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alum
A double salt comprising sulfates of two metals (one monovalent, one trivalent) combined with 12 molecules of water of crystallization: MIMIII(SO4)2.12H2O. The monovalent metal is commonly potassium, sodium, or ammonium; the trivalent metal may be aluminum, chromium, or ferric iron.
Alums are soluble in water and are usually acid. They are used in astringents (styptic pencils), as a mordant in dyes, and in the manufacture of baking powder, antiperspirants, and fire extinguishers. Potash alum (potassium aluminum sulfate, KAl(SO4)2.12H2O) is used in the sizing of paper and in water purification.
Related category
• INORGANIC CHEMISTRY
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