~Determination of Copper in Copper Pyrites.~--Weigh up 2 grams of the
dried and powdered ore, and place in an evaporating dish about four
inches in diameter. Cover with 20 c.c. of nitric acid and put on a hot
plate. Evaporate to dryness without further handling. Allow to cool and
take up with 30 c.c. of hydrochloric acid, boil, dilute, and transfer to
a pint flask, filtering if necessary. Make up the bulk with the washings
to about 150 c.c. Precipitate with sulphuretted hydrogen, filter, and
wash back the precipitate into the flask. Add 15 c.c. of nitric acid,
and boil down rapidly to 10 c.c. Dilute, add 30 c.c. of dilute ammonia,
make up to 150 c.c., and cool. For the standard, weigh up 0.5 gram of
copper, more or less, according to the quantity judged to be present in
the assay. Dissolve in 20 c.c. of dilute nitric acid, boil off nitrous
fumes, add 30 c.c. of dilute ammonia, make up to the same bulk as that
of the assay, and cool. Titrate the two solutions side by side and as
nearly as possible in the same manner.
Since the assay solution is often turbid from the presence of small
quantities of lead and of iron from incomplete washing, and since this
slight precipitate is very slow in settling, the standard can hardly be
compared strictly with the assay. This can be counteracted by
precipitating in both solutions a mixture of ferric and aluminic
hydrates, which settles readily and leaves the supernatant liquor clear.
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