The silver may be got by
difference. Re-cupel the metal thus got with 12 or 15 times its weight
of silver, flatten and part the gold with nitric acid in the usual way
(see under _Gold_), and the platinum will dissolve. The gold may contain
an alloy of osmium and iridium; if so, it should be weighed and treated
with aqua regia. The osmiridium will remain as an insoluble residue,
which can be separated and weighed. Its weight deducted from that
previously ascertained will give the weight of the gold.
When the platinum only is required, the alloy must be dissolved by
prolonged treatment with aqua regia, the solution evaporated to dryness,
and the residue extracted with water. The solution thus obtained is
treated with ammonic chloride in large excess and with some alcohol. A
sparingly soluble[47] yellow ammonic platinum chloride is thrown down,
mixed, perhaps, with the corresponding salts of other metals of the
platinum group. Gold will be in solution. The solution is allowed to
stand for some time, and then the precipitate is filtered off, washed
with alcohol, dried, and transferred (wrapped in the filter paper) to a
weighed crucible. It is ignited, gently at first, as there is danger of
volatilising some of the platinum chloride, and afterwards intensely.
With large quantities of platinum the ignition should be performed in an
atmosphere of hydrogen.
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