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

Electron transfer reactions of sulfate radical anion (SO4.-) and nitrate radical (NO3.) with chlorate ion (ClO3-) have been studied by laser photolysis. Reaction (1) (1) SO4.- + ClO3- reversible arrow SO42- + ClO3. 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) x 10(6) dm(3) mol(-1) s(-1), k(r) = (5 +/- 5) x 10(5) dm(3) mol(-1) s(-1) at ionic strength I = 0], the second-order decay rate constant of ClO3. [2k = (9.0 +/- 1.6) x 10(8) dm(3) mol(-1) s(-1)], and its molar absorption coefficient [epsilon = (1500 +/- 250) dm(3) mol-l cm(-1) at 330 nm]. Reaction (2) (2) NO3. + ClO3- reversible arrow NO3- + ClO3. was also observed and the forward and reverse rate constants were determined to be k(f) = (9.0 +/- 1.2) x 10(3) dm(3) mol(-1) s(-1) and k(r) = (8.3 +/- 1.0) x 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 ClO3., E(0)(ClO3./ClO3-) was deduced to be 2.35 +/- 0.05 V vs. NHE when E(0)(NO3./NO3-) is taken as 2.45 +/- 0.05 V vs. NHE.