An alum // is a type of chemical compound, usually a hydrated double sulfate salt of aluminium with the general formula XAl(SO
2O, where X is a monovalent cation such as potassium or ammonium. By itself, "alum" often refers to potassium alum, with the formula KAl(SO
2O. Other alums are named after the monovalent ion, such as sodium alum and ammonium alum.
The name "alum" is also used, more generally, for salts with the same formula and structure, except that aluminium is replaced by another trivalent metal ion like chromium(III), and/or sulfur is replaced by other chalcogen like selenium. The most common of these analogs is chrome alum KCr(SO
In some industries, the name "alum" (or "papermaker's alum") is used to refer to aluminium sulfate Al
2O. Most industrial flocculation done with "alum" actually uses aluminium sulfate. In medicine, "alum" may also refer to aluminium hydroxide gel used as a vaccine adjuvant.
The most important alums are
Aluminium-based alums have a number of common chemical properties. They are soluble in water, have a sweetish taste, react acid to litmus, and crystallize 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.
Alums crystallize in one of three different crystal structures. These classes are called α-, β- and γ-alums.
The solubility of the various alums in water varies greatly, sodium alum being readily soluble in water, while caesium and rubidium alums are only sparingly soluble. The various solubilities are shown in the following table.
|T||Ammonium alum||Potassium alum||Rubidium alum||Cesium alum|
Aluminium-based alums have been used since antiquity, and are still important in many industrial processes.
The most widely used alum is potassium alum. It was used since antiquity as a flocculant to clarify turbid liquids, as a mordant in dying, and in tanning. It is still widely used to purify piped water, in medicine, for cosmetics (in deodorant and antitranspirants), in food preparation (in baking powder and pickling), and to fire-proof paper and cloth.
Sodium alum is used in substitution to potassium alum in baking powders. Ammonium alum has a few niche uses. Other alums have mostly research interest.
A detailed description of a substance called alumen occurs in Pliny the Elder's Natural History. By comparing this with the account of stupteria given by Dioscorides, it is obvious the two are identical. Pliny informs us that a form of alumen was found naturally in the earth, and calls it salsugoterrae.
Pliny wrote that different substances were distinguished by the name of alumen, but they were all characterised by a certain degree of astringency, and were all employed in dyeing and medicine. Pliny says that there is another kind of alum that the Greeks call schiston, and which "splits into filaments of a whitish colour", From the name schiston and the mode of formation, it appears that this species was the salt that forms spontaneously on certain salty minerals, as alum slate and bituminous shale, and consists chiefly of sulfates of iron and aluminium. One species of alumen was a liquid, which was apt to be adulterated; but when pure it had the property of blackening when added to pomegranate juice. This property seems to characterize a solution of iron sulfate in water; a solution of ordinary (potassium) alum would possess no such property. Contamination with iron sulfate was greatly disliked as this darkened and dulled dye colours. In some places the iron sulfate may have been lacking, so the salt would be white and would be suitable, according to Pliny, for dyeing bright colors.
Pliny describes several other species of alumen but it is not clear as to what these minerals are. The alumen of the ancients then, was not always potassium alum, not even an alkali aluminum sulfate.
The western desert of Egypt was a major source of alum substitutes in antiquity. These evaporites were mainly FeAl
2O and Al
Alum and green vitriol (iron sulfate) both have sweetish and astringent taste, and they a had overlapping uses. Therefore, through the Middle Ages, alchemists and other writers do not seem to have discriminated the two salts accurately from each other. In the writings of the alchemists we find the words misy, sory, and chalcanthum applied to either compound; and the name atramentum sutorium, which one might expect to belong exclusively to green vitriol, applied indifferently to both.
In the early 1700s, Georg Ernst Stahl claimed that reacting sulfuric acid with limestone produced a sort of alum:  The error was soon corrected by Johann Pott and Andreas Marggraf, who showed that the precipitate obtained when an alkali is poured into a solution of alum, namely alumina, is quite different from lime and chalk, and is one of the ingredients in common clay.
Marggraf also showed that perfect crystals with properties of alum can be obtained by dissolving alumina in sulfuric acid and adding potash or ammonia to the concentrated solution. In 1767, Torbern Bergman observed the need for potassium or ammonium sulfates to convert aluminium sulfate into alum, while sodium or calcium would not work.
The composition of common alum was finally determined by Louis Vauquelin in 1797. As soon as Martin Klaproth discovered the presence of potassium in leucite and lepidolite, Vauquelin demonstrated that common alum is a double salt, composed of sulfuric acid, alumina, and potash. In the same journal volume, Jean-Antoine Chaptal published the analysis of four different kinds of alum, namely, Roman alum, Levant alum, British alum and alum manufactured by himself, confirming Vauquelin's result.
Some alums occur as minerals, the most important being alunite.
The most important alums – potassium, sodium, and ammonium – are produced industrially. Typical recipes involve combining aluminium sulfate and the sulfate monovalent cation. The aluminium sulfate is usually obtained by treating minerals like alum schist, bauxite and cryolite with sulfuric acid.
Many trivalent metals are capable of forming alums. The general form of an alum is XM(SO4)2·nH2O, where X is an alkali metal or ammonium, M is a trivalent metal, and n often is 12. The most important example is chrome alum, KCr(SO
2O, a dark violet crystalline double sulfate of chromium and potassium, was used in tanning.
In general, alums are formed more easily 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.
In some cases, solid solutions of alums with different monovalent and trivalent cations may occur.
In addition to the alums, which are dodecahydrates, double sulfates and selenates of univalent and trivalent cations occur with other degrees of hydration. These materials may also be referred to as alums, including the undecahydrates such as mendozite and kalinite, hexahydrates such as guanidinium (CH
3) and dimethylammonium ((CH
2) "alums", tetrahydrates such as goldichite, monohydrates such as thallium plutonium sulfate and anhydrous alums (yavapaiites). These classes include differing, but overlapping, combinations of ions.
A pseudo alum is a double sulfate of the typical formula ASO
2O, where A is a divalent metal ion, such as cobalt (wupatkiite), manganese (apjohnite), magnesium (pickingerite) or iron (halotrichite or feather alum), and B is a trivalent metal ion.
Double sulfates with the general formula A
2O are also known, where A is a monovalent cation such as sodium, potassium, rubidium, caesium, or thallium(I), or a compound cation such as ammonium (NH+
4), methylammonium (CH
3), hydroxylammonium (HONH+
3) or hydrazinium (N
5), 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 possible combinations of univalent cation, trivalent cation, and anion depends on the sizes of the ions.
Double sulfates of the composition A
4, where A is a univalent cation and B is a divalent metal ion are referred to as langbeinites, after the prototypical potassium magnesium sulfate.
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