The
weight, including that of the residue from the silica, is noted as that
of "alumina, &c."
The weighed oxides are next fused with bisulphate of potash for some
hours. The bisulphate should have been first fused, apart, until the
effervescence from the escape of steam has stopped. The melt is
dissolved out with cold water and dilute sulphuric acid, and any
insoluble residue is filtered off, washed, ignited and weighed. The
filtrate is reserved for determinations of iron and titanium. The
residue, after weighing, may be treated with hydrofluoric and sulphuric
acids for any silica,[114] which would be determined by loss. It may be
tested for barium sulphate by treatment with hot strong sulphuric acid;
in which this salt dissolves, but is again insoluble (and so comes out
as a white precipitate) on diluting with cold water; the acid also must
be cold before adding the water. The filtrate containing the iron is
reduced with sulphuretted hydrogen, boiled till free from that gas,
filtered and titrated with a standard solution of permanganate of
potassium. The iron found is calculated to ferric oxide by dividing by
.7. The iron solution after titration serves for the determination of
titanium oxide (TiO_{2}). This is done colorimetrically, by adding
peroxide of hydrogen free from hydrofluoric acid, and comparing the
brown colour produced with that produced by the addition of a standard
solution of titanium to an equal volume of water containing sulphuric
acid.
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