Neutral potassium oxalate is added in excess to the solution of a cobalt
salt, and the clear solution of cobalt potassium oxalate submitted to
electrolysis. The intense red color of this solution is soon changed
into a dark green; the latter diminishing in intensity as the metal is
deposited at the negative electrode. The electric current decomposes the
potassium oxalate into the carbonate, so that a precipitate of cobalt
carbonate is simultaneously formed with the separation of the metallic
cobalt. This precipitate may be dissolved by adding oxalic acid or
dilute sulphuric acid; the further action of the current will change the
solution to an alkaline reaction, upon which the treatment with acid is
repeated until all the cobalt has been separated out in its metallic
condition. The electrolytic separation of cobalt is much more easily
and rapidly effected when the potassium oxalate is substituted by the
corresponding ammonium salt, as the latter forms a soluble double
salt with the cobalt compounds. If the ammonium oxalate added is just
sufficient to form the double salt, a red cobalt oxalate (_which is only
slowly reduced by the current_) will separate out in addition to the
cobalt. In order to obviate this difficulty, the solution to which the
ammonium oxalate had been added in excess is heated, and then three
or four grammes more of solid ammonium oxalate are added. The _hot_
solution, when exposed to the action of the current, deposits the cobalt
as a closely adhering gray film. By the aid of two Bunsen's elements,
0.2 gramme cobalt can be separated in an hour's time. When the reduction
has been completed, and this is best determined by testing a small
sample (removed by a pipette) with ammonium sulphide, the positive
electrode[1] is removed from the solution, and the liquid poured off.
The dish is immediately rinsed several times with water, and the excess
of water removed at first with alcohol, and then with absolute ether.
The cobalt in the dish is dried in the air bath at 100° C., and in the
course of a few minutes a constant weight is obtained.
[Footnote 1: A piece of platinum foil, 4.5 cm. in diameter, is used
for the positive electrode, and a deep platinum dish as the negative
electrode.--_Vide_ "Classen's Quantitative Analysis," 3d Edition, p.
46.]