoniacal
solution, especially if the precaution be taken to raise the
antimony salts present to the higher stage of oxidation by
means of nitric acid. If this precaution be omitted, anti-
mony may be deposited at the kathode with the copper, as
in the case of an antimony trichloride solution, which has
simply been treated with excess of ammonium hydrate.
Schmucker has effected the separation by this method, by
using an oxidised solution of the two metals to which 8
grms. tartaric acid and 30 c.cms. ammonium hydrate have
been added. l The electrolysis is carried out with a current
of '10 ampere density, and five hours is requisite to deposit
10 grm. copper. The latter metal will be obtained quite
free from antimony.
This method may also be used to effect the separation
of copper and tin. The separation of copper from antimony
and tin by electrolytic methods is not, however, of any
technical importance, since this separation is so easily
effected by the chemical method with nitric acid.
Lead from Copper. (See p. 177.)
Lead from Silver. Lead is deposited as peroxide at the
anode from solutions" containing free nitric acid ; and it is
therefore possible to effect separation of lead from those
metals which are deposited at the kathode from such solu-
tions. The method of separation of copper from lead is
based upon this principle, but when one employs a similar
method to effect the separation of silver and lead the silver
exhibits a disturbing characteristic, in that under certain
conditions it separates partly at the anode as peroxide.
This, of course, prevents any quantitative separation of the
two metals, for the lead peroxide deposit is contaminated
1 Jour. Amcr. Chem. 15, 195.
188 THE ELECTEOLYTIC PROCEDURE
Since it is possible, however, under certain conditions
(see ' Silver 3 ) to obtain deposits of this metal by use of
nitric acid solutions without any separation of silver per-
oxide, investigations have been made to discover the condi-
tions which regulate the deposition of the silver in presence
of lead. Luckow has stated that these are the presence of
at least 18 per cent, free nitric acid, and the addition of a
small amount of oxalic acid. l With such an electrolyte the
lead peroxide obtained at the anode is free from silver.
Smith and Moyer state that if 15 c.cms. nitric acid be present
to each 180 c.cms. of the solution, and if a feeble current
be used, equally good results may be obtained. In spite of
these results this method of separation for silver and lead
cannot be regarded as absolutely trustworthy.
Lead from Bismuth. In the attempts that have been
made to effect the electrolytic separation of these two
metals, the same phenomena are found to occur as in the
case of silver and lead. The bismuth is deposited always,
partly as metal at the kathode, and partly as peroxide at
the anode. No separation is therefore possible in a nitric
acid solution ; the lead peroxide deposit, according to Classen
and Ludwig, 2 and Smith and Moyer, 3 always contains
Lead from Mercury. Although in nitric acid solu-
tions lead is always deposited as peroxide, while mercury
is always obtained as metal, it is not always possible to
effect a separation of these two metals in such a solution.
If less than a 15 per cent, excess of free nitric acid
be present, and if moderately strong currents be used,
Smith and Moyer state that part of the lead will be
deposited as an amalgam at the kathode. 4 According to
Heydenreich. the conditions necessary in order to obtain a
complete separation of lead and mercury are the presence
of between 20 and 30 c.cms. free nitric acid in the 150 c.cms.
1 Zeitschr. f. angew. Chemie, 1890, 345.
2 Berichte, 19, 326.
. 3 Jour.f. anal. u. appl. Chem. 1893, 7, 252.
1 Ibid. 1803, 7, 252 ; Zeitschr. f. anorg. Chem. 4, 267.
volume of the electrolyte, and the use of a current of about
20 ampere in density. l
Lead from Arsenic. If a solution containing lead
nitrate, a soluble salt of arsenic acid, and free nitric acid be
electrolysed, the separation of the lead as peroxide at the
anode will be found to be nearly always incomplete.
The deposit obtained during the same time at the
kathode will be a mixture of arsenic and lead, while another
portion of the arsenic will be evolved at the kathode surface
as arseniuretted hydrogen.
