Disulfite

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:

Disulfite salts are used for preserving food and beverages and as antioxidants, with the main species used for this purpose being sodium metabisulfite (E223) and potassium metabisulfite (E224). Sulfites are implicated in asthmatic reactions and may also cause symptoms in non-asthmatic individuals, namely dermatitis, urticaria, flushing, hypotension, abdominal pain and diarrhea, and even life-threatening anaphylaxis.