[C.I. 58000]. 1,2-Dihydroxyanthraquinone; 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 made synthetically from anthracene and used in the manufacture of dyes, e.g., alizarin blue, alizarin orange, Turkey red. As an indicator, it changes from yellow to red at pH 5.5 to 6.8; other modified alizarin’s have other colors and change color at other pH values.
alizarin red S