acid,
followed by hot water. Scorify the residue once more with a little lead
and dissolve the lead button as before, filtering into the beaker
containing the first filtrate. In this liquid precipitate the silver as
AgCl by adding standing XaCl in sufficient quantity; stir well, and if
the amount of silver is small, add about i c. c. of strong
H2SO4 to form a precipitate of lead sulphate. Let the silver chloride,
or the silver chloride plus lead sulphate, settle over night or until
the supernatant liquid is clear; filter through double filter papers;
ignite and scorify the residue of silver chloride with test lead.
If
the amount of palladium contained in the sample is small, the silver
bead obtained by cupeling the lead button obtained by scorifying the
silver chloride may be considered as sufficiently pure for ordinary
purposes. It contains, of course, some palladium, and in accurate
silver determinations the lead button from the first silver-chloride
precipitation should be redissolved and the silver reprecipitated,
filtered, and scorified as before. The amount of palladium retained
after the second precipitation and scorification is so small as to be
negligible.
As
the method described above does not give details for the determination
of all the platinum-group metals, the following paper by Mr. Martin
Schwitter, chief chemist of Goldsmith Brothers' Refinery Co.,1 of Chicago, is also quoted in full:
THE ASSAY OF CRUDE PLATINUM. By Maktin Sohwittee.
Crude
platinum is on the market in the shape of small nuggets, ranging in
weight from 100 mg. down. It may be divided roughly into two portions,
the insoluble and the soluble in aqua regia. The insoluble includes
osmiridium as its only valuable constituent, while the soluble includes
platinum, iridium, palladium, rhodium, and gold. It is desired to
determine each of the above metals with accuracy.
This
original method is now in use by a large buyer of crude platinum and
has been checked up by results obtained in refining the purchased lots
of crude.
The method.—Weigh
25 grams of the sample into a 300 c. c. beaker. Add 200 c. c. of aqua
regia (1 part nitric to 3 parts hydrochloric acid, by volume) and place
on the water bath for about one hour, then place on a thin piece of
asbestos on the hot plate and boil gently for two hours. Remove from
the hot plate and allow to settle for 15 min. Decant the solution into
another 300 c. c. beaker and again boil for about an hour to dissolve
any finely divided metal which may have decanted over. Remove from the
hot plate and fill up the beaker 'with water. Stir well, then settle
over night. Immediately after decanting the aqua regia from the
original beaker, add another 200 c. c. of aqua regia to the residue
therein and repeat the warming and boiling of the acid. Finally remove
from the plate, fill with water, stir, and let settle overnight. Unless
the ore is in unusually large pieces, the solution of the soluble
minerals will be complete with this treatment, which is planned to take
one day.
Decant
the clear solutions from both the beakers into a 1,000 c. c. volumetric
flask, being careful not to allow any of the insoluble to be carried
over. Wash the contents of one of the beakers into the other so as to
have everything in one beaker and to dilute the acid before filtering.
Filter through a 9 c. c. paper into the 1,000 c. c. flask. Wash the
insoluble from the beaker onto the paper and then wash on the paper
till the solution which runs through is perfectly colorless.
Determination of omniridium.—Transfer
the filter paper and its contents to a 2J in. scorifier, add 10 to 15
grams of test lead, fill the scorifier level full with litharge, and
fuse in the muffle for i hour. Allow the scorifier to cool,
break it. and remove the lead button. Clean the lead button thoroughly
with as little hammering as possible, place it in a 150 c. c. beaker
and dissolve out the lead with dilute nitric acid (1 vol. nitric acid
to 3 vol. water). Place on the water bath to assist solution. When the
lead has been all dissolved, filter through an ashless filter and wash
the residue of osmiridium on the filter till free from lead. Dry the
filter, separate the osmiridium from it as completely as possible, and
burn the filter paper. Add the osmiridium to the residue from the
paper, dry thoroughly on the hot plate and weigh as osmiridium.
1 Eng. and Min. Jour., vol. 97, pp. 1249-1250, June 20, 1914.
