Ammonium alum, (NH₄)₂SO₄.Al₂(SO₄)₃

Ammonium alum, (NH₄)₂SO₄.Al₂(SO₄)₃.24H₂O, was formerly manufactured in England to a very large extent, but at the present time it is mainly the potassium salt that is prepared. Commercial alum is often a mixture of potassium and ammonium alums.

Ammonium aluminium sulphate octahydrate, (NH₄)₂SO₄.Al₂(SO₄)₃.8H₂O, may be prepared like the corresponding potassium salt (Marino), or by seeding a supersaturated solution of ammonium alum with a crystal of the corresponding thallic salt, (NH₄)₂SO₄.Tl₂(SO₄)₃.8H₂O.