CONSIDERATIONS ON PROTOACTINIUM REDOX POTENTIALS

The influence of the electronic configurations of Pa2+ (f(3) and f(2)d ) and Pa3+ (f(2) and fd) upon the stability of divalent and trivalent protactinium and their redox potentials is examined. An analysis of th e stabilization of the configuration f(2)d (Pa2+) and fd (Pa3+) leads to predictions on the relative scale for the E(0)(Pa3/2) potentials wh ich depend on the medium and on ligands. For this purpose the relation between the energy (f(3) --> f(2)d) for Pa(II) in water (Pa(2+)aq) an d in solid (Pa2+:CaF2) is established. The values of the redox potenti als are discussed on the basis of the 6d electron stabilization of Pa2 + and destabilization of Pa3+ ions. The standard redox potential E(0)( 3/2) is estimated to be around -4.0 V. This result is confirmed by a m easurement of E(fd) in Pa(4+)aq and by correlation of E(fd) for Pa(2+) aq, Pa(3+)aq and Pa(4+)aq with the corresponding redox potentials.