, purpurin, and the other colouring-matters of madder are hydroxyl
derivatives of a compound derived from anthracene by the replacement of
two atoms of hydrogen by two atoms of oxygen. These oxygen derivatives of
benzenoid hydrocarbons form a special group of compounds known as
quinones. Thus there is quinone itself, or benzoquinone, which is benzene
with two atoms of oxygen replacing two atoms of hydrogen. There are also
isomeric quinones of the naphthalene series known as naphthaquinones. A
dihydroxyl derivative of one of the latter is in use under the somewhat
misappropriate name of "alizarin black." With this exception no other
quinone derivative is used in the colour industry till we come to the
hydrocarbons of the anthracene oil. Phenanthrene forms a quinone which
has been utilized as a source of colouring-matters, but these are
comparatively unimportant. The quinone with which we are at present
concerned is anthraquinone.

The latter is prepared by oxidizing the anthracene--previously reduced by
sublimation to the condition of a very finely-divided crystalline
powder--with sulphuric acid and potassium dichromate. The quinone is
purified, converted into a sulpho-acid, and the sodium salt of the latter
on fusion with alkali gives alizarin, which is dihydroxy-anthraquinone. It
is of interest to note that in this case a monosulpho-acid gives a
dihydroxy-derivative. During the process of fusion potassium chlorate is
added, by which means the yield of alizarin is considerably increased. In
the original process of Graebe and Liebermann, dibromanthraquinone was
fused with alkali; but this method was soon improved upon by the discovery
of the sulpho-acid by Caro and Perkin in 1869, and from this period the
manufacture of artificial alizarin became commercially successful.

In addition to alizarin, other anthracene derivatives are of industrial
importance. The purpurin, discovered among the colouring-matters of madder
in 1826, is a trihydroxy-anthraquinone; it can be prepared by the
oxidation of alizarin, as shown by De Lalande in 1874. Isomeric compounds
known as "flavopurpurin" and "anthrapurpurin" are also made from the
disulpho-acids of anthraquinone by fusion with alkali and potassium
chlorate. These two disulpho-acids are obtained simultaneously with the
monosulpho-acid by the action of fuming sulphuric acid on the quinone, and
are separated by the fractional crystallization of their sodium salts from
the monosulpho-