Kinetics and mechanism of heterogeneous cadmium sulphide and homogeneous manganese(II) catalysed oxidation of sulphur(IV) by dioxygen in acetate buffered medium

Cadmium sulphide and Mn(II) catalysed autoxidation of aqueous sulphur(IV) h as been studied in detail. The CdS catalysed reaction obeys rate law (i), w ith k(3) = 1.17x10(-12) mol L-1 s(-1) -d[S(IV)]/dt = k(3) [S(IV)](2) [H+](-2) + k(4) [CdS] [S(IV)](2) [H+](-1) .. .(i) and k(4) = 4.5 x 10(-7) L-2 mol(-2) g(-1) s(-1) at 30 degreesC in acetate b uffered medium. For manganese(II) catalysed reaction, the overall rate law is given by Eq. (ii) with k(e) = 2 x 10(-4) L mol(-1) s(-1) and k(f) = 6.9 x 10(-2) L-2 mol(-2) s(-1) R-obs = R-o + k(e) [Mn(II)] [S (IV)] [H+](-1) + k(f) [Mn(II)] [S(IV)](2) [H +](-1) ...(ii) at 30 degreesC in acetate buffered medium. R-o term represents the uncataly sed and /or trace metal ion catalysed rate. Ethanol strongly inhibits both the reactions indicating the operation of a radical chain mechanism. A stud y of the effect of ethanol shows that whereas the non-transition metal oxid e catalysed sulphur(IV) autoxidation is severely inhibited, the transition metal oxide catalysed autoxidation is inhibited only slightly. This has bee n interpretated in terms of the operation of a radical chain mechanism in t he case of former and a non-radical mechanism in the case of the latter.