The residue from HN03
parting (which may contain gold, iridium, and rhodium, as well as some
ruthenium, osmium, and undissolved platinum) is treated with 1:5 aqua
regia, dissolving the gold and platinum. The residual platinum metals
are filtered off on a small filter paper, ignited (using "impervite "
crucibles), and weighed. The aqua regia solution, which may contain
gold and platinum, is evaporated to dryness, taken up with HCl and
again evaporated to dryness, then extracted a second time with HCl,
evaporated nearly to dryness, taken up with cold water, and any
residual matter is filtered off. The solution is then boiled with
oxalic acid until the gold is all precipitated. It is well to let the
solution stand overnight before filtering off the gold. The gold is
filtered off and the platinum, recovered by boiling the solution which
has first been neutralized with sodium carbonate with formic acid, is
then filtered off; ignited, and its weight added to that of the platinum already found.
It
is advisable to check results by running a known sample along with the
unknown. This known sample is made by adding to a crucible charge the
approximate quantity of gold, silver, and platinum found in the ore.
The check is treated in every way the same as the sample to be
examined, the latter being corrected if the known sample shows a
discrepancy.
The
resulting platinum andpalladium should be dissolved separately in aqua
regia, taken to dryness twice with HCl, extracted with water, and,
after each solution has been acidified with HCl, tested qualitatively,
KI giving a red color with platinum and a dark-brown precipitate with
palladium.
Typical assay charges used in the estimation of platinum, in different ores.
