If
there is not at least 15 times as much silver as platinum in the ore,
enough of a silver salt—chloride or nitrate—is added to make up the
deficiency. The presence of excess silver is required to render the
platinum soluble in HN03 when parting, to assist in removing
the last traces of lead in cupelling and to lessen any tendency toward
loss in the cupel. The addition of silver in the form of a salt makes
possible a thorough mixing with the charge, and, when reduced, the
numerous silver particles are distributed in a way to be most effective
in collecting the platinum.
To
an unknown ore about 0.05 gram of silver chloride may be added. The
charge is fluxed as in the ordinary fire assay for gold and silver,
except that after the fusion has become quiet the temperature should be
raised somewhat higher than is the usual practice and the heating
continued for about an hour* The crucible is removed from the furnace
without agitating the contents and left to cool. Cooling without
agitation prevents loss of iridium, ruthenium, and osmiridium, which do
not alloy with lead but sink through it and are held mechanically when
the button cools. If the charge is poured, these metals are apt to be
partly lost.
The
button is freed from slag and cupelled at a high temperature. As the
high temperature causes a loss of silver, a check or proof assay should
be run at the same time to be used in correcting this error.
When
platinum makes up over 1.6 per cent of the bead, the latter appears
frosted to the unaided eye. The microscope will detect platinum when it
constitutes 0.3 per cent of the silver bead. Considerable quantities of
platinum make the bead flat and irregular. The roughness caused by
iridium is of finer texture than that due to platinum. Palladium gives
the surface of the bead an embossed appearance. Ruthenium in quantity
turns the surface to a bluish-black color, a black scum being left on
the cupel. After cupelling, the button is then parted with tlN03,
first with 1:4, then with 1:1, and finally with 2:1 acid. If gold,
silver, and all the platinum metals are present in the ore, the silver,
palladium, and platinum are dissolved by this treatment, leaving the
gold, iridium, rhodium, and some ruthenium and osmium. Most of the
osmium and part of the ruthenium are oxidized and lost during
cupellation. Part of the iridium may not collect in the silver and will
be lost on the cupel.
If
considerable platinum is found, some will be left still undissolved.
The residue is filtered off on a 5-centimeter, ashless filter paper,
ignited, and saved for the recovery of any residual platinum as well as
for the determination of iridium and rhodium.
Dilute
HC1 is then added to the filtrate slowly, with constant stirring to
precipitate the silver. After this has set over night, the silver
chloride is filtered off and washed with water, acidulated with HN03.
If the precipitate is pink, it must be redissolved and again
precipitated to remove occluded platinum or palladium. Although
palladium may be separated from silver by one precipitation as the
chloride, in acetic-acid solution, the addition of the ammonia
necessary to neutralize the HN03 present causes a bulk of salts on evaporation, which interfere with the subsequent operations.
Evaporate
the filtrate just to dryness, do not bake; take up withdilute HC1 and
again evaporate just to dryness, then take up with dilute HC1, transfer
to a 30-cubic centimeter beaker and evaporate nearly to dryness (until
there is a dry spot in the center of the beaker). When cold, take up
with a very little cold water and filter on a 5-eentimeter paper to
remove the last traces of silver. It is important that all the silver
be removed; at this point> otherwise silver would come down on boiling with formic acid and vitiate the platinum-palladium result.
The filtrate is made slightly alkaline with Na2C03,
formic acid added, and the solution boiled in a covered beaker until
all the platinum and palladium are precipitated (about 30 minutes).
These finely divided metals are filtered on a small (5-centimeter or
less) paper, washed with hot water, ignited, and weighed. As the
platinum metals adhere to both glazed and unglazed porcelain crucibles
on ignition, '' impervite " crucibles may be used to advantage for the
ignition of the finely divided metals. In transferring the platinum
metals from the beaker to the filter, it has been found convenient to
use a small wash-bottle which will deliver a very fine stream of water.
Funnels that have been cut down so that the small filter papers extend
beyond the rim prevent the fine metals from creeping.
The filtrate should be boiled again with formic acid to confirm a complete precipitation of the platinum metals.
If
the first parting solution is yellow or orange, the presence of
palladium is indicated and the platinum may be separated from the
palladium and determined. Warm the metals with dilute HN03
(1:4) which quickly and completely dissolves the palladium, filter,
wash, ignite, and weigh the pure platinum. The palladium may be found
by difference, or it may be precipitated by formic acid from the
solution obtained by evaporating to dryness with HC1, dissolving in
water, and neutralizing with Na2C03.
