The kinetics of atmospheric autoxidation of S (IV) by Ag(I) in the pH range 4.02-5.25 has been studied. The aqueous phase autoxidation of S (IV) is the major factor responsible for acidification of atmospheric aqueous system. The role of benzamide act as an inhibitor of Ag(I) catalysed autoxidation of S (IV) in acidic medium has been identified, and based on the observed results following rate law given and a free radical mechanismhas been proposed.
d[S(lV)]/dt = (k1+ k2[Ag(I)]) [S(lV)]/1 + B [Benzamide]
Experiments were carried out at 30≤T°C≤40, 4.02≤pH≤5.25, 1×10−3mol/cm3≤ [S(IV)≤10×10−3 mol/cm3, 5×10−6mol/cm3≤[Ag(I)]≤2.5×10−5mol/cm3, 1×10−8 mol/cm3≤[Benzamide]≤5×10-3mol/cm3. Based on the experimental results, rate constants and orders of the reactions were determined.The reaction order in S(IV) were first order for both reactions in the presence and absence of benzamide. The effect of Ag(I) ion and benzamide concentrations as well as an initial pH of the solution on the S(IV) oxidation rate were discussed. It was found that the rate of the S(IV) oxidation depends on the initial pH of the solution but it is independent of the pH change during the reaction. Addition of benzamide leads to the introduction of an induction period and decrease in reaction rate, most likely due to SO4-1 radicals. The value of apparent energy and inhibition constant B were calculated in the presence of benzamide found as17.05 KJ mol-1 and 4.78 x 104mol dm-3 respectively.