Sodium sulfite


Sodium sulfite is the inorganic compound with the chemical formula Na2SO3. A pale yellow, water-soluble solid, it is used commercially as an antioxidant and preservative. A heptahydrate is also known but it is less useful because of its greater susceptibility toward oxidation by air.

Preparation

Sodium sulfite can be prepared by treating a solution of sodium hydroxide with sulfur dioxide. When conducted in warm water, Na2SO3 initially precipitates as a yellow solid. With more SO2, the solid dissolves to give the disulfite, which crystallizes upon cooling.
Sodium sulfite is made industrially by treating sulfur dioxide with a solution of sodium carbonate. The overall reaction is:

Applications

Sodium sulfite is primarily used in the pulp and paper industry.
As an oxygen scavenger agent, it is used to treat water being fed to steam boilers to avoid corrosion problems, in the photographic industry, it protects developer solutions from oxidation and to wash fixer from film and photo-paper emulsions
As a reducing agent it is used in the textile industry as a bleaching, desulfurizing, and dechlorinating agent. Its reducing properties are exploited in its use as a preservative to prevent dried fruit from discoloring, and for preserving meats.
It is used as a reagent in sulfonation and sulfomethylation agent. It is used in the production of sodium thiosulfate.

Reactions

Heptahydrate

If sodium sulfite is allowed to crystallize from aqueous solution at room temperature or below, it does so as a heptahydrate. The heptahydrate crystals effloresce in warm dry air. Heptahydrate crystals also oxidize in air to form the sulfate. The anhydrous form is much more stable against oxidation by air.