Sodium aluminium sulfate, NaAl(SO₄)₂·12H₂O or Na₂SO₄·Al₂(SO₄)₃·24H₂O, also known as soda alum or sodium alum, is a form of alum used in the acidity regulator of food (E521) as well as in the manufacture of baking powder. It is also a common mordant for the preparation of hematoxylin solutions for staining cell nuclei in histopathology.