Preparation of Dithionates

When sulphur dioxide reacts with an aqueous suspension of manganese dioxide, manganese dithionate is formed. When this is treated with barium hydroxide, the barium dithionate is left in solution when manganese(Il) hydroxide separates. Alkali metal or ammonium dithionates are prepared from a solution of the barium dithionate in an ion-exchange column. 

Pass sulphur dioxide through a stirred suspension of 10 g precipitated manganese dioxide in 50 cm water cooled in an ice bath until the powder dissolves (in a few hours). Dilute to 300 cm with water and boil until sulphur dioxide has been expelled. 

When alkali metal sulphites are mixed with silver nitrate solution in equimolar ratio, and the solution heated to boiling, the alkali metal dithionate is formed together with finely divided silver. The initial stage is preparing potassium sulphite. 

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