Dithionite

In its main applications, dithionite is generally prepared in situ by reduction of sulfur dioxide by sodium borohydride, described by the following idealized equation: : Dithionite is a reducing agent. At pH 7, its reduction potential is −0.66 V vs SHE. Its oxidation occurs with formation of sulfite: : + 2 H2O → 2 + 2 e− + 2 H+ Dithionite undergoes acid hydrolytic disproportionation to thiosulfate and bisulfite: : 2 + H2O → + 2 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), 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. 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 () is known to induce pitting corrosion, whereas sulfide (S2−, HS−) is responsible for stress corrosion cracking (SCC). == References ==