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. During the operation the vessel is freely shaken, lumps are
comminuted with a glass rod, and a few drops of the liquid are ultimately
tested with hydrogen sulphide or ammonia. The remainder of zinc dust is
then weighed, and its value deducted from the original weight. Zinc and
cadmium of the filtrate are determined as above. On repeating this method
several times most satisfactory results are obtained.

Another mode of operating is to employ an excess of copper sulphate and to
determine the copper dissolved in the filtrate. The separation of copper
from cadmium being difficult and laborious, and the volumetric estimation
with potassium cyanide not practicable, it is not prudent to apply this
method.