Disulfite

Disulfite ion

Names
IUPAC name disulfite [1]
Other names metabisulfite ion pyrosulfite
Identifiers
PubChem	159940
Properties
Chemical formula	S 2O2− 5
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

A disulfite, commonly known as metabisulfite or pyrosulfite, is a chemical compound containing the disulfite ion (metabisulfite ion) S
2O2−
5.

Structure

Structure of metabisulfite ion

Surprisingly, and in contrast to disulfate (S
2O2−
7), disulfite ion (S
2O2−
5) has two directly connected sulfur atoms, and the structure is described as "thionite-thionate", or [O₂S–SO₃]²⁻, instead of the symmetrical form [O₂S–O–SO₂]²⁻.^[2]

The oxidation state of the sulfur atom bonded to 3 oxygen atoms is +5 while oxidation number of other sulfur atom is +3.

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]

Reactions

Production of the disulfite ion

The disulfite ion is a dimer of the bisulfite ion (HSO−
3). It can arise from:

Dehydration

In aqueous solution, the disulfite ion is formed in minor amounts by dehydration of bisulfite in an equilibrium:

2 HSO−
3(aq) S
2O2−
5(aq) + H₂O (l)

Although the equilibrium lies far to the left, evaporation of a bisulfite salt will produce a substantial amount of disulfite.^[4]

In fact, disulfite is the ion 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

Addition

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

HSO− 3 SO2− 3 + H^+ SO3^2− + SO2 S 2O2− 5

Other reactions

In aqueous solution, disulfite salts decompose with acids:

S
2O2−
5 + H⁺ → HSO−
3 + SO₂

Examples of disulfites

• Sodium metabisulfite (E223) and potassium metabisulfite (E224) are used as a preservative and antioxidant in food.

References

1. International Union of Pure and Applied Chemistry (2005). Nomenclature of Inorganic Chemistry (IUPAC Recommendations 2005). Cambridge (UK): RSC–IUPAC. ISBN 0-85404-438-8. p. 130. Electronic version.
2. I. Lindqvist and M. Mörtsell; "The structure of potassium pyrosulfite and the nature of the pyrosulfite ion". Acta Cryst. (1957). 10, 406–409. doi:10.1107/S0365110X57001322
3. 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
4. Bassam Z. Shakhashiri: Chemical demonstrations: a handbook for teachers of chemistry The University of Wisconsin Press, 1992, p.9