Electrolyte effects on aqueous atmospheric oxidation of sulphur dioxide by ozone

Abstract

The kinetics of sulphur(IV) oxidation by ozone in an aqueous solution were studied in various supporting electrolytes (NaClO 4 , NaCl, NH 4 ClO 4 , Na 2 SO 4 ), using the stopped flow method. The rate data in perchlorate medium (chosen as reference electrolyte) are empirically correlated by the following expression: r = −d[O 3 ]/dt = k [O 3 ] [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 extrapolated to zero ionic strength (1.65 × 10 4 mol −0.5 L 0.5 s −1 ). The rate constants are shown to be higher mainly when ammonium chloride and sodium sulphate are added to the solution. The effect of temperature is studied between 13 ° and 28 °C. The reaction rate is unaffected by the trace presence of metal ion (Mn 2+ , Fe 2+ , Cu 2+ , Fe 3+ , Cr 3+ ). The rate of oxidation of dissolved SO 2 by O 3 , in water droplets under atmospheric conditions (presence of [H 2 O 2 ] = 4.31 × 10 −5 mol L −1 and [O 3 ] = 1.17 × 10 −9 mol L −1 ), is calculated as a function of [H + ] and compared to the oxidation by H 2 O 2 . 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 ionic strength increases (−log [H + ] = 3 in 4 mol L −1 NaCl or 2 mol L −1 Na 2 SO 4 ).