attention is required in order to obtain good results by its
use. Since, however, there are other methods which are
simpler and more conveniently carried out, which always
yield reliable results, this method with double oxalates
cannot be recommended.
Smith l has proposed the use of solutions containing
phosphate of soda and free phosphoric acid.
Heydenreich 2 has, however, pointed out that the deposi-
tion from such solutions occupies a very long time (seventeen
hours), and that the deposits are not bright. The E.M.F. re-
quired is from 2 '4 to 3 volts. Brand 3 has examined into the
effects produced by the use of the alkaline pyrophosphates.
The deposits obtained are fawn-coloured and dull, and the
time required is excessive.
Other solutions that have been recommended are those
containing sodium acetate with free acetic acid, 4 and the
tartrates of the alkali metals and of ammonium. 5
The latter methods offer no advantages over those first
described. The methods which are actually employed in
technical laboratories as substitutes for the ordinary
gravimetric or volumetric processes of analysis arej
exclusively those in which free nitric or sulphuric acid
used. When separations of metals have to be effected, th(
methods with potassium cyanide and excess of ammonia i
1 Amer. Chem. Jour. 12, 329.
2 Berichte, 1896, 1585 ; Zeitschr. f. Elektrochem. 1896, 151.
8 Zeitschr. f. anal. Cliem. 28, 581.
4 Warwick, in Zeitschr. f. anorg. Chem. 1, 285.
5 Smith and others, in Jour. Anal, and Appl. Chem. 5, 488 ;
7, 189, 252.
102 THE ELECTROLYTIC PROCEDURE
With regard to the accuracy of the electrolytic
methods for copper determination, it is proved that these
yield results equal to those of the best purely chemical
processes of analysis. Long practical experience is not
required in order to obtain accuracy with these electrolytic
It is customary to obtain the weight of the deposits to
the fourth decimal place, and a difference of "001 grm.
between successive determinations may be regarded as the
maximum of the deviation that ought to occur.
Iron, in contrast to copper and the noble metals, belongs
to that group of metals which cannot be separated from
moderately acid solutions by the current or E.M.F. which
it is customary to have at one's disposal for analytical
Since iron is precipitated from its solutions as hydroxide
by ammonia and the alkaline hydrates, the use of these
reagents for preparing the iron solution for electrolysis is
also excluded. The choice is therefore restricted to the
neutral salts, the double salts, and some complex substances.
It has, however, been proved that complete deposition is
not possible when neutral salts are electrolysed, and these
are therefore unsuited for analytical purposes.
One of the best methods for the electrolytic separa-
tion of iron is that suggested by Parrodi and Mascazzini ]
and by Classen and von Reiss. 2 This depends upon the
decomposition of the double oxalate of iron and am-
In order to carry out this method, 1 grm. of ferrous
sulphate or of ferrous ammonium sulphate is dissolved in a
small quantity of water, and at the same time 5 to 6 grms. of
ammonium oxalate are dissolved in about TOO c.cms. of water
with the aid of gentle heat. The two solutions are mixed
1 Gazz. Chem. Ital. 1879, B. 8.
2 Berichte, 1881, 14, 1622, 2771.
DEPOSITION FROM PURE SALT SOLUTIONS 103
by pouring the iron salt solution into the other, and the
mixture is stirred until the first-formed precipitate has re-
dissolved. If one proceeds in the reverse manner a ferrous
oxalate will be precipitated, the re-solution of which is
The clear solution, which should measure about 150 c.cms.,
is now electrolysed with a current density of from 1 '0 to 1 '5
amperes, and with an E.M.F. of from 3 -5 to 4 '5 volts. The
electrolysis can either be performed at the normal tempera-
ture or at 50 C. ; in the latter case the rate of deposition
is increased. The length of time necessary to effect the
deposition of the iron contained in the amount of salt named
above will be about four hours with a 1 -ampere current,
and between two and a half and three hours with a current
of from \\ up to 2 amperes.
