cadmium and sodium or potassium formate ; 3 while Smith
and Moore have suggested the double tartrate. 4
These solutions, however, like those that have just
received notice, are not adapted for the requirements of
practical work.
Solutions of cadmium salts containing free ammonium
hydrate give in nearly every case spongy deposits.
Deposits which are not firmly adherent, and of a dead
silver- white colour, cannot be trusted to yield exact results,
The determination of cadmium is not of frequent
1 LcJirbuch dcr Analyse.
* Chem. News, 1886, 53, 209.
3 Zeitschr. f. anorg. Chem. 1, 285.
1 Jour. Anal, and Appl. Chem. 1893, 7, 189.
126 THE ELECTEOLYTIC PKOCEDUKE
occurrence in technical laboratories ; and up to the present
the electrolytic methods have not been used, because the
deposition has not been satisfactorily complete and the
amount of the metal which could be obtained in compact
and adherent form was too small. There is, however, no
longer cause for the exclusion of the electrolytic methods
for determining cadmium from the technical laboratory,
as the result obtained by use of the potassium cyanide
method, and also those obtained with the solutions con-
taining free sulphuric acid, or with the acidified solution
of the double oxalates, are perfectly reliable, and the
methods are easily carried out.
LEAD
Lead belongs to a group of metals, of which manganese,
silver, bismuth, and thallium are the other chief members.
These differ from the metals which have so far received
attention, in their property of separating from many
solutions in a non-metallic form.
This separation occurs as peroxide, or at least as a
higher oxide, at the anode. Frequently the metal
separates in the metallic form at the kathode concurrently
with its deposition as peroxide at the anode.
Manganese and lead, however, differ from the other
metals of the group in the ease with which it is possible to
obtain the deposition of all the metal present in the electro-
lyte, as peroxide at the anode. The methods proposed
for obtaining the quantitative separation of lead as the
metal are numberless. Some of these yield unsatisfactory
results, owing to the deposits of lead occurring not as
uniform firmly adnerent coats, but as growths of needle-
ike or lamellar structure, which extend out toward
the anode and cause short circuiting in the electrolytic
cell.
Other solutions e.g. those of the highly complex salts
which yield the lead as homogeneous and dense deposits
at the kathode, are nevertheless unfitted for use in the
DEPOSITION FROM PURE SALT SOLUTIONS 127
quantitative determination of this metal, because after
washing with water and alcohol some oxidation of the
lead coating occurs during the after drying, whether this
be conducted in the air-bath or in the desiccator. It
is found impossible to prevent this oxidation, and its
occurrence of course leads to incorrect results.
Solutions of the neutral lead salts yield deposits of
lead and lead peroxide. Deposits of metallic lead alone
may be obtained from the following : neutral lead acetate
solution, proposed by Luckow l and Kiliani ; 2 solutions
containing free acetic acid, proposed by Yortmann ; 3
solutions containing an addition of saturated sodium
chloride, suggested by Kiliani 2 and Becquerel ; 4 solutions
to which have been added excess of sodium hydrate,
recommended by Weil, 5 Kiliani, 2 Schiff, 6 Schucht, 7 and
Parrodi and Mascazzini ; 8 solutions containing an addition
of tartrates or acetates of the alkali metals or of ammonium
oxalate, proposed by Classen and v. Reiss ; 9 solutions to
which pyrophosphatea of the alkali metals have been
added, as suggested by Brand ; 10 and, in addition to these,
all solutions which suffer decomposition by means of easily
oxidised (reducing) bodies. In spite, however, of the com-
plete separation of the lead which is possible with the
above solutions, they are not in use for the quantitative
determination of lead.
The separation of lead as peroxide is quite as easily
effected as the separation as metal ; and this can rank with
the very best electrolytic methods in regard to its con-
venience and accuracy. Luckow pointed out, so long ago as
1865, that lead could be completely separated as peroxide
1 Zeitschr. f. anal. Chem. 19, 1.
2 Berg- u. Hiitten-Zeitg. 1883, 285.
3 Berichte, 24, 2758.
1 Compt. rend. 1854, No. 26; Dingl. polyt. Jour. 1854, 213.
5 Tommasi, Electrochemie. 6 Berichte, 10, 1098.
7 Zeitschr. f. anal. Chem. 1883, 22, 287.
8 Ibid. 16, 469. 9 Berichte, 14, 1627.
10 Zeitschr. f. anal. Chem. 28, 581.
128 THE ELECTROLYTIC PROCEDURE
from solutions containing free nitric acid, 1 if there were at
least 10 per cent, by volume of the free acid present in he
electrolyte. 2 In order to carry out such a separation, I grm.
