tory, the above methods are without practical import-
Bismuth amalgam. Bismuth is the metal for the elec-
trolytic determination of which the amalgam method is
advantageous ; for if this separation as an amalgam were
not feasible in the case of bismuth, only very small
amounts of the metal, and even those rarely in the
metallic form, could be electrolytically deposited from its
By use of the amalgam method, amounts up to '60 or
70 grm. bismuth may be separated. The mixed solution
of the bismuth and mercury salts may be subjected to
electrolysis in various forms.
The ammonium oxalate double salt solution has been
found unsuitable for this deposition.
The form of solution in which free nitric acid is present
may be used ; the acid must be present in sufficient amount
DEPOSITION FKOM PUEE SALT SOLUTIONS 168
to keep the basic bismuth salt in solution. Too great an
excess of acid, however, causes a deposition of bismuthic
acid at the anode. In order to carry out an electrolysis
with this solution, '50 grin, bismuth oxide and 2 grms. mer-
curic oxide are dissolved in the required amount of nitric
acid ; and the solution of mixed nitrates is electrolysed
with currents up to 1 ampere in strength. &t the usual
temperature this electrolysis requires an E.M.F. of 3^
volts. The relative amounts of the two salts in the elec-
trolyte must be at least 4 of mercury to 1 of bismuth ; a
silvery-white amalgam will then be obtained. The end of
the electrolysis is determined by use of ammonium sulphide.
The remaining electrolyte must be displaced by water
before the circuit is broken. The washing and drying of
the deposit are effected in the usual manner. The bis-
muth amalgam is not subject to oxidation on exposure
to air, and it is little affected by heat. If too little mer-
cury has been used, a black deposit of bismuth will be
found covering the amalgam. In order to prevent in
all cases the separation of oxygen compounds of bismuth
at the anode, a little tartaric acid is added to the elec-
trolyte. These oxides, when they have once separated at
the anode, are in most cases not to be brought into solution
If a dark band should remain on the platinum electrode
after solution of the amalgam, this may be removed by igni-
tion, or by use of the electrode as anode in a dilute nitric
acid solution, with a stout copper wire or strip of sheet
copper as kathode.
Vortmann has also used solutions containing hydro-
chloric acid. In order to prepare such a solution, he added
potassium iodide and hydrochloric acid to the solution of
the bismuth and mercury salts, until a clear liquid was pro-
duced. On electrolysis this solution yields a gas-holding
scum of iodine upon the surface of the electrolyte. In order
to avoid too great an excess of hydrochloric acid when using
this method, Vortmann added 50 c.cms. 96 per cent, alcohol
164 THE ELECTROLYTIC PROCEDUEE
to the solution of the bismuth and mercury chlorides with
hydrochloric acid. This addition assists the solution of the
The relative proportions of the metals and of the other
reagents which should be employed are therefore as
follows : '20 to '80 grm. bismuth oxide ; 1 to 2 grms. mer-
curic chloride ; hydrochloric acid in sufficient amount to
dissolve the bismuth oxide ; 50 c.cms. 96 per cent, alcohol.
The method yields good results.
In actual work, it is more customary to deposit the
bismuth amalgam from the nitric acid solution than from
that prepared with hydrochloric acid. The reason for this
preference of the nitric acid method apart altogether from
the general objection to the electrolysis of solutions con-
taining chlorides is to be found in the fact that in the
ordinary course of analysis the nitric acid solution is often
obtained, or can easily be prepared. It must certainly be
held to be a great convenience that in the case of bismuth
the amalgam method is sufficiently trustworthy to be used
in place of the unsatisfactory methods of obtaining metal
Antimony amalgam. Formerly the electrolytic deter-
mination of antimony was beset with difficulties, and this
metal was therefore included in the number of those which
it was attempted to separate and to determine as amal-
gams. The experiments in the case of antimony were
made with solutions containing the mixed salts and sodium
sulphide. Now that a simple and easy method is known
by which it is possible to deposit antimony as metal in
sufficiently large amounts and in short periods of time, the
amalgam method has become superfluous.
The experiments made upon arsenic salts showed that
the deposition of this metal as an amalgam was in-
It is seen from the above that at present the amalgam
method of determining the amount of metal in a solution
is only of importance in the case of bismuth.