It is also possible to carry out this electrolysis by means
of a weak current of from -3 to '5 ampere strength, and in this
case the separation can be effected during the night. It is
necessary, however, when the lower current strength is
adopted, to increase the amount of ammonium oxalate in
the solution ; and it is also necessary to increase the current
to a strength of at least 1 ampere at the end of the electro-
lysis, in order to effect the removal of the last traces of
iron from the electrolyte. The ammonium oxalate is
decomposed by the current, carbon dioxide is liberated
at the anode, ammonium carbonate is formed in the
solution, and the electrolyte becomes alkaline with separa-
tion of flakes of ferric hydrate. By use of a large amount
of the ammonium oxalate salt, one can to a very large
extent avoid this result.
If, however, in spite of this increase, ferric hydrate should
be formed, it is possible to bring it into solution again by the
addition of a small amount of oxalic acid to the electrolyte.
Since the decomposition of the ammonium oxalate occurs
more rapidly in hot solutions, it is always preferable to
carry out the electrolysis in cold solutions, with current
densities of from 1 to 1 -5 amperes.
104 THE ELECTKOLYTIC PEOCEDUEE
In order to ascertain if the whole of the iron has been
removed from the solution, a few drops are withdrawn by
means of a small pipette, and to this, after acidifying with
hydrochloric acid, a small portion of sulphocyanide of
potassium solution is added. A red coloration of the
mixture denotes the presence of iron, and occurs with
extremely small amounts of this metal. If it be found
that all the iron is deposited, the liquid is poured out of
the basin, or the jacket electrode is raised out of the solu-
tion, the deposit and its support are rinsed several times
with water, then with alcohol, and are finally dried in the
air bath at 100 C.
The deposit of iron should be of a bright steel- grey
Ferric salts may be treated in an exactly similar way,
but in the case of these it is unnecessary to take any pre-
cautions in mixing the iron and ammonium oxalate solu-
tions. The difference in colour between the complex double
oxalates of iron and the simple salts of iron are worthy of
notice. While the latter give for ferrous salts green
solutions, and for ferric salts solutions reddish brown in
colour, the ferrous double oxalate yields a red and the
ferric double oxalate a green solution.
When the ferric salts are electrolysed as double
oxalates, the colour of the solutions changes from green to
red, and the red gradually fades away to a complete
absence of any colour whatever. Ferric potassium sul-
phate (iron alum), Fe 2 (SO 4 ) 3 .K 2 SO 4 + 24H 2 O, or ferric
potassium oxalate, Fe 2 (C 2 O 4 ) <J 3K 2 C 2 O4 + 6H. 2 O, may be
used as ferric salts, or one may use the ordinary
hydrated ferric chloride. The chlorides may in fact be
safely used when employing this method of electio-
lysis of double oxalates ; but nitrates must be avoided,
since their use almost invariably occasions a separation of
ferric hydrate. If therefore the nitrate salts are to be
analysed, it is best to convert them into sulphates by
heating with excess of sulphuric acid. The chief portion
DEPOSITION FROM PURE SALT SOLUTIONS 105
of the excess must be removed by evaporation, and the
remainder is then neutralised by ammonia.
For the electrolysis of iron, besides the double oxalate
salts, solutions containing citrates and tartrates of the
alkali metals have been recommended and used by Smith. 1
A solution containing 1 grm. of ferrous sulphate is treated
with 2 to 3 grms. of ammonium citrate and a small amount of
citric acid, and is then electrolysed with a current of from '7
to 1 ampere in density. 2 Tt is possible to obtain bright de-
posits of iron in this way, but on dissolving the deposit in
dilute sulphuric acid, particles of carbon will be detected ;
and this is especially the case when high current densities
have been employed for the deposition of the iron. As a
consequence of this, the results obtained are too high.
