Dithionite
Oxyanion of sulfur at the oxidation number +3
From Wikipedia, the free encyclopedia
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.
Production and reactions
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
2O2−
4 + 2 H2O → 2 HSO−
3 + 2 e− + 2 H+
Dithionite undergoes acid hydrolytic disproportionation to thiosulfate and bisulfite:[2]
- 2 S
2O2−
4 + H2O → S
2O2−
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
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
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
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).
