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Alum, (IPA: /ˈæləm/) refers to a specific chemical compound and a class of chemical compounds. The specific compound is the hydrated aluminum potassium sulfate with the formula KAl(SO4)2.12H2O. The class of compounds known as alums have the related stoichiometry, AB(SO4)2.12H2O. Double sulfates with the general formula A2SO4·B2(SO4)3·24H2O, are known where A is a monovalent cation such as sodium, potassium, rubidium, caesium, or thallium(I), or a compound cation such as ammonium (NH4+), methylammonium(CH3NH3+), hydroxylammonium (HONH3+) or hydrazinium (N2H5+), B is a trivalent metal ion, such as aluminium, chromium, titanium, manganese, vanadium, iron (III), cobalt(III), gallium, molybdenum, indium, ruthenium, rhodium or iridium.[Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.] The specific combinations of univalent cation, trivalent cation and anion depends on the sizes of the ions. For example, unlike the other alkali metals the small lithium ion does not form alums, and there is only one sodium alum. In some case solid solutions of alums occur.
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