The separation of iron from this solution also occurs
very slowly, six to seven hours being necessary to deposit
the iron contained in 1 grm. of ferrous sulphate.
Solutions containing the tartrates of the alkali metals
behave similarly. For example, a complete separation of
the iron is possible from an ammonium tartrate solution, 3
but the deposit contains carbon. Luckow has recom-
mended the ammonium fluoride double salt. 4
Moore has proposed solutions containing sodium phos-
phate, r> Brand solutions containing pyrophosphates of the
alkali metals, 6 for use in obtaining deposits of iron. These
solutions demand a very high E.M.F., and much time for their
decomposition ; and the deposits obtained are not very good.
[Nicholson and Avery have suggested an improvement
of Classen's oxalate method by the addition of sodium
borate to the ferrous ammonium oxalate solution. 7
1 Amer. Ghent. Jour. 10, 330.
2 v. Miiller and Kiliani, Lehrbuch der Analyse.
3 Jour. Anal, and Appl. Chem. 1891, 5, 488.
4 Zeitschr. f. anal. Chem. 19, 1.
5 Chem. News, 1886, 53, 209.
* Zeitschr. f. anal. Chem. 28, 581.
' Jour. Amer. Chem. Soc. 18. 654.
106 THE ELECTROLYTIC PROCEDURE
The method which depends upon the use of the double
oxalate salts is the only simple and safe one to employ for
the analytical electrolysis of solutions of iron. This method
has been used in determining the atomic weight of iron.
For technical purposes the electrolytic determination of iron
is of little importance, since it is unlikely to be used in
place of the volumetric method with permanganate of
potash. It may, however, be employed in standardising
such solutions of permanganate.
Nickel, like iron, is one of that group of metals which are
not separated by the customary currents of from 1 to 2
amperes from strongly acid solutions. The separation
from neutral salt solutions is only incomplete. Luckow,
however, states l that this drawback is avoided by the
addition of a small quantity of acetic acid, and Riche
states <2 that the same effect is produced by other acids.
The method proposed by Gibbs, 3 and by Fresenius and
Bergmann, 4 depends upon the use of a solution containing
free ammonia and ammonium sulphate, and this has proved
to be the most convenient and neat. Either the sulphate
or chloride of nickel may be employed. A solution of
1 grm. of nickel sulphate in a little water is prepared, and to
this a solution of from 5 to 10 grms. of ammonium sulphate is
added, together with 30 to 40 c.cms. of ammonium hydrate.
This solution is electrolysed at the normal temperature,
with a current density of '5 to 1*5 amperes and an E.M.F. of
2-8 to 3-3 volts. The deposition will be complete in about
two hours. If the solution be heated to 50 to 60 C. the
deposition can be effected in fifty to sixty minutes, by use
of a current density of 1'5 amperes and an E.M.F. of 3'4 to
3-8 volts. The deposit obtained is bright and shining, and
resembles in appearance rolled platinum ; and it is to some
extent proof against the action of dilute acids.
1 Zeitschr.f. anal Chem. 1880, 19, 1. 2 Ibid. 21, 11(5.
3 Ibid. 1864, 3, 334. 4 Ibid. 1880, 19, 320.
DEPOSITION FEOM PUKE SALT SOLUTIONS 107
This method can be employed for solutions containing a
larger amount of nickel than that given above. Care must,
however, be given to the maintenance of an excess of
ammonium hydrate in such cases, as when this is not
present, a coating of nickel, bad in colour, is obtained, and
a crusting of black nickel oxide forms upon the anode. Too
great an excess of ammonia retards the deposition. The
metal separated from these solutions is silver-grey in colour,
and adheres firmly to the kathode. Winkler l states that
large amounts of nickel may be deposited in this way. The
last traces of nickel, as in the case of iron, are difficult to
separate from the solution. On this account it is not
advisable to work with currents of less than 1 ampere in
density, or, in case such weaker currents have been used, it is
necessary to increase the density to 1 ampere towards the end
of the electrolysis, and to allow this current to pass through
the solution for a period of from fifteen to thirty minutes.
