Reduction of tellurium (IV) by ascorbic acid in acid perchlorate solutions. A kinetic and mechanistics approach on the nature of electron-transfer process

Abstract

The kinetics and mechanism of ascorbic acid (AH2) reduction of tellurium (IV) ion in acid perchlorate solutions at a constant ionic strength of 0.1 moldm−3 has been investigated, spectrophotometrically, in terms of the nature of electron-transfer and transition states in the rate-determining step. The experimental observations indicated that the reaction kinetics was of complexity nature. The pseudo-first-order plots were found to be of sigmoidal S-shape nature which indicated the presence of two distinct stages. The initial part of the curves was relatively slow, followed by an increase in the reaction rates at longer time periods. This complicated behavior indicates that the rate-law expression obeys the law-expression (Dt−D∞) = A0e−kst + B0e−kft where ks and kf are corresponding to the observed pseudo-first-order rate constants of the initial induction and final autoacceleration periods, respectively. The experimental results indicated first-order dependence in [AH2] and fractional-first-order kinetics in [Te (IV)] for the initial induction period. The hydrogen ion dependence of the rate constants indicates the oxidation reaction is acid-inhibition with inverse fractional first order in [H+]. A kinetic evidence for formation of 1:1 intermediate complex between the reactants prior to the rate-determining step has been revealed from the Michaelis-Menten relationship. The reaction was found to proceed via free-radical intervention mechanism. The oxidation was found to proceed by one electron-transfer of inner-sphere nature throughout the entire course of reaction. Tellurium (0) of nanoparticle size was synthesized.The ionization constants of tellurium (IV) (H2TeO3) were evaluated at various temperatures, kinetically. The activation parameters and the ionization constant of Te(IV) have been studied at four temperatures. A tentative reaction mechanism consistent with the kinetic data is suggested and discussed.