Alum is both a specific chemical compound and a class of chemical compounds. The specific compound is the hydrated potassium aluminium sulfate(potassium alum) with the formulaKAl(SO4)·12H2O. More widely, alums are double sulfate salts, with the general formula AM(SO4)2·12H2, where A is amonovalent cation such as potassium orammonium and M is a trivalent metal ion such as aluminium or chromium(III).When the trivalent ion is aluminium, the alum is named after the monovalent ion.
Alums are useful for a range of industrial processes. They are soluble in water, have a sweetish taste, react acid to litmus, andcrystallize in regular octahedra. In alums each metal ion is surrounded by six water molecules. When heated, they liquefy, and if the heating is continued, the water of crystallization is driven off, the salt froths and swells, and at last an amorphous powder remains. They are astringent and acidic.
- Alum is used in the treatment of canker sores in the mouth, as it has a significant drying effect to the area and reduces the irritation felt at the site.
- Alum was the major adjuvant used to increase the efficacy of vaccines, and has been used since the 1920s.But it has been almost completely replaced by aluminium hydroxide and aluminium phosphate in commercial vaccines.
- Alum has been used to stop bleeding in cases of hemorrhagic cystitis
Many trivalent metals are capable of forming alums. The general form of an alum is AMIII(SO4)2·nH2O, where A is analkali metal or ammonium, MIII is a trivalent metal, and n often is 12. In general, alums are easier formed when the alkali metal atom is larger. This rule was first stated by Locke in 1902, who found that if a trivalent metal does not form a caesium alum, it neither will form an alum with any other alkali metal or with ammonium.
Double sulfates with the general formula A2SO4·B2(SO4)3·24H2O, are known whereA is a monovalent cation such as sodium,potassium, rubidium, caesium, orthallium(I), or a compound cation such asammonium (NH4+),methylammonium(CH3NH3+</sup), hydroxylammonium (HONH3+) orhydrazinium (N2H5+), B is a trivalent metal ion, such as aluminium, chromium,titanium, manganese, vanadium, iron(III),cobalt(III), gallium, molybdenum, indium,ruthenium, rhodium, or iridium.Analogous selenates also occur. The specific combinations of univalent cation, trivalent cation, and anion depends on the sizes of the ions. For example, unlike the other alkali metals the smallest one, lithium, does not form alums, and there is only one known sodium alum. In some cases, solid solutions of alums occur.
Alums crystallize in one of three different crystal structures. These classes are called α-, β- and γ-alums.
Aluminum potassium sulfate, also known as potash alum, KAl(SO4)2·12H2O is used as an astringent and antisepsis in various food preparation processes such as pickling and fermentation and as a flocculant for water purification, among other things. A common method of producing potash alum is leaching ofalumina from bauxite, which is then reacted with potassium sulfate. As a naturally occurring mineral, potassium alum is known as alum-(K). Other potassium aluminium sulfate minerals are alunite(KAl(SO4)2·2Al(OH)3) and kalinite(KAl(SO4)2·11H2O). It is also used in so-called “crystal” deodorants.
Soda alum, NaAl(SO4)2·12H2O, mainly occurs in nature as the mineral mendozite. It is very soluble in water, and is extremely difficult to purify. In the preparation of this salt, it is preferable to mix the component solutions in the cold, and to evaporate them at a temperature not exceeding 60 °C. 100 parts of water dissolve 110 parts of sodium alum at 0 °C, and 51 parts at 16 °C.Soda alum is used in the acidulent of food as well as in the manufacture of baking powder.
Ammonium alum, NH4Al(SO4)2·12H2O, a white crystalline double sulfate of aluminium, is used in water purification, in vegetable glues, in porcelain cements, in deodorants, in tanning, dyeing and in fireproofing textiles.