Alum

Alum /ˈæləm/ is both a specific chemical compound and a class of chemical compounds. The specific compound is the hydrated potassium aluminium sulfate (potassium alum) with the formula KAl(SO
4)₂·12H
2O. More widely, alums are double sulfate salts, with the general formula A
2(SO
4).M
2(SO
4)
3.24H
2O, where A is a monovalent cation such as potassium or ammonium and M is a trivalent metal ion such as aluminium or chromium(III). When the trivalent ion is aluminium, the alum is named after the monovalent ion.