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(ăl`əm), any one of a series of isomorphous double salts that are hydrated sulfatessulfate,
chemical compound containing the sulfate (SO4) radical. Sulfates are salts or esters of sulfuric acid, H2SO4, formed by replacing one or both of the hydrogens with a metal (e.g., sodium) or a radical (e.g., ammonium or ethyl).
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 of a univalent cation (e.g., potassium, sodium, ammonium, cesium, or thallium) and a trivalent cation (e.g., aluminum, chromium, iron, manganese, cobalt, or titanium). The name alum commonly refers to potassium aluminum sulfate dodecahydrate, or potash alum, KAl(SO4)2·12H2O, a colorless-to-white, crystalline compound. It is used in water purification, leather tanning, mordant dyeing, as an astringent, and in baking powder; it occurs in nature as the mineral kalunite. Sodium aluminum sulfate, or soda alum, NaAl(SO4)2·12H2O, is also used in baking powder. Ammonium aluminum sulfate, or ammonia alum, NH4Al(SO4)2·12H2O, is used in tanning, in dyeing and fireproofing textiles, in vegetable glues and porcelain cements, and in water purification. Chromium potassium sulfate, or chrome alum, KCr(SO4)2·12H2O, is used as a mordant in dyeing, in tanning, and in photographic fixing baths to harden gelatin films and plates. Aluminum sulfate, Al2(SO4)3·18H2O, is also called alum. A pseudoalum is a double sulfate salt of a divalent cation (e.g., magnesium or calcium) and a trivalent cation (e.g., aluminum).



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.


(inorganic chemistry)
Any of a group of double sulfates of trivalent metals such as aluminum, chromium, or iron and a univalent metal such as potassium or sodium.
aluminum sulfate; ammonium aluminum sulfate; potassium aluminum sulfate
KAl(SO4)2·12H2O A colorless, white, astringent-tasting evaporite mineral.


A chemical compound added to gypsum plaster to make the plaster harden faster.


On drawings, abbr. for aluminum.


charm against evil eye. [Egyptian Folklore: Leach, 40]


1. a colourless soluble hydrated double sulphate of aluminium and potassium used in the manufacture of mordants and pigments, in dressing leather and sizing paper, and in medicine as a styptic and astringent. Formula: K2SO4.Al2(SO4)3.24H2O
2. any of a group of isomorphic double sulphates of a monovalent metal or group and a trivalent metal. Formula: X2SO4.Y2(SO4)3.24H2O, where X is monovalent and Y is trivalent