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.
When calcined zinciferous pyrites have to be examined, the estimation of
zinc is similar to that employed in the analysis of zinc ore. The sample
is exhausted with water, filtered, and, to eliminate calcium sulphate and
basic iron sulphate, evaporated to dryness. It is then dissolved in a
small quantity of alcohol and water, refiltered, and the filtrate
decomposed with ammonium carbonate. The original residue is treated with a
solution of ammonium carbonate, which dissolves arsenious acid and basic
zinc sulphate, filtered, and united with the first filtrate.
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