practice ; though, as regards the other methods, the remarks
made upon this point under ' Nickel J are also true for
If a solution contains both nickel and cobalt, all the
methods described above will lead to a simultaneous
separation of the two metals at the kathode.
Zinc belongs in accordance with its position in the
voltaic series of metals to the same group as iron, nickel,
and cobalt ; that is to say, the quantitative separation of
zinc from solutions containing more than a very small
percentage of free acid is not possible by the ordinary
In many respects zinc behaves similarly to the metals
whose separation has been already described, but it differs
from these in its tendency to separate in the spongy form.
Such spongy deposits are obtained especially by the electro-
lysis of neutral salt solutions ; and in order to avoid this
objectionable feature Reinhardt and Ihle have recommended
the addition of a small amount of acetic acid to the
electrolyte, 1 while Luckow (I.e.), Parrodi and Mascazzini, 2
1 Jour. f. prakt. Chem. 24, 195.
2 Gazz. chim. Ital. 1877, iv. v. 222 ; Berichte, 10, 1098.
114 THE ELECTEOLYTIC PEOCEDUEE
Riche, 1 Millot, 2 Reinhardt and Ihle, 3 and Riidorff 4 have
recommended the addition of sodium acetate for the same
purpose. Neither of these additions, however, effectually
prevents the occurrence of spongy deposits. It is necessary
to note here that the deposition of zinc upon the usual
platinum electrodes leads to unpleasant results ; for on dis-
solving the dried and weighed deposit of zinc a black
powdery coat remains, as a rule beneath the whole of the
deposit but at least about the edges of the electrode,
which is neither soluble in hot hydrochloric acid nor in hot
Vortmann states that this black coat consists of finely
divided platinum, 5 and that a mere mechanical rubbing
with sand will remove it. This is of course an objection-
able treatment for the platinum electrode. On this
account it is customary to coat the electrode which is to
serve as kathode with some other metal before its use for
zinc depositions. Copper, silver, or tin is most generally
used for this purpose.
The procedure described under 'Copper,' in which
nitric acid is used in the electrolyte, is especially suitable
for obtaining such a coating of copper. The current is
only allowed to pass for a few minutes ; the electrode is
then washed, dried, and weighed. This coating of other
metal, used to protect the platinum of the electrode, may
lead to incorrect results during the after electrolysis of the
zinc salt, owing to minute drops of the electrolyte being
carried or spirted, by the bubbles of gas which escape from
the liquid, on to the portion of the coating which is not
immersed. The copper may in this way be oxidised ; the
other metals may even be dissolved. To guard against this
source of error, the coating upon the inner surface of the
basin, or upon the jacket electrode, is made only very slightly
higher than the level to which the electrolyte will reach.
1 Compt. rend. 85, 226 ; Zeitschr. f. anal. Chem. 17, 218.
2 Bull, de la Soc. Chim. 1882, 37, 339.
3 Jour. f. prakt. Chem. 24, 195.
4 Zeitschr. f. angew. Chem. 1892, 179. 5 Berichte, 24, 2753.
DEPOSITION FKOM PUKE SALT SOLUTIONS 115
This is most simply attained by measuring the volume
of the zinc solution of which the electrolysis is to be made,
and by using a volume of silver or copper solution 5 to 10 c.c.
In order to avoid the trouble involved in preparing
these coatings, nickel basins or jacket electrodes have been
used. These are of course subject to the action of the
acid when the zinc is dissolved.
The zinc deposit obtained from any solution is only
fitted for analytical work when it is of a pale greyish-blue
colour, and is firmly adherent to the kathode.
Dark deposits are to be regarded with distrust, while
deposits which are partly or wholly spongy in character
must be rejected. At the commencement of the electro-
lysis most solutions yield a deposit of the desired character,
but it is from a few only that absolutely trustworthy
deposits can be obtained when the electrolysis lasts for a
considerable period of time.
