Topic summary
Alum
An alum () is a type of chemical compound, usually a hydrateddoublesulfatesalt of aluminium with the general formulaXAl(SO4)2·12H2O, such that X is a monovalentcation such as potassium or ammonium. By itself, alum often refers to potassium alum, with the formula KAl(SO4)2·12H2O. Other alums are named after the monovalent ion, such as sodium alum and ammonium alum.
The name alum is also used, more generally, for salts with the same formula and structure, except that aluminium is replaced by another trivalent metal ion like chromium, or sulfur is replaced by another chalcogen like selenium. The most common of these analogs is chrome alumKCr(SO4)2·12H2O.
In most industries, the name alum (or papermaker's alum) is used to refer to aluminium sulfate, Al2(SO4)3·nH2O, which is used for most industrial flocculation (the variable n is an integer whose size depends on the amount of water absorbed into the alum). For medicine, the word alum may also refer to aluminium hydroxide gel used as a vaccine adjuvant.