Dithionic acid

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.

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:


 * 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:


 * BaS2O6 (aq) + H2SO4 (aq) -> H2S2O6 (aq) + BaSO4 (s)↓