One of these latter is the solution of the double cyanide
of zinc and potassium, recommended by Luckow, 1 Beilstein
and Jawein, 2 and Millot. 3
The solution for electrolysis is prepared by dissolving
1 grm. zinc sulphate in a little water, and by adding to
this a solution of pure potassium cyanide in small portions
at a time, until the precipitate of zinc cyanide, which first
forms, has dissolved in the excess of potassium cyanide.
If other salts of zinc containing free acid be used, it is
necessary to neutralise this acid with sodium hydrate, or
to make the solution slightly alkaline before adding the
The clear colourless solution of the double cyanide is
made up to 150 c.cms., and may be electrolysed with either
strong or weak currents, at the normal or at a higher
temperature. Under all conditions and without any
attention, it yields on electrolysis, homogeneous, pale blue
1 Zeitschr. f. anal Chem. 19, 1. 2 Berichte, 12, 446.
8 Bull, de la Soc. Chim. 1882, 37, 339.
116 THE ELECTROLYTIC PROCEDUEE
deposits of zinc which are firmly adherent to the
At the normal temperature an E.M.F. of 5-8 volts
will be required for a current of *5-ampere density, but
this E.M.F. will fall during the electrolysis owing to the
heating of the electrolyte by the ^current. The deposition
will occupy between two and two and a half hours.
If the electrolysis be conducted at 50 C. } the E.M.F.
required is reduced to 5 volts and the time to two hours
for the above current density. It is possible to obtain
equally good deposits when using a current density of
1 ampere ; in this case, at a temperature of 50 "* to 60 C.,
all the zinc is deposited in from an hour and a half
to an hour and three-quarters with an E.M.F of from
5 to 5-2 volts.
This method with the double cyanide of potassium and
zinc is also adapted for obtaining depositions during the
night with weak currents.
The recognition of the complete deposition of the zinc
is attained by use of potassium ferrocyanide solution.
The presence of zinc is proved by the formation of a
white insoluble precipitate, insoluble in hydrochloric acid,
or a white cloudiness of ferrocyanide of zinc. In order to
apply this test, the small test-portion of the electrolyte
the withdrawal of which demands care, lest any should be
sucked into the mouth is treated with a few drops of hydro-
chloric acid, and after warming is mixed with potassium
For some solutions which are already alkaline, or the
test-portion of which has been made so by the addition of
ammonia, sodium or ammonium sulphide solution may be
used to determine the presence of zinc. In. this case white
zinc sulphide is formed.
When the deposition has been proved to be complete,
the cone is removed from the solution, or the basin is
washed out before breaking the circuit, and the deposit of
zinc is washed with water and alcohol and dried in the air-
DEPOSITION FROM PURE SALT SOLUTIONS 117
bath at 100 to 1 10 C., as previously described. The deposit
should be of a pale greyish-blue colour.
Reinhardt and Ihle l and Classen and v. Reiss 2 have
recommended the use of the double oxalates for obtaining
useful deposits of zinc. In order to prepare such a solution
1 grm. zinc sulphate is dissolved in water, and to this is
added a solution containing 4 grms. ammonium oxalate.
The precipitate of zinc oxalate which first forms dissolves
in the excess of ammonium oxalate, which is present in
much greater amount than that requisite to form the
double salt. This neutral solution may be electrolysed at
the normal temperature with a current density of '5 ampere.
An E.M.F. of from 3-8 to 4-1 volts will be required, and
the deposition will be completed in about four hours.
With smaller amounts of zinc, and current densities
which do not exceed '5 ampere, it is possible to obtain
bright deposits ; but spongy deposits under these circum
stances may also occur.
