Effect of micelles on the oxidation of oxalic acid by chromium(VI) in the presence and absence of manganese(II)

The effects of cationic cetyltrimethylammonium bromide (CTAB), anionic sodium dodecyl sulphate (SDS) and non-ionic Triton X-100 (TX-100) surfactant micelles on the kinetics of the oxidation of oxalic acid by chromium(VI) is studied spectrophotometrically in the absence and presence of manganese(II). The reaction follows second-order kinetics with respect to oxalic acid in absence of manganese(II), which shifts to first-order in presence of manganese(II). In the latter case, the reduction of chromium(VI) to chromium(III) occurs in a single-step without passing through chromium(IV) as an intermediate. The reaction is catalysed by CTAB and TX-100 micelles but SDS has no effect. The observed catalyses are explained with the model proposed by Menger and Portnoy. The activation parameters are determined in water and in CTAB. The effect of added inorganic salts (NaCl, NaNO3, Na2SO4 and NaBr) on the CTAB catalysed reaction is also reported.