A sulfate is a salt or ester of sulfuric acid (H2SO4). Sulfate salts contain the ion –SO42–. A common example is copper (II) sulfate, CuSO4.
Inorganic sulfates are formed by reaction of the acid with metals, their oxides or carbonates, or by oxidation of sulfides or sulfites (see sulfur). Most sulfates are soluble in water, the main exceptions being calcium, strontium, barium, and lead sulfates. They decompose at high temperatures to give sulfur trioxide and dioxide. Sulfates form ligand complexes and soluble salts. Many sulfate minerals occur in nature, often as evaporites or from oxidation of sulfides (see anhydrite; barite; Epsom salts; gypsum).
Bisulfates contain the ion HSO4; they are acid, and are converted to pyrosulfates (S2O7)21) on heating.
Organic sulfates have the formula R2SO4, where R is an organic group.
Alum is a double salt comprising sulfates of two metals (one monovalent, one trivalent) combined with 12 molecules of water of crystallization: MIMIII(SO4)2.12H2O. The monovalent metal is commonly potassium, sodium, or ammonium; the trivalent metal may be aluminum, chromium, or ferric iron.
Alums are soluble in water and are usually acid. They are used in astringents (styptic pencils), as a mordant in dyes, and in the manufacture of baking powder, antiperspirants, and fire extinguishers. Potash alum (potassium aluminum sulfate, KAl(SO4)2.12H2O) is used in the sizing of paper, as a mordant in dyeing, and in water purification.