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 Tschermigite is a mineral form of ammonium alum, formula N H4 Al( S O4)2·12(H2O). It is found in burning coal seams, bituminous shale and fumarole A fumarole (or fumerole) is a vent in the surface of the Earth or other rocky planet fro ...

Production and basic properties

Ammonium alum is made from

aluminium hydroxide Aluminium hydroxide, Al(OH)3, is found in nature as the mineral gibbsite (also known as hydrargillite) and its three much rarer polymorphs: bayerite, doyleite, and nordstrandite. Aluminium hydroxide is amphoteric, i.e., it has both basic an ...

, sulfuric acid and

ammonium sulfate Ammonium sulfate (American English and international scientific usage; ammonium sulphate in British English); (NH4)2SO4, is an inorganic salt with a number of commercial uses. The most common use is as a soil fertilizer. It contains 21% nitrogen a ...

. It forms a

solid solution A solid solution, a term popularly used for metals, is a homogenous mixture of two different kinds of atoms in solid state and have a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The wor ...

with

potassium alum Potassium alum, potash alum, or potassium aluminium sulfate is a chemical compound: the double sulfate of potassium and aluminium, with chemical formula KAl(SO4)2. It is commonly encountered as the dodecahydrate, KAl(SO4)2·12H2O. It crystalli ...

Pyrolysis The pyrolysis (or devolatilization) process is the thermal decomposition of materials at elevated temperatures, often in an inert atmosphere. It involves a change of chemical composition. The word is coined from the Greek-derived elements ''py ...

leaves alumina. Such alumina is used in the production of grinding powders and as precursors to synthetic gems.

Uses

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

deodorant A deodorant is a substance applied to the body to prevent or mask body odor due to bacterial breakdown of perspiration or vaginal secretions, for example in the armpits, groin, or feet. A subclass of deodorants, called antiperspirants, prevents ...

s 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. Ammonium alum is a common ingredient in animal repellent sprays..

References

{{DEFAULTSORT:Ammonium Aluminium Sulfate Aluminium compounds Ammonium compounds Sulfates Double salts Astringent flavors E-number additives