In order to satisfy oneself that all the nickel has been
deposited, a few drops of electrolyte are tested by means of
an ammonium sulphide or sodium sulphide solution. The
formation of a brown colouring in the mixture is proof of
the presence of nickel. Potassium sulphocarbonate may
also be employed for this purpose ; in this case the presence
of nickel causes a rose-red coloration to appear. When it
has been proved that all the nickel is deposited, the removal
of the electrolyte and the washing of the deposit are effected
without breaking the circuit in the manner described under
Copper. The electrode and its deposit are then washed
with water and alcohol, and dried at 100 C. As already
remarked, the metallic coating of nickel obtained in this
way possesses the noteworthy property of being but slowly
attacked by sulphuric or hydrochloric acids. On this account
it is best to use nitric acid for removal of the deposit from
the electrode. This method for the separation of nickel
always gives reliable and good results. The nitrate salt
interferes with the satisfactory course of the electrolysis,
1 Zeitschr. /. anorg. Chem. 1894, 8.
108 THE ELECTEOLYTIC PROCEDURE
so that it is necessary, when this salt is to be analysed,
to convert it into the sulphate by means of sulphuric acid.
It has frequently been asserted that the presence of
chlorides as, for example, ammonium chloride has also a
disadvantageous influence upon the separation of nickel.
Oettel has contradicted this, 1 and has shown that useful
deposits of nickel may be obtained from chloride solutions,
when attention is given to the following points. The nickel
chloride solution must be strongly alkaline ; at least 10 per
cent, of ammonium hydrate (sp. gr. *92) being required
in the solution to prevent the separation of the black nickel
oxide at the anode.
In addition to this, at least sufficient ammonium chloride
must be present to form the double chloride of nickel and
ammonium ; a larger amount is not injurious, but such an
addition of ammonium chloride only compensates for a
shortness of ammonium hydrate when high current densities
are employed. Insufficiency of ammonium hydrate in-
creases the time required for the separation, and also the
danger that the nickel may be partly separated as oxide.
In order to carry out an electrolysis by this method, 1 grm.
nickel chloride and 2 to 4 grms. ammonium chloride are
dissolved in 100 c.cms. water, 40 c.cms. ammonium hydrate
are added, and the solution is electrolysed at the normal
temperature with a current density of -5 ampere. The
deposition of the nickel is complete in four to five hours.
Larger amounts of nickel can also be obtained as firmly
adhering deposits by use of this solution. Thus 1 grm.
nickel may be completely deposited under the above con-
ditions in six to seven hours, or with a current density of '1 1
ampere in fourteen hours. When using a flat kathode of sheet
metal, Oettel recommends the use of a fork-shaped anode,
in order to obtain an equal current density upon the two
sides of the kathode, which is fixed in the centre of the
anode. Although this arrangement was originally sug-
gested for nickel deposition, it is one which can be recom-
1 Zeitschr.f. Elektrocliem. 1894, 1, 194.
DEPOSITION FROM PURE SALT SOLUTIONS 109
mended for adoption in all cases in which a metal has to be
deposited upon a flat electrode. Nitrates exert a disturbing
influence upon the course of this electrolysis.
Good deposits of nickel are also easily obtained by use
of solutions of the double oxalates of nickel and am-
monium, recommended by Classen and v. Reiss, 1 and also
by Classen. 2
The solution is prepared similarly to that of iron, by
dissolving 1 grm. nickel sulphate in water, adding a solu-
tion of 5 to 6 grms. ammonium oxalate, and by diluting this
mixture until it measures 150 c.cms.
The solution is then electrolysed with a current density
of 1 ampere. If the solution be heated to 50 or 60 C. the
E.M.F. required will be 2'8to 3'3 volts, and the deposition
will occupy about four hours ; if the separation be effected at
the normal temperature, the E.M.F. will be increased to
between 3'5 and 4*2 volts, the time to five or six hours.
