Sodium metabisulfite or sodium pyrosulfite (IUPAC spelling; Br. E. sodium metabisulphite or sodium pyrosulphite) is aninorganic compound of chemical formula Na2S2O5. The substance is sometimes referred to as disodium (metabisulfite). It is used as an antioxidant and preservative agent. Preparation Sodium metabisulfite can be prepared by evaporating a solution of sodium bisulfite saturated with sulfur dioxide: 2 HSO3- ⇌ H2O + S2O52- which leaves solid Na2S2O5 behind. Chemical structure The anion is a hybrid of dithionite (S2O42-) and dithionate (S2O62-). The anion consists of an SO2 group linked to an SO3 group, with the negative charge more localized on the SO3 end. The S-S bond length is 2.22 Å and the "thionate" and "thionite" S-O distances are 1.46 and 1.50 Å, respectively. Uses Food additive It is used as a preservative and antioxidant in food and is also known as E223. It may cause allergic reactions in those who are sensitive to sulfites, including respiratory reactions in asthmatics, anaphylaxis and other allergic reactions in sensitive individuals. Sodium metabisulfite and potassium metabisulfite are the primary ingredients in Campden tablets, used for wine and beer making. The acceptable daily intake is up to 0.7 mg per kg of body weight. Sodium metabisulfite has no side effects; it is oxidised in the liver to harmless sulfate and excreted in urine.[8][clarification needed] Sanitization and cleaning agent It is commonly used in homebrewing and winemaking to sanitize equipment. It is used as a cleaning agent for potable water reverse osmosis membranes in desalination systems. It is also used to remove chloramine from drinking water after treatment. Other uses Added to local anaesthetic(lidocaine etc.) solutions to prevent oxidation of vasoconstrictor adrenaline and thus improve the shelflife of the solution It is used in photography.
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