Dithionic acid

Dithionic acid
IUPAC name	Dithionic acid Bis(hydro​xidodioxi​dosulfur)​(S–S)
Identifiers
InChI	InChI=1/H2O6S2/c1-7(2,3)8(4,5)6/h(H,1,2,3)(H,4,5,6)
InChIKey	RMGVZKRVHHSUIM-UHFFFAOYAM
Standard InChI	InChI=1S/H2O6S2/c1-7(2,3)8(4,5)6/h(H,1,2,3)(H,4,5,6)
Standard InChIKey	RMGVZKRVHHSUIM-UHFFFAOYSA-N
CAS number	[14970-71-9]
ChemSpider	25128
Properties
Chemical formula	H2S2O6
Molar mass	162.14 g/mol
Related compounds
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)

Dithionic acid, H₂S₂O₆, is a chemical compound known only in solution.

Salts

Dithionic acid is dibasic and salts called dithionates are known. No acid salts have been discovered. All dithionates are readily soluble in water. They are mild oxidizing and mild reducing agents. The structure of dithionate ion is like ethane, but two SO₃ groups adopt an almost eclipsed conformation. The S–S length is 215 pm; S–O bonds are rather short with bond length of 143 pm.

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 MnO₂:

2MnO₂ + 3SO₂ → MnS₂O₆ + MnSO₄

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

Ba²⁺ + MnS₂O₆ + MnSO₄ → BaSO₄ ↓ + BaS₂O₆·2H₂O

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

BaS₂O₆ + H₂SO₄ → H₂S₂O₆ (aq) + BaSO₄ ↓