lead nitrate is dissolved in a little water, from 20 to
30 c.cms. nitric acid are added, and the mixture is diluted
to 150 c.cms. The cell connections are then made, care
being taken that the electrode of greatest surface area i.e.
the basin or the jacket electrode is used as anode. It is
advantageous to employ a dulled, or at least a much-used
electrode for the separation of lead as peroxide, as the
deposit adheres more firmly to such than to a new and
perfectly smooth surface. The current densities employed
in this separation may rise to 2 amperes without injury
to the deposit. With a current of '50 ampere, at the normal
temperature, an E.M.F. of from 2'Oto 2'4 volts is requisite ;
and between two and two and a half hours suffice to effect the
complete separation of the lead as peroxide. If the solu-
tion be heated to 50 or 60 C., and a current density of 1*5
amperes be employed, the E.M.F. required will be from 2'1
to 2 '5 volts, and the time will be reduced to about an hour.
A higher temperature is not to be recommended, since the
adherence of the deposit to the electrode is unfavourably
affected by temperatures exceeding 60 C. If the amount
of nitric acid present has been too small, part of the lead
will be found to have separated as metal at the kathode.
The deposit of peroxide is golden-yellow or reddish in
colour when only small amounts of lead are present in the
solution ; the deposit is, however, dark brown or black, even
from the commencement, when larger amounts are present.
The deposit of lead peroxide obtained in this way is not
represented by the formula PbO 2 , but contains water. The
deposit cannot be reduced to the anhydrous condition by
drying at the usual temperature in the air-bath ; to effect
this it is necessary to dry at 180 to 200 C. The weight of
anhydrous peroxide found, multiplied by '866, will give the
1 Dingl. polyt. Jour. 1865, 177, 178.
2 Zeitschr.f. anal. Chem. 19, 1.
DEPOSITION FKOM PURE SALT SOLUTIONS 129
weight 'of metallic lead present in the salt used for the elec-
trolysis. The deposition of the lead as peroxide cannot
be effected from solutions containing chlorides.
The deposit is redissolved if the electrolysis be allowed
to continue for too great a period of time ; but in all
probability this only occurs when the amount of free acid
present is insufficient.
In order to test whether all the lead has been deposited
from the solution, the reaction with sodium sulphide or
sulphuretted hydrogen gas may be used.
The test with potassium bichromate is, however, more
sensitive and less troublesome. The test-portion of the
electrolyte is neutralised with ammonium hydrate, acidi-
fied with acetic acid, and then treated with a solution of
potassium bichromate. Mere traces of lead cause a cloudi-
ness, or a precipitation of yellow lead chromate.
When the electrolysis is completed, the acid liquid must
be washed out of the basin, if this has been used as anode,
before breaking the circuit.
The deposits of the metals which have hitherto been
dealt with are easily removed from the electrodes by means
of nitric acid. This method is useless for deposits of lead
peroxide. In order to effect the removal of these, one may
either use the dilute nitric acid solution to which oxalic
acid or potassium nitrite has been added, which has then
been heated, or one may use the same dilute nitric acid
solution with a strip of copper or zinc to form the second
element of a galvanic couple.
The latter is the simpler plan, and results in the rapid
solution of the deposited peroxide.
This method of determining lead as peroxide is fre-
quently used in technical laboratories ; it is not only sim-
pler and more accurate than the gravimetric methods of
determination, but it offers the further advantage that a
separation of lead from other metals is at the same time
effected. This employment of the method will, however,
receive a fuller notice under ' Separations' in Part III, C.
K
130 THE ELECTROLYTIC PROCEDURE
MANGANESE
This metal, which resembles iron very closely in its
chemical properties, behaves on electrolysis very differently
from iron, and much more resembles lead. As with the
latter metal, so manganese may be separated from certain
of its solutions by the current, in the form of metal ; from
others, as metal and peroxide ; while from others it may
be obtained in the form of peroxide alone.
To obtain deposits of the metal, Moore 1 and Smith and
Frankel 2 have recommended the use of solutions to which
potassium sulphocyanide has been added ; for both metal
and peroxide, Warwick has suggested the use of the
acetate ; 3 but the same deposits can be obtained at times
from neutral salt solutions or solutions containing a small
excess of nitric acid, the acid in the latter being converted
into ammonia by the action of the current.
Since metallic manganese decomposes water, these
proposals are of no value for the quantitative determination
of the metal, for the after washing and weighing of the
separated metal is quite impossible.
