The precipitate is filtered off, and
digested with a solution of ammonium acetate or of sodium hyposulphite
at 50° or 60° C. to dissolve out any lead sulphate. The residue is
filtered off, washed, dried, and ignited. The ignited substance is mixed
with four or five times its weight of "fusion mixture," and fused in a
platinum-dish over the blowpipe for a few minutes. When cold, it is
extracted with cold water, filtered, and washed. The residue is
dissolved in dilute hydrochloric acid, and (if necessary) filtered. The
solution contains the barium as baric chloride mixed, perhaps, with
salts of strontium or lime. To separate these, ammonia is added till the
solution is alkaline, and then acetic acid in slight excess. Chromate of
baryta is then thrown down, by the addition of bichromate of potash, as
a yellow precipitate. It is allowed to settle, filtered and washed with
a solution of acetate or of nitrate of ammonia. It is dried, ignited
gently, and weighed. It is BaCrO_{4}, and contains 60.47 per cent. of
baryta.
GRAVIMETRIC DETERMINATION.
The gravimetric determination of baryta, when lime and strontia are
absent, is as follows:--The solution, if it contains much free acid, is
nearly neutralised with ammonia, and then diluted to 100 or 200 c.c. It
is heated to boiling, and dilute sulphuric acid is added till no further
precipitation takes place.
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