d, followed by the
evaporation of the solution so obtained. The residue is then heated in a
current of superheated steam, in which the boric acid volatilizes and
distils over. It may also be obtained by the decomposition of boracite
with hot hydrochloric acid. In small quantities, it may be prepared by
the addition of concentrated sulphuric acid to a cold saturated solution
of borax.

Na2B4O7 + H2SO4 + 5H2O = Na2SO4 + 4H3BO3.

Boric acid crystallizes from water in white nacreous laminae belonging
to the triclinic system; it is difficultly soluble in cold water, but
dissolves readily in hot water. It is one of the "weak" acids, its
dissociation constant being only 0.08169 (J. Walker, _Jour. of Chem.
Soc._, 1900, lxxvii. 5), and consequently its salts are appreciably
hydrolysed in aqueous solution. The free acid turns blue litmus to a
claret colour. Its action upon turmeric is characteristic; a turmeric
paper moistened with a solution of boric acid turns brown, the colour
becoming much darker as the paper dries; while the addition of sodium
or potassium hydroxide turns it almost black. Boric acid is easily
soluble in alcohol, and if the vapour of the solution be inflamed it
burns with a characteristic vivid green colour. The acid on being
heated to 100 deg. C. loses water and is converted into _metaboric
acid_, HBO3; at 140 deg. C., _pyroboric acid_, H2B4O7, is produced; at
still higher temperatures, boron trioxide is formed. The salts of the
normal or orthoboric acid in all probability do not exist; metaboric
acid, however, forms several well-defined salts which are readily
converted, even by carbon dioxide, into salts of pyroboric acid. That
orthoboric acid is a tribasic acid is shown by the formation of ethyl
orthoborate on esterification, the vapour density of which corresponds
to the molecular formula B(OC2H5)3; the molecular formula of the acid
must consequently be B(OH)3 or H3BO3. The metallic borates are
generally obtained in the hydrated condition, and with the exception
of those of the alkali metals, are insoluble in water. The most
important of the borates is sodium pyroborate or borax (q.v.).

Borax and boracic acid are feeble but useful antiseptics. Hence they
may be used to preserve food-substances, such as milk and butter (see
ADULTERATION). In medicine boracic acid is used in solution to relieve
itching, but its chief use is as a mild antiseptic