DEPOSITION FROM PURE SALT SOLUTIONS 165
SEPARATION OF THE METALS
In Chapter VII. of Part I. it was shown that when
the current is passed through a liquid containing two
or more metal salts in solution, the metals are, according
to circumstances, deposited either together as alloys or
amalgams, or the deposition is only partial, and certain of
the metals in the mixture are deposited, while others
remain in the solution. The investigation of this
phenomenon, in so far as it relates to its application to
analytical purposes, is chiefly confined to the discovery of
the conditions under which certain metals are deposited
separately from mixed solutions. The various separations
possible by this mode of procedure may be divided into the
four following groups :
Group I. Separations of the metals from mixtures
containing two or more different metals, by deposition of
the one as metal at the kathode, of the other as peroxide
at the anode. The separation of copper from lead in a
nitric acid solution is an example of an application of the
method, which is much used in technical laboratories.
Group II. Separations of different metals by the
maintenance of the electric current used for the electro-
lysis, at a definite maximum, as regards E.M.F."(ref. p. 35).
Since the decomposition values of different salts vary, it is
possible to effect separations by means of an E.M.F. vari-
able at will, if the two values lie sufficiently far apart.
Kiliani and Freudenberg have shown that if the E.M.F.
of the current be kept below that required to effect the
electrolysis of the salt with the higher decomposition value,
the one metal will be deposited, while the other will remain
Group III. The separations in this group are effected
by artificially increasing the decomposition value of the one
metal salt. This may be achieved either by raising the salt
to a higher level of oxidation, or by converting it into a
complex salt through the addition of other salts to the
166 THE ELECTROLYTIC PROCEDURE
solution. In either case the metal passes into the anion
group on electrolysis ; and its separation from this only
occurs in a secondary manner by use of an increased
E.M.F., or in some cases does not occur at all.
An example of this class of separation is to be seen
in the method adopted to part antimony and arsenic
in a sodium sulphide solution. The arsenic is previously
raised to the arsenic acid stage of oxidation. As an ex-
ample of the complex salt method of separation, the
parting of iron and cobalt from zinc after addition of
potassium hydrate may be cited.
Group IV. The separation of certain metals from others
may be effected by the addition of strong mineral acids to
their salt solutions.
In this way the deposition of iron, cobalt, nickel, cad-
mium, and zinc is prevented. Since those metals are in all
cases first separated for which the least E.M.F. is required,
it will be the noble metals, gold, silver, copper, and mercury,
that are first deposited ; while if a considerable excess of
acid be present the remainder of the series of metals given
on page 35 will not be deposited, a liberation of the hydro-
gen of the acid being produced instead.
These methods of separation are not all applicable in the
case of every metal ; neither are the four groups given
above sharply divided the one from the other in all cases.
In many instances a combination of the methods given
under two or more of the above groups is used.
For example, one may have a solution containing copper,
zinc, lead, and iron for analysis that is, a solution of a
brass containing lead and iron as impurities.
In this case the separation of the copper and lead is
effected in a nitric acid solution by the method of Group I. ;
but at the same time the copper is being separated from
the iron and zinc by the method of Group IV., since these
cannot be deposited from a strongly acid solution.
They remain in the electrolyte, and after the copper has
been removed they are determined by some special method.
SEPARATION OF METALS 167
The different separations that are possible will be found
described under the headings of the individual metals. The
current strengths given in these descriptions of the methods
are again based upon a kathode or anode surface area of
100 sq. c.m., even when this is not expressly stated to be
the case. In those cases in which no details are given con-
cerning the weights of the salts of the individual metals
which are to be used in carrying out these methods, those
given in Part III., B, may be taken. Reference to the pre-
vious division of the work may also be made in those cases
in which nothing is stated concerning the character or the
treatment of the metallic deposits.
This metal one of the group known as the noble
metals can very easily be deposited in useful form from
solutions containing free mineral acids (ref. p. 93). This
characteristic makes the elaboration of a method of sepa-
ration from the metals zinc, iron, nickel, and cadmium
possible, while it also indicates that a separation from
those metals which form peroxide deposits at the anode can
be carried out.