The greater the amount of arsenic in the solution, the
less will be the amount of lead separable at the anode
as peroxide ; the excess of nitric acid present also affects
this result. If the electrolysis be continued, after all the
lead has been separated from the solution either as metal
at the kathode or as peroxide at the anode, part of the
former redissolves and migrates to the anode, where it is
deposited as peroxide ; but the author has made experi-
ments which prove that the separation of the lead at the
anode is never complete. 2
Lead from Manganese. From acid solutions manganese
separates as peroxide at the anode, and, since lead yields a
similar deposit in nitric acid solutions, one would surmise
that the electrolysis of a solution of the mixed salts of
these metals containing free nitric acid would yield a
mixed deposit of peroxides at the anode. This is found by
experiment to be the case. If the excess of nitric acid
added to the electrolyte be, however, over 4 per cent.,
no deposition of manganese peroxide occurs at the anode ;
in place of this there is a formation of permanganic acid,
recognisable by the pink coloration which it produces
round the anode. If a solution of the two salts containing
about 20 per cent, nitric acid be electrolysed at the normal
temperature by means of a weak current, the deposition of
lead peroxide will take place slowly, and the solution will
1 Zeitschr. f. Elektrochem. 1896, 3, 151.
2 Chem. Zeitg. 1896, 20, No. 39.
190 THE ELECTROLYTIC PROCEDURE
remain colourless. If, however, the electrolyte be heated
to 60 or 70 0., and the electrolysis be carried out with a
current of from 1-5 to 2*0 amperes in density (E.M.F.
2-5 to 2-7 volts), the whole of the lead will be deposited as
peroxide in a short time, and the liquid will assume a rose
colour owing to the formation of permanganic acid and its
salts. The method yields approximately accurate results
when carried out as described above, and when the amount
of manganese present does not exceed '03 grm. for 150 c.cms.
of the electrolyte. If the manganese present exceeds this
amount, or if the electrolysis be permitted to continue for
too long a time, the author has found that a flocculent pre-
cipitate of a hydrated manganese peroxide is formed, and
that the lead peroxide deposit is no longer free from the
other metal. 1
Lead from Zinc, Iron, Nickel, Cobalt, and Cadmium.
Lead can be separated in a very simple manner from all
those metals placed above hydrogen in the list given on
p. 35 which cannot be deposited in acid solutions. The
solution of the mixed salts simply requires to be acidified
with 15 to 20 per cent. cone, nitric acid, and to be electro-
lysed with the current conditions given under ' Lead ' (see
Lead peroxide will be deposited at the anode, while the
other metal remains in solution. The separation is com-
plete. After deposition of the whole of the lead, the
remaining liquid, which will still contain much free nitric
acid, is treated with the chemical reagents necessary to
produce the salt of the metal present, that is recommended
for use under the single metal separations. In few cases
only is neutralisation sufficient ; and a conversion of the
nitrates into sulphates will be found to be necessary in the
greater number of instances.
1 Chem. Zeitg. 1896, 20, No. 39.
SEPARATION OF METALS 191
Silver from Copper. (See p. 181.)
Silver from Lead. (See p. 187.)
Silver from Bismuth. Freudenberg has stated that if
a solution of the nitrates of silver and bismuth (about *30
grm. each metal) be treated with 2 to 3 c.cms. nitric acid,
and, after addition of 2 to 4 grms. ammonium nitrate and
dilution to 150 c.cms., the mixture be electrolysed with a
current the E.M.F. of which does not exceed 1'3 volts, an
electrolytic separation of these two metals will be obtained.
If the electrolysis be permitted to continue through the
night, '30 to '40 grm. silver may be easily deposited. The
remaining electrolyte which contains the bismuth is used
for the deposition of the latter by the amalgam method.
Silver from Mercury and Gold. This separation
cannot be effected either by the use of nitric acid solutions
or by the use of cyanide solutions, since the ' decomposition
values ' of these salts of the concerned metals lie too close
one to the other.
The electrolytic determination of silver and mercury
may, however, be carried out as follows. The two metals
are deposited together from a solution at the normal
temperature by means of a current of *50 ampere density.
The E.M.F. required will lie between 1*7 and 2'2 volts;
and for '30 grm. silver about four and a half hours will be
necessary to effect complete deposition. After drying, the
weight of the combined metals on the electrode is deter-
mined ; the mercury is then driven off by ignition, and the
weight of the remaining silver obtained. The deposit of
the two metals is grey in colour and spongy in character ;
but in spite of this the method yields correct results.
