Dithionic acid

Structural formula of the dithionic acid molecule

Ball-and-stick model of the dithionic acid molecule

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 2.15 Å; S—O bonds are rather short with 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 MnO₂:

2MnO₂ + 3SO₂ → 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)↓