Copper from Zinc. The separation of these two metals
can be effected in various acid solutions, provided that a
sufficient amount of acid be present. The mineral acids
are found to yield the best results, and of these, nitric and
sulphuric acids are most generally used.
The solution of '50 grm. of each of the salts, zinc
sulphate and copper sulphate, is treated with 1 to 2 c.cms.
cone, sulphuric acid, diluted to about 150 c.cms. and
electrolysed with a current density of between '50 and I'O
ampere. The E.M.F. required will be from 2 -5 to 2 -8
volts, and the electrolysis may be carried out either at the
normal temperature or at 50 C. The deposition of the
copper will be complete in from one and a half to two
hours ; but the last traces of the copper are difficult to
168 THE ELECTROLYTIC PROCEDURE
remove from the solution, and on this account the time
required is often greater than that named.
The remarks made on p. 95 concerning the character
and the treatment of the deposit apply in this case also.
The remaining acid zinc solution is neutralised after
the complete deposition of the copper, and bhe zinc is de-
posited by one of the trustworthy methods of electrolytic
determination given under " Zinc." If the copper deter-
mination has been carried out in the basin electrode, and
the liquid displaced before breaking the circuit, it is neces-
sary to evaporate the excess of water, in order that after the
addition of the necessary reagents the zinc solution may be
electrolysed in the same basin electrode, This is now pro-
vided with the required coating of copper, and the weight
is already known. If the electrolysis has been conducted
in a beaker with the jacket form of electrode, the evapora-
tion of the diluted solution is unnecessary, and the electro-
lytic deposition of the zinc may be directly proceeded with. 1
Nitric acid may be used in place of sulphuric acid in
effecting this separation. In this case the deposition of
the copper occurs rather more slowly.
The solution of the two sulphates is treated with about
5 c.cms. cone, nitric acid, diluted to the usual volume, and
after heating to about 50 C. it is electrolysed with a
current density of from *50 to I'O ampere. The E.M.F. re-
quired will be between 2 -5 and 3-0 volts, and the deposition
will demand about three hours.
Since, during the electrolysis of the copper salt, part of
the nitric acid is converted into ammonia by the action of
the current, a too lengthy duration of the electrolytic
separation, or an insufficiency of acid in the solution, may
cause the electrolyte to lose its acid reaction through the
formation of ammonium nitrate. If this should occur, the
zinc may be deposited with the copper.
The liquid containing the zinc salt, after the complete
1 When a jacket kathode is used, so little wash water is produced
that evaporation is unnecessary.
SEPARATION OF METALS 169
separation of the copper, should not be used directly after
neutralisation of any remaining free acid for deposition of
the zinc. The presence of nitrates in solution is not
favourable to the attainment of good metallic deposits of
It is most advantageous to evaporate this solution with
sulphuric acid, and then to convert the resulting sulphate
of zinc into one of the forms given under ' Zinc ' as most
suitable for the electrolytic deposition of the metal. The
further conduct of the electrolytic separation is then exactly
as described under 'Zinc.'
Smith has recommended the use of a solution of the
sulphate salts of copper and zinc, to which 30 c.cms. of a
saturated solution of sodium phosphate, and 3 c.cms. phos-
phoric acid, have been added.
Very weak currents must be employed with this solution.
The copper is deposited first quite free from admixture with
zinc. The deposition takes place very slowly, and the
method is not so simple as the two methods already de-
scribed, in which sulphuric or nitric acid solutions are used.
Classen has effected the separation of copper and zinc
by means of his oxalate method. 1
The salt solutions of the two metals receive an addition
of ammonium oxalate, and are thereby converted into the
double oxalate form. The electrolysis is conducted with a
neutral or feebly acid solution, and the E.M.F. required is
about 2 volts. The method suffers from the disadvantages
already discussed under ' Copper ' (see p. 100).
The electrolytic method of separation of copper and zinc
is made use of in technical laboratories for the analysis of
brass. In Part III., D, further reference will be made to
this use of the method.
The first-named methods of separation i.e. those effected
in solutions acidified with the mineral acids are the only
two which are of technical importance.
