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Sodium alum

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Sodium alum
IUPAC name
Aluminium sodium bis(sulfate) — water (1:12)
Other names
  • Sodium alum
  • Soda alum
  • E521
3D model (JSmol)
ECHA InfoCard 100.030.239 Edit this at Wikidata
EC Number
  • 233-277-3
E number E521 (acidity regulators, ...)
  • InChI=1S/Al.Na.2H2O4S.12H2O/c;;2*1-5(2,3)4;;;;;;;;;;;;/h;;2*(H2,1,2,3,4);12*1H2/q+3;+1;;;;;;;;;;;;;;/p-4
  • InChI=1/Al.Na.2H2O4S.12H2O/c;;2*1-5(2,3)4;;;;;;;;;;;;/h;;2*(H2,1,2,3,4);12*1H2/q+3;+1;;;;;;;;;;;;;;/p-4
  • [O-]S(=O)(=O)[O-].[O-]S(=O)(=O)[O-].[Na+].[Al+3]
Molar mass 458.28 g/mol
Appearance white crystalline powder
Density 1.6754 (20 °C)
Melting point 61 °C (142 °F; 334 K)
208 g/100 ml (15 °C)
Cubic, cP96
Pa3, No. 205
a = 1221.4 pm
Octahedral (Na+)
Octahedral (Al3+)
Flash point non-flammable
Related compounds
Other cations
Ammonium aluminium sulfate
Potassium aluminium sulfate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Sodium aluminium sulfate is the inorganic compound with the chemical formula NaAl(SO4)2·12H2O (sometimes written Na2SO4·Al2(SO4)3·24H2O). Also known as soda alum, sodium alum, or SAS, this white solid is used in the manufacture of baking powder and as a food additive. Its official mineral name is alum-Na (IMA symbol: Aum-Na[3]).


Like its potassium analog, sodium aluminum sulfate crystallizes as the dodecahydrate in the classical cubic alum structure.

Sodium alum is very soluble in water, and is extremely difficult to purify. In the preparation of this salt, it is preferable to mix the component solutions in the cold, and to evaporate them at a temperature not exceeding 60 °C. 100 parts of water dissolve 110 parts of sodium alum at 0 °C, and 51 parts at 16 °C.[4]

Production and natural occurrence[edit]

Sodium aluminum sulfate is produced by combining sodium sulfate and aluminium sulfate. An estimated 3000 ton/y (2003) are produced worldwide.

The dodecahydrate is known in mineralogy as alum-(Na).[5][6] Two other rare mineral forms are known: mendozite (undecahydrate)[7] and tamarugite (hexahydrate).[8]


In the US, some brands combine sodium aluminum sulfate with sodium bicarbonate and monocalcium phosphate in formulations of double acting baking powder.[9] Kawahara et al. 1994 noted that aluminum is “a suspected risk factor in Alzheimer's disease” and that “aluminum directly influences the process of Alzheimer′s disease”.[10] More recent research however disputes the alleged link between aluminum and Alzheimer's disease [11][12][13] and The Alzheimer’s Society concluded that “No convincing relationship between aluminium and the development of Alzheimer's disease has been established.”[14][15]

Sodium alum is also used as an acidity regulator in food, with E number E521.

Sodium alum is also a common mordant for the preparation of hematoxylin solutions for staining cell nuclei in histopathology.[citation needed]

It is also used as a flocculant in water treatment and disinfection, but its relatively crude, caustic action makes it more suitable for industrial applications.[16]


  1. ^ Weast, Robert C., ed. (1981). CRC Handbook of Chemistry and Physics (62nd ed.). Boca Raton, FL: CRC Press. p. B-146. ISBN 0-8493-0462-8..
  2. ^ Cromer, D. T.; Kay, M. I.; Larson, A. C. (1 February 1967). "Refinement of the alum structures. II. X-ray and neutron diffraction of NaAl(SO4)2·12H2O, γ-alum". Acta Crystallographica. 22 (2): 182–187. doi:10.1107/S0365110X67000313.
  3. ^ Warr, L.N. (2021). "IMA-CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  4. ^ Chisholm 1911, p. 767[full citation needed].
  5. ^ Burke, Ernst A.J. (2008). "Tidying up mineral names: an IMA-CNMNC scheme for suffixes, hyphens and diacritical marks" (PDF). Mineralogical Record. 39 (2): 131–35. CiteSeerX Gale A177553581 ProQuest 211734059. Archived from the original (PDF) on 2012-03-26. Retrieved 2009-11-29.
  6. ^ Alum-(Na), WebMineral.com, retrieved 2009-11-28.Alum-(Na), Mindat.org, retrieved 2009-11-28.
  7. ^ Mendozite, WebMineral.com, retrieved 2009-11-28.Mendozite, Mindat.org, retrieved 2009-11-28.
  8. ^ Tamarugite, WebMineral.com, retrieved 2009-11-28.Tamarugite, Mindat.org, retrieved 2009-11-28.
  9. ^ Helmboldt, Otto; Keith Hudson, L.; Misra, Chanakya; Wefers, Karl; Heck, Wolfgang; Stark, Hans; Danner, Max; Rösch, Norbert. "Aluminum Compounds, Inorganic". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a01_527.pub2. ISBN 978-3527306732.
  10. ^ Kawahara, M.; Muramoto, K.; Kobayashi, K.; Mori, H.; Kuroda, Y. (January 1994). "Aluminum Promotes the Aggregation of Alzheimer′s Amyloid β-Protein in Vitro". Biochemical and Biophysical Research Communications. 198 (2): 531–535. doi:10.1006/bbrc.1994.1078. PMID 7507666.
  11. ^ Lidsky, T. I. (2014). "Is the Aluminum Hypothesis Dead?". Journal of Occupational and Environmental Medicine. 56 (5 Suppl): S73–S79. doi:10.1097/JOM.0000000000000063. PMC 4131942. PMID 24806729.
  12. ^ Santibáñez, M.; Bolumar, F.; García, A. M. (2007). "Occupational risk factors in Alzheimer's disease: A review assessing the quality of published epidemiological studies". Occupational and Environmental Medicine. 64 (11): 723–732. doi:10.1136/oem.2006.028209. PMC 2078415. PMID 17525096.
  13. ^ Killin, L. O.; Starr, J. M.; Shiue, I. J.; Russ, T. C. (2016). "Environmental risk factors for dementia: A systematic review". BMC Geriatrics. 16 (1): 175. doi:10.1186/s12877-016-0342-y. PMC 5059894. PMID 27729011.
  14. ^ "Metals and the risk of dementia | Alzheimer's Society".
  15. ^ https://www.alz.org/alzheimers-dementia/what-is-alzheimers/myths
  16. ^ "Products of the Sodium Hydroxide Tree" (PDF). WorldChlorine.org. Retrieved 17 June 2019.

Works cited[edit]