Silver precipitates
carry about 2.5 per cent. of zinc.
GASOMETRIC METHOD.
Metallic zinc is readily soluble in dilute hydrochloric or sulphuric
acid, hydrogen being at the same time evolved.[74] The volume of the
hydrogen evolved is obviously a measure of the amount of zinc present in
the metallic state. The speed with which the reaction goes on (even in
the cold) and the insolubility of hydrogen renders this method of assay
a convenient one. It is especially applicable to the determination of
the proportion of zinc in zinc dust. The apparatus described in the
chapter on gasometric method is used. The method of working is as
follows: Fill the two burettes with cold water to a little above the
zero mark, place in the bottle about 0.25 gram of the substance to be
determined, and in the inner phial or test tube 5 c.c. of dilute
sulphuric acid; cork the apparatus tightly and allow to stand for a few
minutes; then bring the water to the same level in the two burettes by
running out through the clip at the bottom. Read off the level of the
liquid in the graduated burette. Turn the bottle over sufficiently to
spill the acid over the zinc, and then run water out of the apparatus so
as to keep the liquid in the two burettes at the same level, taking care
not to run it out more quickly than the hydrogen is being generated.
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