Dithionite

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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, 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⁻ + H₂O → S₂O₃²⁻ + 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.

Dithionite is often used in conjunction with complexing agent (e.g., citric acid) to reduce Fe-III oxy-hydroxide into soluble Fe-II compounds and to remove amorphous Fe-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 (S₂O₃²⁻) is well known to induce pitting corrosion while dissolved free sulfide (Na₂S) is responsible for stress corrosion cracking (SCC).

[edit] References

• Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements, 2nd Edition, Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4. 

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Dithionites