ALIZ'ARIN ( probably from Ar. al, the 'aCtrali, juice extracted from a plant), times called madder extract. An exceedingly valuable coloring matter. With the oxides of aluminium, iron. and most other metals, it gives a series of beautifully colored insoluble salts or lakes. It was first obtained from madder (the root of rubia tinetorum) by heating with strong sulphuric acid. In 1868 Graebe and Liebermann discovered a process by which it could be factured from anthracene, thus for the first time a natural coloring sub- stance. Their method. since modified with view to economy, consists in the following operations: (1) Anthracene, a hydro-carbon contained in the refuse coal-tar of gas works, is oxidized with potassium bichromate to yield the compound anthraquinone • (2) when anthraquinone is heated with sulphuric acid, quinonc-sulphonie acids are produced; (3) if the sodium salts of these acids are kept for forty eight hours, in closed cylinders, with caustic soda and potassium chlorate, at a temperature of 180° C., the sodium salts of alizarin and sim ilar compounds, called purpurins, are obtained; (4) the sodium salts of alizarin and purpurin are then dissolved in water, and, by addition of hydrochloric acid, alizarin itself and the pur purins are set free and precipitated in crystal line form. The precipitate is collected in filter presses, thoroughly washed with water, and brought into the market in the form of a rather thin paste usually containing either 10 or 20 per cent. of alizarin. Pure alizarin can be read
il• obtained from this paste by sublimation. Alizarin was first made on an industrial scale by Perkin in 1869.
Alizarin is to sonic extent soluble in hot water. In the dyeing processes, fabrics are first soaked in a solution of the required mordant, and after the latter has been decomposed by steam or with alkali, leaving an insoluble metallic hydroxide in the fibre, alizarin solution is applied to produce the desired "lake." Chemically, alizarin is a dioxy-anthraqui none, having the structural formula Nitro-alizarin (commercial alizarin orange), which is itself a coloring matter, yields, when heated with glycerin and sulphuric acid, an other color, alizarin blue, useo in calico printing. Alizarin carmine is another important alizarin color: it is much used as a substitute for cochineal.
The artificial production of alizarin on a large industrial scale has naturally brought about im portant changes in the agriculture of the coun tries where madder used to be extensively cul tivated. Consult Gnebm, Die Antlsracen farbstoffe (Brunswick, 1897).