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]

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]

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.^[10]

•  This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Alum". Encyclopædia Britannica. Vol. 1 (11th ed.). Cambridge University Press. pp. 766–767.