1. a red dye that occurs in the root of madder (Rubia tinctorum and other Rubiaceae) in glucose combination (ruberythric acid) as orange needles, slightly soluble in water; used by the ancients as a dye. Now produced synthetically from anthracene and used in the manufacture of dyes, alizarin blue, alizarin orange, Turkey red. As an indicator, it is yellow below pH 5.5 and red above pH 6.8; other modified alizarins have other colors and change color at other pH values.
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