Similar results are obtained by use of the potassium
zinc oxalate salt recommended by Reinhardt and Ihle, ' and
by v. Miller and Kiliani. 3 This solution is prepared by
dissolving 1 grm. zinc sulphate, 4 grms. potassium oxalate,
and 3 grms. potassium sulphate ; the best results are ob-
tained when it is electrolysed at the normal temperature
with a current density of *3 ampere. The E M.F. required
is from 3 -9 to 4-2 volts, and the time from three to four
hours. Classen has shown that in order to obtain with
certainty reliable deposits of zinc from the solutions of the
double oxalates, it is absolutely necessary to maintain the
electrolyte acid during the electrolysis, by means of the
addition of small amounts of free organic acids. 4 Either
oxalic, tartaric, or lactic acid may be used for this pur-
pose. It is most convenient to use a 5 per cent, solution
of tartaric acid, which is less readily decomposed than
oxalic acid ; 1 to 2 c cms. of this solution are added to the
1 Jour. /. prakt. Chem. 24, 195. 2 Berichte, 1881, 14, 1630.
3 Quant. Analyse. * Zeitschr.f. Elektrochem. 1894, 1, 280.
118 THE ELECTEOLYTIC PROCEDURE
ammonium zinc oxalate solution prepared as described
above, at the commencement of the electrolysis.
During the deposition of the zinc small amounts continue
to be added, and tests with litmus paper are made in order
to have proof that the electrolyte has been kept acid. Too
great an excess of acid delays the deposition. By use of
this method dense and bright deposits of zinc can be ob-
tained ; but the electrolysis demands some attention during
its course. The ammonium zinc oxalate solution is best
electrolysed at a temperature of 50 to 60 C. with '5 ampere ;
the E.M.F. required will be from 3-5 to 4-0 volts, and the
time two hours. When 1 ampere is used, the E.M.F. rises
to between 4'7 and 4'8 volts, and the time is reduced to
an hour and a half. The deposit obtained is pale bluish-
grey in colour. If the electrolyte has not been kept acid
during the electrolysis, grey zinc sponge is nearly always
When tartaric acid has been used as recommended
above, in order to maintain the electrolyte in the acid state
the test with potassium ferrocyanide is not applicable. It
is necessary to wash the electrolyte from the basin or cone
before breaking the circuit, when using this method with
the double oxalates.
Jordis has shown that good deposits can be obtained
wnen ammonium lactate and free lactic acid are used in
place of ammonium oxalate and oxalic or tartaric acid. 1
The solution is prepared by dissolving 1 grm. zinc sulphate,
2 grms. ammonium sulphate, and 6 grms. ammonium
lactate in 150 c.cms. water, and by adding to this solution
10 drops of lactic acid.
The electrolysis may be carried out with current densi-
ties varying between -5 and 1 ampere ; in the latter case,
with solutions heated to 50 to 60 C., the deposition
occupies an hour and a half, while the E.M.F. required
is from 3 '8 to 4 '5 volts.
The zinc deposit obtained is pale blue. It would be
1 Zeitschr. f. Elektrochem. 1895, 2, 656.
DEPOSITION FROM PURE SALT SOLUTIONS 119
erroneous to draw the conclusion, from the similarity which
in other respects exists between zinc, and cobalt or nickel,
that the solution of the double sulphate of ammonium and
zinc, to which excess of ammonium hydrate has been added,
will yield good deposits of zinc. A solution containing
1 grin, zinc sulphate and 6 grms. ammonium sulphate, with
a small addition of ammonium hydrate, will give bright de-
posits under certain conditions with weak currents. From
neutral solutions of zinc ammonium sulphate it is possible
to separate all the zinc contained in 1 grm. zinc sulphate
with a current density of from -3 to -5 ampere in an hour,
as a rule, in a useful form ; but occasionally the deposit is
spongy. The E.M.F. required varies between 3 and 4 volts.
