Thermodynamic Study of Ce4+ /Ce3+ Redox Reaction in Aqueous Solutions at Elevated Temperatures: 1. Reduction Potential and Hydrolysis Equilibria of Ce4+ in HCIO4 Solutions

Abstract The redox potential (E) of the couple Ce 4+ /Ce 3+ has been determined up to 368 K by means of cyclic voltammetric measurement in aqueous HClO 4 solutions with c HClO4 decreasing from 7.45 to 0.023 mol kg -1 . A constant potential of (1.741 V) 298 K , resp. (1.836 V) 368K , indicating the existence of pure unhydrolysed Ce 4+ was obtained at c HClO4 ≥ 6.05 m. At lower HClO 4 concentration, the potential as a function of the HClO 4 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. Δ S and Δ H of the redox reaction were determined as functions of T at the different HClO 4 concentrations. ΔS is positive at c HClO4 > 1.85 m and turns to be negative at lower concentrations. ΔH is negative at all HClO 4 concentrations studied. The cumulative formation constants ß i , of the Ce(IV) hydroxo complexes and the corresponding hydrolysis constants (K h ) i were calculated. An unusual decrease of ß i with increasing temperature has been discussed