Dithionite

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

A ball-and-stick model of the dithionite ion.

The dithionite anion ([S₂O₄]²⁻), is an oxoanion of sulfur ^[1] formally derived from dithionous acid, H₂S₂O₄.

Chemistry

Dithionous acid has not been detected either as a pure compound or in solution. Dithionite ions undergo both acid and alkaline hydrolysis to thiosulfate and bisulfite, and sulfite and sulfide, respectively

2 S₂O2−
4 + H₂O → S₂O2−
3 + 2 HSO−
3

3 Na₂S₂O₄ + 6 NaOH → 5 Na₂SO₃ + Na₂S + 3 H₂O

Uses

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

Dithionite is often used in conjunction with complexing agent (e.g., 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 (S₂O2−
3) is well known to induce pitting corrosion while dissolved free sulfide (Na₂S) is responsible for stress corrosion cracking (SCC).

References

• Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0-08-037941-9. 

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