alum

n. double sulfate (Chemistry)

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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Noun
1. a white crystalline double sulfate of aluminum: the ammonium double sulfate of aluminum
(synonym) ammonia alum, ammonium alum
(hypernym) aluminum, aluminium, Al, atomic number 13
2. a white crystalline double sulfate of aluminum: the potassium double sulfate of aluminum
(synonym) potassium alum, potash alum
(hypernym) aluminum, aluminium, Al, atomic number 13
3. a person who has received a degree from a school (high school or college or university)
(synonym) alumnus, alumna, graduate, grad
(hypernym) scholar, scholarly person, student
(hyponym) Ivy Leaguer
4. a double sulphate of aluminum and potassium that is used as an astringent (among other things)
(hypernym) astringent, astringent drug, styptic

(v. t.)
To steep in, or otherwise impregnate with, a solution of alum; to treat with alum.
  

(n.)
A double sulphate formed of aluminium and some other element (esp. an alkali metal) or of aluminium. It has twenty-four molecules of water of crystallization.
  

A type of immune adjuvant (a substance used to help boost the immune response to a vaccine). Also called aluminum sulfate.