The trioxide contains 31.4
per cent. of boron (B). Since magnesia does not form a soluble hydrate
it cannot satisfactorily be used instead of lime.
[Illustration: FIG. 82.]
The apparatus required is shown in fig. 82. It consists of a small
retort or evaporating vessel made out of a pipette of 200 c.c. capacity.
This is heated by means of a paraffin-bath at 130° or 140° C. It is
connected with an upright condenser, at the lower end of which is a
small flask which serves as a receiver.
The quantity of the borate taken should contain not more than 0.2 gram
of the trioxide. Insoluble compounds are "dissolved in nitric acid at
once, or, if necessary, first fused with sodium carbonate." With soluble
and alkaline borates sufficient nitric acid is added to render it
faintly acid. The solution is then introduced into the retort.
"The lime, to retain the boric acid in the distillate, is ignited in the
crucible in which the evaporation of the distillate is to be made
subsequently." It is then cooled in the desiccator for ten minutes, and
weighed. The lime is transferred to the receiving flask and slaked with
a little water. The retort is lowered into the bath so that "only the
rear dips below the surface." The evaporation is carried to dryness,
the retort being lowered further into the bath as the evaporation
proceeds.
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