Disulfite

Not to be confused with Bisulfite.

Disulfite ion

File:Structure of metabisulfite ion.png
File:Metabisulfite-ion-from-xtal-3D-bs-17.png
Names
IUPAC name disulfite
Systematic IUPAC name pentaoxido-1κ3O,2κ2O-disulfate(S—S)(2−)
Other names metabisulfite ion pyrosulfite
Identifiers
CAS Number	23134-05-6 checkY
3D model (JSmol)	Interactive image
ChemSpider	140610
ECHA InfoCard
E number
PubChem CID	159940
UNII	7992SO049K
CompTox Dashboard (EPA)	{{#property:P3117}}
InChI InChI=1S/H2O5S2/c1-6(2)7(3,4)5/h(H,1,2)(H,3,4,5)/p-2 Key: WBZKQQHYRPRKNJ-UHFFFAOYSA-L
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). checkY verify (what is checkY☒N ?) Infobox references

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

Structure

In contrast to disulfate (S
₂O²⁻
₇), disulfite ion (S
₂O²⁻
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.

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.

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.

2 HSO−3 File:Equilibrium left.svg S₂O2−5 + H₂O

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

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 File:Equilibrium left.svg SO2− 3 + H+ SO32− + SO2 File:Equilibrium left.svg S 2O2− 5

File:Disulfite-Synthese.png

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) and potassium metabisulfite (E224). 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.