There is only one series of boron compounds which have any importance.
These are the borates in which the trioxide (B_{2}O_{3}) acts the part
of a weak acid. The addition of any acid liberates boric acid, which
separates out in cold solutions as a crystalline precipitate. Boric acid
is soluble in alcohol and in hot water. On evaporating these solutions
it is volatilised, although the anhydrous oxide is "fixed" at a red
heat. The borates are mostly fusible compounds, and are soluble in acids
and in solutions of ammonic salts.
~Detection.~--Boron in small quantities will escape detection unless
specially looked for, but there is no difficulty in detecting its
presence. Heated in the Bunsen-burner flame with "Turner's test," it
gives an evanescent yellowish-green colour, due to fluoride of boron
(BF_{3}). "Turner's test" is a mixture of 5 parts of bisulphate of
potash and 1 part of fluor spar. Boric acid itself imparts a
characteristic green colour to the flame, which gives a spectrum made up
of four well-marked and equidistant lines, three in the green and one in
the blue. Solutions of boric acid give with "turmeric paper," which has
been dipped into it and dried, a characteristic red tint. This is a very
delicate test, but in trying it a blank experiment should be carried out
alongside with a solution made up of the same re-agents which have been
used in liberating the boric acid in the sample.
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