Dithionite

The dithionite anion ([S2O4]2−), is an oxoanion of sulfur.[1] It is commonly encountered as colorless salts.

The unusual structure of the dithionite anion. It has a remarkably long sulfur-sulfur bond.
A ball-and-stick model of the dithionite ion.

Reactions

Dithionite is a reducing agent. At pH 7, the potential is −0.66 V vs NHE. Redox occurs with formation of sulfite:[2]

S
2O2−
4 + 2 H2O → 2 HSO−
3 + 2 e− + 2 H+

Dithionite undergoes acid hydrolytic disproportionation to thiosulfate and bisulfite:

2 S
2O2−
4 + H2O → S
2O2−
3 + 2 HSO−
3

It also undergoes alkaline hydrolytic disproportionation to sulfite and sulfide:

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

It is formally derived from dithionous acid (H2S2O4).

Use and occurrence

Sodium dithionite finds widespread use in industry as a reducing agent.

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 (S
2O2−
3) is known to induce pitting corrosion, whereas sulfide (S2−, HS−) is responsible for stress corrosion cracking (SCC).

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References
  1. International Union of Pure and Applied Chemistry (2005). Nomenclature of Inorganic Chemistry (IUPAC Recommendations 2005). Cambridge (UK): RSC–IUPAC. ISBN 0-85404-438-8. p. 130. Electronic version.
  2. Mayhew, S. G. (2008). "The Redox Potential of Dithionite and SO2− from Equilibrium Reactions with Flavodoxins, Methyl Viologen and Hydrogen plus Hydrogenase". European Journal of Biochemistry. 85: 535–547. doi:10.1111/j.1432-1033.1978.tb12269.x.
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