these
metals, as they are soluble in dilute nitric acids; crude platinum can
be dissolved only in concentrated aqua regia, a mixture of 3 parts of
hydrochloric (muriatic) acid and 1 part of nitric acid.
In
some placer deposits the grains of platinum are coated with a dark film
and somewhat resemble the grains of the dark minerals chromite,
magnetite, or ilmenite, from which they are separated by careful
panning, as the specific gravity of platinum is greater than that of
any of those minerals.
Platinum
will not amalgamate with quicksilver alone, but will amalgamate if
sodium is added. In ordinary quicksilver amalgamation the flakes of
platinum float on the surface and can be removed. If sodium amalgam is
used, the platinum may be separated from gold by agitating the amalgam
with water until all the sodium is used up to form sodium hydroxide;
then the platinum will come out on the surface of the amalgam,
provided, of course, the amalgam is sufficiently liquid.
Platinum
has a hardness of 4 to 5, and can be scratched with a knife. It is so
malleable that it can be pounded without heating into very thin sheets.
It is practically infusible; the grains can not be melted together, as
particles of gold can.
A
relatively simple chemical test can be made to determine the platinum.
The metallic particles are dissolved by boiling in concentrated aqua
regia and the resulting solution is allowed to remain on the stove till
dry. The residue is dissolved again in hydrochloric acid and evaporated
by boiling till the solution is thick but not quite dry. This mass is
dissolved in distilled water and a few drops of sulphuric acid and of
potassium iodide solution are added, which, in the presence of
platinum, causes the solution to turn a very characteristic wine-red,
if much of the metal is present, or to a reddish pink in thepresence of
small quantities of platinum.
The
test outlined above is fairly delicate, but it can not be used to
detect traces of platinum in the presence of large quantities of iron
or other elements.
A
second test may be applied to the aqua regia solution after the
resolution in hydrochloric acid outlined above. In this test potassium
chloride (KC1) is added to the solution, which precipitates yellow
crystals of potassium platinic chloride (K2PC1„), if platinum is present.
A third test may be applied. Add ammonium chloride (NH4C1) to the aqua regia solution, which will precipitate yellow crystals of ammonium platinic chloride, if platinum is present.
The
precipitates from tests 2 and 3 are both insoluble in alcohol but are
soluble in water and may be reduced to platinum sponge by heating.
All
these tests are comparatively simple and positive when made on single
grains, but they can not be rehed upon when various other elements are
present in the material tested. It is, therefore, recommended that
their use be restricted to grains of a single mineral picked from the
concentrates obtained by panning a sample of either rock or gravel.
