An alum (/ˈæləm/) is a type of chemical compound sulfate with the general formula XAl(SO 4) 2·12H 2O, where X is a monovalent such as potassium By itself, "alum" often refers to potassium alum, with the formula KAl(SO 4) 2·12H 2O. Other alums are named after the monovalent ion, such as sodium alum.
The name "alum" is also used, more generally, for salts with the same formula and structure, except that aluminium is replaced by another trivalent, and/or sulfur is replaced by another chalcogen like selenium The most common of these analogs is chrome alum KCr(SO 4) 2·12H 2O.
In most industries, the name "alum" (or "papermaker's alum") is used to refer to aluminium sulfate Al 2(SO 4) 3·nH 2O, which is used for most industrial flocculation. In medicine, "alum" may also refer to aluminium hydroxide