The dry assay of zinc can only be made indirectly, and is
unsatisfactory. Zinc is volatile, and at the temperature of its
reduction is a gas. It is impracticable to condense the vapour so as to
weigh the metal, consequently its amount is determined by loss. The
following method gives approximate results: Take 10 grams of the dried
and powdered ore and roast, first at a low temperature and afterwards at
a higher one, with the help of carbonate of ammonia to decompose the
sulphates formed; cool and weigh. The metals will be present as oxides.
Mix with 2 grams of powdered charcoal and charge into a black-lead
crucible heated to whiteness, cover loosely, and leave in the furnace
for about a quarter of an hour. Uncover and calcine the residue, cool
and weigh. The loss in weight multiplied by 8.03 gives the percentage of
zinc in the ore.
WET METHODS.
Solution and separation may be effected as follows: Treat 1 or 3 grams
of the substance with 10 or 30 c.c. of hydrochloric acid or aqua regia;
evaporate to dryness; take up with 10 c.c. of hydrochloric acid and
dilute to 100 c.c.; heat nearly to boiling; saturate with sulphuretted
hydrogen; filter, and wash with water acidulated with hydrochloric acid.
Boil off the sulphuretted hydrogen and peroxidise with a few drops of
nitric acid. Cool; add caustic soda till nearly, but not quite,
neutralised, and separate the iron as basic acetate by the method
described under _Iron_.
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