|IUPAC name||Aluminium sulfate|
|Other names||Cake alum|
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|Molar mass||342.15 g/mol as anhydrous salt|
|Appearance||white crystalline solid|
|Density||2.672 g/cm³, solid|
|Crystal structure||monoclinic (hydrate)|
|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.
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.
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.
It is also used in styptic pencils.
- 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.