The
application of ammonium carbonate as solvent has the advantage, over the
application of ammonia, that it is a far better solvent, that it
decomposes insoluble basic sulphates, and that the remaining carbonates
are readily dissolved by acids.
The decomposition of zinc dust is accompanied by a lively evolution of
gas; it is therefore necessary to continue the digestion of the sample
till no more hydrogen is given off. Zinc dust contains both metals and
their oxides, and methods which, from the volume of hydrogen generated,
determine indirectly the percentage of metallic zinc do not give the real
composition of the zinc dust. For the determination of the metallic
components the material is digested with a solution of copper sulphate,
which dissolves zinc and cadmium; the liquid is filtered, acidified, and
decomposed with hydrogen sulphide, or treated with a solution of ammonium
carbonate. The use of cupric chloride is not advisable, as it corrodes
lead, and gives rise to the formation of soluble chloride of lead, which
complicates the separation of zinc from cadmium. The best mode of
operation is the following: Both copper sulphate and zinc dust are weighed
separately, the former is dissolved in water and the latter introduced
into the solution of copper sulphate in small portions until it appears
colorless.
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