alum, any one of a series of isomorphous double salts that are hydrated sulfates of a univalent cation (e.g., potassium, sodium, ammonium, cesium, or thallium) and a trivalent cation (e.g., aluminum, chromium, iron, manganese, cobalt, or titanium). The name alum commonly refers to potassium aluminum sulfate dodecahydrate, or potash alum, KAl(SO₄)₂·12H₂O, a colorless-to-white, crystalline compound. It is used in water purification, leather tanning, mordant dyeing, as an astringent, and in baking powder; it occurs in nature as the mineral kalunite. Sodium aluminum sulfate, or soda alum, NaAl(SO₄)₂·12H₂O, is also used in baking powder. Ammonium aluminum sulfate, or ammonia alum, NH₄Al(SO₄)₂·12H₂O, is used in tanning, in dyeing and fireproofing textiles, in vegetable glues and porcelain cements, and in water purification. Chromium potassium sulfate, or chrome alum, KCr(SO₄)₂·12H₂O, is used as a mordant in dyeing, in tanning, and in photographic fixing baths to harden gelatin films and plates. Aluminum sulfate, Al₂(SO₄)₃·18H₂O, is also called alum. A pseudoalum is a double sulfate salt of a divalent cation (e.g., magnesium or calcium) and a trivalent cation (e.g., aluminum).