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Dithionic acid

Names
IUPAC name dithionic acid[1]
Other names hypodisulfuric acid
Identifiers
CAS Number	14970-71-9 N
3D model (JSmol)	Interactive image
ChEBI	CHEBI:29208 Y
ChemSpider	25128 Y
PubChem CID	26985
UNII	9MP5KH5623 Y
CompTox Dashboard (EPA)	DTXSID50164368
InChI InChI=1S/H2O6S2/c1-7(2,3)8(4,5)6/h(H,1,2,3)(H,4,5,6) Y Key: RMGVZKRVHHSUIM-UHFFFAOYSA-N Y InChI=1/H2O6S2/c1-7(2,3)8(4,5)6/h(H,1,2,3)(H,4,5,6) Key: RMGVZKRVHHSUIM-UHFFFAOYAM
SMILES O=S(=O)(O)S(=O)(=O)O
Properties
Chemical formula	H2S2O6
Molar mass	162.13 g·mol−1
Acidity (pKa)	−3.4 (estimated)[2]
Conjugate base	Dithionate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

Dithionic acid, H₂S₂O₆, is the inorganic compound with the formula H₂S₂O₆. It is the doubly protonated derivative of dithionate, a well-characterized dianion. Dithionic acid is mainly observed and characterized as an aqueous solution.^[3]

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:

2 MnO₂ + 3 SO₂ → MnS₂O₆ + MnSO₄

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

Ba²⁺ (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:

BaS₂O₆ (aq) + H₂SO₄ (aq) → H₂S₂O₆ (aq) + BaSO₄ (s)↓

• Sodium dithionate
• Hypophosphoric acid, the phosphorus equivalent.