The solution is filtered. The residue may consist of unremoved
silica, and oxides of tantalum, niobium, and, perhaps, chromium. On the
prolonged boiling of the filtrate, the oxide of titanium (and oxide of
zirconium, if any) is precipitated.
Any titanium dissolved by the first extraction with acids is recovered
in the following way:--Sulphuretted hydrogen is passed into the acid
solution, and any precipitate that may be formed is filtered off. The
filtrate is oxidised, and the iron, aluminium, and titanium are
separated as basic acetates (see under _Iron_). The precipitate is dried
and fused with bisulphate of potash. The "melt" is extracted with cold
water, filtered if necessary, and the solution rendered first faintly
alkaline with ammonia, then very slightly acid with sulphuric acid. 30
or 40 c.c. of a saturated solution of sulphurous acid is added, and the
oxide of titanium precipitated by prolonged boiling. It is filtered off,
added to the precipitate previously got, ignited with ammonic carbonate
towards the end, and then weighed.
~Detection.~--Titanium is detected in an insoluble residue by fusing the
residue for some time in a bead of microcosmic salt. In the reducing
flame it gives a violet colour, which becomes reddish-brown if much iron
is present. In the oxidising flame it gives a colourless or whitish
bead.
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