Disulfite

Disulfite ion
Names
	IUPAC name disulfite
	Systematic IUPAC name pentaoxido-1κ3O,2κ2O-disulfate(S—S)(2−)
	Other names metabisulfite ion; pyrosulfite
Identifiers
3D model (JSmol)	Interactive image;
PubChem CID	159940;
CompTox Dashboard (EPA)	DTXSID70945813 ;
	SMILES [O-][S+2]([O-])([O-])[S+]([O-])[O-];
Properties
Chemical formula	S; 2O2−; 5
Conjugate acid	Disulfurous acid
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). verify (what is ?) Infobox references

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

Structure

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]

Production

Salts of disulfite ion are produced by dehydration of salts of bisulfite ion (HSO⁻
₃). Attempted crystallization of sodium bisulfite gives disodium bisulfite. In aqueous solution, the disulfite ion is formed in minor amounts by dehydration of bisulfite in an equilibrium:

2 HSO⁻
₃

S
₂O²⁻
₅ + H₂O

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

Dsulfite 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 SO²⁻ ₃ + H⁺ SO₃²⁻ + SO₂ S ₂O²⁻ ₅

Reactions

In aqueous solution, disulfite salts decompose with acids:

S
₂O²⁻
₅ + H⁺ → HSO⁻
₃ + SO₂

Examples of disulphites

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

References

^ ^a ^b 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.
^ I. Lindqvist and M. Mörtsell; "The structure of potassium pyrosulfite and the nature of the pyrosulfite ion". Acta Crystallogr. (1957). 10, 406–409. doi:10.1107/S0365110X57001322
^ 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
^ Bassam Z. Shakhashiri: Chemical demonstrations: a handbook for teachers of chemistry The University of Wisconsin Press, 1992, p.9

[redbookname-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 Crystallogr. (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

[Bassam-4] Bassam Z. Shakhashiri: Chemical demonstrations: a handbook for teachers of chemistry The University of Wisconsin Press, 1992, p.9

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