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What Is Sodium Sulfite?
Sodium sulfite is an inorganic compound, represented by the formula Na2SO3. It forms as sodium sulfite heptahydrate when sodium hydroxide and sulfur dioxide react and cool, transitioning to anhydrous sodium sulfite upon heating. This compound can also be synthesized from sodium hydrogen sulfite and sodium carbonate, commonly produced industrially by sulfur dioxide recovery from heavy oil combustion flue gases.
Uses of Sodium Sulfite
As an antioxidant in wine and a preservative in cosmetics and hair color, sodium sulfite finds extensive use in cosmetics, foods, and pharmaceuticals. It prevents fading in dried fruits and acts as a bleaching agent in foods like kanpyo and cooked beans. In photography, it serves as a retaining agent to prevent developer oxidation and to remove the fixing solution from film.
Properties of Sodium Sulfite
With a pH of approximately 9, sodium sulfite solutions are slightly alkaline. Exposed to air, sodium sulfite solutions and heptahydrate crystals gradually oxidize to sulfate, with the anhydrous form being more air stable. It reacts with weak acids, releasing sulfur dioxide.
Structure of Sodium Sulfite
Sodium sulfite exists as a colorless monoclinic crystal in its heptahydrate form, with a density of 1.56 g/cm3, and as a hexagonal crystal in its anhydrous form, with a density of 2.63 g/cm3. X-ray crystallography reveals a pyramidal SO32- ion structure.
Other Information on Sodium Sulfite
1. Hazards of Sodium Sulfite
Linked to various adverse reactions including respiratory issues in asthmatics, sodium sulfite’s inclusion in fresh foods is restricted by the FDA, with required labeling for concentrations above 10 ppm. It can also cause skin reactions in cosmetics and medications.
2. Related Compounds of Sodium Sulfite
Sodium hydrogen sulfite, formed from sulfur dioxide and sodium sulfite, shares properties and uses with sodium sulfite, including its gradual oxidation to sulfate in air. Sodium bisulfite, another related compound, is used interchangeably with sulfite in solutions.