The following experiments
test the method of separation. Ten grams of copper were treated as above
and precipitated with 1.5 gram of "soda;" the precipitate contained 0.6
milligram of bismuth (= 0.006 per cent.). The filtrate treated with
another 1.5 gram of "soda" gave a precipitate which was free from
bismuth. To the filtrate from this was added 1.0 milligram of bismuth,
and another fraction was precipitated with 1.5 gram of "soda." In this
precipitate was found 1.0 milligram of bismuth. To the filtrate another
milligram of bismuth was added and the separation with "soda" repeated.
The bismuth was separated from this precipitate with ammonic carbonate
before determination, and 0.9 milligram was found.
~Arsenic.~--The proportion of arsenic in copper varies from 0.01 to 0.75
per cent. whilst in coarse copper it may amount to 2 or even 3 per cent.
To determine it, dissolve 5, 10, or 20 grams of the copper (according to
the amount of arsenic present) in 18 c.c., 35 c.c., or 70 c.c. of nitric
acid, and an equal volume of water. Boil off the nitrous fumes, dilute
to 100 c.c. and neutralise with soda; add 1.5 or 2 grams of carbonate of
soda dissolved in a little water, and boil. Filter (washing is
unnecessary) and dissolve back into the flask with a little dilute
hydrochloric acid; add 30 c.c. of dilute ammonia and 25 c.
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