c. of nitric acid, boil off nitrous fumes, dilute
to 100 c.c. with water, and then electrolyse.
~General Considerations.~--In the preliminary work with the copper
sulphide there is a small loss owing to its imperfect removal in washing
the filter paper, and another small loss in dissolving in nitric acid
owing to the retention of particles in the fused globules of sulphur. To
determine its amount the filter-papers and sulphur were collected from
forty assays, and the copper in them determined. The average amount of
copper in each assay was 0.175 gram; that left on the filter paper was
0.00067 gram; and that retained by the sulphur 0.00003 gram; thus
showing an average loss from both sources of 0.00070 gram. The
determinations from another lot of forty-two similar assays gave on an
average
Copper left on filter paper 0.00070 gram
Copper retained by sulphur. 0.00004 "
The loss from these sources is trifling, and need only be considered
when great accuracy is required.
The deposition of the copper under the conditions given is satisfactory,
but, as already stated, if the solution contain more than 10 per cent.
of nitric acid it is not thrown down at all; or if a stronger current is
used, say that from three Bunsen cells, it will be precipitated in an
arborescent brittle form, ill adapted for weighing.
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