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The unusual structure of the dithionite anion. It has a remarkably long sulfur-sulfur bond.

The dithionite anion, S2O42−, is a sulfur oxoanion formally derived from the hypothetical compound dithionous acid, H2S2O4. Dithionous acid has not been detected either as a pure compound or in solution.

Dithionite ions undergo both acid and alkaline hydrolysis:

2 S2O42− + H2OS2O32− + 2 HSO3
3 Na2S2O4 + 6 NaOH → 5 Na2SO3 + Na2S + 3 H2O

The sodium salt of dithionous acid, sodium dithionite, finds widespread use in industry as a reducing agent.