Equally good deposits can be obtained by the employment of
weaker currents ; but it will be necessary to increase these
to 1 ampere or higher towards the end of the electrolysis, in
order to effect the separation of the last traces of the metal.
The nickel obtained in this way is bright and steel- grey
in colour, with a reddish tinge. By this method it is un-
necessary to wash out the electrolyte before breaking the
In the same manner that solutions of the neutral nickel
salts to which oxalates of the alkali metals have been
added are used for obtaining useful deposits of nickel,
solutions of neutral nickel salts containing tartrates,
citrates, and acetates of the alkali metals have been
proposed by Luckow, 3 Wrightson, 4 Ohl, 5 Schweder, 6 and
Smith and Muhr. 7 Good metallic deposits are obtained ;
1 Beriche, 1881, 14, 1622.
2 Zeitschr.f. Elektrochem. 1894, 1, 280 ; Berichte, 27, 2072.
8 Dingl.polyt. Jour. 117, 225.
4 Zeitschr. f. anal. Chem. 15, 300.
5 Ibid. 18, 523. 6 Ibid. 16, 344.
Jour. Appl. Chem. 1891, 5, 488 ; 1893, 7, 189.
110 THE ELECTROLYTIC PROCEDURE
but when using the tartrates and citrates, there is the same
tendency, observed with iron, for carbon to be deposited
with the rnetal. Solutions containing an excess of
potassium cyanide may also be used for nickel deposition,
according to Ohl, 1 Schweder, 2 Wrightson, 3 and Luckow. 4
The author has obtained, however, under the most varying
condition, only dark-coloured non-adherent deposits from
this solution. Von Foregger 5 states that useful deposits of
nickel may be obtained from solutions of nickel salts to
which ammonium carbonate has been added. To prepare
such a solution 1 grm. nickel sulphate and about 15 grms,
ammonium carbonate are dissolved in 150 c.cms. water.
This is heated to 50 or 60 C., and a current of from
1-0 to 1-5 ampere indensity is used to decompose it, with an
E.M.F. of 3-5 to 4-0 volts.
The complete separation of the nickel in the usual
bright metallic form will be effected in 1^ hrs. Useful
deposits of nickel can be obtained from solutions that have
merely received an addition of 10 c.cms. of ammonium
hydrate, on electrolysing them at the normal temperature
with currents of from *1 to '5 ampere in density.
Similar results can be obtained from solutions containing
an excess of pyrophosphoric acid and ammonium carbonate.
A solution of nickel sulphate is treated with 25 c.cms.
ammonium hydrate and 25 c.cms. of a saturated solution of
sodium pyrophosphate ; this is then electrolysed, either at
the normal temperature or after heating. With a current
of from '3 to '5 ampere the deposition will occupy sixteen
hours, with -5 to *8 ampere, nine hours.
Stronger currents may also be employed.
The deposits obtained are good, but the deposition
takes place too slowly, and the solution has a tendency to
permit nickel oxide to separate at the anode.
[Nicholson and Avery have recommended the use of
1 Zeitschr.f. anal Chem. 17, 215.
2 Ibid. 16, 344. 3 Ibid. 15, 300.
4 Ibid. 19, 1. 5 Dissertation, 1896, Berne.
DEPOSITION FROM PURE SALT SOLUTIONS 111
solutions containing the metal as sulphate, with addition
of ammonium oxalate and sodium borate an improvement
of Classen's oxalate method. 1 Translator's note.]
The method which finds frequent and exclusive employ-
ment in technical laboratories is that first described, de-
pending upon the use of ammonium hydrate and ammonium
sulphate. Nevertheless, the methods in which ammonium
chloride, ammonium oxalate, and ammonium carbonate are
used may occasionally be employed with advantage.
