Where a large number of
results are wanted quickly it is a very convenient method. The iodide
process is very satisfactory when worked under the proper conditions.
CYANIDE METHOD.
The process is based upon the facts--(1) that when ammonia is added in
excess to a solution containing cupric salts, ammoniacal copper
compounds are formed which give to the solution a deep blue colour; and
(2) that when potassic cyanide is added in sufficient quantity to such a
solution the colour is removed, double cyanides of copper and potassium
or ammonium being formed.[54] In the explanation generally given the
formation of cuprous cyanide is supposed[55]; but in practice it is
found that one part of copper requires rather more than four parts of
cyanide, which agrees with the former, rather than the latter,
explanation.
Reliance on the accuracy of the process cannot rest upon the
supposition that the cyanide required for decoloration is proportional
to the copper present, for varying quantities of ammonia salts, ammonia
and water, and differences of temperature have an important effect. The
results are concordant and exact only when the cyanide is standardised
under the same conditions as it is used. It is best to have the assay
solution and that used for standardising as nearly as possible alike,
and to titrate the two solutions side by side.
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