Ammonium aluminium sulfate
3D model (JSmol)
CompTox Dashboard (EPA)
|Molar mass||237.15 g/mol (anhydrous)|
453.33 g/mol (dodecahydrate)
|Density||2.45 g/cm3 (anhydrous)|
1.64 g/cm3 (dodecahydrate)
|Melting point||93.5 °C (200.3 °F; 366.6 K) (dodecahydrate)|
|Boiling point||120 °C (248 °F; 393 K) dehydr. (dodecahydrate)|
|15 g/100 ml (20 °C, dodecahydrate)|
|Safety data sheet||External MSDS|
|GHS signal word||WARNING|
|P264, P280, P302+352, P305+351+338, P321, P332+313, P337+313, P362|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is ?)(|
Ammonium aluminium sulfate, also known as ammonium alum or just alum (though there are many different substances also called "alum"), is a white crystalline double sulfate usually encountered as the dodecahydrate, formula (NH4)Al(SO4)2·12H2O. It is used in small amounts in a variety of niche applications. The dodecahydrate occurs naturally as the rare mineral tschermigite.
Production and basic properties
Ammonium alum is made from aluminium hydroxide, sulfuric acid and ammonium sulfate. It forms a solid solution with potassium alum. Pyrolysis leaves alumina. Such alumina is used in the production of grinding powders and as precursors to synthetic gems.
Ammonium alum is not a major industrial chemical or a particularly useful laboratory reagent, but it is cheap and effective, which invites many niche applications. It is used in water purification, in vegetable glues, in porcelain cements, in deodorants and in tanning, dyeing and in fireproofing textiles. The pH of the solution resulting from the topical application of ammonium alum with perspiration is typically in the slightly acid range, from 3 to 5.
Alum was once a common pickling ingredient used to promote crispness in preserved vegetables, due to the way it reacts with natural pectin. It has fallen out of use from a suspected link to Alzheimer's Disease, and is no longer recommended for pickling, but is still known as E number E523.
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