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Unusual rate inhibition of manganese(II) assisted oxidation of citric acid by chromium(VI) in the presence of ionic micelles

Research paper by Kabir-ud-Din, Khaled Hartani, Zaheer Khan

Indexed on: 01 Aug '00Published on: 01 Aug '00Published in: Transition Metal Chemistry



Abstract

The kinetics and mechanism of citric acid oxidation by CrVI; catalyzed by MnII, has been studied in H2O and in the presence of anionic and cationic surfactants. A linear correlation between kobs−1 and [MnII]−1 was found, satisfying the Michaelis–Menten kinetics. The rate-determining step is the decomposition of complex HCrO4–citric acid–MnII formed between citric acid–MnII and CrVI. 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‡ and ΔS‡) were significantly affected by the presence of 10.0 × 10−4 mol dm−3 of CTAB or CPB.