Aluminium sulfate

Aluminium sulfate, written as Al₂(SO₄)₃ or Al₂O₁₂S₃, is a widely used industrial chemical. It is sometimes incorrectly referred to as alum, as it is closely related to this group of compounds. It occurs naturally as the mineral alunogenite. It is frequently used as a flocculating agent in the purification of drinking water^[1][2] and waste water treatment plants, and also in paper manufacturing.

Aluminium sulfate is rarely, if ever, encountered as the anhydrous salt. It forms a number of different hydrates, of which the hexadecahydrate Al₂(SO₄)₃•16H₂O and octadecahydrate Al₂(SO₄)₃•18H₂O are the most common.

It can also be very effective as a molluscicide, killing spanish slugs. [1]

Recent research suggests aluminum sulfate may contribute to dementia and Alzheimer's disease.

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Preparation

Aluminium sulfate may be made by dissolving aluminium hydroxide, Al(OH)₃, in sulfuric acid, H₂SO₄:

2Al(OH)₃ + 3H₂SO₄ + 3H₂O → Al₂(SO₄)₃·6H₂O

Uses

Aluminium Sulfate is used in water purification and as a mordant in dyeing and printing textiles. In water purification, it causes impurities to coagulate which are removed as the particulate settles to the bottom of the container or more easily filtered. This process is called coagulation or flocculation.

When dissolved in a large amount of neutral or slightly-alkaline water, aluminium sulfate produces a gelatinous precipitate of aluminium hydroxide, Al(OH)₃. In dyeing and printing cloth, the gelatinous precipitate helps the dye adhere to the clothing fibers by rendering the pigment insoluble.

Aluminium sulfate is sometimes used to reduce the pH of garden soil, as it hydrolyzes to form the aluminium hydroxide precipitate and a dilute sulfuric acid solution.

Aluminium sulfate is the active ingredient of some antiperspirants; however, beginning in 2005 the US Food and Drug Administration no longer recognized it as a wetness reducer.

Aluminum Sulfate is usually found in baking powder.

In construction industry it is used as waterproofing agent and accelerator in concrete. Another use is a foaming agent in fire fighting foam.

It is also used in styptic pencils, and pain relief from stings and bites; it is the active ingredient in popular pain relief products such as Stingose.

See also

• Camelford, a town in Cornwall (UK) where the local water supplies were accidentally contaminated with aluminium sulfate.

References

1. ^ Global Health and Education Foundation (2007). Conventional Coagulation-Flocculation-Sedimentation. Safe Drinking Water is Essential. National Academy of Sciences. Retrieved on 2007-12-01.
2. ^ Kvech S, Edwards M (2002). "Solubility controls on aluminum in drinking water at relatively low and high pH". WATER RESEARCH 36 (17): 4356-4368. PMID 12420940.

• Pauling, Linus (1970). General Chemistry. W.H. Freeman: San Francisco. ISBN 0-486-65622-5.