Potassium alum

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Potassium aluminium sulfate
Aluminium potassium sulfate
Identifiers
CAS number 10043-67-1 YesY
7784-24-9 (dodecahydrate)
PubChem 24856
Jmol-3D images Image 1
Properties
Molecular formula KAl(SO4)2
Molar mass 258.205 g/mol
Appearance white small crystals
Odor watery metallic
Density 1.725 g/cm3
Melting point 92–93 °C
Boiling point 200 °C
Solubility in water 14.00 g/100 mL (20 °C)
36.80 g/100 mL (50 °C)
Solubility insoluble in acetone
Refractive index (nD) 1.4564
Hazards
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 noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Potassium alum, potash alum, tawas, 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)2.12(H2O). 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,[citation needed] 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.[4]

Characteristics[edit]

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[edit]

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.[5] Alunite is an associate and likely potassium and aluminium source.[3][6] 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)2·11(H2O).[7]

Uses[edit]

Chemical[edit]

  • in tanning of leather to prepare the hide
  • as a mordant for dyes
  • in the clarifying of turbid liquids, including post-storm treatment of lakes to precipitate contaminants[8]
  • as a fire retardant in textile products
  • (historically) as a hardener for photographic emulsions (films and papers), usually as part of the fixer. Modern alternatives are superior.

Medicinal[edit]

Culinary[edit]

  • As an additive to baking powder to provide a second leavening phase at high temperatures.

Traditional[edit]

Toxicology and safety[edit]

Deodorant crystals containing synthetically made potassium alum are a weak irritant to the skin.[10]

See also[edit]

References[edit]

  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. ^ a b "Alum-(K) Mineral Data". Mineralogy Database. Retrieved 2013-04-19. 
  4. ^ a b Helmenstine, Anne Marie. "What is Alum?". About.com. Retrieved 2013-04-19. 
  5. ^ Bottomley (2010) p. 35
  6. ^ "Alum-(K) mineral data and informaton". MinDat. Retrieved 2013-04-19. 
  7. ^ "Kalinite Mineral Data". MinDat. Retrieved 2013-04-19. 
  8. ^ http://www.engr.wisc.edu/news/archive/2013/aug20.html
  9. ^ http://tcm.health-info.org/Herbology.Materia.Medica/mingfan-properties.htm Uses of Alum in Traditional Chinese Medicine
  10. ^ 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[edit]

  • Bottomley, L.; Bottomley, L.A. (2010). School of Chemistry & Bichemistry, Georgia Institute of Technology, Chemistry 1310: Laboratory Manual. Plymouth, MI: Hayden-McNeil Publishing. ISBN 978-0-7380-3819-3. 

External links[edit]