Considerations on protactinium redox potentials

The influence of the electronic configurations of Pa2+ (f3 and f2d) and Pa3+ (f2 and fd) upon the stability of divalent and trivalent protactinium and their redox potentials is examined. An analysis of the stabilization of the configuration f2d (Pa2+) and fd (Pa3+) leads to predictions on the relative scale for the E0(Pa3/2) potentials which depend on the medium and on ligands. For this purpose the relation between the energy (f3 → f2) for Pa(II) in water (Pa2+aq) and in solid (Pa2+:CaF2) is established. The values of the redox potentials are discussed on the basis of the 6d electron stabilization of Pa2+ and destabilization of Pa3+ ions. The standard redox potential E0(3/2) is estimated to be around -4.0 V. This result is confirmed by a measurement of Efd in Pa4+aq and by correlation of Efd for Pa2+aq, Pa3+aq and Pa4+aq with the corresponding redox potentials. (orig.)