Dithionite

Not to be confused with dithionate.

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

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).