Disulfite

Structure

In contrast to disulfate (S₂O₇²⁻), disulfite ion (S₂O₅²⁻) has an unsymmetrical structure with an S-S bond. The oxidation state of the sulfur atom bonded to 3 oxygen atoms is +5 while oxidation number of other sulfur atom is +3.

The anion consists of an SO₂ group linked to an SO₃ group, with the negative charge more localized on the SO₃ end. The S–S bond length is 2.22 Å, and the "thionate" and "thionite" S–O distances are 1.46 and 1.50 Å respectively.

Production

Salts of disulfite ion are produced by dehydration of salts of hydrogensulfite ion (HSO₃⁻). When solutions of sodium hydrogensulfite or potassium hydrogensulfite are evaporated, sodium metabisulfite and potassium metabisulfite result.

2 HSO₃⁻ S₂O₅²⁻ + H₂O

Although the equilibrium lies far to the left, evaporation of a bisulfite salt will produce a substantial amount of disulfite.

Disulfite is the conjugate base of disulfurous acid (pyrosulfurous acid), which originates from sulfurous acid in accordance with the dehydration reaction above:

2 H₂SO₃ → 2 HSO₃⁻ + 2 H⁺ → H₂S₂O₅ + H₂O

The disulfite ion also arises from the addition of sulfur dioxide to the sulfite ion: