Equilibrium Studies and Redox Kinetics of the Peroxo Complex of Zirconium(IV) in Acidic Perchlorate Solution

摘要

The complexation of zirconium (IV) by hydrogen peroxide has been studied in aqueous solutions containing M Zr(IV), M H202, and M HClO4. The peroxozirconium(IV) complex does not exhibit detectable absorption bands in the visible- or ultraviolet-wavelength regions. Therefore, the equilibrium concentrations of uncomplexed Zr(4+) and H2O2 were measured by the use of the indicators 2-thenoyltrifluoroacetone and titanium(IV), respectively. The principal complex formed is a tetrameric species (4:2) containing four Zr(IV) and two peroxide ions. The value of the formation constant is given. The thermodynamic parameters Delta H sub f degrees = 11.2 kcal/mol and Delta S sub f degrees = 130 cal/(deg mol) were calculated from the temperature dependence of K4:2 in 1.96 M HClO4. The variation of the formation constant with acidity at an ionic stength of 2.0 M allows the tentative formulation of the peroxozirconium(IV) complex as Zr4(O2)2(OH)4(8+). The results of concentration-jump kinetic experiments have been interpreted. Kinetic studies of the oxidation of peroxozirconium(IV) by cerium(IV) and the reduction by sulfur(IV) in 1.96 M HClO4 yielded a common rate expression. According to our interpretation, both systems proceed by a rate-determining ring-opening process on peroxozirconium(IV), followed by rapid attack on an exposed pendant -O2H unit by the external reagent.