Potassium alum

Potassium aluminium sulfate
IUPAC name
Aluminium potassium sulfate dodecahydrate[1][2]
Other names
Potassium alum
Potash alum
10043-67-1 YesY
7784-24-9 (dodecahydrate) N
3D model (Jmol) Interactive image
ECHA InfoCard 100.030.116
E number E522 (acidity regulators, ...)
PubChem 24856
Molar mass 474.3884 g/mol
Appearance white small crystals
Odor watery metallic
Density 1.725 g/cm3
Melting point 92 to 95 °C (198 to 203 °F; 365 to 368 K)
Boiling point 200 °C (392 °F; 473 K)
14.00 g/100 mL (20 °C)
36.80 g/100 mL (50 °C)
Solubility insoluble in acetone
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g., chloroform Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazards (white): no codeNFPA 704 four-colored diamond
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YesYN ?)
Infobox references

Potassium alum, potash alum, or potassium aluminum sulfate is a chemical compound: the potassium double sulfate of aluminium. Its chemical formula is KAl(SO4)2 and it is commonly found in its dodecahydrate form as KAl(SO4)2·12H2O. Alum is the common name for this chemical compound, given the nomenclature of potassium aluminum sulfate dodecahydrate. It is commonly used in water purification, leather tanning, dyeing,[4] fireproof textiles, and baking powder. It also has cosmetic uses as a deodorant, as an aftershave treatment and as a styptic for minor bleeding from shaving.[5]


Octahedral potassium alum crystal with unequal distribution of the face area

Potassium alum crystallizes in regular octahedra with flattened corners and is very soluble in water. The solution reddens litmus and is an astringent. When heated to nearly a red heat, it gives a porous, friable mass, which is known as "burnt alum". It fuses at 92 °C (198 °F) in its own water of crystallization. "Neutral alum" is obtained by the addition of as much sodium carbonate to a solution of alum as will begin to cause the separation of alumina.

Mineral form and occurrence

Potassium alum or alum-(K) is a naturally occurring sulfate mineral, which typically occurs as encrustations on rocks in areas of weathering and oxidation of sulfide minerals and potassium-bearing minerals. In the past, alum was obtained from alunite, a mineral mined from sulfur-containing volcanic sediments source.[6] Alunite is an associate and likely potassium and aluminium source.[3][7] It has been reported at Vesuvius, Italy; east of Springsure, Queensland; Alum Cave, Tennessee; Alum Gulch, Santa Cruz County, Arizona and the Philippine island of Cebu. A related mineral is kalinite, a fibrous mineral with formula KAl(SO4)2·11H2O.[8]






Toxicology and safety

Potassium alum may be a weak irritant to the skin.[11]

See also


  1. International Union of Pure and Applied Chemistry (2005). Nomenclature of Inorganic Chemistry (IUPAC Recommendations 2005). Cambridge (UK): RSCIUPAC. ISBN 0-85404-438-8. Electronic version.
  2. "Aluminium potassium sulfate dodecahydrate". ChemExper. Retrieved 2013-04-19.
  3. 1 2 "Alum-(K) Mineral Data". Mineralogy Database. Retrieved 2013-04-19.
  4. Editors, The. "alum | chemical compound". Britannica.com. Retrieved 2016-01-18.
  5. 1 2 Helmenstine, Anne Marie. "What is Alum?". About.com. Retrieved 2013-04-19.
  6. Bottomley (2010) p. 35.
  7. "Alum-(K) mineral data and information". MinDat. Retrieved 2013-04-19.
  8. "Kalinite Mineral Data". MinDat. Retrieved 2013-04-19.
  9. "Storm water treatment will strip phosphorus from Arboretum pond, College of Engineering @ The University of Wisconsin-Madison, initiatives in energy, health, nanotechnology, security, and information technology". Engr.wisc.edu. Retrieved 2016-01-18.
  10. Uses of Alum in Traditional Chinese Medicine
  11. Gallego H, Lewis EJ, Crutchfield CE 3rd (July 1999). "Crystal deodorant dermatitis: irritant dermatitis to alum-containing deodorant". Cutis. 64 (1): 65–6. PMID 10431678.

Further reading

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