Dithionites

Dithionites refer to the salts of the dithionite dianion, S2O4^2-, and to the ion itself. The most common examples are sodium dithionite (Na2S2O4) and potassium dithionite (K2S2O4); calcium dithionite (CaS2O4) also occurs in some applications. The two sulfur atoms are connected by a single S–S bond, and each sulfur has an oxidation state of +3. In water, dithionite is a relatively strong reducing agent, but it is sensitive to air and to acids, and it tends to oxidize to sulfite and related sulfur oxyanions when exposed to oxygen or low pH.

Preparation and properties in solution vary by method, but industrially the sodium salt is typically generated by reducing sulfite solutions with sulfur dioxide under alkaline conditions. Dithionite solutions are best maintained under inert or reducing conditions; contact with air accelerates oxidation, and acidification promotes decomposition, sometimes releasing sulfur dioxide.

Uses and applications are primarily as a reducing agent. In textile and paper industries, dithionites are employed for bleaching and decolorization, including reduction of vat dyes and other colored components. In organic synthesis, they serve as mild, water-soluble reductants for various carbonyl or other oxidized functionalities. Dithionites have also been used in laboratory settings for redox workups and selective reductions.

Safety and handling considerations: dithionites are reactive and can irritate skin, eyes, and the respiratory tract. They can release sulfur dioxide on acidification and may form hazardous sulfur-containing species if degraded or mixed with incompatible oxidants. Solutions should be stored refrigerated, kept away from oxidants, metals, and acids, and handled in well-ventilated areas.