[115] The alumina is determined by difference. From the weight of
the combined precipitate which has been recorded as "Alumina, &c.,"
deduct (1) the residue, insoluble, after fusion with bisulphate; (2) the
ferric oxide; (3) the titanium oxide; and (4) the phosphoric oxide
(P_{2}O_{5}), the amount of which is subsequently determined in a
separate portion. This gives the alumina.
~Manganous oxide, &c.~--The filtrate from the "alumina, &c." contained
in a 6 or 8-ounce flask, which it nearly fills, is made slightly
alkaline with ammonia and treated with a small excess of ammonium
sulphide; the flask is then corked and placed on one side for some time
(a day or so) so that the manganese sulphide may separate. The
precipitate is filtered off and washed with water containing ammonium
chloride and a few drops of ammonium sulphide. The filtrate is reserved
for lime, &c. The precipitate is digested with sulphuretted hydrogen
water, to which one-fifth of its volume of strong hydrochloric acid has
been added; this dissolves the sulphides of zinc and manganese; any
black residue should be tested for copper and perhaps nickel. The
solution is evaporated to dryness, taken up with a little water and
treated with a small excess of solution of carbonate of soda. It is
boiled and again evaporated, washed out with hot water and filtered on
to a small filter, dried, ignited, and weighed as Mn_{3}O_{4}.
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