alum(redirected from Alum compounds)
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salts of the general formula
MeI MeIII (SO4)2 · 12H2O
where MeI is a univalent cation (for example, Na+, K+, NH4+) and MeIII, a trivalent cation (Al3+, Cr3+, Fe3+). In other words, alum is the crystalline hydrate of double sulfates. All types of alum have a sour, astringent taste (hence the Russian kvastsy, from the old Slavic kysati, “to turn sour,” 15th century). Alums are complex compounds of the double salt type, so that the formula is often written MeI [MeIII (SO4)2]·12H2O.
Alum is completely stable under normal conditions. When heated it loses its water of crystallization and is converted to a compound known as burnt alum. Alum is readily soluble in water and undergoes nearly complete dissociation into simple ions in weak aqueous solutions. Alum can be obtained by mixing hot aqueous solutions containing equimolar quantities of univalent and trivalent metal sulfates. The alum crystals precipitate upon cooling.
Alum serves as a tanning agent in the leather and photographic industries and also as a mordant in dyeing textiles. The most widely used types of alum are potash alum, K2SO4·AL2-(SO4)3·24H2O; chrome alum, K2SO4·Cr2(SO4)3·24H2O; and ammonium ferric alum, (NH4)2SO4·Fe2(SO4)3·24H2O.
In medicine, potash alum is used externally as a styptic and cauterizing agent (styptic pencils), and as an astringent in mouthwashes, irrigants, lotions, and douches. Burnt alum is used as an astringent and also as a dessicant in dusting powders.