Evaporate
the aqua-regia solution to dryness, take up the residue with HC1 and
evaporate again to dryness to remove all HNOs. Take up the residue with
two or three drops of HC1 and about 2 c. e. of H2O. The solution is
usually perfectly clear, but it may be slightly cloudy owing to the
presence of a little AgCl in it. No attention need be paid to this,
however. Add 5 to 10 c. c. of a saturated solution of NH4C1,
stir well and allow to stand over night. Platinum is precipitated as
ammonium-platinum chloride—(NHihPtCle- Filter and wash the precipitate
with 20 per cent NILC1 solution. All Pd passes into the filtrate which
is reserved (solution B). Dissolve the Pt precipitate in boiling hot 5 per cent H2S04; heat the liquid to actual boiling and precipitate with H2S as before, filtering and washing with H2S
water. Burn the filter and precipitate at a low temperature in a
scorifier; add six times as much Ag as Pt, scorifying with lead, cupel
and part the silver bead containing the platinum with H2SO4; decant off
the silver solution and wash once with strong H2S04, followed by 50 per cent H2S04,
until practically all silver is washed away; finally wash with water,
anneal, and weigh. A minute quantity of Ag is retained with the
platinum, but it can usually be neglected. In very important work,
where the amount of platinum is large, dissolve in aqua regia,
evaporate the solution to dryness, take up with a drop of HC1, dilute
largely with water, and let the AgCl settle over night; filter on a
small paper, cupel it with a little sheet lead, and deduct the weight
from the weight of platinum. This refinement need not be considered in
materials running less than 15 or 20 ounces to the ton.
It
may seem an unnecessary step to precipitate the platinum as sulphide,
scorify it with silver, and part it as described in the foregoing.
General practice has been to ignite the ammonium-platinum-chloride
precipitate and weigh the metallic residue. When this is done, however,
there is danger of losing considerable platinum, which is carried away
mechanically during the decomposition of the compound; furthermore, it
is extremely difficult (if not impossible) to collect the finely
divided residue for weighing, and the precipitate invariably contains
lead and silver. Precipitation as sulphide, scorification, and
eupellation with excess silver and parting with sulphuric acid overcome
the difficulties inherent in handling the ammonium precipitate.
The palladium is all contained in the filtrate and washings from the platinum-ammonium-chloride precipitates (solution B). Add
to this solution at least seven times as much di-methylglyoxime as
there is Pd present (in any case, at least 0.1 gm. glyoxime). The
precipitant should be dissolved in a mixture of two-thirds strong HC1
and one-third water. Dilute the liquid to 250-300 c. c, heat on a steam
bath for half an hour and let stand over night. I'd is precipitated as
a voluminous, yellow, easily filtered glyoxime compound (Cs,Hi4X404) 2Pd,
containing, when dried at 110° 0., 31.680 per cent of Pd. Filter the Pd
precipitate on a weighed Gooch crucible and wash it, first, with
dilute HC1, half-and-half, then with warm water and finally with
alcohol; dry it at 110° to 115° C. and weigh. The disadvantage of
weighing palladium on a Gooch crucible is overcome—at least to some
extent—by the fact that the Td compound contains a relatively small
amount of Pd—less than one-third of its weight. This compound may also
be weighed on carefully counterpoised papers; but it is better to use
Gooch crucibles, if they are available, because of the relatively
strong acid which is required for washing. The object in using
half-and-half hydrochloric acid as a wash liquid is to dissolve out
any excess of the glyoxime precipitant. This is easily soluble in
moderately strong HC1, but is substantially insoluble in wa^er.
DETERMINATION OF SILVER IN ORES AND CONCENTRATES CONTAINING PLATINUM AND PALLADIUM.
By A. M. Smoot.
Make
the usual crucible fusion on one-quarter, one-half, or full assay ton,
according to the amount of silver present. Instead of cupeling the lead
button, hammer it free from slag and dissolve it in dilute nitric acid.
Host of the silver passes into solution together with palladium, and
perhaps a trace of platinum; but gold and most of the platinum remain
insoluble. The gold and platinum retain an appreciable proportion of
silver which can not be washed out. Filter out the insoluble residue
and wash it thoroughly with hot dilute nitric
