{{Chembox | Verifiedfields = changed | verifiedrevid = 477000976 | ImageFile1 = Dithionic-acid-2D.png | ImageFile2 = Dithionic-acid-3D-balls.png | ImageAlt2 = Ball-and-stick model of dithionic acid | IUPACName = dithionic acid<ref>{{RedBookRef|page=130}}</ref> | OtherNames = hypodisulfuric acid |Section1={{Chembox Identifiers | ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}} | ChemSpiderID = 25128 | InChI = 1/H2O6S2/c1-7(2,3)8(4,5)6/h(H,1,2,3)(H,4,5,6) | InChIKey = RMGVZKRVHHSUIM-UHFFFAOYAM | StdInChI_Ref = {{stdinchicite|correct|chemspider}} | StdInChI = 1S/H2O6S2/c1-7(2,3)8(4,5)6/h(H,1,2,3)(H,4,5,6) | StdInChIKey_Ref = {{stdinchicite|correct|chemspider}} | StdInChIKey = RMGVZKRVHHSUIM-UHFFFAOYSA-N | CASNo_Ref = {{cascite|changed|??}} | CASNo = 14970-71-9 | UNII_Ref = {{fdacite|correct|FDA}} | UNII = 9MP5KH5623 | PubChem = 26985 | ChEBI_Ref = {{ebicite|correct|EBI}} | ChEBI = 29208 | SMILES = O=S(=O)(O)S(=O)(=O)O }} |Section2={{Chembox Properties | Formula = {{chem2|H2S2O6}} | MolarMass = 162.14&nbsp;g&nbsp;mol<sup>−1</sup> | ConjugateBase =Dithionate | pKa = −3.4 (estimated)<ref name=P82db>{{cite book|title=Ionisation Constants of Inorganic Acids and Bases in Aqueous Solution|editor-first=D.&nbsp;D.|editor-last=Perrin|edition=2nd|series=IUPAC Chemical Data|issue=29|publisher=Pergamon|location=Oxford|year=1982|publication-date=1984|orig-date=1969|lccn=82-16524|isbn=0-08-029214-3|at=Entry 63}}</ref> }} |Section3={{Chembox Hazards | MainHazards = | FlashPt = | AutoignitionPt = }} }}

'''Dithionic acid''', {{chem2|H2S2O6}}, is the inorganic compound with the formula {{chem2|H2S2O6}}. It is the doubly protonated derivative of dithionate, a well-characterized dianion. Dithionic acid is mainly observed and characterized as an aqueous solution.<ref name=G&E>{{Greenwood&Earnshaw}} pp. 715-716</ref>

== Synthesis == Dithionates can be made by oxidizing a sulfite (from the +4 to the +5 oxidation state), but on a larger scale they are made by oxidizing a cooled aqueous solution of sulfur dioxide with manganese dioxide:

:{{chem2|2 MnO2 + 3 SO2 -> MnS2O6 + MnSO4}}

The manganese dithionate solution formed can then be converted to dithionate salts of other metals by metathesis reactions:

:{{chem2|Ba^{2+} (aq) + MnS2O6 (aq) + MnSO4 (aq) -> BaSO4 (s)↓ + BaS2O6 * 2 H2O (aq)}}

Concentrated solutions of dithionic acid can subsequently be obtained treating a barium dithionate solution with sulfuric acid:

:{{chem2|BaS2O6 (aq) + H2SO4 (aq) -> H2S2O6 (aq) + BaSO4 (s)↓}}

== See also == *Sodium dithionate *Hypophosphoric acid, the phosphorus equivalent.

== References == {{reflist}}

{{Hydrogen compounds}}

{{DEFAULTSORT:Dithionic Acid}} Category:Dithionates Category:Sulfur oxoacids