Chrome Alums

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.