To obtain deposits of pure peroxide, Luckow has re-
commended neutral salt solutions 4 ; Riidorff 5 and Riche 6
have suggested neutral salt solutions to which dilute
sulphuric acid has been added ; Luckow, 7 Classen and von
Reiss, 8 Riche, 6 and Schucht 9 have proposed the same with
nitric acid in place of sulphuric acid ; Becquerel 10 and
Classen 1 1 the same, with acetic acid in place of the mineral
acids ; while Classen and von Reiss 8 have recommended the
1 Chem. News, 1886, 53, 209.
2 Chem. Zeitg. Eep. 1889, 13, 257.
Zeitschr. f. anorg. Chem. 1, 285.
4 Zeitschr. f. anal. Chem. 19, 1.
Zeitschr. f. angew. Chem. 1892, 3, 197.
6 Compt. rend. 85, 226. 7 Zeitschr. f. anal Chem. 8, 24.
8 Berichte, 14, 1626. 9 Zeitschr. f. anal Chem. 22, 492.
10 Anal Chim. phys. 1830, 43, 380.
11 Zeitschr. f. Elektrochem. 1894, 1, 280.
DEPOSITION FROM PURE SALT SOLUTIONS 131
double oxalate of potassium and manganese, and Brand l has
suggested the double pyrophosphate salt. Though it is
possible to obtain from all of these solutions especially
from those containing free acid deposits of manganese
peroxide, their use suffers from the disadvantage that only
very small amounts can be obtained in adherent form at
the anode ; about '15 grm. calculated as metal. To carry
out the electrolysis of one of these solutions, one may
dissolve '30 grm. manganese nitrate in water, and to this
solution add 2 c.cms. nitric acid. The mixture is then
diluted to 150 c.cms., and is electrolysed at a temperature
of 50 to 60 C. with a current density of '30 ampere. The
connections must be so made that the basin or the jacket
electrode functions as anode. In this case, as in that of
lead, a dulled electrode surface is most suitable for the
reception of the deposit. The E.M.F. required will be from
3'0 to 3 - 5 volts ; the deposition will demand about two hours.
If the amount of free acid present should exceed 3 per
cent., no peroxide is formed ; permanganic acid will be
produced instead. During the electrolysis the nitric acid
will be decomposed and partly converted into ammonia ;
on this account an addition of nitric acid must be made
during its course. The peroxide will be found not to
adhere very well to the electrode. In place of the nitrate,
one may use "30 grm. manganese sulphate, the solution of
which has been acidified with 10 drops of concentrated
sulphuric acid. At a temperature of 60 to 70 C., a current
of from '40 to '60 ampere will suffice to deposit all the man-
ganese from this solution in three and a half or four hours.
The E.M.F. required will be 4 volts. In this case there is no
necessity to add sulphuric acid during the course of the
electrolysis. This method gives better results than that
with nitric acid, but the deposit in this case is still un-
satisfactory as regards its adherence to the electrode.
Latterly acetic acid has been again recommended for
employment in place of the above two acids. In order to
1 Zeitschr.f. anal. Chem. 23, 581.
K2
132 THE ELECTROLYTIC PROCEDURE
carry out an electrolysis with this acid, '30 grm. manganese
sulphate is dissolved in about 125 c.cms. water, and 25 c.cms.
60 per cent, acetic acid is added. The acid solution is
heated to 50 or 60 C., and is electrolysed with a current
of -30 ampere. The E.M.F. required under these con-
ditions will be from 4*3 to 4-9 volts, and the whole of the
manganese will be separated as peroxide in from two to
two and a half hours. The deposit is no better as regards
adherence to the electrode than that obtained from the
preceding solution.
The same results are obtained by use of a solution con-
taining excess of sodium pyrophosphate and free ammonium
hydrate. All the manganese will be deposited in about
two hours with an E.M.F. of 4*1 volts and a current of '30
ampere, but in this case the deposit is just as liable to
part from the electrode as in the previous examples.
Additions of free tartaric, oxalic, milk, or phosphoric acids
delay the deposition of the peroxide.
In order to obtain adherent deposits of larger amounts
of manganese peroxide, Engels has recently suggested a
method of aiding the separation by the addition of other
chemicals. 1
To a solution of 1 grm. manganese sulphate in water, a
solution of 10 grms. ammonium acetate and of 1*5 to 2 grms.
chrome alum is added ; the mixture is made up to 150
c.cms., and after heating to 80 C. it is electrolysed with a
current of from '50 to *60 ampere density. Under these
conditions the E.M.F. will be from 2-8 to 3'1 volts ; if the
current density be increased to 1-0 ampere, the E.M.F. will
rise to between 3'7 and 4-1 volts. The deposition will
require from an hour and a quarter to an hour and a half.
