The filtrate is now ready for the
gravimetric determination.
GRAVIMETRIC ASSAY.
Pass a current of sulphuretted hydrogen through the solution containing
the antimony to which a little tartaric acid has been previously added.
Pass the gas till the precipitate becomes dense, and the antimony is all
down. The solution must not be too strongly acid. Filter off the
precipitate, wash with hot water, dry in the water oven, transfer to a
weighed porcelain dish, and cautiously treat with fuming nitric acid.
Continue the action on the water bath till the sulphur and antimony are
completely oxidised. Evaporate; ignite, gently at first, then strongly
over the blast; cool, and weigh. The residue is a white infusible
powder, and consists of antimony tetroxide, Sb_{2}O_{4}, containing
78.94 per cent. of the metal.
~Determination of Antimony as Bigallate.~--What appears to be a very
good method has been worked out by M.A. Guyard, and is described in
Crookes' _Select Methods_, p. 398.
The antimony must be in solution as antimonious chloride, and must not
be accompanied by an excess of hydrochloric acid. To ensure these
conditions, the solution is treated with potassium iodide until no more
iodine is evolved, and is then evaporated to remove the excess of
hydrochloric acid. To the concentrated, and nearly neutral, solution a
freshly-prepared solution of gallic acid is added in slight excess.
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