To effect this, boil the nitric acid solutions with 30 c.c. of a
solution containing 15 grams each of alum and ferrous sulphate to the
litre. In an actual determination 2 grams of the ore were taken and
compared with 0.5 gram of copper. The assay required 57.7 c.c. of
cyanide and the standard 52.5 c.c.
52.5 : 0.5 :: 57.7 : 0.5495
This on 2 grams of ore = 27.47 per cent.; the same sample by
electrolysis gave 27.60 per cent. of copper.
~Determination without Previous Separation.~--Dissolve up 2 grams as
before, but, instead of passing sulphuretted hydrogen, add 30 c.c. of
dilute ammonia, shake well, and cool. Prepare a standard by dissolving
0.5 gram of copper in 1 c.c. of nitric acid, add 0.6 gram of iron in the
form of ferric chloride and 20 c.c. of hydrochloric acid, dilute to
about 150 c.c., add 30 c.c. of dilute ammonia, and cool. Titrate the two
solutions side by side. In a determination on the sample last used, 58
c.c. were required for the assay and 53 c.c. for the standard, which
indicates 27.3 per cent. of copper.
This method of working is somewhat rough.
IODIDE METHOD.
This is based upon the fact that when potassic iodide in excess is added
to a strong solution of a cupric salt in a faintly acid solution,
cuprous iodide is formed and an equivalent of iodine liberated.
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