Sodium metabisulfite or sodium pyrosulfite (sodium metabisulphite or sodium pyrosulphite) is an inorganic compound of chemical formula Na2S2O5. The substance is sometimes referred to as disodium metabisulfite. It is used as a disinfectant, antioxidant, and preservative agent.[2] When dissolved in water it forms sodium bisulfite.
Sodium metabisulfite can be prepared by treating a solution of sodium hydroxide with sulfur dioxide.[3] When conducted in warm water, Na2SO3 initially precipitates as a yellow solid. With more SO2, the solid dissolves to give the disulfite, which crystallises upon cooling.[4]
- SO2 + 2 NaOH → Na2SO3 + H2O
- SO2 + Na2SO3 → Na2S2O5
which yields a residue of colourless solid Na2S2O5.
Upon dissolution in water, bisulfite is generated:
- Na2S2O5 + H2O → 2 Na+ + 2 HSO3−
- Sodium bisulfite (or sodium bisulphite, sodium hydrogen sulfite) is a chemical mixture with the approximate chemical formula NaHSO3. Sodium bisulfite is not a real compound,[2] but a mixture of salts that dissolve in water to give solutions composed of sodium and bisulfite ions. It appears in form of white or yellowish-white crystals with an odor of sulfur dioxide. Sodium bisulfite is used in a variety industries such as a food additive with E number E222 in the food industry. It is a reducing agent in the cosmetic and in the bleaching applications.[3][4][5]
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Sodium bisulfite solutions can be prepared by treating a solution of suitable base, such as sodium hydroxide or sodium bicarbonate with sulfur dioxide.
- SO2 + NaOH → NaHSO3
- SO2 + NaHCO3 → NaHSO3 + CO2
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Attempts to crystallize the product yield sodium metabisulfite (also called sodium disulfite), Na2S2O5.[6]
Upon dissolution of the metabisulfite in water, bisulfite is regenerated:
- Na2S2O5 + H2O → 2 Na+ + 2 HSO3−
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Sodium bisulfite is formed during the Wellman-Lord process.[7]
