THERMODYNAMIC STUDY OF CE(4+)/CE(3+) REDOX REACTION IN AQUEOUS-SOLUTIONS AT ELEVATED-TEMPERATURES .1. REDUCTION POTENTIAL AND HYDROLYSIS EQUILIBRIA OF CE(4+) IN HCLO4 SOLUTIONS

The redox potential (E) of the couple Ce4+/Ce3+ has been determined up to 368 K by means of cyclic voltammetric measurement in aqueous HClO4 solutions with c(HClO4) decreasing from 7.45 to 0.023 mol kg-1. A constant potential of (1.741 V)298K, resp. (1.836 V)368K, indicating the existence of pure unhydrolysed Ce4+ was obtained at c(HClO4) greater-than-or-equal-to 6.05 m. At lower HClO4 concentration, the potential as a function of the HClO4 molality, 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)4 occurred. 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. DELTAS and DELTAH of the redox reaction were determined as functions of T at the different HClO4 concentrations. DELTAS is positive at c(HClO4) > 1.85 m and turns to be negative at lower concentrations. DELTAH is negative at all HClO4 concentrations studied. The cumulative formation constants beta(i) of the Ce(IV) hydroxo complexes and the corresponding hydrolysis constants (K(h))i were calculated. An unusual decrease of beta(i) with increasing temperature has been discussed.