E521 - Aluminium sodium sulphate

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. - Wikipedia