Silver from Antimony and Arsenic. The separation of
silver from these metals is possible by electrolytic methods
if solutions containing free nitric and tartaric acids be used,
and if the antimony and arsenic present be previously
raised by chemical methods to the higher stage of oxidation.
192 THE ELECTROLYTIC PROCEDURE
Under these conditions it is not safe, however, to exceed an
E.M.F. of 1-5 volts.
The deposit of silver obtained is not very well suited
for correct weighing. In the case of arsenic the E.M.F.
used may be slightly greater than in the case of antimony ;
but 1*7 volts must not be exceeded even in this case.
Silver may also be separated from arsenic and antimony
in a solution which contains free ammonium hydrate and
ammonium sulphate, since from such a solution silver can
be deposited by means of an E.M.F. of 1'20 or 1-30 volts.
This low E.M.F. causes, however, the deposit of silver to
be but loosely adherent to the platinum basin.
The separation of these metals may also be carried out
with solutions containing 1*0 grm. pure potassium cyanide
for each "10 grm. metal present. Freudenburg has stated
that from such a solution the silver can be obtained as a
firmly adherent deposit. The E.M.F. used may be some-
what higher than in the case of the nitric acid solution, but
it must not exceed 2-4 volts. As before, it is best to raise
the arsenic and antimony to the higher stage of oxidation
before commencing the electrolysis. Smith states that the
separation of silver from these metals by this method suc-
ceeds perfectly when tartaric acid is used in excess in the
Silver from Platinum and Palladium. In order to
effect the separation of silver from the first of these metals,
the solutions of their mixed salts is neutralised, and after
the addition of 2 or 3 grms. potassium cyanide it is electro-
lysed with a current, the E.M.F. of which does not exceed
The separation of silver from palladium is not possible
in this way.
Silver from Cadmium, Zinc, Cobalt, Nickel, and
Iron. The separation of silver from the metals which
cannot be deposited in an acid solution, of which those
named above are examples, is conveniently carried out by
1 Amer. Chem. Jour. 12, 428.
SEPAKATION OF METALS 193
electrolysis of the solution of the mixed salts, after acidify-
ing with nitric acid.
The E.M.F. used should lie between 2'0 and 2 "2
Solutions of the double cyanide salts containing an
excess of potassium cyanide (2 to 3 grms. pure KCN) may
also be used to effect the separation of silver from the
metals named above.
This method is to be preferred to that first described
since the deposit of silver obtained by it is more satisfactory.
The separation of silver from zinc can be effected in such
a solution if the E.M.F. used does not exceed 2*5 volts.
The current density possible with this E.M.F. is between
05 and '08 ampere ; the temperature should be 60 C.
The same conditions apply in the separation of silver from
nickel, but in this case if the electrolysis be allowed to con-
tinue for too long a period nickel may be deposited with
the silver. The presence of a cobalt salt in the electrolyte
renders it more difficult to effect the separation of the silver ;
and in this case the E.M.F. used may rise to a maximum of
2*7 volts. In the case of cadmium the ' decomposition values '
of the two double cyanide salts lie very near together, and in
order to obtain the silver free from cadmium it is necessary
to use an E.M.F. of only 1'9 volt. The current obtained
by use of this E.M.F. will be only -04 ampere in density. .
In all these cases it is best to use the solutions at a
temperature of between 50 and 60 C. In order to deter-
mine by electrolytic methods the amount of the second
metal in the solution, it is necessary after complete deposi-
tion of the silver to treat the remaining electrolyte with
sulphuric acid under the draught-hood, and then to apply
the method which is most strongly recommended for the
concerned metal in Part III., B. .
This metal is Closely .related to silver in its electrolytic
characteristics ; and, its separation from the other metals
194 THE ELECTEOLYTIC PROCEDURE
is effected by methods very similar to those used for silver.
In nitric acid solutions an E.M.F. of only 1'3 volts suffices
to produce a deposit of mercury.
Mercury from Copper. See p. 184.
Mercury from Lead. See p. 188.
Mercury from Silver. See p. 191.
Mercury from Bismuth. In spite of all assertions to
the contrary, the separation of these two metals can be
effected in solutions of their nitrates containing an excess
of nitric acid, if, as Freudenberg has pointed out, the
E.M.F. of the current used does not exceed 1'30 volt.
