Abstract
The kinetics and mechanism of citric acid oxidation by Cr^sup VI^; catalyzed by Mn^sup II^, has been studied in H^sub 2^O and in the presence of anionic and cationic surfactants. A linear correlation between k ^sub obs^ ^sup -1^ and [Mn^sup II^]^sup -1^ was found, satisfying the Michaelis-Menten kinetics. The rate-determining step is the decomposition of complex HCrO^sub 4^-citric acid-Mn^sup II^ formed between citric acid-Mn^sup II^ and Cr^sup VI^. Based on kinetic data, a one-step three-electron oxidation mechanism has been proposed. The rate decreased with increase in concentration of the cationic surfactants cetyltrimethylammonium bromide (CTAB) and cetylpyridinium bromide (CPB), while anionic sodium dodecyl sulphate (SDS) had no effect on the rate. The data have been interpreted in terms of reaction in the aqueous phase. The effect of added anions, such as chloride, bromide, nitrate, and sulphate, has been studied and discussed. The activation parameters (ΔH ^sup [double dagger]^ and ΔS ^sup [double dagger]^) were significantly affected by the presence of 10.0 × 10^sup -4^ mol dm^sup -3^ of CTAB or CPB.[PUBLICATION ABSTRACT]