Cobalt so closely resembles nickel in most of its
properties, that those electrolytic methods which are found
useful for separating nickel are also applicable for cobalt. In
most cases the proposals made by various experimenters for
the deposition of nickel cover at the same time that of cobalt.
To carry out an electrolytic determination of cobalt,
1 grm. cobalt sulphate and 5 grms. ammonium sulphate are
dissolved in 100 to 120 c.cms. water, and 30 to 40 c.cms.
ammonium hydrate are added. This solution is then elec-
trolysed with a current density of from -5 to 1 -5 ampere,
either at the normal temperature or at 50 to 60 C.
The E.M.F. and time required for this deposition are
practically the same as in the case of nickel ; and
ammonium sulphide or potassium sulphocarbonate is like-
wise used to determine the completion of the deposition.
Nitrates produce a disturbing effect upon the electrolysis.
The remarks made under ' Nickel ' regarding the use of the
chloride with ammonium chloride are also correct for
cobalt ; but as cobalt is in general more difficult to separate
from its solutions than nickel, it is necessary to use a
weight of ammonium chloride equal at least to four times
that of the cobalt present ; and the ammonium hydrate
added must equal one -fifth of the total volume of the liquid.
The electrolysis requires less time the greater the pro-
portion of ammonium hydrate present in the electrolyte.
1 Jour. Amer. CJiem. Soc. 18, 654.
OF THE -.
112 THE ELECTROLYTIC PROCEDURE
The solution is therefore made up as follows : 1 grm.
cobalt chloride and 5 grms. ammonium chloride are dis-
solved in water, and the solution, after addition of 30 c.cms.
ammonium hydrate, is made up to 150 c.cms. It is then
electrolysed with a current density of 1*5 amperes. The
time required to complete the separation is five to six hours ;
and as a general rule it is more difficult to remove the last
traces of cobalt from the solution than those of nickel.
The method with solutions of the double oxalates gives
equally good results with salts of cobalt as with those of
nickel. The colour of the deposit of cobalt is slightly dif-
ferent to that of the deposit of nickel ; but the metallic coat
is equally bright. The separation from a solution containing
1 grm. cobalt sulphate and 5 to 6 grms. ammonium oxalate
in 150 c.cms. water is complete in four hours, when the
electrolysis is conducted at a temperature of 50 to 60 C. ;
at the normal temperature six to seven hours are requisite
to complete the deposition. The E.M.F. in the former
case is from 3'2 to 3'8 volts ; in the latter, 3-8 to 4-2 volts.
When small currents have been used to effect the
separation, an increase of the current strength is absolutely
necessary in order to remove the last traces of the metal
from the solution.
The method with ammonium carbonate has also been
suggested by von Foregger for use with cobalt salts. l
Quantitatively exact results are obtained, but the metal
deposit is not so bright as that obtained when using the
first method described. The solution is prepared by dis-
solving 1 grm. cobalt sulphate and 15 grms. ammonium
carbonate in water, adding a few cubic centimetres of am-
monium hydrate, and making up to a volume of 150 c.cms.
by addition of water. This solution is then heated to 50
or 60 C., and is electrolysed with a current density of
1 ampere. The E.M.F. required is from 3-7 to 3-9 volts ; the
time varies between 2^ and 3J hours.
From solutions containing an excess of potassium
1 Dissertation, 1896, Berne.
DEPOSITION FROM PUKE SALT SOLUTIONS 113
cyanide a quantitative separation of cobalt is no more
possible than in the case of nickel, in many cases cobalt
oxide separating at the anode.
The method with the pyrophosphates of the alkali
metals gives the same results as with nickel.
A cobalt solution containing 3 grms. sodium pyrophos-
phate and 100 grms. ammonium hydrate requires seven
hours for the separation of '1 to -2 grm. of the metal, and is
therefore unfitted for practical use.
The method with ammonium sulphate and ammonium
hydrate is that which alone finds employment in actual