On weak complex formation: Re-interpretation of literature data on the Np and Pu nitrate complexation

The problems encountered when analyzing data on weak complex formation are discussed. To study such systems at a constant ionic strength, it is necess ary to increase the ligand concentration replacing a substantial part of th e medium anions. A general methodology using the specific ion interaction a pproaches (e.g. SIT) is presented which allows the extrapolation of data on weak complex formation obtained at constant ionic strength to "pure" ionic media (and trace concentrations of the ligand). The method is applied to t he Np and Pu nitrate systems, showing that no complexes of penta- and hexav alent Np and Pu can be inferred from the studies in the literature. For Np( IV) and Pu(IV) only one aqueous complex is needed to describe the experimen tal literature data at 1 less than or equal to 6 M and at [NO3-](TOT) less than or equal to 2 M, namely: PuNO33+, and NpNO33+. Equilibrium constants a t infinite dilution are reported. Other complexes, like Pu(NO3)(6)(2-), are nevertheless stable outside the range of applicability of the SIT method. The method outlined here may also be used in data evaluation and/or experim ental planning in other systems, where important changes in the composition of the ionic medium have to be made.