Disulfite

Not to be confused with Bisulfite.

Disulfite ion
IUPAC name disulfite [1]
Other names metabisulfite ion pyrosulfite
Identifiers
PubChem	159940
Properties
Molecular formula	S2O5
Y (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

A disulfite, commonly known as metabisulfite, is a chemical compound containing the disulfite ion (metabisulfite ion) [S₂O₅²⁻].

Chemistry

Production of the disulfite ion

The disulfite ion is a dimer of the bisulfite ion (HSO₃⁻). It can arise from:

dehydration

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

2 HSO₃⁻ (aq) S₂O₅²⁻ (aq) + H₂O (l)

Although the equilibrium lies far to the left, evaporation of a bisulfite salt will produce a substantial amount of disulfite.^[2]

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 HSO₃⁻ + 2 H⁺ → H₂S₂O₅ + H₂O

addition

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

HSO3− SO32− + H+ SO32− + SO2 S2O52−

Other reactions

In aqueous solution, disulfite salts decompose with acids:
S₂O₅²⁻ + H⁺ → HSO₃⁻ + 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. ^ Bassam Z. Shakhashiri: Chemical demonstrations: a handbook for teachers of chemistry The University of Wisconsin Press @Google Books, 1992, p.9