A KINETIC-STUDY OF THE H2O2 OXIDATION OF AQUEOUS SO2 BY AMPEROMETRY

Atmospheric studies of the kinetic rate law of the oxidation of dissol ved sulphur dioxide by hydrogen peroxide are generally carried out by laboratory measurements using reactant concentrations 10(2)-10(4) time s higher than those in tropospheric water (clouds, fog and rain). The present amperometric technique, based on detection of H2O2 With a rota ting platinum anode (measurement of the oxidation current at 0.4 V/SCE ), allows us to study the kinetics at low reactant concentrations. The results (in NaClO4 medium, at 25 degrees C) are consistent with our p revious study. On the basis of the derived rate law, v = k(H2O2) [H+] [HSO3-] [H2O2] and the influence of ionic strength on the third-order rate constant (k(H2O2)), we propose a mechanism with a fast pre-equili brium step in which some of the reactants are ionic. This is followed by specific acid catalysis (rate-determining step). The variation of k (H2O2) was determined as a function of ionic strength (0 less than or equal to I less than or equal to 3 mol L(-1)).