Abstract
The kinetics of manganese (II) catalysed oxidation of lactic acid by HCrO4-; in aqueous solution at 50 degreesC was investigated spectrephotometrically in: (a) the absence of surfactant, (b) the presence of cationic surfactants (cetyltrimethylammonium bromide, CTAB and cetylpyridinium bromide, CPB) and, in part, (c) the presence of anionic surfactant (sodium dodecyl sulphate, SDS). The oxidation follows first and second order kinetics with respect to HCrO4-, and lactic acid, respectively. In the presence of manganese(II), the second order kinetics shifted to first order with respect to lactic acid. The dependence on [Mn-II] suggests one-step three-electron oxidation. The oxidation rate increased in the presence of CTAB or CPB, while SDS had no effect on the rate. The oxidation of lactic acid by HCrO4- in the presence and absence of CTAB, CPB and SDS is routed through the same mechanism as shown by the kinetic study. The results are best accounted for by pseudo-phase model proposed by Menger and Portnoy, with the modification given by Bunton. The effect of added inorganic salts (NaBr, NaCl, NaNO3, and Na2SO4) shows that the substrate orientation at the micellar surface has no significant kinetic role.