Aluminium sulfate

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Template:Chembox ECNumberTemplate:Chembox E numberTemplate:Chembox SolubilityInWater
Aluminium sulfate
IUPAC name Aluminium sulfate
Other names Cake alum
Filter alum
Papermaker's alum
Aluminum sulfate
Aluminium sulphate
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RTECS number BD1700000
Molar mass 342.15 g/mol as anhydrous salt
Appearance white crystalline solid
Density 2.672 g/cm³, solid
Melting point
Crystal structure monoclinic (hydrate)
Related compounds
Other cations {{{value}}}
Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)

Infobox disclaimer and references

Aluminium sulfate 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 and waste water treatment plants, and also in paper manufacturing.[citation needed]

Aluminium sulfate is rarely, if ever, encountered as the anhydrous salt. It forms a number of different hydrates, of which the hexadecahydrate is the most common.

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


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

2Al(OH)3 + 3H2SO4 + 10H2O → Al2(SO4)3·16H2O


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.

It is also used in styptic pencils.

See also

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


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

External links

de:Aluminiumsulfat ku:Bafûnsulfat nl:Aluminiumsulfaat qu:Millu sr:Алуминијум сулфат fi:Alumiinisulfaatti sv:Aluminiumsulfat