Sodium metabisulfite or sodium pyrosulfite (IUPAC spelling; Br. E. sodium metabisulphite or sodium pyrosulphite) is an inorganic compound of chemical formula Na2S2O5. The substance is sometimes referred to as disodium metabisulfite. It is used as a disinfectant, antioxidant, and preservative agent.\
The anion metabisulfite is a hybrid of dithionite (S2O42) and dithionate (S2O62). The anion consists of an SO2 group linked to an SO3group, with the negative charge more localized on the SO3 end. The SS bond length is 2.22 , and the "thionate" and "thionite" SO distances are 1.46 and 1.50 respectively.