Ammonium Chromium Alum, (NH₄)₂SO₄.Cr₂(SO₄)₃

Ammonium Chromium Alum, (NH₄)₂SO₄.Cr₂(SO₄)₃.24H₂O, crystallises from a suitable mixture of solutions of its component salts in efflorescent octahedra of density 1.73, melting at 100° C. with loss of 18H₂O.

Ammonium chromium alum is soluble in water to the extent, at 25° C., of 21.21 grams of the hydrated salt in 100 c.c. of water, yielding a bluish-violet solution which becomes green at 70° to 80° C. A study of the equilibria existing in such a solution makes it appear that the change from the violet to the green modification does not take place at all at 0° C., but that at 40° C. the solution contains about 40 per cent, of the green alum.