A double sulfate of a monovalent metal or radical (as sodium, potassium, or ammonium) with a trivalent metal (as aluminum, iron, or chromium), thus of the formula A2SO4·B2(SO4)3·24H2O, where A is the monovalent radical and B the trivalent metal. It is used as an astringent, as an emetic, and in the manufacture of baking powders, dyes, and paper. The most common form is potassium aluminum sulfate, K2SO4·Al2(SO4)3·24H2O.