Redox Reaction in Aqueous Solutions at Elevated Temperatures: 1. Reduction Potential and Hydrolysis Equilibria of Ce + in HCIO4 Solutions

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 H C 1 0 4 solutions with cHC104 decreasing from 7.45 to 0.023 mol kg" 1 . A constant potential of (1.741 V)2 9 8 K , resp. (1.836 V) 3 6 8 K , indicating the existence of pure unhydrolysed Ce 4 + was obtained at c H c l 0 4 > 6.05 m. At lower HC10 4 concentration, the potential as a function of the H C 1 0 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-, triand 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 HC10 4 concentrations. AS is positive at cHC104 > 1.85 m and turns to be negative at lower concentrations. AH is negative at all H C 1 0 4 concentrations studied. The cumulative formation constants ß, of the Ce(IV) hydroxo complexes and the corresponding hydrolysis constants (Kh); were calculated. An unusual decrease of ßt with increasing temperature has been discussed.