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

Abstract

The kinetics and mechanism of Cr(VI) oxidation of formaldehyde in the presence and absence of picolinic acid (PA) in aqueous acid media have been carried out under the conditions, [formaldehydeh]_T » [Cr(VI)]_T at different temperatures. Under the kinetic conditions, the monomeric species 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 active oxidant. In the PA catalysed path, Cr(VI)-PA complex receives a nucleophilic attack by the substrate to form a ternary complex which subsequently experiences a redox

decomposition (through 2<i>e </i>transfer) leading to formic acid and Cr(IV)-PA complex. Then Cr(IV)-PA complex participates further in the oxidation of formaldehyde and ultimately is converted into Cr(III)-PA complex. In the uncatalysed path, Cr(VI)-substrate ester experiences an acid catalysed redox decomposition (2<i>e </i>transfer) at the rate determining step. The uncatalysed

path shows a second order dependence on [H⁺] while the PA catalysed path shows a zeroth order dependence on [H⁺]. Both the uncatalysed and PA catalysed paths show first-order dependence on [formaldehyde]_T and [Cr(VI)]_T. The PA catalysed path is first order in [PAh. All these observations <i>(i.e. </i>dependence patterns on the reactants) remain unaltered in the presence of externally added surfactants. Effect of cationic surfactant (<i>i.e. </i>cetylpyridinium chloride, CPC) and anionic surfactant (<i>i.e. </i>sodium dodecyl sulphate, SDS) has been studied both in the presence and absence of PA. CPC inhibits the

reactions while SDS catalyses the reactions. The observed micellar effects have been explained by considering the partitioning of the reactants between the micellar and aqueous phase. Applicability of different kinetic models, <i>e.g. </i>pseudo-phase ion exchange (PIE) model, Menger-Portnoy model, Piszkiewicz cooperative model, has been tested to explain the observed micellar effects.