the chemical determination of platinum and allied metals the following articles1 by Mr. A. M. Smoot, chief chemist of Ledoux & Co., New York, are published in full:
DETERMINATION OF PLATINUM, PALLADIUM, AND GOLD. By A. M. Smoot.
Scorify
the lead buttons from two or more i-a.t. crucible fusions together,
adding at least six times as much silver as the combined weight of the
Pt, Pd, and Au present, and cupel hot. In rich materials, such
as slimes or concentrates, two J-a.t. fusions suffice, but low-grade
ores may require 10 or more J-a.t. fusions combined for each
determination.
Part the silver beads with HN03
(1:6), followed by stronger parting acia (1:1) and wash with water as
usual. All Pd goes into solution, together with considerable Pt. The
residue consists of Au plus some Pt. Dissolve residue ir. strong aqua
regia and reserve the solution (solution A). Precipitate the
silver in the nitric-acid solution—containing Ag, Pd, and some Pt—with
HC1. Practically all the Pt will remain in solution; but the
precipitated AgCl is pink ana contains considerable Pd. Filter off the
AgCl, scorify and cupel it and part again with HNOs (1:6) ; all should
dissolve. Reprecipitate the Ag with HC1. The liquid now contains most
of the remaining Pd, but some is coprecipitated with AgCl. Filter off
the AgCl and add the filtrate to the first filtrate from AgCl. Again
scorify and cupel the silver chloride, dissolving the silver in nitric
acid as before and reprecipitating the silver as chloride. In most
cases the filtrate from this silver chloride contains all the remaining
Pd. If, however, the AgCl is distinctly pink, another separation must
be made.
Unite
all filtrates from AgCl precipitations and evaporate to small bulk,
adding the aqua-regia solution of the Au and Pt (solution A). The
liquid now contains all the Au, Pt, and Pd present in the original ore,
together with traces of Ag due to solubility in AgCl in excess of HC1,
and also traces of Pb gathered from the lead retained in the silver
buttons from the several recupellations.
Evaporate
the liquid to dryness on the steam bath; take up with dilute HC1 (1:3)
and evaporate again to dryness; take up with five drops of HC1 and 40
c.c H20. Pay no attention to any insoluble residue of AgCl or PbCl-.a
Precipitate gold by,adding, say, 3 gm. of oxalic acid to the solution
and boiling it. Let stand overnight and filter off the Au. If Pt and Pd
are high, it is necessary to redissolve the Au in aqua regia,
evaporating with HC1 to dryness and repeating the oxalic-acid
precipitation, uniting the filtrate with that from the first gold
precipitation. Burn the filter containing the gold and scorify it with
six times its weight of silver and a little test lead; cupel, part, and
weigh the gold as usual.
To
the oxalic-acid filtrates from Au add 5 c. c. of HC1 and make volume up
to 150 c. c.; heat to boiling and precipitate Pt and Pd with a rapid
current of HiS in hot solution, passing the current of gas for
some time and keeping the solution hot during precipitation. Filter and
wash the Pt and Pd sulphides with H2S water containing a
little HC1. Wash the precipitate from the filter with a fine water jet
into an original beaker; spread the filter paper (which will contain a
small amount of precipitate impossible to wash off) with the
precipitate side down over the lower side of a watch-glass cover. Add
aqua regia to the precipitate in the beaker and place the cover on the
beaker; warm gently to dissolve the Pt and Pd sulphides. The fumes
arising from the acid dissolve the traces of Pt and Pd adhering to the
filter paper. When solution is complete and the filter paper is white,
remove the watch-glass cover and wash the paper with hot dilute HC1
thrown against it in a fine stream.
1 Eng. and Min. Jour., vol. 99, pp. 700-701, Apr. 17, 1915.
2 In materials rich In palladium the small amount of AgCl + PbCl2
may be distinctly pink in color and retain weighable quantities of Pd.
If this is the case, the Pd may be recovered in the solution from the
nitric acid parting of the gold. To do this, precipitate the silver in
this liquid by adding HC1, filter off the silver chloride and
evaporate the filtrate to dryness. Take up with a drop of HC1 and a
little water, let stand overnight and filter through a very small
filter. This liquid may be added to solution B before precipitating
palladium with glyoxime.
