double sulfates of Cr (III) and ammonium or any of the alkali metals. Chrome alums take the form of crystal hydrates. They are readily soluble in water; solubility increases in the series Cs, Rb, K, and Na.
The most important chrome alum is potassium chromium sulfate, which is formed by the interaction of hot K2SO4 and Cr2(SO4)3 solutions. On cooling, it is isolated in the form of dark violet crystals having the formula KCr(SO4)2·12H2O. Potassium chromium sulfate has a density of 1.83 g/cm3 and a melting point of 89°C; its solubility in water is 18.3 percent at 20°C. Solutions of potassium chromium sulfate are used for the tanning of leather, the impregnation of textiles, the manufacture of film, and various photographic purposes; they are also used as mordants in dyeing.