alizarin

alizarin , or 1,2-dihydroxyanthraquinone, mordant vegetable dye obtained originally from the root of the madder plant ( Rubia tinctorum ), in which it occurs as a glucoside. The term also includes a group of synthetic dyestuffs prepared from coal-tar derivatives. A method for the synthesis of alizarin was first discovered (1868) by Karl Graebe and Karl Liebermann, German chemists. With salts of metals the compound forms brilliant lakes , although by itself it is a poor dye. Turkey red is produced with an aluminum mordant, other shades of red with calcium and tin salts, dark violet with iron mordants, and brownish red with chromium. Purpurin, also used in dyeing, occurs with alizarin in madder and is produced synthetically.

stains and staining techniques Various chemical staining techniques are used to identify minerals. The procedure followed is to etch the specimen, and then to expose it to a range of organic and/or inorganic compounds which form distinctive coloured complexes with certain minerals. Feldspars can be identified by etching with hydrofluoric acid and then treating with sodium cobaltinitrate, barium chloride, and potassium acid rhodizonate. Plagioclases stain red and K-feldspars are stained yellow after this treatment. For carbonates, a range of stains, including alizarin red S, Feigl's solution, potassium ferricyanide alizarine cyanic green, and titan yellow are used. A number of staining combinations allow the differentiation of calcite, high-Mg calcite, dolomite, anhydrite, and gypsum.