Sodium metabisulfite

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Sodium metabisulfite
Sodium metabisulfite
Structure of sodium metabisulfite
CAS number 7681-57-4 YesY
PubChem 656671
EC number 231-673-0
RTECS number UX8225000
Jmol-3D images Image 1
Molecular formula Na2S2O5, Na-O-(S=O)-O-(S=O)-O-Na
Molar mass 190.107 g/mol
Appearance white to yellow powder
Odor faint SO2
Density 1.48 g/cm3
Melting point 170 °C (338 °F; 443 K) decomposition begins at 150 °C
Solubility in water 45.1 g/100 mL (0 °C)
65.3 g/100 mL (20 °C)
81.7 g/100 mL (100 °C)
Solubility very soluble in glycerol
soluble in ethanol
MSDS Mallinckrodt MSDS
EU Index 016-063-00-2
EU classification Harmful (Xn)
Irritant (Xi)
R-phrases R22 R31 R41
S-phrases (S2) S26 S39 S46
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g., chloroform Reactivity code 1: Normally stable, but can become unstable at elevated temperatures and pressures. E.g., calcium Special hazards (white): no codeNFPA 704 four-colored diamond
Related compounds
Other anions Sodium sulfite
Sodium bisulfite
Other cations Potassium metabisulfite
Related compounds Sodium dithionite
Sodium thiosulfate
Sodium sulfate
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 N (verify) (what is: YesY/N?)
Infobox references

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.


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.[1]

Chemical structure[edit]

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.[2]


Food additive[edit]

It is used as a preservative and antioxidant in food and is also known as E223.[3]

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.[4][5]

Sodium metabisulfite and potassium metabisulfite are the primary ingredients in Campden tablets, used for wine and beer making.[6]

The acceptable daily intake is up to 0.7 mg per kg of body weight.[7] 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[edit]

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[edit]

  • Added to local anaesthetic (lidocaine etc.) solutions to prevent oxidation of vasoconstrictor adrenaline and thus improve the shelf life of the solution
  • It is used in photography.[9]
  • 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.[10]
  • It is also used as an excipient in some tablets, such as paracetamol. Approximately 0.5 mg is used in epinephrine autoinjectors such as the EpiPen.
  • 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 SO2 source (mixed with air or oxygen) for the destruction of cyanide in commercial gold cyanidation processes.

Chemical properties[edit]

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:[11]

Na2S2O5 → Na2SO3 + SO2

See also[edit]


  1. ^ Catherine E. Housecroft; Alan G. Sharpe (2008). "Chapter 16: The group 16 elements". Inorganic Chemistry, 3rd Edition. Pearson. p. 520. ISBN 978-0-13-175553-6. 
  2. ^ K. L. Carter, T. A. Siddiquee, K. L. Murphy, D. W. Bennett "The surprisingly elusive crystal structure of sodium metabisulfite" Acta Cryst. (2004). B60, 155-162. doi:10.1107/S0108768104003325
  3. ^
  4. ^ Dean D. Metcalfe, Ronald A. Simon, Food allergy: adverse reactions to food and food additives, Wiley-Blackwell 2003, pp 324-339
  5. ^
  6. ^ Milne, George W. A. (2005). Gardner's commercially important chemicals: synonyms, trade names, and properties. New York: Wiley-Interscience. p. 568. ISBN 0-471-73518-3. 
  7. ^
  8. ^ : E-numbers : E223: Sodium disulphite
  9. ^
  10. ^
  11. ^ Sodium sulfite.

External links[edit]