Alizarin (from Alizari, a variety of madder, rose madder) C14H8O4 or C6H4.CO.CO.C6H2(OH)2, red pigment, is found in madder as a product of the decomposition of the ruberythric acid contained in the fresh root, which is split by ferments into alizarin and sugar. Alizarin was discovered by Robiquet and Colin in 1826, and in 1868 Graebe and Liebermann recognized alizarin as a dioxyanthraquinone and thereupon based the synthetic production from anthracene C14H10. This first synthesis of a plant pigment has become very important for the industry and agriculture of several countries. In 1869 Perkin commercialized the first synthetic alizarin (1 ton) and in 1898 Germany exported 9,320 tons of alizarin worth 16,874,000 marks.

To prepare alizarin, oxidize anthracene with chromate of potash and diluted sulfuric acid to anthraquinone C14H8O2, treat this with fuming sulfuric acid and precipitate from the resulting mixture of sulphonic acids and caustic soda firstly anthraquinone monosulphonic acid, then, when completely neutralized, anthraquinone disulphonic acid soda. The former is processed to alizarin while the latter provides flavopurpurin and isopurpurin by the same treatment. When heated with caustic soda and some chlorate of potash under high pressure to 180°, the anthraquinone monosulphonate soda yields alizarin sodium C14H7O2.ONa+NaOH = C14H6O2(ONa)2+2H. Alizarin is precipitated from the solution of the melt by acid. It is washed with water and sold as 10-20 % pasta.

Alizarin forms reddish-yellow prisms, melts at 290°, sublimes to orange-red needles, dissolves easily in alcohol and ether, a little in hot water, with a dark red colour in concentrated sulfuric acid, with a crimson colour in alkalis. The solutions are precipitated red by alum and tin salts, black-violet by iron oxide salts, and violet-brown by chromium salts, and its use in dyeing and printing is based on this property of forming coloured compounds with metal oxides, where it has gradually replaced madder. A solution of alizarin in acetic acid provided nitroalizarin C14H7(NO2)O4, which can be crystallized with nitric acid and is used as alizarin orange for dyeing cotton and wool, and when heated with glycerol and sulfuric acid provides alizarin blue (dioxyanthraquinonequinoline) C17H19NO4. This forms metallically lustrous, blue-violet needles, melts at 270°, sublimes at higher temperatures and dissolves in alcohol and benzene, hardly in water. Since it is discoloured by zinc dust, dextrose, etc., but regenerates in the air, it is suitable for vat dyeing. It is especially used for calico printing. Alizarin powder (alizarin carmine) is alizarin monosulphonate soda, which creates many nuances on wool, of which the scarlet ones are absolutely resistant to light and air. Nitric acid and alizarin form phthalic acid, and burning with zinc dust creates anthracene. Many colourants are commercially available under the name alizarin which are not derived from anthracene, but are similar in constistancy to alizarin and can be used like it in dyeing.

Source: Meyers Großes Konversations-Lexikon, 6. Auflage 1905–1909

Model Paints