Warm and filter, wash and redissolve in nitric acid, neutralise
with ammonia, add ammonic carbonate, boil and filter. The copper freed
from impurities will be in the solution. Acidulate and reprecipitate
with sulphuretted hydrogen. When the nature of the impurities will allow
it, this process may be shortened to first filtering off the gangue,
then precipitating with sulphuretted hydrogen and washing the
precipitate on the filter first with water and then with ammonium
sulphide.
Having separated the copper as sulphide, its weight is determined as
follows. Dry and transfer to a weighed porcelain crucible, mix with a
little pure sulphur, and ignite at a red heat for 5 or 10 minutes in a
current of hydrogen. Allow to cool while the hydrogen is still passing.
Weigh. The subsulphide of copper thus obtained contains 79.85 per cent.
of copper; it is a greyish-black crystalline mass, which loses no weight
on ignition if air is excluded.
Copper may be separated from its solutions by means of sodium
hyposulphite. The solution is freed from hydrochloric and nitric acids
by evaporation with sulphuric acid; diluted to about a quarter of a
litre; heated nearly to boiling; and treated with a hot solution of
sodium hyposulphite (added a little at a time) until the precipitate
settles and leaves the solution free from colour.
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