Sulfurous and disulfurous acids, H₂SO₃ and H₂S₂O₅ 

   Neither ‘sulfurous acid’  nor ‘disulfurous acid’ has been isolated as a free acid. Salts containing the sulfite ion, [SO₃]^2- , are well established (e.g. β-Na₂SO₃ and K₂SO₃ are commercially available) and are quite good reducing agents . Applications of sulfites include those as food preservatives, e.g. an additive in wines . The [SO₃]^2- ion has a trigonal pyramidal structure with delocalized bonding (S^_O = 151 pm, ∠O^_S^_O = 106 ⁰). There is evidence from ¹⁷O NMR spectroscopic data that protonation of [SO₃]^2- occurs to give a mixture of isomers as shown in equilibrium 1.1.

                                     (1.1) 

   Although the [HSO₃]^- ion exists in solution, and salts such as NaHSO₃ (used as a bleaching agent) may be isolated, evaporation of a solution of NaHSO₃ which has been saturated with SO₂ results in the formation of Na₂S₂O₅ (equation 1.2).

                                          (1.2)

Table 1.1 Selected oxoacids of sulfur.

(1.1)

The [S₂O₅]^2- ion is the only known derived anion of disulfurous acid and possesses structure 1.1 with a long, weak S^_S bond.