Reduction of water-soluble colloidal manganese dioxide by thiourea : a kinetic and mechanistic study

Kinetic data for the colloidal MnO^sub 2^-thiourea redox system are reported for the first time. The reduction of water-soluble colloidal MnO^sub 2^ by thiourea (sulfur containing reductant) in aqueous perchloric acid medium has shown that it proceeds in two stages, i.e., a fast stage followed by a relatively slow second stage. The log (absorbance) versus time plot deviates from linearity. The kinetics of both the stages was investigated spectrophotometrically. The first-order kinetics with respect to [thiourea] at low concentration shifts to zero-order at higher concentration. The reaction rate increases with [HClO^sub 4^] and the kinetics reveals complex order dependence in [HClO^sub 4^]. Addition of P^sub 2^O^sub 7^^sup 4-^ and F^sup -^ in the form of Na^sub 4^P^sub 2^O^sub 7^ and NaF, respectively, has inhibitory effect on the reaction rate. The reaction proceeds through the fast adsorption of thiourea on the surface of the colloidal MnO^sub 2^. A mechanism involving the protonated thiourea as the reactive reductant species is proposed. The observed results are discussed in terms of Michaelis-Menten/Langmuir-Hinshelwood model. From the observed kinetic data, colloidal MnO^sub 2^-thiourea adsorption constant (K^sub ad1^) and rate constant (k^sub 1^) were calculated to be 1.25×10^sup 10^ mol^sup -1^ dm^sup 3^ and 3.1×10^sup -4^ s^sup -1^, respectively. The variation of temperature does not have any effect on the reaction rate.[PUBLICATION ABSTRACT]