Copper from Iron. The separation of these two metals
1 Berichte, 17, 2467.
170 THE ELECTROLYTIC PROCEDURE
can, in a similar manner to that of the two former ones, be
effected in an acid solution, one containing free sulphuric
acid having been found to be the most suitable. In order
to prepare such a solution of the mixed salts, about 1 grm.
each of cupric and ferrous sulphates are dissolved in water,
the solution is treated with 3 c.cms. cone, sulphuric acid,
and after dilution to 150 c.cms. it is electrolysed at the
normal temperature, with a current density of 1 ampere.
The E.M.F. required will be from 2-75 to 3-0 volts ; the
time necessary for the complete deposition of the copper
will be from two to two and a half hours. The remaining
electrolyte must be washed out of the basin before breaking
the circuit ; after evaporating to a suitable volume, it is
neutralised with ammonium hydrate, and between 4 and 6
grms. ammonium oxalate are added to the solution.
If a jacket electrode is to be used, this evaporation is un-
necessary. The electrolysis is carried out at a temperature
of 30 to 40 C., with a current density of between I'O and
1-5 ampere, and an E.M.F. of from 3'4 to 3'8 volts. The
iron will require between three and four hours for complete
deposition. The results obtained by this method are good.
The ammonium sulphate which results from the neutralisa-
tion of the excess acid by ammonium hydrate is without
prejudicial influence upon the separation of the iron from
the double oxalate salt solution.
If 5 c.cms. cone, nitric acid be added to the solution of
the two salts, and, after the usual dilution, the electrolysis
be carried out at the normal temperature with a current
density of 1 ampere, and an E.M.F. of from 2'9 to 3-3 volts,
the complete deposition of the copper will require from four
to five hours. The remaining electrolyte must be displaced
before the current connections are broken.
If one proceeds, as before, to electrolyse the iron solu-
tion after neutralisation of the free acid with ammonium
hydrate, and addition of from 4 to 6 grms. ammonium
oxalate, a separation of ferric hydrate will occur in the
electrolyte during the deposition of the iron. This precipi-
SEPARATION OF METALS 171
tate of iron can be dissolved by the use of oxalic acid, but
such an addition always has a prejudicial influence upon
the results, and in very many cases the deposition of the
iron is incomplete. It is, on this account, necessary, when
a nitric acid solution has been used to effect the separation
of the copper, to remove the nitric acid by evaporation
with sulphuric acid, before proceeding to this electrolytic
separation of the iron.
Classen has used his oxalate method to effect separations
of these two metals. The solution containing the two
metals as sulphates, together with between 6 and 8 grms.
ammonium oxalate in 150 c.cms. water, is treated with oxalic,
tartaric, or acetic acid, and small quantities of the acid
are added from time to time during the course of the
electrolysis in order to maintain the acid reaction of the
The electrolysis is conducted at a temperature of
between 50 and 60 C. with a current density of 1 ampere.
The E.M.F. required under these conditions will be from
2 '9 to 3-4 volts ; and the complete deposition of the copper
(which separates quite free from iron) will demand about
three hours, if 1 grm. copper sulphate has been used.
If sufficient care be not given during the electrolysis to
the maintenance of the acidity of the electrolyte, oxalic acid
being especially easily decomposed by the current, iron will
be deposited with the copper at the kathode. When the
deposition of the copper has been completed satisfactorily,
the remaining electrolyte may be simply neutralised with
ammonium hydrate, and the iron deposited forthwith from
this solution at the normal temperature by means of a
current of from 1*0 to 1*5 ampere density.
The E.M.F. required will be from 3'0 to 3-3 volts, and
the time about three hours. When the iron is to be
estimated in this way, only oxalic acid, of those named,
may be employed for acidifying the electrolyte during the
copper deposition; the electrolysis demands constant super-
vision, and does not always give satisfactory results.
172 THE ELECTROLYTIC PROCEDURE
Vortmann has recommended the use of ammonium
sulphate and ammonium hydrate with the sulphate salts
of the two metals, when the amount of iron is considerable.
A precipitate of flocculent ferrous or ferric hydrate of
course occurs in such a solution, but this is said not to be
detrimental to the deposit of copper. A current density of
between '10 and *60 ampere is employed.
Apart from the objection that exists to the use of solu-
tions containing precipitates in suspension for electrolysis,
it is by no means certain that by the use of this method
small amounts of iron will not be deposited with the copper.
