Dithionic acid electrolyte

Dithionites. Although the free-dithionous acid, H2S2O4, has never been isolated, the salts of the acid, in particular zinc [7779-86-4] and sodium dithionite [7775-14-6] have been prepared and are widely used as industrial reducing agents. The dithionite salts can be prepared by the reaction of sodium formate with sodium hydroxide and sulfur dioxide or by the reduction of sulfites, bisulfites, and sulfur dioxide with metallic substances such as zinc, iron, or zinc or sodium amalgams, or by electrolytic reduction (147). 

Dithionic acid tends to decompose at normal cell operating temperatures to H2SO4 and SO2. The sulfate will then precipitate lead and SO2 wiU be reduced at the cathode to H2S, which in turn will precipitate PbS. Sulfamic acid is also unstable at higher current densities and tends to break down to form ammonium sulfate, in turn precipitating lead sulfate. Hence, the most suitable practical alternative electrolytes are fluosilicic and fluoboric acid systems. 

Dithionates may be prepared by the electrolytic or chemical oxidation of sulfurous acid and sulfites. Chemical oxidation is accomplished by the action of chlorine, iodine, hydrogen peroxide, or oxygen in an acid medium by the action of chromates or permanganates in neutral solution - ... 

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