Although the current density obtained with this
E.M.F. is extremely small, and as a consequence the time
demanded for the electrolysis is rather long, the method is
a practicable one. The deposit of'mercury obtained is not
composed of minute globules, but is a smooth metallic
If stronger currents be used the two metals will be
simultaneously deposited as an amalgam, a fact which is made
use of in the electrolytic method for determining bismuth.
Mercury from Arsenic and Antimony. The separation
of mercury from these two metals can be effected, if the
electrolysis be carried out with solutions containing free
nitric acid by means of currents, the E.M.F. of which does
not exceed 1*8 volts. A solution of the mixed salts con-
taining tartaric acid and an excess of ammonium hydrate
may also be used, if the arsenic and antimony are present
in the form of their higher oxides.
The current conditions in this case are as above. In
order to carry out such a separation, the chlorides are
dissolved with the addition of 1 grm. tartaric acid,
the solution is diluted, and after neutralisation with
ammonium hydrate a further 20 c.cms. of this reagent is
added. The mixture is then electrolysed by a current the
E.M.F. of which is kept between 1'60 and 1'70 volts.
In order to determine the antimony in the electrolyte
remaining when the first method is used, the excess of
SEPAEATION OF METALS 195
nitric acid must be carefully evaporated, and sulphuretted
hydrogen then passed through the diluted solution. The
precipitate of antimony pentasulphide is then dissolved in
sodium sulphide, and the electrolysis of the resulting solu-
tion is conducted as described under ' Antimony.'
Mercury from Tin. It was stated under ' Mercury '
that it was possible to completely deposit that metal from
the alkaline solutions of its sulphide in sodium sulphide,
while under ' Tin ' it was noted that the latter metal could
not be deposited from such solutions. A method of separa-
tion may therefore be based upon this difference.
If both metals be present in solution in presence of free
alkali and excess of sodium sulphide, it is merely necessary
to employ the current conditions given under ' Mercury '
(see p. 142) in order to obtain a complete separation.
The remaining electrolyte containing the tin must be
boiled with 30 grms. ammonium sulphate, in order to con-
vert the sodium sulphide into ammonium sulphide before the
deposition of the tin can be proceeded with. This method
of depositing tin will be found more fully described under
* Antimony and Tin 5 (see p. 201).
A separation of mercury from tin can also be effected
by the method with tartaric acid and ammonium hydrate
described under * Mercury and Antimony.' In this case, a
few grams tartaric acid and 30 c.cms. ammonium hydrate
are added to the mixed salts solution, and the electrolysis
is carried out with a feeble current and an E.M.F. not
exceeding 1*70 volts.
Mercury from Gold. This separation can only be
effected in solutions containing an excess of potassium
cyanide by means of currents the E.M.F. of which does
not exceed 1*90 volts. It is also necessary that the
electrolysis should be stopped when all the mercury is
deposited, otherwise the mercury deposit will be found to
contain some gold. Smith states that the deposition of the
mercury under these conditions is extremely slow. !
1 Amcr. Chcm. Jour. 11, 264, 352 ; 12, 428 ; 13, 417.
196 THE ELECTROLYTIC PROCEDURE
Mercury from Palladium, Platinum, and Osmium. If
the solution of the salts of any one of these metals and
mercury ('20 grm. of each metal) be treated with an excess
of potassium cyanide, and then be electrolysed with a
current of '20 ampere in density, a separation of the metals
will be found to occur.
The mercury will be obtained as a deposit at the
kathode, whereas the other metal will remain in solution.
According to Smith, 1 and to Smith and Frankel, 2 the sepa-
ration requires from fourteen to sixteen hours.
Mercury from Manganese. From a solution containing
free sulphuric acid, mercury can be separated as a metal
and manganese as peroxide. This form of solution can
therefore be used to effect the separation of these two metals.
The electrolysis is carried out under the conditions described
under ' Manganese.' It is, however, necessary to note here
that only very small amounts of either metal must be present
in the solution, on account of the tendency of large amounts
of manganese to separate from such solutions in a non-adherent
form at the anode. The use of the larger electrode surface
as anode does not remove this difficulty. With regard to
mercury, the deposition of larger amounts is attended by a
running together of the minute globules and formation of small
balls, which are easily detached from the kathode surface.
