Spectroscopy and decay kinetics of the sulfite radical anion in aqueous solution

Abstract

The sulfite radical anion, SO^–₃, has been proposed as a chain carrier in the aerobic free-radical chain oxidation of sulfur dioxide in aqueous solution. The absorption spectrum of SO^–₃ has been obtained following the laser photolysis of argon-saturated sodium dithionate solutions at 193 nm. Using kinetic spectrophotometric and ionchromatographic techniques, it has been shown that the laser photolysis of aqueous dithionate solutions generates both the sulfite anion, SO^2–₃, and the sulfite radical anion, SO^–₃. the decay of SO^–₃ in aqueous solution is second-order with an overall rate coefficient of (4.0 ± 0.3)× 10⁸ dm³ mol^–1 s^–1 at 293 K in the zero ionic strength limit. The reaction proceeds by two channels; recombination to produce dithionate and electron or O^– transfer to produce S^IV and S^VI. SO^–₃+ SO^–₃ [graphic omitted] S₂O^2–₆, SO^–₃+ SO^–₃ [graphic omitted] SO^2–₃+ SO₃.

Product analysis gives a value of 0.8 ± 0.2 for the ratio k_a/k_b.