Dithionite

For historical reasons, it is sometimes called hydrosulfite, but it contains no hydrogen and is not a sulfite.

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] Dithionite is a reducing agent.

Its oxidation occurs with formation of sulfite:[4] Dithionite undergoes acid hydrolytic disproportionation to thiosulfate and bisulfite:[2] It also undergoes alkaline hydrolytic disproportionation to sulfite and sulfide:[2] It is formally derived from dithionous acid (H2S2O4), but this acid does not exist in any practical sense.

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

[3] 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).