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PLATINUM AND ALLIED METALS.
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993
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inexperience
and how much to deliberate fraud. The most ridiculous statements are
often made. A statement was made, for instance, in 1911 of a discovery
of an "immense bed of osmium ore near Boise, Idaho." The analysis was
given as follows:
Gold..........................................ounces per ton.. 2. 3
Iridium...............................................do.....1
Osmium................................................do___20. 0
Tungsten oxide......................................per cent.. 5. 0
Tin...................................................do.....25
Cobalt.................................................do.....5
Nickel.................................................do___ 1.0
Other
statements were current of a discovery of a rock containing tin and
platinum near Merlin, Oreg. It was claimed to contain from 1 to 65
ounces of platinum per ton and said to occur " in a mineralized zone
600 feet wide, with 15 feet of spherulitic gangue and 8 feet of
pitchblende." It is unfortunate that such statements should find even
temporary credence.
The
usual method of determining the platinum metals is by the crucible or
scorifier assay and subsequent determination of the metals in the
silver button.
By
the old method the silver alloy first obtained in the regular course of
assays ia parted in strong sulphuric acid and the residual metal
weighed. This is realloyed with silver in a second cupellation and
parted in nitric acid, the residual metal being again weighed. Any
difference shown between the two weighings is assumed to be and is
called platinum. Sometimes it is so, and if any considerable amount of
platinum be present there will be a decided difference between the two
weighings; but a slight difference is no evidence whatever of the
presence of platinum. On the other hand, the second weight may equal or
possibly exceed the first, even when traces of platinum are present.
Again, other members of the platinum group may go into solution in
nitric acid. If present, these would be called platinum and escape
detection. The method does not provide any direct tests whatever as to
the presence or absence of platinum. It is often indecisive, and
sometimes gives erroneous results. It is, therefore, -quite
unsatisfactory. * * * In the regular course of assaying gold is parted
from silver by dissolving the silver in nitric acid. If platinum be
present m small amounts only, it will readily go into solution in the
nitric acid. If now a limited amount of hydrogen sulphide be added to
the solution from parting, any platinum present will be precipitated as
sulphide, along with some silver sulphide. On filtering off the
precipitate (which generally is sufficiently washed by the operations
necessary to transfer it from the precipitating dish to the filter),
the moist filter is transferred to a small porcelain crucible, dried at
a low heat, and burned off by gentle ignition. This transforms the
sulphide precipitate into a metallic sponge, which is wrapped in a
small piece of thin lead foil and cupelled. The resulting bead is then
parted in strong sulphuric acid, when the platinum will be left as a
dark residue, generally collected in spongy form, even when minute in
quantity. This sponge, after reboiling in fresh acid, if necessary, is
suitably washed by decantation, annealed, and weighed.
This excellent method, as quoted above, has been proposed by F. P. Dewey.1
However, it does not take into account the possible presence of the
other platinum metals. Osmium and iridium are always present, and it is
well known that some copper ores contain more palladium than platinum.
For exact determinations the crude platinum obtained must be separated
according to standard analytical methods. Another method for the same
purpose has been suggested by A. Steinmann.2
By
using slightly diluted acid the silver can be extracted without loss of
platinum. The following method is proposed. About 250 milligrams of
platinum alloy are mixed
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