Smith has also recommended the use of solutions con-
taining sodium phosphate and free phosphoric acid for
effecting the electrolytic separation of copper and iron.
The remarks made upon this method as applied to the
separation of copper and zinc are also applicable in this case.
A consideration of the separate methods described above
shows that the method with free sulphuric acid is clearly
superior both in simplicity and in reliability to any of the
It is necessary to note, in conclusion, that the separation
of the two metals copper and iron by the electrolytic
method is attended with difficulties when the latter metal
is present in considerable amount. This is especially the
case when a nitric acid solution is employed. Not only
does the already deposited copper partly redissolve, but,
according to Schweder, the deposition remains incomplete. 1
Copper from Cobalt or Nickel. This separation can be
effected in a manner precisely similar to that described for
the separation of copper from zinc or from iron namely, by
means of solutions of the salts containing an excess of free
mineral acids, from which copper alone will be deposited.
In this case also sulphuric acid is found to be eminently
fitted for use as the acidifying agent. In order to prepare
a solution of the mixed salts for electrolysis, 1 grm.
each of copper and nickel sulphates (or cobalt sulphate)
1 Berg- u. Hiittenzeitg. 30, 5, 11, 31.
SEPARATION OF METALS 173
are dissolved in the necessary amount of water, and 3 c.cms.
cone, sulphuric acid are added. After dilution to the usual
volume the solution is electrolysed with a current density
of about 1 ampere at the normal temperature. From two
and a half to three hours will be requisite to effect the
complete removal of the copper from the electrolyte. The
further treatment of the deposit and of the electrolyte is as
described under the separations Copper- Zinc and Copper-
In place of the addition of sulphuric acid, 5 c.cm. of
nitric acid may be used, in which case the deposition of the
copper occurs under approximately the same conditions as
those given above.
In either case the deposit of copper is perfectly free
from nickel (or cobalt). Of the two methods, that with
sulphuric acid is to be preferred, since, after the electro-
lyte has been removed from the basin electrode and
the washings have been added, it is only necessary to add
ammonium hydrate in excess to the solution, now contain-
ing only nickel (or cobalt), and to decompose the heated
solution by means of a strong current, as described under
The nitric acid solution requires a previous evaporation
with sulphuric acid in order to convert the nitrate salts into
sulphates, if later disturbing influences upon the electrolytic
process are to be avoided ; whereas the sulphuric acid solu-
tion is, after addition of ammonium hydrate, at once ready
for the deposition of the nickel or cobalt.
Classen has recommended the use of the ammonium
oxalate double salt, with the addition of oxalic or tartaric
acid, for effecting the separation of these metals. A
quantitative separation is, however, only possible if the
E.M.F. be kept at or below 1-3 volts, since a higher E.M.F.
produces an alloy of the two metals at the kathode. This
low E.M.F. will only produce a very small current, and
consequently the deposition of the copper occupies much
174 THE ELECTROLYTIC PROCEDURE
Heydenreich has stated that about four hours are
requisite to deposit -25 grm. copper. 1
The deposition of copper from a solution of a copper salt
to which ammonium oxalate has been added in excess only
commences when an E.M.F. of I'l volts has been attained,
and the margin between this and that named above is ex-
tremely small. The conditions are precisely similar in the
case of either metal. The oxalate solution when freed from
its copper contents may be used directly for the electrolytic
separation of the nickel (or cobalt) ; neutralisation of the
free acid by means of ammonium hydrate being alone
necessary to prepare it for this further electrolysis. On
account of the time required to carry out the deposition of
the copper, this method cannot be regarded as a convenient
or useful one.
Smith has also recommended the use of his sodium
phosphate method for effecting the separation of these
metals. The remarks already made concerning this method
under Copper-Zinc apply in this case also, and it is unneces-
sary to repeat them here.
The method of separation of copper from nickel or
cobalt by use of a sulphuric acid solution of the metals is
the only one of technical importance. It is noteworthy
that it was from this solution that Gibbs, in the year 1864,
separated the two metals nickel and copper in an examina-
tion of ' mint nickel/ and thus gave proof of the applicability
of electrolysis to practical analytical purposes.-