Ten c.c. of methyl alcohol are introduced upon the residue,
and the evaporation again started. Six such portions of alcohol are thus
distilled and 2 c.c. of water are introduced and evaporated between the
second and third, as also between the fourth and fifth distillations. If
acetic acid is used instead of nitric in the first instance this
addition of water is unnecessary.
The distillate is evaporated in the crucible ignited over the blowpipe,
cooled in the desiccator for ten minutes and weighed. The increase in
weight gives the boron trioxide. The results tend to be from 1 to 2
milligrams too high.
VOLUMETRIC METHOD.
This method is applicable to the indirect determination of boric acid in
borax and similar compounds. It is based on the measurement of the
quantity of normal solution of acid required to replace the boric acid,
and, consequently, is rather a measure of the soda present. The process
is an alkalimetric one, and is carried out as follows:--Weigh up 3 grams
of the sample and dissolve in water. Tint with methyl orange, and run in
from an ordinary burette normal solution of sulphuric acid until a pink
tint is got. 100 c.c. of the normal solution of acid are equal to 7.0
grams of boron trioxide (B_{2}O_{3}), or 10.1 grams of anhydrous borax
(Na_{2}B_{4}O_{7}).
~Examination of Borax.
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