Sodium metabisulfite

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]

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

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⁻

Sodium bisulfite is formed during the Wellman-Lord process.^[7]