ELECTROLYTE EFFECTS ON AQUEOUS ATMOSPHERIC OXIDATION OF SULFUR-DIOXIDE BY OZONE

The kinetics of sulphur(IV) oxidation by ozone in an aqueous solution were studied in various supporting electrolytes (NaClO4, NaCl, NH4ClO4 , Na2SO4), using the stopped flow method. The rate data in perchlorate medium (chosen as reference electrolyte) are empirically correlated b y the following expression: r = -d[O3]/dt = k[O3][S(IV)][H+]-1/2. For each supporting electrolyte studied, the rate constant varies linearly with the ionic strength. The value of the rate constant is extrapolat ed to zero ionic strength (1.65 x 10(4) mol-0.5 L0.5 s-1). The rate co nstants are shown to be higher mainly when ammonium chloride and sodiu m sulphate are added to the solution. The effect of temperature is stu died between 13-degrees and 28-degrees-C. The reaction rate is unaffec ted by the trace presence of metal ion (Mn2+, Fe2+, Cu2+, Fe3+, Cr3+). The rate of oxidation of dissolved SO2 by O3, in water droplets under atmospheric conditions (presence of [H2O2] = 4.31 x 10(-5) mol L-1 an d [O3] = 1.17 x 10(-9) mol L-1), is calculated as a function of [H+] a nd compared to the oxidation by H2O2. At zero ionic strength the ozone reaction becomes faster than hydrogen peroxide reaction above -log [H +] = 4.70. This effect appears at higher H+ concentration when the ion ic strength increases (-log[H+] = 3 in 4 mol L-1 NaCl or 2 mol L-1 NaS O4).