Oxidation of hydrogen peroxide by tungstate ion: formation and decay of a long-lived intermediate

The kinetics of the reaction between tungstate ion and hydrogen peroxide in aqueous medium containing phosphate ions has been followed spectrophotometrically at 225 nm. This wavelength led to two different kinds of absorbance-time plots, showing either an increasing-maximum-decreasing temporal pattern or a continuously decreasing one, depending on the medium pH. This allowed to carry out two independent kinetic studies, one at high pH (first reaction stage) concerning the formation of a long-lived intermediate, thought to be W(V), and the other at low pH (second reaction stage) concerning its decay. The kinetic tool chosen to obtain the quantitative information was that of the initial rate method. The results indicated that, whereas both reaction stages were of first order in hydrogen peroxide, the rate dependence on the concentration of tungstate ion differed for the two stages: an apparent kinetic order intermediary between 1 and 2 for the first stage, and a well-defined order 1 for the second stage. There was also a difference between the dependences of the initial rates of the two stages on the concentrations of phosphate ions (the rate of the first stage independent and that of the second decreasing) and of the background electrolyte KCl (for the first stage an increasing effect and for the second a decreasing one). Both stages showed catalysis by hydrogen, copper(II), zinc and manganese(II) ions, the latter three probably acting as superoxide radical scavengers. Although the activation energy of the first stage was unusually close to zero (1.0 ± 1.2 kJ mol⁻¹), that of the second stage was considerably higher (28 ± 3 kJ mol⁻¹). Finally, a mechanism coherent with the available experimental information, and where the solvent cage effect plays an important role, has been proposed for each reaction stage.

Graphical abstract