Role of Organics in Atmospheric Catalytic Autoxidation of Aqueous Sulphur Dioxide in Acidic Medium

The kinetics of atmospheric autoxidation of sulphur(IV) by silver(I) in the pH range 4.02−5.25 was studied. The aqueous phase autoxidation of sulphur(IV) was the major factor responsible for acidification of atmospheric aqueous system. The role of organics which act as inhibitors of silver(I) catalysed autoxidation of sulphur(IV) in the acidic medium was identified and based on the observed results following rate law and a free radical mechanism was proposed: d[S(lV)]/dt = (k1+ k2[Ag(I)]) [S(lV)]/1 + B [organics].Experiments were carried out at 30 ≤ tºC ≤ 40, 4.02 ≤ pH ≤ 5.25, 1 × 10−3 mol/dm3 ≤ [S(IV) ≤ 10 × 10−3 mol/dm3, 5 × 10−6 mol/dm3 ≤ [Ag(I)] ≤ 2.5 × 10−5 mol/dm3, 1 × 10−8 mol/dm3 ≤ [organics] ≤ 8 × 10−4 mol/dm3. Based on the experimental results, rate constants and orders of the reactions were determined. The reaction order in sulphur(IV) was first order for both reactions in the presence and absence of organics. The effect of silver(I) ion and organics concentrations as well as an initial pH of the solution on the sulphur(IV) oxidation rate were discussed. It was found that the rate of the sulphur(IV) oxidation depended on the initial pH of the solution, but it was independent of the pH change during the reaction. The addition of organics led to the introduction of an induction period and decrease in reaction rate, most likely due to SO4–1 radicals. The value of the apparent energy and inhibition constant B were calculated in the presence of organics selected as formic acid, isoamyl alcohol, sodium benzoate, benzamide, isopropyl alcohol, and aniline. Out of the six organics undertaken for study in the present work, isoamyl alcohol showed higher inhibiting capacity as compared to other five organics. The order of inhibition of selected organics are as aniline < isopropyl alcohol < formic acid < sodium benzoate < benzamide < isoamyl alcohol.