ANTHRAQUINONE, or OXYANTHRAOENE. Ci4E802.—By acting upon anthracene with oxidizing agents, anthraquinone is produced. Bromine, nitric acid, and chromic acid, have been employed for the purpose. It is obtained by the action of chromic acid, set free by adding strong sulphuric acid to a solution of biehromate of potash ; the anthracene is dissolved in boiling acetic acid, or more generally in ordinary oil of vitriol free from nitric acid, and diluted with twice its bulk of water, in lead-lined vats. The bichromate is gradually added to the solution, when the heat produced is sufficient to bring about the complete reaction. When acetic acid is used, some method must be adopted for its recovery. The anthraquinone is precipitated by the addition of water, and well washed ; it separates in the form of light, silky, almost colourless needles, which are dried iu thin layers on trays, ready for subliming ; the retorts used are similar to those for subliming orude anthracene.
On the largo scale, thoroughly purified anthracene is converted into anthraquinone, by placing it with a little water in large lead-lined wooden vats, adding bichromate potash, or chromic acid, boiling, and adding sulphuric', acetic, or nitric acid. The boiling is continued for several hours, or even a day or more, by injected steam, which gives the requisite agitation. It is then allowed to settle, and the clear liquid is siphoned off ; the precipitate is well washed for some days with boiling water, and after settling and cooling, the water is again drawn off, and received in other tanks for further subsidence for several days. The washed anthraquinone is dried in a filter press, and is then a yellowish-white, silky, crystalline powder. The solution of the chromium salts is recon verted into hichromate, or used in making chrome-alum.
The methods which have been proposed for converting anthracene into anthraquinone are :— (1) Anthracene, 1 part ; bichromate of potash, 2i parts ; concentrated acetic acid, 10-15 parts ; heated in a clay or glass vessel at 100°-120° (212°-248° F.), till nearly all the bichromate is
dissolved, and the liquid has acquired a deep-green colour.
(2) Acetic acid is replaced by sulphuric acid diluted with 1-2 parts water.
(3) Anthraoene, 1 part ; glacial acetic acid, 10 parts ; heated to 100° (212° F.); nitric acid (sp. gr. 1.3) is added in small portions at a time till the violent reaction ceases ; the acetic aoid is .partly recovered by distillation. The objection to nitric acid is the alleged formation of nitro compounds, which is said not only to impede the purification of the product, but involve a loss, from the conversion of the anthracene into compounds incapable of yielding alizarine.
(4) The method proposed by F. Baeyer, to heat anthracene with 5 parts manganese to 200° (392° F.), answers very well on a small scale, and yields the sublimed anthraquinone at one operation.
The crude anthraquinone thus obtained is washed with water, dried, heated with sulphuric acid to 120° (248° F.), and the carbonized matter is removed by washing and filtering. The anthraquinone is thus raised to 93-96 per cent., and is purified by sublimation in iron retorts fixed over a naked fire, the sublimate being blown over into the subliming chambers by superheated steam. 1VIuch of the substance is decomposed and leaveia fine hard coke, the loss being about 30 per cent. This coke is, according. to Bolley and Kopp, rich in chromium oxides ; it may therefore be possible that the loss is due to imperfect washing of the anthraquinone : on the small scale, it sublitnes without leaving any residue, which may be attributed to the more easily controlled heating, and may suggest its being sublimed on a large scale by supplying it continuously to the retorts. The sublimate has a fine, silky, crystalline texture, a light-yellow colour, and contains streaks of reddisb, crystalline matter. It is dried on canvas in heated rooms, and mixed by sifting, when it is ready for conversion into bromo-, chloro-, and nitro- compounds.