For alum meaning "graduate," see
Alumnus. Alum, (
IPA: ) (aluminium potassium sulfate,) in
chemistry, is a term given to the crystallized double
sulfates of the typical formula M+2SO4·M3+2(SO4)3·12H2O, where M+ is the sign of an alkali metal (or generally monovalent cation) (
lithium,
sodium,
potassium,
rubidium, or
caesium), and M3+ denotes one of the trivalent metal ions (typically
aluminium,
chromium, or
iron (III)). The
ammonium ion (NH4+) also occurs in the M+ position.
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chim. alum
chim. alumen