Dithionite is a reducing agent. At pH 7, the potential is -0.66 V vs NHE. Redox occurs with formation of sulfite:
It is formally derived from dithionous acid (H2S2O4), but this acid does not exist in any practical sense.
Dithionite is used in conjunction with complexing agent (for example, citric acid) to reduce iron(III) oxy-hydroxide into soluble iron(II) compounds and to remove amorphous iron(III)-bearing mineral phases in soil analyses (selective extraction).
The decomposition of dithionite produces reduced species of sulfur that can be very aggressive for the corrosion of steel and stainless steel. Thiosulfate is known to induce pitting corrosion, whereas sulfide (S2-, HS-) is responsible for stress corrosion cracking (SCC).