Generally a small, but
decided, precipitate of alumina comes down, together with sulphides of
any zinc or manganese which is present. The precipitate is allowed to
settle, dried, ignited, and weighed. The metals are separately
determined in it; and the residue is counted as alumina, and added to
that already found. The mixed precipitate amounts to from 1 to 2 per
cent. of the sample.
~Lime.~--The filtrate from the last is treated with ammonic oxalate,
boiled for a few minutes, allowed to settle, and filtered. The
precipitate is washed with hot water; dried; ignited; and weighed as
carbonate, after gentle ignition; or as lime, after strong ignition in
the muffle.
~Magnesia.~--The filtrate from the lime is treated with sodic phosphate
and ammonia. It is well mixed by stirring, and allowed to stand
overnight. The precipitate is washed with dilute ammonia, dried,
ignited, and weighed as pyrophosphate.
~Soda and Potash.~--These are determined in the remaining half of the
filtrate from the tin. The solution is rendered ammoniacal with ammonia;
and treated, first with sulphuretted hydrogen, and then with ammonium
oxalate. The precipitate is filtered off and rejected. The filtrate is
evaporated in a small porcelain dish over a Bunsen burner, or on the
sand bath; and towards the close (or earlier if the evaporation is not
proceeding well) nitric acid is added.
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