After the separation of the silver salt (see above) obtained on sulphonating anthraquinone, the remaining acid liquid gives on treatment with calcium carbonate the calcium salt of anthraquinone 2.6 disulphonic acid (anthraquinone-a-disulphonic acid).
It is then oxidized to anthraquinone by means of sodium dichromate and sulphuric acid in leaden vats, steam heated so that the mixture can be brought to the boil.
In order to synthesize alizarin, they converted anthracene into anthraquinone and then brominated the quinone.
KOnigs, and the observation that anthraquinone yielded oxyanthraquinones when treated in the cold with strong sulphuric acid, and the recent introduction of fuming sulphuric acid for the oxidation of naphthalene to phthalic acid, a process of great value in the manufacture of artificial indigo, may be noted.
Various sulphonic acids of anthraquinone are known, as well as oxy-derivatives, for the preparation and properties of which see Alizarin.
A more delicate method consists in adding a very little anthraquinone and sodium amalgam; absolute alcohol gives a green coloration, but in the presence of minute traces of water a red coloration appears.
In a similar manner anthrapurpurin is prepared by alkali fusion of anthraquinone 2.8 disulphonic acid.