Solutions of cobalt are pink, whilst those
of nickel are green.
The detection of cobalt, even in very small quantity, is rendered easy
by the strong blue colour which it gives to the borax bead, both in the
oxidising and in the reducing flame. It is concentrated from the ore in
the same way as nickel, and should be separated from that metal by means
of potassic nitrite in the way described. The dry assay of cobalt has
been given under _Nickel_.
GRAVIMETRIC METHOD.
The yellow precipitate from the potassium nitrite, after being washed
with the acetate of potash, is washed with alcohol, dried, transferred
to a weighed porcelain crucible, and cautiously ignited with an excess
of strong sulphuric acid. The heat must not be sufficient to decompose
the sulphate of cobalt, which decomposition is indicated by a blackening
of the substance at the edges. The salt bears a low red heat without
breaking up. If blackening has occurred, moisten with sulphuric acid,
and ignite again. Cool and weigh. The substance is a mixture of the
sulphates of cobalt and potash (2CoSO_{4} + 3K_{2}SO_{4}), and contains
14.17 per cent. of cobalt.
Cobalt is also gravimetrically determined, like nickel, by electrolysis,
or by precipitation with sodic hydrate. In the latter case, the ignited
oxide will be somewhat uncertain in composition, owing to its containing
an excess of oxygen.
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