A sulfite is a salt or ester of sulfurous acid (H₂SO₃). Sulfurous acid is known systematically as trioxosulfuric (IV) acid and its salts, the sulfites, contain the trioxosulfate (IV) ion (SO₃^2–). Many sulfites are reducing agents, used as dininfectants and bleaches. Hydrogensulfates (IV) (bisulfites, HSO₃^–) also exist.