Mercury from Iron, Cadmium, Nickel, Cobalt, and
Zinc. The separation of mercury from these metals can be
effected by the method used to separate copper and silver
from the same group of metals namely, by the electrolysis
of a nitric acid solution. The E.M.F. used to effect the
deposition of the mercury in such a solution may rise to a
maximum of 2'4 volts. The deposition occurs easily, and
the separation is complete.
The remaining solution after deposition of the whole of
the mercury should be treated with sulphuric acid in order
to convert the nitrates into sulphates, since only in few
cases can a nitrate solution be used without harmful results
1 Amer. Chem. Jour. 11, 264, 352 ; 12, 428 ; 13, 417. - Ibid. 12, 428.
SEPAKATION OF METALS 197
for the electrolytic determination of the above -named metals.
For the details of this treatment with sulphuric acid,
see p. 169.
A solution containing tartaric acid and ammonium
hydrate may also be used to effect the separation of these
metals, in place of the nitric acid solution. The E.M.F.
required is practically the same as that named above.
The fact that mercury can be deposited in very satis-
factory form from solutions containing an excess of
potassium cyanide by means of almost any E.M.F. or
current density has already been noted ; and the solution
of the double cyanide salt may also be used to effect the
separation of mercury from the metals named above.
The ' decomposition value ' of the double cyanide salt of
mercury and potassium is equivalent to about 1'60 volts ;
and the E.M.F. of the current used to effect the separation
may be allowed to rise to 2 '50 volts without any of the other
metals named being deposited with the mercury at the
If from 2 to 3 grms. pure potassium cyanide be added
to the neutral solution of the mixed salts, a current of '08
ampere density can be obtained from the above E.M.F.
The solution is heated to 50 or 60 C. before electrolysis,
and the time required to deposit '50 grm. mercury is
between five and six hours.
When cobalt is present the time required to deposit
the mercury is increased. When using the double cyanide
solution for the separation of mercury and cadmium it is
necessary to keep the E.M.F. used for the electrolysis at
from 1-80 to 1'90 volts, and in this case the deposition
of the mercury is conveniently carried out at night. The
separation from cadmium in an acid solution demands,
however, less time and attention.
Mercury may also be separated from aluminium,
magnesium, and the alkali metals in acid or cyanide
solutions by use of any E.M.F., or current density, that
may be thought suitable by the experimenter.
198 THE ELECTEOLYTIC PROCEDURE
Gold from Silver. See p. 191.
Gold from Mercury. See p. 195.
Gold may be deposited from a hydrochloric acid solution
by means of a current with an E.M.F. of only one volt, but
the deposit so obtained is not very adherent, and it is much
the better plan always to make use of the double cyanide
solution. This on electrolysis yields an even and bright
coating of the metal upon the kathode surface.
The ' decomposition value ' of the double cyanide of gold
and potassium is somewhat higher than those of the corre-
sponding salts of silver and mercury. In such a solution
the separation of gold from zinc, copper, nickel, cobalt, and
iron may be effected by means of a current the E.M.F. of
which does not exceed 2-5 volts.
If the amount of each metal present be about -10 grm.,
and if from I'O to 2'0 grms. pure potassium cyanide have
been used in preparing the solution, a current density of
between -05 and '10 ampere may be used. According to
Smith and Wallace, 1 from three to three and a half hours
will be requisite to complete the deposition. If a lower
current density be used, it is perfectly feasible and safe to
allow the electrolysis to continue through the night.
Smith and Muhr 2 state that gold may also be separated
from palladium, platinum, and osmium by this method.
In this case, from 2 to 2| grms. potassium cyanide are used
with a current density of about -05 ampere, and the time
required is between twelve and fourteen hours.
Gold can also be deposited from a sodium sulphide
solution, and this fact renders it possible to effect a separa-
tion by electrolysis of gold and arsenic.
A similar method cannot be applied to effect the separa-
tion of gold from antimony and tin. It was hoped that
as gold cannot be deposited from an ammonium sulphide
1 Jour. Amer. Chem. Soc. 1895, 17, 612.
2 Berichte, 1891, 2171.
SEPARATION OF METALS 199
solution, a separation from tin would be effected by electro-
lysis of such solutions ; the experiments, however, did not
yield successful results. Arsenic may not only be sepa-
rated from gold in the sulpho-salt solution described above,