The presence of chlorides or of ammonium hydrate in the
electrolyte increases, as a general rule, the tendency to form
The use of zinc sulphate solutions to which sodium or
ammonium acetate and free acetic or citric acid have
been added has been proposed by Riche, 1 Rudorff, 2 and
Parrodi and Mascazzini. 3 1 grm. zinc sulphate and 3 grms.
sodium acetate are dissolved in water in order to prepare
such a solution, and 20 drops of acetic acid are added.
In order to electrolyse this solution at the normal tem-
perature with a current of -5 ampere, an E.M.F. of from
5 -9 to 6 '3 volts is required.
The deposit, which is at first bright, becomes later
spongy in character ; if a current of only '2 to '3 ampere
be, however, employed, a useful deposit can be obtained ; the
time required will be about eight hours. If such a solution
be heated to 50 or 60 C., and a current of -5 ampere be
again used, a good deposit of zinc, bluish- white in colour,
can be obtained. The E.M.F. required will in this case be
reduced to about 5 volts, and the time required will be
between three-quarters and one hour.
1 Compt. rend. 85, 226 ; Zeitschr. f. anal Chem. 17, 208.
2 Zeitschr. f. angew. Chem. 1892, 179.
3 Gaza. chim. Ital. 1877, iv. v. 222 ; Berichte, 10, 1098.
120 THE ELECTROLYTIC PROCEDURE
Insufficiency of acetic acid increases the tendency to
form a spongy deposit ; it should therefore be added in
small portions at a time during the electrolysis. Excess of
this acid delays the deposition. It is necessary to wash
without breaking the circuit, when using this method. This
electrolysis yields precisely similar results when ammonium
acetate is used, in place of sodium acetate, with the zinc
sulphate solution. In order to prepare such a solution
1 grm. zinc sulphate is dissolved in water, and ammonium
hydrate is added until the precipitate which first forms
has redissolved. Acetic acid is now added until a feebly
acid reaction is produced. From this solution, heated to
50 or 60 C., the zinc can be wholly deposited in about an
hour by use of a current of '5 ampere. An E.M.F. of from
3*5 to 4 volts is required. The deposit is bright and firmly
adherent to the electrode.
Yortmann has recommended the use of zinc solutions
containing tartrates of the alkali metals. 1 The solution of
zinc sulphate is prepared in this case for electrolysis by
adding 5 to 6 grms. sodium potassium tartrate and 2 to 2^
grms. caustic soda, and it is then electrolysed at the normal
temperature with a current of from -40 to -70 ampere in
density. The complete deposition requires from two to
three hours ' } the character of the deposit obtained is satis-
If an excess of sodium hydrate solution be added to a
solution of a zinc salt, a solution of sodium zincate is
formed, which has also been recommended for electrolysis
by Millot 2 and Kiliani and v. Foregger. 3 This solution
does not give bright deposits in all cases.
Satisfactory results may be obtained if solutions of 1
grm. zinc sulphate and from 2'5 to 4'0 grms. caustic soda
are mixed, and, after dilution and heating to 50 to 60 C.,
are electrolysed with currents of from '70 to 1 *5 amperes
1 Monats.f. Chem. 1893, 14, 546.
2 Bull de la Soc. CUm. 1882, 37, 339.
3 Dissertation, 1896, Berne.
DEPOSITION FROM PURE SALT SOLUTIONS 121
in density. The E.M.F. required is from 3'9 to 4'5 volts ;
complete deposition is effected in two hours. Increase in
the amount of caustic soda improves the character of the
Solutions containing sodium or ammonium chloride give,
under certain conditions, good deposits ; but on account of
the unpleasant nature of the gas liberated at the anode
chlorine they are but little used. Little use is also made
of a solution which contains only 10 c.cms. ammonium
hydrate in addition to the zinc salt.
In this latter case a current of from '10 to '30 ampere
in density requires five to six hours to complete the de
position of the zinc.
