to
a porcelain crucible and ignite in the muffle at a good red heat. Cool,
moisten with a few drops of dilute formic acid to reduce oxide, and dry
thoroughly on the hot plate. Transfer to the pan of the assay balance
and weigh the palladium.
Textbooks
usually describe palladous iodide as easily soluble in excess of KI.
This is true, but the presence of a large amount of HC1 probably
prevents any excess of KI being present, hydriodic acid and potassium
chloride being formed. This precipitation is complete. The strong
hydrochloric-acid solution also prevents the precipitation of lead and
copper iodides if only a slight excess of KI is used.
Dilute formic acid reduces palladium oxides to metal.
Determination of rhodium.—To the filtrate from Pdl2
and c.p. zinc till the solution is colorless, heating the solution
while doing so. Allow all zinc to dissolve and the precipitate to
settle. Decant the solution and wash the precipitate three times by
decantation with 100 c. c. portions of hot water. Add 25 c. c. of
dilute nitric acid (1 acid: 3 water) and place on top of the water bath
for 10 minutes. Filter through an ashless filter, wash two or three
times with water, ignite, reduce in hydrogen, and weigh as rhodium plus
traces of iridium.
Zinc
reduces all the platinum metals from solution. In this case only
rhodium and traces of iridium are present. The nitric acid dissolves
any base metals which may be present, but as freshly precipitated
rhodium is slowly soluble in nitric acid, the treatment must not be too
much prolonged.
A
representative assay of crude met;)I would be the following:
Osmiridium, 20 fine; Pt (containing 3 per cent Ir), 830; Pd, 5; Au, 30;
Rh, 20 fine.
The
strongly colored solutions furnished by all of the platinum metals,
platinum giving the least color, is a good guide for washing, zinc
precipitation, etc.
FIELD TESTS FOB PLATINUM.
The
articles quoted are of greatest use to assayers, chemists, and those
having access to well-equipped chemical laboratories. For the
prospector and those with slight chemical equipment the identification
of platinum and allied metals is not always certain. The following
tests are believed to be most easily applied, and, if the material is
carefully handled, they will give fairly reliable results.
Platinum
as found in the placers of the Western States usually occurs as small
scales or flakes, but in some placers it occurs as irregular nuggets.
The distinctly metallic mineral has a color ranging from silvery white
to steel-gray, its shade depending on the quantity of impurities
present. In some placer deposits the grains of platinum are coated with
a dark film and somewhat resemble grains of ilmenite or magnetite, from
which, however, they are separated by careful washing, as platinum has
a specific gravity equal to or greater than gold, and so stays in the
pan with the gold.
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 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, when
the platinum will come out on the surface of the amalgam, provided, of
course, that 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 infusible, can not be run together as gold can, and is insoluble
in all acids except aqua regia, a mixture of two parts hydrochloric
(muriatic) acid and one part nitric acid. This solution (platinum
chloride?) is yellow, but its color is changed to deep red by the
addition of metallic tin. From an aqua regia solu-
