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350 MINERAL RESOURCES, 1914----PART I.
The
scorification with lead is for the purpose of cleaning the osmiridium
from sand, heavy oxides, etc. The osmiridium is collected by the lead
and the impurities are slagged off. Some care is required in igniting
the residue, as osmiridium will lose osmium if heated to redness in the
air. For this reason the paper is burnt after separating from the
osmiridium.
Determination of platinum.—Fill
the 1,000 c. c. flask containing the solution of the ore to the mark.
Mix thoroughly by pouring into a dry beaker and back into the flask
four times. Taking a 25 c. c. pipette, which has been carefully
standardized against the 1,000 c. c. flask, rinse it out with a portion
of the solution and then pipette two portions to 75 e. c. beakers.
Evaporate these portions just to dryness on the water bath. Add to each
5 c. c. of dilute hydrochloric acid (1 acid : 9 water) and warm for
five or 10 min. Allow to cool and unless perfectly clear filter through
5£ cm. papers into similar beakers. Wash with water till the filter
papers are perfectly colorless. This can be done easily with less than
20 c. c. of water.
Make
up each portion so that it contains about 25 c. c. Add to each 8 grams
of c. p. ammonium chloride and heat on the water bath for one hour with
occasional stirring. Remove from the bath and let stand at least three
hours, or over night if convenient. Decant the clear solutions through
a 9 cm. ashless filter into 150 c. e. beakers. Allow all the solution
to run through and then wash the precipitates of ammonium
chlorplatinate, (NH4)2PtCls, onto the filters
with a 20 per cent solution of ammonium chloride. Allow all the
solution to run through and then wash repeatedly around the edge of
the papers with small quantities of the ammonium-chloride solution till
the filtrate is colorless and the papers are white. Finally rinse
around the edge of the papers with alcohol. After all the solution has
run through place the filters with their points down into size B
Battersea annealing cups. Carefully fold over the edges of the papers
so as completely to inclose the precipitate in an envolope of paper.
Cover the crucibles and bring to a good red heat by very slowly
increasing the temperature. Finally remove the covers from the
crucibles till the papers are burnt off. After cooling, transfer the
platinum to the pan of the assay balance and weigh. (The duplicate
should not vary more than two milligrams. The average is taken as
correct.)
The precipitation of (NH4)PtCle
is usually made with the addition of alcohol to reduce the supposed
solubility of the precipitate. I am sure that by the above method I can
take 0.0004 gram of platinum in the given size solution. 25 c. c.
precipitate it, wash with the usual quantity of wash solution, and get
back more than half of the original platinum. The solubility under the
conditions given is negligible for this assay.
The
solutions are heated on the water bath for two reasons: First, to get
all the ammonium chloride rapidly into solution and, second, to change
the precipitate from an amorphous to a sandy, crystalline precipitate,
which is easily filtered and washed.
The
precipitate of platinum usually contains iridium as (NEU^IrCle.
Iridium alloyed with platinum in small quantities, less than 10 per
cent, dissolves with the platinum in aqua regia. It precipitates with
the platinum, but not quite so completely. The traces which pass the
precipitation are afterward estimated with rhodium.
Pure
(NILhPtCIc is a pure yellow. The iridium compound has an intense red
color. If A per cent of iridium is present it gives a decided red tint
to the platinum precipitate. The quantity of iridium present, if not
more than 5 per cent, can De judged by the color of the precipitate
quite accurately enough for setting a value on the ore.
There
is a slight loss in igniting the platinum precipitate. This loss is
lessened by keeping inclosed in the filter paper and by slow heating.
Determination of gold.—Combine
the filtrate from the platinum precipitation and add 5 grams of ferrous
sulphate in water solution plus HC1. Allow to stand till the gold has
settled. Filter and wash on the filter, transfer to an annealing cup,
and burn off the paper in front of the muflie. When cool, wrap in lead
foil and cupel as usual for gold.
Determination of palladium.—Add to the filtrate from the gold about i its
volume of hydrochloric acid. Add a 10 per cent solution of potassium
iodide five drops at a time till no further precipitation of black
palladous iodide (Pdl2) takes place. Heat on the hot plate
nearly to boiling. Cool, filter, and wash with dilute hydrochloric acid
(1 acid : 4 water). Wash free from iron. Transfer
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