SO42

SO4^2−, commonly called the sulfate ion, is a dianion in which sulfur is in the +6 oxidation state and is bound to four oxygen atoms. It is the conjugate base of sulfuric acid and represents the most common oxidized form of sulfur in natural environments and many chemical processes.

The sulfate ion has a tetrahedral geometry. Through resonance, the four S–O bonds share double and single-bond character, giving all bonds a similar average length and distributing the negative charge over the four oxygens. The sulfur–oxygen framework is robust, and the ion behaves as a relatively stable inorganic anion in solution.

Sulfate occurs widely in nature and in minerals. Gypsum (CaSO4·2H2O) and anhydrite (CaSO4) are common calcium sulfates, while barite (BaSO4) and celestine (SrSO4) are notable sulfates of heavier metals. In aqueous environments, many sulfate salts such as sodium sulfate and calcium sulfate are present in seawater and freshwaters; solubility depends on the associated cation and temperature.

In solution, sulfate acts as a weak base and is the conjugate base of bisulfate (HSO4−). It can form complexes with metal ions under certain conditions and participates in various redox and acid–base processes in geochemical and biological contexts. Under anaerobic conditions, certain microorganisms reduce sulfate to sulfide, integrating sulfate into the global sulfur cycle.

Industrially, sulfates have numerous uses: gypsum is widely employed in construction materials, barite serves as a drilling fluid weighting agent, and various sulfate salts are used in fertilizers, pigments, and chemical manufacturing. Sulfates are also encountered in environmental monitoring and water treatment.