{{Distinguish|Bisulfite}} {{chembox | Watchedfields = changed | verifiedrevid = 423158544 | Name = Disulfite ion | ImageFile = Structure of metabisulfite ion.png | ImageSize = 190px | ImageFile1 = Metabisulfite-ion-from-xtal-3D-bs-17.png | IUPACName = disulfite<ref name=redbookname>{{RedBookRef|page=130}}</ref> | SystematicName = pentaoxido-1''κ''<sup>3</sup>''O'',2''κ''<sup>2</sup>''O''-disulfate(S—S)(2−)<ref name=redbookname/> | OtherNames = metabisulfite ion<br />pyrosulfite | Section1 = {{Chembox Identifiers | CASNo = 23134-05-6 | CASNo_Ref = {{cascite|correct|CAS}} | ChemSpiderID = 140610 | PubChem = 159940 | RTECS = | UNII = 7992SO049K | SMILES = [O-][S+2]([O-])([O-])[S+]([O-])[O-] | StdInChI=1S/H2O5S2/c1-6(2)7(3,4)5/h(H,1,2)(H,3,4,5)/p-2 | StdInChIKey = WBZKQQHYRPRKNJ-UHFFFAOYSA-L }} | Section2 = {{Chembox Properties | Formula = {{chem|S|2|O|5|2−}} | MolarMass = | Appearance = | Density = | Solubility = | MeltingPt = | BoilingPt = | ConjugateAcid = Disulfurous acid }} }}

A '''disulfite''', commonly known as '''metabisulfite''' or '''pyrosulfite''', is a chemical compound containing the ion {{chem|S|2|O|5|2−}}. 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 {{chem|H|S|O|3|−}} anion. These salts act equivalently to sodium hydrogensulfite or potassium hydrogensulfite.<ref>{{cite book|doi=10.1002/9780470132333.ch49|isbn=9780470132333|chapter=Sulfites and Pyrosulfites of the Alkali Metals|year=1946|last1=Johnstone|first1=H. F.|title=Inorganic Syntheses|pages=162–167|volume=2}}</ref>

==Structure== In contrast to disulfate ({{chem|S|2|O|7|2−}}), disulfite ion ({{chem|S|2|O|5|2−}}) 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.<ref>{{cite journal|first1=I.|last1=Lindqvist|first2=M.|last2=Mörtsell|title=The Structure of Potassium Pyrosulfite and the Nature of the Pyrosulfite Ion". |journal=Acta Crystallographica |date= 1957|volume=10 |issue=6 |pages=406–409 |doi=10.1107/S0365110X57001322 |doi-access=free}}</ref>

The anion consists of an SO<sub>2</sub> group linked to an SO<sub>3</sub> group, with the negative charge more localized on the SO<sub>3</sub> end. The S–S bond length is 2.22&nbsp;Å, and the "thionate" and "thionite" S–O distances are 1.46 and 1.50&nbsp;Å respectively.<ref>{{cite journal | last1 = Carter | first1 = K. L. | last2 = Siddiquee | first2 = T. A. | last3 = Murphy | first3 = K. L. | last4 = Bennett | first4 = D. W. | year = 2004 | title = The surprisingly elusive crystal structure of sodium metabisulfite | url = | journal = Acta Crystallogr | volume = B60 | issue = 2| pages = 155–162 | doi = 10.1107/S0108768104003325 }}</ref>

===Production=== Salts of disulfite ion are produced by dehydration of salts of hydrogensulfite ion ({{chem|HSO|3|−}}). When solutions of sodium hydrogensulfite or potassium hydrogensulfite are evaporated, sodium metabisulfite and potassium metabisulfite result.<ref>{{Ullmann|doi=10.1002/14356007.a25_477|isbn=3527306730|title=Sulfites, Thiosulfates, and Dithionitesl Chemistry|year=2000|last1=Barberá|first1=José Jiménez|last2=Metzger|first2=Adolf|last3=Wolf|first3=Manfred}}</ref> :{{chem2|2 HSO3−}} 40px {{chem2|S2O5(2−) + H2O}}

Although the equilibrium lies far to the left, evaporation of a bisulfite salt will produce a substantial amount of disulfite.<ref name=Bassam>Bassam Z. Shakhashiri: [https://books.google.com/books?id=btNF9aLXtPcC&pg=PA9&lpg=PA9 ''Chemical demonstrations: a handbook for teachers of chemistry''] The University of Wisconsin Press, 1992, p.9</ref>

Disulfite is the conjugate base of disulfurous acid (pyrosulfurous acid), which originates from sulfurous acid in accordance with the dehydration reaction above:<br /> :2 H<sub>2</sub>SO<sub>3</sub> → 2 {{chem|HSO|3|−}} + 2 H<sup>+</sup> → H<sub>2</sub>S<sub>2</sub>O<sub>5</sub> + H<sub>2</sub>O

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

{| class="wikitable" |- |{{chem|HSO|3|−}} 40px {{chem|SO|3|2−}} + H<sup>+</sup><br /><br />SO<sub>3</sub><sup>2−</sup> + SO<sub>2</sub> 40px {{chem|S|2|O|5|2−}} || &nbsp; || center|300px |}

==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)<ref>{{Cite journal |last1=Noorafshan |first1=A. |last2=Asadi-Golshan |first2=R. |last3=Monjezi |first3=S. |last4=Karbalay-Doust |first4=S. |date=2014 |title=Sodium metabisulphite, a preservative agent, decreases the heart capillary volume and length, and curcumin, the main component of Curcuma longa, cannot protect it |journal=Folia Biologica |volume=60 |issue=6 |pages=275–280 |issn=0015-5500 |pmid=25629268 |doi=10.14712/fb2014060060275|doi-access=free }}</ref> and potassium metabisulfite (E224).<ref>{{Cite web |last=PubChem |title=Potassium metabisulfite |url=https://pubchem.ncbi.nlm.nih.gov/compound/28019 |access-date=2024-04-19 |website=pubchem.ncbi.nlm.nih.gov |language=en}}</ref> 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.<ref name="pmid24834193">{{cite journal |vauthors=Vally H, Misso NL |title=Adverse reactions to the sulphite additives |journal=Gastroenterol Hepatol Bed Bench |volume=5 |issue=1 |pages=16–23 |date=2012 |pmid=24834193 |pmc=4017440 |doi= |url=}}</ref>

==References== {{reflist}}

Category:Metabisulfites Category:Sulfur oxyanions