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.
3D model (JSmol)
|E number||E223 (preservatives)|
CompTox Dashboard (EPA)
|Molar mass||190.107 g/mol|
|Appearance||White to yellow powder|
|Melting point||170 °C (338 °F; 443 K) decomposition begins at 150 °C|
|Solubility||Very soluble in glycerol |
Slightly soluble in ethanol
|Safety data sheet||Mallinckrodt MSDS|
|GHS Signal word||Danger|
|P264, P270, P280, P301+312, P305+351+338, P310, P330, P501|
|NFPA 704 (fire diamond)|
|NIOSH (US health exposure limits):|
|TWA 5 mg/m3|
IDLH (Immediate danger)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
- 2 HSO3− ⇌ H2O + S2O52−
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.
Food preservation and additive
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 monochloramine from drinking water after treatment.
- Sodium metabisulphite is added as an excipient to medications which contain adrenaline (epinephrine), in order to prevent the oxidation of adrenaline. For example, it is added to combination drug formulations which contain a local anaesthetic and adrenaline, and to the formulation in epinephrine autoinjectors, such as the EpiPen. This lengthens the shelf life of the formulation, although the sodium metabisulphite reacts with adrenaline, causing it to degrade and form epinephrine sulphonate.
- It is used in photography.
- Concentrated sodium metabisulfite can be used to remove tree stumps. Some brands contain 98% sodium metabisulfite, and cause degradation of lignin in the stumps, facilitating removal.
- It is also used as an excipient in some tablets, such as paracetamol.
- 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 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:
- Na2S2O5 → Na2SO3 + SO2
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- 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. doi:10.1107/S0108768104003325
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