The addition of the chrome-alum solution gives to the
deposit of manganese peroxide at the anode a physical
character differing from that observed in deposits from
acid solution. The chief distinction is that, even in
comparatively large amounts, it is firmly adherent to the
1 Zeitschr.f. Elektrochem. 1895, 2, 410.
DEPOSITION FROM PURE SALT SOLUTIONS 133
anode. For still larger amounts of manganese than that
named above, the addition of chrome alum must also be
increased. Alcohol may be used as a substitute for
chrome alum. To prepare such a solution, -50 grm. man-
ganese sulphate and 10 grms. ammonium acetate are dis-
solved in water, the mixture is diluted to about 140 c.cms.,
and from 5 to 10 c.cms. alcohol are added.
The solution is heated to 70 to 80 C. and is electro-
lysed with a current density of 1 ampere.
The E.M.F. required under these conditions will be
from 4-0 to 4*2 volts, and the time about an hour and a
quarter.
In order to ascertain if all the manganese has been
separated from the solution, the best and most sensitive
test is that with lead peroxide. The reaction with am-
monium sulphide is not applicable. The small test- sample
of the electrolyte is heated with lead peroxide and a few
drops of concentrated nitric acid. A purple coloration,
due to the formation of permanganic acid, will be produced
if manganese be present in the solution in even the
smallest amounts.
The brown or blackish-brown deposit of manganese per-
oxide which has been obtained by these various methods
upon the anode is no better fitted for direct weighing after
drying than the deposit of lead peroxide, since it also
separates in a hydrated form.
Riidorff has stated that if the deposit of manganese
peroxide be first dried over sulphuric acid and then at
60 C., it will be found to possess the constant composition
represented by the formula Mn0 2 -f H 2 O.
Groger has, however, proved by the iodine method that
the constitution of the deposit dried under these conditions
is only approximately represented by this formula. 1 Clas-
sen has shown that, if the peroxide be converted into the
lower oxide (Mn 3 O 4 ) by ignition, a compound of constant
composition will be obtained, the weight of which multiplied
1 Zeitschr. f. angew. Chem. 1895, 253.
134 THE ELECTEOLYTIC PEOCEDUEE
by -720 will yield the weight of metallic manganese pre-
sent in the electrolyte.
The electrolytic method for the determination of man-
ganese, which until very recently suffered under the
disadvantage that only very small amounts of manganese
could be obtained as an adherent deposit of the peroxide,
was naturally not fitted to compete with the gravimetric or
volumetric processes for determining this metal.
The two electrolytic methods last described remove this
disadvantage, it is true ; but the volumetric process for
manganese determination is so simple that one can hardly
expect these improved electrolytic methods to replace it in
technical laboratories.
SILVER
Silver is classed as one of the noble metals, and there-
fore one can prophesy that it will be possible to deposit
it from solutions containing free acid. This prophecy is
found to be correct ; but the deposition of silver from such
solutions is, from a practical point of view, attended by
several objectionable features.
In the first place, silver, under certain conditions,
separates concurrently as metal at the kathode and as per-
oxide at the anode. A further difficulty is caused by the
character of the metallic deposit, which is compact, smooth,
and bright only in exceptional cases, unless extremely
feeble currents have been employed in electrolysing these
acid solutions.
The neutral salts yield flocculent bulky deposits of a
brown colour even with the feeblest currents and most
dilute solutions. Luckow states that similar deposits are
obtained from solutions to which ammonium hydrate and
ammonium carbonate have been added, but in this case silver
peroxide is deposited at the same time at the anode.
If free nitric acid be added to a solution of silver
nitrate, the electrolysis of this mixture will yield, at times,
adherent and bright deposits of the metal ; but quite as
DEPOSITION FROM PURE SALT SOLUTIONS 135
frequently greyish-brown non- adherent deposits will be
obtained, with peroxide formation at the anode. The
addition of lactic or tartaric acid prevents the peroxide
separation.
Fresenius and Bergmaim 1 have shown, however, that
even with this addition it is only possible to obtain useful
deposits from this solution with any degree of certainty,
when using very feeble currents and a very dilute elec-
trolyte.
On this account the time required to complete the
separation of the metal is very great. In order to carry
out such an electrolysis, a maximum of '50 grin, silver
nitrate or silver sulphate is dissolved in water, and after
addition of 5 to 6 c.cms. nitric acid the mixture is diluted
to 125 or 150 c.cms., and is electrolysed at a temperature of
50 to 60 C. with a current density of "04 to '05 ampere.