Other additions that have been suggested are am-
monium phosphate, by Moore, 1 and sodium pyrophosphate
and ammonium carbonate, by Brand. 2 Solutions prepared
with these salts seldom give good deposits. [Nicholson and
A very recommend the use of zinc solutions containing the
metal as sulphate with the addition of formic acid in
excess, and sodium hydrate. 3 Translator's noteJ]
The traces of zinc that remain in the electrolyte
towards the end of the electrolysis of zinc salt solutions are
very difficult to remove. In this respect zinc resembles
iron and nickel. In order to effect their deposition, it is
necessary towards the end of the electrolysis to increase
the current density if this be feasible, or otherwise to allow
the current to continue to pass through the electrolyte for
a fairly long period of time.
In technical laboratories the electrolytic methods of
determining zinc are little used ; this is especially true of
those laboratories in which a large number of zinc estima-
tions have to be made concurrently. This does not signify,
however, that in particular instances the electrolytic
methods are not the most suitable. When these methods
1 Chem. News, 1886, 53, 209.
2 Zeitschr. f. anal. Chem. 28, 581.
8 Jour. Amer. Chem. Soc. 18, 654.
122 THE ELECTROLYTIC PROCEDURE
are employed, that first described with the double cyanide
of zinc and potassium is the most to be recommended,
since it yields with certainty good deposits, and permits the
use of fairly strong currents.
This metal is closely related to zinc, not only in its
electrical but in its chemical and other properties. Like
iron, nickel, and zinc, it cannot be deposited from strongly
acid solutions, and it especially resembles the latter metal
in the tendency that it exhibits, even more strongly than
zinc, to separate in a spongy or loose form. The presence
of from 1J to 2 per cent, mineral acid in the solution com-
pletely stops the deposition. Bright silvery white metallic
deposits can only be obtained with regularity from com-
paratively few solutions ; and the amount of the metal
which can be separated in this form is, further, very
For these reasons it is especially necessary, in the
electrolysis of cadmium salt solutions, to take great care
that the electrodes are perfectly clean, and also that the
form of the electrodes used for carrying out the electrolysis
is such that the current density will be practically the
same at all points of the kathode surface. The electrodes
are most satisfactorily cleaned by boiling with acids or by
immersion in fused potassium bisulphate.
One of the best deposits of cadmium is obtained by the
electrolysis of the double cyanide of cadmium and potas-
sium, as recommended by Beilstein and Jawein l and by
Wallace and Smith. 2
This electrolysis is carried out in the same manner as
that of the corresponding salt of zinc. '50 grm. cadmium
sulphate is dissolved in water, and to this solution pure
potassium cyanide solution is added until the first-formed
precipitate of cadmium cyanide has redissolved. The solu-
tion is then diluted to 150 c.cms., and is electrolysed at the
1 Berichte, 1879, 12, 446. - Ibid. 1892, 25, 779.
DEPOSITION FEOM PURE SALT SOLUTIONS 123
normal temperature with a current of '50 ampere. The
E.M.F. required will be from 4*75 to 5-0 volts, and the
time from six to seven hours. The presence of an excess of
potassium cyanide in the solution is advisable, in order to
lessen the tendency to form a spongy deposit.
In order to test whether the deposition is complete, sul-
phuretted hydrogen is used with a feebly acid test-portion of
the electrolyte. A yellow precipitate or a yellow colouring of
the solution indicates the presence of cadmium. When the
electrolyte contains cyanides, as in the above case, it is first
necessary to destroy these by boiling the test-portion with
excess of dilute sulphuric acid (under a draught-hood) ;
the solution is then nearly neutralised, and sulphuretted
hydrogen gas is passed through it.
Good deposits may also be sometimes obtained by use
of neutral salt solutions ; but it has been found that the
deposits are denser and show less tendency to a spongy
formation if some free acid be added to the liquid during
the electrolysis. Luckow l and Smith 2 have recommended
sulphuric, nitric, or acetic acid for this purpose ; while
Warwick 3 has recommended formic acid.
