Dithionic acid

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	162.14 g mol⁻¹
Acidity (pK_a)	-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

Chemical compound

Dithionic acid, H₂S₂O₆, is a chemical compound known only in solution.^[3]

Salts

Dithionic acid is diprotic and salts called dithionates are known. No acid salts (i.e.: one proton lost) have been discovered. All dithionates are readily soluble in water.^[3] They are mild oxidizing and mild reducing agents. The shape of the dithionate ion is like ethane, but two SO₃ groups adopt an almost eclipsed conformation. The S—S bond length is about 2.15 Å; the S—O bonds are rather short with a bond length of 1.43 Å.

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:

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)↓ + BaS₂O₆·2H₂O(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)↓

References

^ International Union of Pure and Applied Chemistry (2005). Nomenclature of Inorganic Chemistry (IUPAC Recommendations 2005). Cambridge (UK): RSC–IUPAC. ISBN 0-85404-438-8. p. 130. Electronic version.
^ Perrin, D. D., ed. (1982) [1969]. Ionisation Constants of Inorganic Acids and Bases in Aqueous Solution. IUPAC Chemical Data (2^nd ed.). Oxford: Pergamon (published 1984). Entry 63. ISBN 0-08-029214-3. LCCN 82-16524.
^ ^a ^b Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8. pp. 715-716