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|Jmol-3D images||Image 1|
|Molar mass||342.15 g/mol (anhydrous)
666.42 g/mol (octadecahydrate)
|Appearance||white crystalline solid
|Density||2.672 g/cm3 (anhydrous)
1.62 g/cm3 (octadecahydrate)
770 °C (decomp, anhydrous)
|Solubility in water||31.2 g/100 mL (0 °C)
36.4 g/100 mL (20 °C)
89.0 g/100 mL (100 °C)
|Solubility||slightly soluble in alcohol, dilute mineral acids|
|Refractive index (nD)||1.47 |
|Crystal structure||monoclinic (hydrate)|
|Other cations||Gallium sulfate
|Related compounds||See Alum|
|Supplementary data page|
|n, εr, etc.|
Solid, liquid, gas
|Spectral data||UV, IR, NMR, MS|
| (what is: / ?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Aluminium sulfate, alternatively spelt either aluminum or sulphate, is a chemical compound with the formula Al2(SO4)3. It is soluble in water and is mainly used as a flocculating agent in the purification of drinking water and waste water treatment plants, and also in paper manufacturing.
Aluminium sulfate is sometimes referred to as a type of alum. Alums are a class of related compounds typified by AB(SO4)2.12H2O. The anhydrous form occurs naturally as a rare mineral millosevichite, found e.g. in volcanic environments and on burning coal-mining waste dumps. 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. The heptadecahydrate, whose formula can be written as [Al(H2O)6]2(SO4)3•5H2O, occurs naturally as the mineral alunogen.
- 2 Al(OH)3 + 3 H2SO4 → Al2(SO4)3·6H2O
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. An example of what changing the pH level of soil can do to plants is visible when looking at the Hydrangea macrophylla. The gardener can add aluminium sulfate to the soil to reduce the pH level which in turn will result in the flowers of the Hydrangea turning a different color.
Aluminium sulfate is usually found in baking powder, where there is controversy over its use due to concern regarding the safety of adding aluminium to the diet.
It is also used in styptic pencils, and pain relief from stings and bites.
Chemical reactions 
- Al2(SO4)3 + 6 NaHCO3 → 3 Na2SO4 + 2 Al(OH)3 + 6 CO2
The carbon dioxide is trapped by the foam stabilizer and creates a thick foam which will float on top of hydrocarbon fuels and seal off access to atmospheric oxygen, smothering the fire. Chemical foam was unsuitable for use on polar solvents such as alcohol, as the fuel would mix with and break down the foam blanket. The carbon dioxide generated also served to propel the foam out of the container, be it a portable fire extinguisher or fixed installation using hoselines. Chemical foam is considered obsolete in the United States and has been replaced by synthetic mechanical foams, such as AFFF which have a longer shelf life, are more effective, and more versatile, although some countries such as Japan and India continue to use it.
- Pradyot Patnaik. Handbook of Inorganic Chemicals. McGraw-Hill, 2002, ISBN 0-07-049439-8
- Global Health and Education Foundation (2007). "Conventional Coagulation-Flocculation-Sedimentation". Safe Drinking Water is Essential. National Academy of Sciences. Retrieved 2007-12-01.
- Kvech S, Edwards M (2002). "Solubility controls on aluminum in drinking water at relatively low and high pH". WATER RESEARCH 36 (17): 4356–4368. doi:10.1016/S0043-1354(02)00137-9. PMID 12420940.
- International Chemical Safety Card 1191
- NIOSH Pocket Guide to Chemical Hazards
- WHO Food Additive Series No. 12
- Aluminum and health
- Government of Canada Fact Sheets and Frequently Asked Questions: Aluminum Salts