Kinetics of the oxidation of hydrogen sulfite by hydrogen peroxide in aqueous solution: ionic strength effects and temperature dependence

Conductometry was used to study the kinetics of the oxidation of hydrogen sulfite, HSO3-, by hydrogen peroxide in aqueous non-buffered solution at the low concentration level of 10(-5)-10(-6) M, typically found in cloud water. The kinetic data confirm that the rate law reported for the pH range 3-6 at higher concentration levels, rate = k(H).[H+].[HSO3-].[H2O2], is valid at the low concentration level and at low ionic strength I-c. At 298 K and I-c= 1.5 x 10(-4) M, third-order rate constant k(H) was found to be k(H) = (9.1 +/- 0.5) x 10(7) M-2 s(-1). The temperature dependence of k(H) led to an activation energy of E-a = 29.7 +/- 0.9 kJ mol(-1). The effect of the ionic strength (adjusted with NaCl) on rate constant kH was studied in the range I-c = 2 x 10(-4)-5.0 M at pH = 4.5-5.2 by conductometry and stopped-flow spectrophotometry. The dependence of k(H) on I-c can be described with a semi-empirical relationship, which is useful for the purpose of comparison and extrapolation. The kinetic data obtained are critically compared with those reported earlier. (C) 1999 Elsevier Science Ltd. All rights reserved.