The solution of '30 grm. cadmium sulphate in 150 c.cms.
water receives an addition of from 1 to 2 c.cms. of dilute sul-
phuric acid, and after heating to 70 or 80C. is electrolysed
with a current of from '60 to 1-0 ampere. The E.M.F.
required to produce this current varies between 2 '5 and 5'0
volts according to the amount of free acid present the
deposition demands three hours. A silver- white deposit is
Heydenreich 4 has stated that a solution of -30 grm. cad-
mium sulphate containing free acetic acid yields, with cur-
rents of from '10 to '40 ampere in density, a bright deposit
of a crystalline lamellar character, and not particularly
1 Zeitschr. f. anal. Chem. 19, 1.
2 Chem. Jour. 10, 330.
3 Zeitschr. f. anorg. Chem. 1, 285.
4 Zeitschr. f. Elektrochem. 1896, 3, 151.
124 THE ELECTROLYTIC PROCEDURE
adherent to the kathode. The E.M.F. required is from 4 -5
to 7'5 volts.
Solutions made alkaline with ammonium hydrate can-
not be recommended for use ; the metal separates in the
spongy form, even after addition of ammonium sulphate.
Classen and v. Reiss } found that good results could be
obtained by use of the double oxalate salt of cadmium and
ammonium. The whole of the metal can be deposited from
a solution containing '30 grm. cadmium sulphate and from
8 to 10 grms. ammonium oxalate, with a current of '60
ampere, in about two hours.
At a temperature of 50 to 70 C., the E.M.F. required
is from 2'7 to 3'4 volts. The deposit is bright and firmly
adherent to the electrode.
The addition of a little free acid to the electrolyte is
found (as in the case of zinc) to increase the density of the
deposited metal, and to lessen the tendency to sponge
formation even at higher current densities. In order to
carry out the electrolysis in this way, the solution of the
double salt, as described above, is again prepared, and
during the electrolysis of the hot solution a few cubic
centimetres of an oxalic acid solution are added from time
to time, so that the electrolyte is kept slightly acid.
Tartaric acid, which is more stable than oxalic acid,
may be used with equally good results.
The current density can, under these conditions, be
allowed to vary between '50 and 1'5 ampere without any
danger to the character of the deposit. The E.M.F.
required for a current of from "60 to '70 ampere at 70 C.
is between 2'7 and 3-2 volts, and the time about three hours
and a half ; while for 1 ampere the E.M.F. is between 2*75
and 3-3 volts, and the duration of the electrolysis about three
hours. When tartaric acid has been used to acidify the elec-
trolyte, the E.M.F. required is slightly increased, being from
3-0 to 3'4 volts ; while the deposition takes place rather more
slowly, and on this account less close attention is necessary.
1 Berichte, 1881, 14, 1622.
DEPOSITION FKOM PUKE SALT SOLUTIONS 125
V. Miller and Kiliani have recommended the use of
solutions containing sodium acetate and free acetic acid
for cadmium depositions. 1
Such a solution is prepared by dissolving "50 grm.
cadmium sulphate and 3*0 grms. sodium acetate in water
and mixing the solutions. A little free acetic acid is
added to the mixture, and after diluting and heating to
50 C. it is electrolysed with a current of from "20 to '70
ampere. The complete separation of the metal from this
solution requires between five and eight hours. The current
densities which may be safely employed in this method are
too small for practical work \ and there is the further dis-
advantage, noticed by the authors, that the deposit has a
tendency to pass into the spongy form.
Moore has found that solutions of cadmium to which
sodium phosphate and phosphoric acid have been added,
yield non-metallic deposits unfitted for quantitative work. 2
If, however, the cadmium be precipitated from its solution
by means of a solution of sodium pyrophosphate, and the
precipitate be dissolved in an excess of ammonium hydrate,
the electrolysis of this solution with currents of from '10
to '30 ampere will yield useful deposits.
Warwick has recommended the use of the double salt of