Chrome Alum

Chrome alum, or chromium(III) potassium sulfate, has the chemical formula KCr(SO₄)₂, most commonly as the dodecahydrate KCr(SO₄)₂·12H₂O. It appears as dark purple needles, violet crystals, or greyish-brown powder (anhydrous), with a molar mass of 499.4 g/mol (dodecahydrate), density of 1.83 g/cm³, and melting point around 89-90°C (decomposes). The compound forms regular octahedral crystals, is highly soluble in water (forming acidic, astringent solutions that redden litmus), and solutions shift from violet to green above 50°C due to complex formation.​

It is produced by reducing potassium dichromate with SO₂ in sulfuric acid or from ferrochromium alloys. Chrome alum exhibits efflorescence, losing water in air, and is isomorphous with other alums like potash alum.​

Widely used in leather tanning (chrome tanning), textile mordanting, photography fixing baths, and analytical chemistry. It is irritant and toxic, requiring protective handling.​