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.
The anion metabisulfite 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.
It is used as a preservative and antioxidant in food and is also known as E223.
A very important health related aspect of this substance is that it can be added to a blood smear in a test for sickle cell anaemia (and other similar forms of haemoglobinmutation). The substances causes defunct cells to sickle (through a complex polymerisation) hence confirming disease.
It is used as the source of SO2 in wine. An important anti-oxidant and bactericide
It is also used to precipitate gold from auric acid (gold dissolved in aqua regia).
It is used in waste treatment to chemically reduce hexavalent chromium to trivalent chromium which can then be precipitated and removed from an aqueous waste stream.
It is used as a bleaching agent in the production of Coconut cream
It is used as a reducing agent to break sulfide bonds in shrunken items of clothing made of natural fibers, thus allowing the garment to go back to its original shape after washing
It is used as an SO2 source (mixed with air or oxygen) for the destruction of cyanide in commercial gold cyanidation processes.
It is used in the oil and gas industry as a corrosion inhibitor/oxygen scavenger.
It is used in the water treatment industry to quench chlorine residual
It is used in tint etching iron-based metal samples for microstructural analysis. 
When mixed with water, sodium metabisulfite releases sulfur dioxide (SO2), a pungent, unpleasant smelling gas that can also cause breathing difficulties in some people. For this reason, sodium metabisulfite has fallen from common use in recent times, with agents such as hydrogen peroxide becoming more popular for effective and odorless sterilization of equipment. Released sulfur dioxide however makes the water a strong reducing agent.
Sodium metabisulfite releases sulfur dioxide in contact with strong acids:
Na2S2O5 + 2 HCl -> 2 NaCl + H2O + 2 SO2
On heating to high temperature, it releases sulfur dioxide, leaving sodium sulfite behind:
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