Pyrosulfite

Pyrosulfite is the name of the pyrosulfite anion, S2O5^2-, also known as disulfite. It is the condensation product of two sulfite units and is the conjugate base of pyrosulfurous acid, H2S2O5. In practice, pyrosulfite is encountered in salts such as sodium metabisulfite Na2S2O5 and potassium metabisulfite K2S2O5, which dissolve in water to give the pyrosulfite dianion.

In aqueous solution, metabisulfite salts rapidly equilibrate with bisulfite and sulfite species depending on pH. The dianion S2O5^2- can hydrolyze toward two bisulfite ions: S2O5^2- + H2O ⇌ 2 HSO3^−. Acidifying sulfite-containing solutions releases sulfur dioxide (SO2), a property exploited in various applications.

Preparation of metabisulfite salts typically involves reacting a sulfite salt with sulfur dioxide: Na2SO3 + SO2 → Na2S2O5, and similarly for potassium. These salts are used commercially as sources of sulfite/bisulfite and, upon dissolution, can supply pyrosulfite species in solution.

Uses of pyrosulfite and its salts include food preservation and antioxidant functions, most notably in wines, dried fruits, and other processed foods. They serve as reducing agents and buffering sources, and are also employed in water treatment and certain industrial processes. The ability of metabisulfites to release SO2 under acidic conditions underpins both their preservative action and potential antimicrobial effects.

Safety and regulatory considerations are important: sulfites can trigger hypersensitivity in some individuals, a risk addressed by labeling requirements in many jurisdictions. In handling, care is taken to avoid inhalation of dust and to manage emissions of sulfur dioxide upon acidification or reaction.