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Jai Ganesh

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Registered: 2005-06-28

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Alum

Alum

Gist

Alum is a natural crystalline compound with antiseptic, astringent, and deodorizing properties, commonly known as a double sulfate salt of aluminum. It is used for water purification, in skincare for acne and tightening, and in traditional medicine for issues like bleeding gums and as a deodorant. It is also used in the food and textile industries.

Alum is a crystalline double sulfate salt of a monovalent cation (like potassium) and aluminium, which has properties like antiseptic, astringent, and deodorizing effects. It is used in various applications including water purification, food, and medicine.

Summary

An alum is a type of chemical compound, usually a hydrated double sulfate salt of aluminium with the general formula XAl(SO4)2·12H2O, such that X is a monovalent cation such as potassium or ammonium. By itself, alum often refers to potassium alum, with the formula KAl(SO4)2·12H2O. 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 chromiumIII, or sulfur is replaced by another chalcogen like selenium. The most common of these analogs is chrome alum KCr(SO4)2·12H2O.

In most industries, the name alum (or papermaker's alum) is used to refer to aluminium sulfate, Al2(SO4)3·n2O, which is used for most industrial flocculation  (the variable n is an integer whose size depends on the amount of water absorbed into the alum). For medicine, the word alum may also refer to aluminium hydroxide gel used as a vaccine adjuvant.

Production

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.

Details

Alum is a type of chemical compound that is commonly used in everyday and industrial applications. You'll find alum in everything from baking powder and toothpaste to cosmetics and some fire extinguishers, but there are also various types of alum with different use cases.

Usually, when you hear about alum it is in reference to potassium alum, which is the hydrated form of potassium aluminum sulfate and has the chemical formula KAl(SO4)2·12H2O.

However, any of the compounds with the empirical formula AB(SO4)2·12H2O are also considered to be alum. Sometimes alum is seen in its crystalline form, although it is most often sold as a powder. Potassium alum is a fine white powder that you can find sold with kitchen spices or pickling ingredients. It is also sold as a large crystal as a "deodorant rock" for underarm use.

Key Takeaways: Alum

* Alum refers to a collection of chemical compounds that are hydrated sulfate salts of aluminum and usually one other metal.
* Common forms of alum include hydrated potassium aluminum sulfate, ammonium aluminum sulfate, and sodium aluminum sulfate.
* The different compounds have different functions. Alum finds use in vaccines, baking powder, tanning agents, deodorants, and antiseptics.

What Are the Types of Alum?

* Potassium Alum: Potassium alum is also known as potash alum or tawas. It is aluminum potassium sulfate. This is the type of alum that you find in the grocery store for pickling. It is also used in leather tanning, as a flocculant in water purification, as an ingredient in aftershave, and as a treatment for fireproof textiles. Its chemical formula is KAl(SO4)2.
* Soda Alum: Soda alum has the formula NaAl(S O4)2·12H2O. It is used in baking powder and as an acidulant in food.
* Ammonium Alum: Ammonium alum has the formula NH4Al(SO4)2·12H2O. Ammonium alum is used for many of the same purposes as potassium alum and soda alum. Ammonium alum finds applications in tanning, dyeing textiles, purifying water, and making textiles flame retardant. It's also used in the manufacture of porcelain cement, vegetable glues, and some deodorants.
* Chrome Alum: Chrome alum or chromium alum has the formula KCr(S O4)2·12H2O. This deep violet compound is used in tanning and can be added to other alums to grow lavender or purple crystals.
* Selenate Alums: Selenate alums occur when selenium takes the place of sulfur so that instead of a sulfate you get a selenate, (SeO42-). The selenium-containing alums are strong oxidizing agents, so they can be used as antiseptics, among other uses.
* Aluminum Sulfate: This compound is also known as papermaker's alum. However, it is not technically an alum.

What Is Alum Used for?

Alum has several household and industrial uses. Potassium alum is used most often, although ammonium alum, ferric alum, and soda alum may be used for many of the same purposes.

* purification of drinking water as a chemical flocculant
* in styptic pencil to stop bleeding from minor cuts
* the adjuvant in vaccines ( a chemical that enhances the immune response)
* deodorant "rock"
* pickling agent to help keep pickles crisp
* flame retardant
* the acidic component of some types of baking powder
* an ingredient in some homemade and commercial modeling clay
* an ingredient in some depilatory (hair removal) waxes
* skin whitener
* ingredient in some brands of toothpaste.

Alum in Science Projects

Several interesting science projects use alum. In particular, it is used to grow stunning non-toxic crystals. Clear crystals result from potassium alum, while purple crystals grow from chrome alum.

How Is Alum Produced?

Several minerals are used as the source material to produce alum, including alum schist, alunite, bauxite, and cryolite.

The specific process used to obtain the alum depends on the original mineral. When alum is obtained from alunite, the alunite is calcined. The resulting material is kept moist and exposed to air until it turns to a powder, which is lixiviated, or extracted, with sulfuric acid and hot water. The liquid is decanted, and the alum crystallizes out of the solution.

Additional Information

Alum is any of a group of hydrated double salts, usually consisting of aluminum sulfate, water of hydration, and the sulfate of another element. A whole series of hydrated double salts results from the hydration of the sulfate of a singly charged cation (e.g., K+) and the sulfate of any one of a number of triply charged cations (e.g., Al3+). Aluminum sulfate can thus form alums with sulfates of the singly charged cations of potassium, sodium, ammonium, cesium, and other elements and compounds. In similar fashion, sulfates of the triply charged cations of iron, chromium, manganese, cobalt, and other metals may take the place of aluminum sulfate. The most important alums are potassium aluminum sulfate, ammonium aluminum sulfate, and sodium aluminum sulfate. Potassium aluminum sulfate, also known as potassium alum or potash alum, has a molecular formula of K2(SO4)·Al2(SO4)3·24H2O or KAl(SO4)2·12H2O.

Alums can easily be produced by precipitation from an aqueous solution. In producing potassium alum, for example, aluminum sulfate and potassium sulfate are dissolved in water, and then upon evaporation the alum crystallizes out of the solution. A more common production method is to treat bauxite ore with sulfuric acid and then with potassium sulfate. Ammonium alum is produced by the evaporation of a water solution containing ammonium sulfate and aluminum sulfate. It can also be obtained by treating a mixture of aluminum sulfate and sulfuric acid with ammonia. Alums occur naturally in various minerals. Potassium alum, for example, is found in the minerals kalinite, alunite, and leucite, which can be treated with sulfuric acid to obtain crystals of the alum.

Most alums have an astringent and acid taste. They are colourless, odourless, and exist as a white crystalline powder. Alums are generally soluble in hot water, and they can be readily precipitated from aqueous solutions to form large octahedral crystals.

Alums have many uses, but they have been partly supplanted by aluminum sulfate itself, which is easily obtainable by treating bauxite ore with sulfuric acid. The commercial uses of alums mainly stem from the hydrolysis of the aluminum ions, which results in the precipitation of aluminum hydroxide. This chemical has various industrial uses. Paper is sized, for example, by depositing aluminum hydroxide in the interstices of the cellulose fibres. Aluminum hydroxide adsorbs suspended particles from water and is thus a useful flocculating agent in water-purification plants. When used as a mordant (binder) in dyeing, it fixes dye to cotton and other fabrics, rendering the dye insoluble. Alums are also used in pickling, in baking powder, in fire extinguishers, and as astringents in medicine.

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