A disulfite, commonly known as metabisulfite or pyrosulfite, is a

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

containing the ion . 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 anion. These salts act equivalently to sodium hydrogensulfite or potassium hydrogensulfite.

Structure

In contrast to disulfate (), disulfite ion () 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 (). When solutions of sodium hydrogensulfite or potassium hydrogensulfite are evaporated, sodium metabisulfite and potassium metabisulfite result. : Equilibrium left

Although the equilibrium lies far to the left, evaporation of a bisulfite salt will produce a substantial amount of disulfite.Bassam Z. Shakhashiri
''Chemical demonstrations: a handbook for teachers of chemistry''
The University of Wisconsin Press, 1992, p.9 Disulfite is the conjugate base of disulfurous acid (pyrosulfurous acid), which originates from

sulfurous acid Sulfuric(IV) acid (United Kingdom spelling: sulphuric(IV) acid), also known as sulfurous (UK: sulphurous) acid and thionic acid, is the chemical compound with the chemical formula, formula . Raman spectroscopy, Raman spectra of solutions o ...

in accordance with the dehydration reaction above:
:2 H₂SO₃ → 2 + 2 H⁺ → H₂S₂O₅ + H₂O The disulfite ion also arises from the addition of

sulfur dioxide Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is r ...

to the sulfite ion:

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 Dermatitis is a term used for different types of skin inflammation, typically characterized by itchiness, redness and a rash. In cases of short duration, there may be small blisters, while in long-term cases the skin may become thickened ...

, urticaria, flushing,

hypotension Hypotension, also known as low blood pressure, is a cardiovascular condition characterized by abnormally reduced blood pressure. Blood pressure is the force of blood pushing against the walls of the arteries as the heart pumps out blood and is ...

, abdominal pain and

diarrhea Diarrhea (American English), also spelled diarrhoea or diarrhœa (British English), is the condition of having at least three loose, liquid, or watery bowel movements in a day. It often lasts for a few days and can result in dehydration d ...

, and even life-threatening anaphylaxis.

References

{{reflist Sulfur oxyanions