Alum, any of a group of hydrated double salts, usually consisting of aluminum sulfate, water of hydration, and the sulfate of another element. A whole series of hydrated double salts results from the hydration of the sulfate of a singly charged cation (e.g., K+)
and the sulfate of any one of a number of triply charged cations (e.g., Al3+). Aluminum sulfate can thus form alums with sulfates of the singly charged cations of potassium,
sodium, ammonium, cesium, and other elements and compounds. In similar fashion, sulfates of the triply charged cations of iron, chromium
manganese, cobalt, and other metals may take the place of aluminum sulfate.The most important alums are potassium aluminum sulfate, ammonium aluminum sulfate, and sodium aluminum sulfate. Potassium aluminum sulfate, also known as potassium alum or potash alum, has a molecular formula of K2(SO4)·Al2(SO4)3·24H2O or KAl(SO4)2·12H2O.
