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 (SO ₄ ^- ) and nitrate radical (NO ₃ ) with chlorate ion (ClO ₃ ^- ) have been studied by laser photolysis. Reaction (1) leads to the formation of chlorine trioxide radical (ClO ₃ ) 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)×10 ⁶ dm ³ mol ^-1 s ^-1 , k _r =(5±5)×10 ⁵ dm ³ mol ^-1 s ^-1 at ionic strength I=0], the second-order decay rate constant of ClO ₃ [2k=(9.0±1.6)×10 ⁸ dm ³ mol ^-1 s ^-1 ], and its molar absorption coefficient [ε=(1500±250) dm ³ 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)×10 ³ dm ³ mol ^-1 s ^-1 and k _r =(8.3±1.0)×10 ² dm ³ 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 ClO ₃ , E ⁰ (ClO ₃ /ClO ₃ ^- ) was deduced to be 2.35±0.05 V vs. NHE when E ⁰ (NO ₃ /NO ₃ ^- ) is taken as 2.45±0.05 V vs. NHE.