alum - 12

Alum (pronounced /ˈæləm/) 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 formula KAl(SO4)2.12H2O. The wider class of compounds known as alums have the related empirical formula, AB(SO4)2.12H2O.

DETAILED DESACRIPTION
Alum (pronounced /ˈæləm/) 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 formula KAl(SO4)2.12H2O. The wider class of compounds known as alums have the related empirical formula, AB(SO4)2.12H2O.
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Uses
Industrial
Alums are useful for a range of industrial processes. They are soluble in water; have an astringent, acid, and sweetish taste; react acid to litmus; and crystallize in regular octahedra. 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.
Potassium alum is the common alum of commerce, although soda alum, ferric alum, and ammonium alum are manufactured.
Alum is also used in purification of drinking water in industries. In a holding tank, some alum (phitkari) is added to the water so that the negatively charged light colloidal parts stick together and get heavy (flocculate) when alum makes the colloidal particles neutralized by making its aluminum ions get loaded with the colloidal parts . When the colloidal parts get heavy they can be easily separated from the tank for further chlorination of water in the process of purification of water.
Cosmetic
• Alum in block form (usually potassium alum) is used as an aftershave, rubbed over the wet, freshly shaved face.
• Styptic pencils containing aluminium sulfate or potassium aluminium sulfate are used as astringents to prevent bleeding from small shaving cuts.
• Alum was used as a base in skin whiteners and treatments during the late 16th century. A recipe for one such compound was given thus:
"For the Freckles which one getteth by the heat of the Sun: Take a little Allom beaten small, temper amonst it a well brayed white of an egg, put it on a milde fire, stirring it always about that it wax not hard, and when it casteth up the scum, then it is enough, wherewith anoint the Freckles the space of three dayes: if you will defend your self that you get no Freckles on the face, then anoint your face with the whites of eggs." —Christopher Wirzung, General Practise of Physicke, 1654.
Medicinal
• Alum is used in many subunit vaccines as an adjuvant to enhance the body's response to immunogens. Such vaccines include hepatitis A, hepatitis B and DTaP.
• Alum in powder or crystal form, or in styptic pencils, is sometimes applied to cuts to prevent or treat infection.
• Powdered alum is commonly cited as a home remedy for canker sores.
• Preparations containing alum are used by pet owners to stem bleeding associated with animal injuries caused by improper nail clipping.
• Alum is listed as an ingredient of some brands of toothpaste or toothpowder.
Culinary
• Alum powder, found in the spice section of many grocery stores, may be used in pickling recipes as a preservative to maintain fruit and vegetable crispness.
• Alum is used as the acidic component of some commercial baking powders.
• Alum was used by bakers in England during the 1800s to make bread whiter[1]. In 1875, the Sale of Food and Drugs Act prevented this and other adulterations.[2]
Flame retardant
• Solutions containing alum may be used to treat cloth, wood and paper materials to increase their resistance to fire.
• Alum is also a component of foamite.
• Alum is also used in fire extinguishers to smother chemical and oil fires.
Chemical flocculant
• Alum is used to clarify water by neutralizing the electrical double layer surrounding very fine suspended particles, allowing them to flocculate (stick together). After flocculation, the particles will be large enough to settle and can be removed.
• Alum may be used to increase the viscosity of a ceramic glaze suspension; this makes the glaze more readily adherent and slows its rate of sedimentation.
• Alum is an ingredient in some recipes for homemade modeling compounds intended for use by children. (These are often called "play clay" or "play dough" for their similarity to "Play-Doh", a trademarked product marketed by American toy manufacturer Hasbro).
History
In antiquity
The word "alumen" occurs in Pliny's Natural History. In the 52nd chapter of his 35th book, he gives a detailed description.[3] By comparing this with the account of stupteria given by Dioscorides in the 123rd chapter of his 5th book, it is obvious that the two are identical. Pliny informs us that alumen was found naturally in the earth. He calls it salsugoterrae. Different substances were distinguished by the name of "alumen"; but they were all characterized by a certain degree of astringency, and were all employed in dyeing and medicine, the light-colored alumen being useful in brilliant dyes, the dark-colored only in dyeing black or very dark colors.[citation needed] One species was a liquid, which was apt to be adulterated; but when pure it had the property of blackening when added to pomegranate juice.
Alchemical and later discoveries and uses
In the 18th century, J. H. Pott and Andreas Sigismund Marggraf demonstrated that alumina was a constituent. Pott in his Lithogeognosia showed that the precipitate obtained when an alkali is poured into a solution of alum is quite different from lime and chalk, with which it had been confounded by G.E. Stahl. Marggraf showed that alumina is one of the constituents of alum, but that this earth possesses peculiar properties, and is one of the ingredients in common clay. He also showed that crystals of alum can be obtained by dissolving alumina in sulfuric acid and evaporating the solutions, and when a solution of potash or ammonia is dropped into this liquid, it immediately deposits perfect crystals of alum.
Torbern Bergman also observed that the addition of potash or ammonia made the solution of alumina in sulfuric acid crystallize, but that the same effect was not produced by the addition of soda or of lime, and that potassium sulfate is frequently found in alum.
Early uses in industry
Egyptians reportedly used the coagulant alum as early as 1500 B.C. to reduce the visible cloudiness (turbidity) in the water. Alum was imported into England mainly from the Middle East, and, from the late 15th century onwards, the Papal States for hundreds of years. Its use there was as a dye-fixer (mordant) for wool (which was one of England's primary industries, the value of which increased significantly if dyed). These sources were unreliable, however, and there was a push to develop a source in England especially as imports from the Papal States were ceased following the excommunication of King Henry VIII.
Production
From alunite
In order to obtain alum from alunite, it is calcined and then exposed to the action of air for a considerable time. During this exposure it is kept continually moistened with water, so that it ultimately falls to a very fine powder. This powder is then lixiviated with hot water and sulfuric acid, the liquor decanted, and the alum allowed to crystallize. The alum schists employed in the manufacture of alum are mixtures of iron pyrite, aluminium silicate and various bituminous substances, and are found in upper Bavaria, Bohemia, Belgium, and Scotland. These are either roasted or exposed to the weathering action of the air. In the roasting process, sulfuric acid is formed and acts on the clay to form aluminium sulfate, a similar condition of affairs being produced during weathering.
From clays or bauxite
In the preparation of alum from clays or from bauxite, the material is gently calcined, then mixed with sulfuric acid and heated gradually to boiling; it is allowed to stand for some time, the clear solution drawn off and mixed with acid potassium sulfate and allowed to crystallize. When cryolite is used for the preparation of alum, it is mixed with calcium carbonate and heated. By this means, sodium aluminate is formed; it is then extracted with water and precipitated either by sodium bicarbonate or by passing a current of carbon dioxide through the solution. The precipitate is then dissolved in sulfuric acid, the requisite amount of potassium sulfate added and the solution allowed to crystallize.
Crystal chemistry
Double sulfates with the general formula A2SO4•B2(SO4)3•24H2O, are known where A is a monovalent cation such as sodium, potassium, rubidium, caesium, or thallium(I), or a compound cation such as ammonium (NH4+), methylammonium (CH3NH3+), hydroxylammonium (HONH3+) or hydrazinium (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.[7] 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.