Thermodynamic Study of Ce4+/Ce3+ Redox Reaction in Aqueous Solutions at Elevated Temperatures: 1. Reduction Potential and Hydrolysis Equilibria of Ce4+ in HC104Solutions

Abstract

The redox potential (E) of the couple Ce4+/Ce3+has been determined up to 368 K by means of cyclic voltammetric measurement in aqueous HC104solutions with cHC104decreasing from 7.45 to 0.023 mol kg−1. A constant potential of (1.741 V)298 K, resp. (1.836 V)368 K, indicating the existence of pure unhydrolysed Ce4+ was obtained at cHC104> 6.05 m. At lower HC104concentration, the potential as a function of the HC104molality, as well as of the pH shows 4 further distinct steps. At 298 K, for instance, the potential became nearly constant at pH values of 0.103,0.735,1.115, after which it drastically decreased, respectively at 1.679, just before the precipitation of Ce(OH)4occurred. The curves indicate obviously the stepwise formation of the Ce(IV) mono-, di-, tri- and tetrahydroxo complexes. The slope of the curves E vs. pH increased gradually with increasing temperature. AS and AH of the redox reaction were determined as functions of T at the different HC104concentrations. AS is positive at cHC104> 1.85m and turns to be negative at lower concentrations. AH is negative at all HC104concentrations studied. The cumulative formation constants βiof the Ce(IV) hydroxo complexes and the corresponding hydrolysis constants (Kh)iwere calculated. An unusual decrease of βiwith increasing temperature has been discussed. © 1992, Verlag der Zeitschrift für Naturforschung. All rights reserved.