Disulfite

A disulfite, commonly known as metabisulfite or pyrosulfite, is a chemical compound containing the ion S2O2−5. It is a colorless dianion that is primarily marketed in the form of sodium metabisulfite or potassium metabisulfite. When dissolved in water, these salts release the hydrogensulfite HSO−3 anion. These salts act equivalently to sodium hydrogensulfite or potassium hydrogensulfite.[1]

## Contents

* 1 Structure
* 1.1 Production
* 2 Use
* 3 References

## Structure

In contrast to disulfate (S2O2−7), disulfite ion (S2O2−5) has an unsymmetrical structure with an S-S bond. The oxidation state of the sulfur atom bonded to 3 oxygen atoms is +5 while oxidation number of other sulfur atom is +3.[2]

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.[3]

### Production

Salts of disulfite ion are produced by dehydration of salts of hydrogensulfite ion (HSO−3). When solutions of sodium hydrogensulfite or potassium hydrogensulfite are evaporated, sodium metabisulfite and potassium metabisulfite result.[4]

2 HSO−
3 40px S
2O2−
5 \+ H

Although the equilibrium lies far to the left, evaporation of a bisulfite salt will produce a substantial amount of disulfite.[5]

Disulfite is the conjugate base of disulfurous acid (pyrosulfurous acid), which originates from sulfurous acid in accordance with the dehydration reaction above:

2 H2SO3 → 2 HSO−3 \+ 2 H+ → H2S2O5 \+ H2O

The disulfite ion also arises from the addition of sulfur dioxide to the sulfite ion:

HSO−3 40px SO2−3 \+ H+

SO32− \+ SO2 40px S2O2−5 |  |

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## Use

Disulfite salts are used for preserving food and beverages and as antioxidants, with the main species used for this purpose being sodium metabisulfite (E223)[6] and potassium metabisulfite (E224).[7] Sulfites are implicated in asthmatic reactions and may also cause symptoms in non-asthmatic individuals, namely dermatitis, urticaria, flushing, hypotension, abdominal pain and diarrhea, and even life-threatening anaphylaxis.[8]

## References

1. ↑ Johnstone, H. F. (1946). "Sulfites and Pyrosulfites of the Alkali Metals". Inorganic Syntheses. 2. pp. 162–167. doi:10.1002/9780470132333.ch49. ISBN 9780470132333.
2. ↑ Lindqvist, I.; Mörtsell, M. (1957). "The Structure of Potassium Pyrosulfite and the Nature of the Pyrosulfite Ion".". Acta Crystallographica 10 (6): 406–409. doi:10.1107/S0365110X57001322.
3. ↑ Carter, K. L.; Siddiquee, T. A.; Murphy, K. L.; Bennett, D. W. (2004). "The surprisingly elusive crystal structure of sodium metabisulfite". Acta Crystallogr B60 (2): 155–162. doi:10.1107/S0108768104003325.
4. ↑ Barberá, José Jiménez; Metzger, Adolf; Wolf, Manfred (2000). "Ullmann's Encyclopedia of Industrial Chemistry". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a25_477.
5. ↑ Bassam Z. Shakhashiri: Chemical demonstrations: a handbook for teachers of chemistry The University of Wisconsin Press, 1992, p.9
6. ↑ Noorafshan, A.; Asadi-Golshan, R.; Monjezi, S.; Karbalay-Doust, S. (2014). "Sodium metabisulphite, a preservative agent, decreases the heart capillary volume and length, and curcumin, the main component of Curcuma longa, cannot protect it". Folia Biologica 60 (6): 275–280. doi:10.14712/fb2014060060275. ISSN 0015-5500. PMID 25629268.
7. ↑ PubChem. "Potassium metabisulfite" (in en). https://pubchem.ncbi.nlm.nih.gov/compound/28019.
8. ↑ "Adverse reactions to the sulphite additives". Gastroenterol Hepatol Bed Bench 5 (1): 16–23. 2012. PMID 24834193.

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