The solution contains
suspended sulphur. The precipitate is easily washed, and under the
proper conditions the separation is complete, but the separation with
sulphuretted hydrogen is more satisfactory, since the conditions as to
acidity, &c., need not be so exact.
Zinc or iron is sometimes used for separating copper from its solutions,
but they are not to be recommended.
ELECTROLYTIC ASSAY.
The separation of copper by means of a current of electricity is largely
made use of, and forms the basis of the most satisfactory method for the
determination of this metal. If the wire closing an electric circuit be
broken, and the two ends immersed in a beaker of acidulated water or
solution of any salt, the electricity will pass through the liquid,
bringing about some remarkable changes. Hydrogen and the metals will be
liberated around that part of the wire connected with the zinc end of
the battery, and oxygen, chlorine, and the acid radicals will be set
free around the other. Different metals are deposited in this way with
varying degrees of ease, and whether or not any particular metal will be
deposited depends--(1) on the conditions of the solution as regards acid
and other substances present, and (2) on the _intensity_ of the current
of electricity used. For analytical purposes the metal should be
deposited not only free from the other metals present, but also as a
firm coherent film, which may afterwards be manipulated without fear of
loss.
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