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Alum is a salt that in chemistry is a combination of an alkali metal, such as sodium, potassium, or ammonium and a trivalent metal, such as aluminum, iron, or chromium.
It refers to a specific chemical compound and a class of chemical compounds. The specific compound is the hydrated aluminum potassium sulfate with the formula KAl(SO4)2.12H2O. The wider class of compounds known as alums have the related stoichiometry, AB(SO4)2.12H2O.
The most common form, potassium aluminum sulfate, or potash alum, is one form that has been used in food processing. Another, sodium aluminum sulfate, is an ingredient in commercially produced baking powder. (Have you never noticed the faint metallic taste in baking powder? It comes from the alum.)
The potassium-based alum has been used to produce crisp cucumber and watermelon-rind pickles as well as maraschino cherries, where the aluminum ions strengthen the fruits’ cell-wall pectins.
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.
Aluminium sulfate is sometimes called alum in informal contexts, but this usage is not regarded as technically correct. Its properties are quite different from those of the set of alums formally described above.
Early uses in industry:-
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. With state financing, attempts were made throughout the 16th century, but without success until early on in the 17th century. An industry was founded in Yorkshire to process the shale which contained the key ingredient, aluminium sulfate, and made an important contribution to the Industrial Revolution. Alum (known as turti in local Indian languages) was also used for water treatment by Indians for hundreds of years.
Alum is used in vaccines as an adjuvant. Alum is commonly used as a coagulant in water treatment.
Alum 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, 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. The mass is now systematically extracted with water, and a solution of aluminium sulfate of specific gravity 1.16 is prepared. This solution is allowed to stand for some time (in order that any calcium sulfate and basic ferric sulfate may separate), and is then evaporated until ferrous sulfate crystallizes on cooling; it is then drawn off and evaporated until it attains a specific gravity of 1.40. It is now allowed to stand for some time, decanted from any sediment, and finally mixed with the calculated quantity of potassium sulfate (or if ammonium alum is required, with ammonium sulfate), well agitated, and the alum is thrown down as a finely-divided precipitate of alum meal. If much iron should be present in the shale then it is preferable to use potassium chloride in place of potassium sulfate.
Alum 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.
Types of alum:-
Sodium alum, Na2SO4·Al2(SO4)3·24H2O, 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.
Ammonia alum, NH4Al(SO4)2·12H2O, a white crystalline double sulfate of aluminium, is used in water purification, in vegetable glues, in porcelain cements, in natural deodorants (though potassium alum is more commonly used), in tanning, dyeing and in fireproofing textiles.
*Alum in block form (usually potassium alum) is used as an aftershave, rubbed over the wet, freshly shaved face.
*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.
*Alum may be used in depilatory waxes used for the removal of body hair, or applied to freshly waxed skin as a soothing agent.
*In the 1950s, men sporting crewcut or flattop hairstyles sometimes applied alum to their front short hairs as an alternative to pomade. When the hair dried, it would stay up all day.
*Alum’s antibacterial properties contribute to its traditional use as an underarm deodorant. It has been used for this purpose in Europe; Mexico; Thailand, where it is called Sarn-Som; throughout Asia; and in the Philippines, where it is called Tawas. Today, potassium alum is sold commercially for this purpose as a “deodorant crystal,” often in a protective plastic case.
*Alum is used in vaccines as an adjuvant to enhance the body’s response to immunogens.
*Styptic pencils containing aluminium sulfate or potassium aluminium sulfateare are used as astringents to prevent bleeding from small shaving cuts.
*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 powder, found in the spice section of many grocery stores, may be used in pickling recipes and as a preservative to maintain fruit and vegetable crispness.
*Alum is used as the acidic component of some commercial baking powders.
As a 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, used in fire extinguishers to smother chemical and oil fires.
As a Chemical Flocculant
Alum is used to clarify water by catching the very fine suspended particles in a gel-like precipitate of aluminum hydroxide. This sinks to the bottom of the containing vessel and can be removed in a variety of ways.
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).
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 (CH6N3+) and dimethylammonium (CH3)2NH2+) “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 ASO4·B2(SO4)3·22H2O, 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.
A Tutton salt is a double sulfate of the typical formula A2SO4·BSO4·6H2O, where A is a univalent cation, and B a divalent metal ion.
In popular culture
Gags in which someone ingests alum, either accidentally self-administered or surreptitiously administered by another, resulting in exaggerated effects, are a traditional staple of comedy. In live-action comedies, effects on the victim usually include extreme puckering of the mouth and lips and tightening of the throat. An example of this is in the Three Stooges short “No Census, No Feeling” when Curly is making a fruit punch and thinking it was sugar, puts alum in the fruit punch.
In animated cartoons, the effects are normally expanded to include extreme shrinking of the head. One example would be in the Merrie Melodies cartoon Long-Haired Hare featuring Bugs Bunny in which he plays a prank on a pompus opera singer named Giovanni Jones by lacing his atomizer with liquid alum. This causes Jones’ head to shrink and his voice to squeak. (Please see the link to the cartoon for a more complete synopsis.) Another such use is Back Alley Op-Roar (Freleng, 1945), in which Elmer feeds Sylvester Pussycat alum-laced milk, shrinking his head and driving his voice up several octaves while singing Figaro.
Also, Thomas Pynchon borrows the joke in chapter 16 of his 1963 novel V., in a scene where alum is slipped into the beer of a jazz trumpet player.