The dithionite is the oxyanion with the formula [S2O4]2−.[1] It is commonly encountered as the salt sodium dithionite. For historical reasons, it is sometimes called hydrosulfite, but it contains no hydrogen and is not a sulfite.[2] The dianion has a steric number of 4 and trigonal pyramidal geometry.

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

Production and reactions

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In its main applications, dithionite is generally prepared in situ by reduction of sulfur dioxide by sodium borohydride, described by the following idealized equation:[3]

NaBH4 + 8 SO2 + 8 NaOH → 4 Na2S2O4 + NaBO2 + 6 H2O

Dithionite is a reducing agent. At pH 7, its reduction potential is −0.66 V vs SHE. Its oxidation occurs with formation of sulfite:[4]

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

Dithionite undergoes acid hydrolytic disproportionation to thiosulfate and bisulfite:[2]

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

It also undergoes alkaline hydrolytic disproportionation to sulfite and sulfide:[2]

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

It is formally derived from dithionous acid (H2S2O4), but this acid does not exist in any practical sense.

Use and occurrence

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Sodium dithionite finds widespread use in industry as a reducing agent. It is for example used in bleaching of wood pulp and some dyes.[3]

Chemical analyses

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Dithionite is used in conjunction with complexing agents (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).

Harmful properties

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The decomposition of dithionite produces reduced species of sulfur that can be very aggressive for the corrosion of steel and stainless steel. Thiosulfate (S
2
O2−
3
) is known to induce pitting corrosion, whereas sulfide (S2−, HS) is responsible for stress corrosion cracking (SCC).

References

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  1. ^ International Union of Pure and Applied Chemistry (2005). Nomenclature of Inorganic Chemistry (IUPAC Recommendations 2005). Cambridge (UK): RSCIUPAC. ISBN 0-85404-438-8. p. 130. Electronic version.
  2. ^ a b c José Jiménez Barberá; Adolf Metzger; Manfred Wolf (2000). "Sulfites, Thiosulfates, and Dithionites". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a25_477. ISBN 978-3527306732.
  3. ^ a b Wietelmann, Ulrich; Felderhoff, Michael; Rittmeyer, Peter (2016-09-29) [2002]. "Hydrides". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim, Germany: Wiley-VCH Verlag GmbH & Co. KGaA. doi:10.1002/14356007.a13_199.pub2. ISBN 978-3-527-30673-2. OCLC 751968805.
  4. ^ 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 (2): 535–547. doi:10.1111/j.1432-1033.1978.tb12269.x. PMID 648533.

Further reading

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