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.
Preparation
Sodium metabisulfite can be prepared by treating a solution of
sodium hydroxide with
sulfur dioxide.
When conducted in warm water, Na
2SO
3 initially precipitates as a yellow solid. With more SO
2, 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 Na
2S
2O
5.
Chemical structure
The anion
metabisulfite consists of an SO
2 group linked to an SO
3 group, with the negative charge more localised on the SO
3 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.
[K. L. Carter, T. A. Siddiquee, K. L. Murphy, D. W. Bennett "The surprisingly elusive crystal structure of sodium metabisulfite" Acta Crystallogr. (2004). B60, 155–162. ]
Reactivity
Upon dissolution in water, bisulfite is generated:
- Na2S2O5 + H2O → 2 Na+ + 2 HSO3−
Uses
Sodium and potassium metabisulfite have many major and niche uses. It is widely used for preserving food and beverages.
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Sodium metabisulphite is one of the main ingredients in "Drywhite", a composition used to prevent chipped potatoes oxidising during storage prior to use in many fish and chip shops.
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Sodium metabisulfite is added as an excipient to medications which contain adrenaline (epinephrine), in order to prevent the oxidation of adrenaline.
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In combination with sodium hydrosulfite it is used as a rust-stain remover
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It is used in photography as an antioxidant in photographic film development.
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Concentrated sodium metabisulfite can be used to remove . Some brands contain 98% sodium metabisulfite, and cause degradation of lignin in the stumps, facilitating removal.
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It is also used as an excipient in some tablets, such as paracetamol.
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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 haemoglobin mutation). The substance causes defunct cells to sickle (through a complex polymerisation) hence confirming disease.
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It is used as a bleaching agent in the production of coconut cream.
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It (or liquid SO2) is commonly used as an antimicrobial and antioxidant in winemaking; bottled wine indicates its use with the label "Contains Sulfites" in the US.
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It is used as a reducing agent to break sulfide bonds in shrunken items of clothing made of natural fibres, thus allowing the garment to go back to its original shape after washing.
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It is used as an SO2 source (mixed with air or oxygen) for the destruction of cyanide in commercial gold cyanidation processes.
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It is used as an SO2 source (mixed with air or oxygen) for the precipitation of elemental gold in chloroauric (aqua regia) solutions.
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It is used in the water treatment industry to quench residual chlorine.
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It is used in tint etching iron-based metal samples for microstructural analysis.
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It is used as a fungicide for anti-microbe and mould prevention during shipping of consumer goods such as shoes and clothing. Plastic stickers and packaging (such as Micro-Pak trademark symbol) containing the anhydrous, sodium metabisulfite solid active ingredient are added prior to shipping. The devices absorb moisture from the atmosphere during shipping and release low levels of sulfur dioxide.
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It is used for preserving fruit during shipping.
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It is used as an additive in the extraction of starch from tubers,
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It is used as a pickling agent to treat high pressure reverse osmosis and nanofiltration water desalination membranes for extended storage periods between uses.
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It is used to create a bisulfite adduct from ketones to aid in separation of the ketone product. The usage of metabisulfite versus the sulfite is also more entropically favourable.
Safety
Sodium metabisulfite, despite not being flammable, decomposes in 150 °C of heat releasing
Sulphur dioxide when decomposed. It is corrosive when dissolved in water. Some people who are sulfite sensitive may experience an allergic reaction to sodium metabisulfite, sometimes severe, resulting in labeling requirements for food safety.
In 2024, it was named ‘allergen of the year 2024’ by the American Contact Dermatitis Society.
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