Laser photolysis study on reactions of sulfate radical and nitrate radical with chlorate ion in aqueous solutions Formation and reduction potential of ClO₃ radical

Abstract

Electron transfer reactions of sulfate radical anion (SO₄^-) and nitrate radical (NO₃) with chlorate ion (ClO₃^-) have been studied by laser photolysis. Reaction (1) leads to the formation of chlorine trioxide radical (ClO₃) which has an absorption peak at 330 nm. Simulation work gave estimated results including: the forward and reverse rate constants of reaction (1), [k_f=(4.0±0.5)×10⁶ dm³ mol^-1 s^-1, k_r=(5±5)×10⁵ dm³ mol^-1 s^-1 at ionic strength I=0], the second-order decay rate constant of ClO₃ [2k=(9.0±1.6)×10⁸ dm³ mol^-1 s^-1], and its molar absorption coefficient [ε=(1500±250) dm³ mol^-1 cm^-1 at 330 nm]. Reaction (2) was also observed and the forward and reverse rate constants were determined to be k_f=(9.0±1.2)×10³ dm³ mol^-1 s^-1 and k_r=(8.3±1.0)×10² dm³ mol^-1 s^-1. The equilibrium constant K of reaction (2) was determined to be 42±6, from the observed absorbance of the radicals at equilibrium. The reduction potential of ClO₃, E⁰(ClO₃/ClO₃^-) was deduced to be 2.35±0.05 V vs. NHE when E⁰(NO₃/NO₃^-) is taken as 2.45±0.05 V vs. NHE.