~Solution and Separation.~--The solution presents no difficulty, but the
separation is troublesome. The best method is that of Gooch; who, if
necessary, first fuses with carbonate of soda, and after the removal of
chlorides and fluorides (by nitrate of silver or a lime salt),
evaporates the aqueous extract with nitric or acetic acid to dryness in
a retort and, subsequently, with repeated doses of 10 c.c. each of
methyl alcohol. The distillate contains the boron as boric acid. Half a
gram of the trioxide (B_{2}O_{3}) is completely carried over by two
evaporations, each with 10 c.c. of the alcohol; but if water or foreign
salts are present, more than this is required. In ordinary cases six
such evaporations are sufficient for 0.2 gram of the oxide.[123]
GRAVIMETRIC DETERMINATION.
Before the introduction of Gooch's process it was usual to determine the
boron trioxide "by difference." If the alcoholic distillate containing
the boric acid is digested with about 1 gram (a known weight) of lime
for ten or fifteen minutes, the alcohol can be evaporated off without
danger of loss. Either calcium nitrate or acetate (which will be formed
at the same time) yields lime upon subsequent ignition. Consequently,
the increase in weight, after ignition, upon that of the lime taken
gives the amount of boron trioxide present.
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