The separation will demand four to five hours.
The electrolysis with this solution may be carried out at
the normal temperature, if a current density of from *10 to
20 ampere be not exceeded. The E.M.R required in this
case will be about 2 volts. Higher current densities than
these, or insufficiency of nitric acid, cause the formation
of peroxide and of non-adherent deposits of the metal.
The well-known reaction with chlorides is made use of
to ascertain the completion of the electrolysis of the silver
salt. The acid liquid must be washed out of the basin before
breaking the circuit ; the deposit of silver must be dried at
100 C. The colour of deposits of silver obtained from
nitric acid solutions is white with a metallic lustre, and
much resembles that of platinum when the electrolysis has
been successfully carried out. Deposits that are a light
greyish-brown in colour are untrustworthy.
The method proposed by Luckow, 2 in which the silver
is deposited from the double cyanide salt of silver and
potassium, is much to be preferred to that described above.
1 Zeitschr. f. anal. Chem. 19, 316.
- Ibid. 1.
136 THE ELECTROLYTIC PROCEDURE
The solution for this electrolysis is prepared by dissolv-
ing amounts not exceeding 1 grm. in weight of silver nitrate
or silver sulphate in water, and by adding to this solution a
freshly prepared solution of pure potassium cyanide until
the precipitate of Ag(CN) 2 , which first formed, has dissolved
in the excess of the potassium cyanide. Rather more than
the exact amount necessary to obtain a clear solution is
added. From 2 to 3 grms. solid potassium cyanide will be
requisite. The solution is then diluted to 150 c.cms. It is
advisable to make use of the purest potassium cyanide that
can be obtained, since the use of the impure commercial
product leads to a less satisfactory deposit of silver at the
kathode. The current density employed with this solu-
tion may rise to 1 ampere without injury to the character
of the deposit. If feeble currents of from -20 to '30
ampere be employed to effect the deposition at the normal
temperature, the E.M.F. required will be between 3*3 and 3-5
volts, and the complete separation will demand four to five
hours ; if the currents be increased to '50 or '60 ampere,
an E.M.F. of from 4*0 to 4*6 volts will be requisite, and the
time will be reduced to from two to two and a half hours.
The electrolysis may also be carried out with a heated elec-
trolyte, without any danger to the character of the deposit.
Using a current density of 1 ampere and an E.M.F. of
5 '8 volts, a solution containing '50 grm. of the silver salt
heated to 60 C. will have all the silver deposited as a dead-
white coating upon the kathode in half an hour. The E.M.F.
required for such a heated solution when the current is
reduced to -60 ampere is only 4'8 volts. This potassium
cyanide method may also be used with extremely feeble
currents from '10 to '20 ampere at the normal tempera-
ture, and on this account it may be employed for performing
the electrolysis at night. The E.M.F. required in this case
is 3-3 volts.
The deposit obtained from the double cyanide solution
is of a dead silver- white colour, and therefore differs in
this respect from that obtained from the acid solutions.
DEPOSITION FKOM PUEE SALT
The deposit appears to be partly crystalline in structure,
but in spite of this it adheres firmly to the electrode. The
use of roughened or well-used electrodes as kathodes is
found to be advantageous. The dead -white appearance of
the deposit is at times found to give place to patches of
brown. This indicates that at these spots the current
density has been too great, a result that can easily arise
with certain forms of electrodes. When a basin electrode
has been employed, the liquid must be washed out before
breaking the circuit. This and the remaining washing and
drying operations are carried out exactly as described for
copper and the other metals of this group.
Krutwig has recommended an ammoniacal solution of
silver containing ammonium sulphate, for obtaining deposits
of this metal. 1 In order to prepare such a solution '50 grm.
silver nitrate or silver sulphate is dissolved in water,
and to this solution 25 c.cms. ammonium hydrate and a
solution of 6 grms. ammonium sulphate are added. This
mixture is heated, and, according to v. Miller and Kiliani, 2
should be electrolysed with a current density of from -02 to
'05 ampere. The E.M.F. required will be about 2*5 volts.
The results obtained by use of this method are uncertain.
Stronger current densities result always in the separation
of the silver in a loose flocculent form, greyish brown in
colour, at the kathode, a result which cannot be avoided by
lessening the amount of ammonium hydrate. The deposit
of metal also encloses ammonium sulphate, and for the
removal of this a very careful washing with water is
required. The method therefore cannot be recommended.