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

The kinetics and mechanism of citric acid oxidation by Cr-VI; catalyzed by Mn-II, has been studied in H2O and in the presence of anionic and cationic surfactants. A linear correlation between k(obs)(-1) and [Mn-II](-1) was fo und, satisfying the Michaelis-Menten kinetics. The rate-determining step is the decomposition of complex HCrO4-citric acid-Mn-II formed between citric acid-Mn-II and Cr-VI. Based on kinetic data, a one-step three-electron oxi dation mechanism has been proposed. The rate decreased with increase in con centration 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 chlorid e, bromide, nitrate, and sulphate, has been studied and discussed. The acti vation parameters (Delta H double dagger and Delta S double dagger) were si gnificantly affected by the presence of 10.0 x 10(-4) mol dm(-3) of CTAB or CPB.