Micellar effect on the reaction of chromium(VI) oxidation of formaldehyde in the presence and absence of picolinic acid in aqueous acid media: A kinetic study

The kinetics and mechanism of Cr(VI) oxidation of formaldehyde in the prese nce and absence of picolinic acid (PA) in aqueous acid media have been carr ied out under the conditions, [formaldehyde](r) much greater than [Cr(VI)IT at different temperatures. Under the kinetic conditions, the monomeric spe cies of Cr(VI) has been found kinetically active in the absence of PA while in the PA catalysed path, Cr(VI)-PA complex has been suggested as the acti ve oxidant. In the PA catalysed path, Cr(VI)-PA complex receives a nucleoph ilic attack by the substrate to form a ternary complex which subsequently e xperiences a redox decomposition (through 2e transfer) leading to formic ac id and Cr(IV)-PA complex. Then Cr(IV)-PA complex participates further in th e oxidation of formaldehyde and ultimately is converted into Cr(III)-PA com plex. In the uncatalysed path, Cr(VI)-substrate ester experiences an acid c atalysed redox decomposition (2e transfer) at the rate determining step. Th e uncatalysed path shows a second order dependence on [H+] while the PA cat alysed path shows a zeroth order dependence on [H+]. Both the uncatalysed a nd PA catalysed paths show first-order dependence on [formaldehyde]T and [C r(VI)](r). The PA catalysed path is first order in [PA]T. All these observa tions (i.e. dependence patterns on the reactants) remain unaltered in the p resence of externally added surfactants. Effect of cationic surfactant (i.e . cetylpyridinium chloride. CPC) and anionic surfactant tie. sodium dodecyl sulphate, SDS) has been studied both in the presence and absence of PA. CP C inhibits the reactions while SDS catalyses the reactions. The observed mi cellar effects have been explained by considering the partitioning of the r eactants between the micellar and aqueous phase. Applicability of different kinetic models, e.g. pseudo-phase ion exchange (PIE) model, Menger-Portnoy model, Piszkiewicz cooperative model, has been tested to explain the obser ved micellar effects.