Sodium metabisulfite
chemical compound
Sodium metabisulfite or sodium pyrosulfite (IUPAC spelling; Br. E. 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. When dissolved in water it forms sodium bisulfite.
Preparation
Sodium metabisulfite 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 crystallises upon cooling.
SO2 + 2 NaOH → Na2SO3 + H2O
SO2 + Na2SO3 → Na2S2O5
which yields a residue of colourless solid Na2S2O5.
Chemical structure
The anion metabisulfite consists of an SO2 group linked to an SO3 group, with the negative charge more localised on the SO3 end.
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