A radiochemical study of chemical equilibria by partition methods

Abstract

A radiochemical study has been made of complex formation in aqueous solutions containing trivalent lanthanide and actinide metal ions and selected ligands. Partition methods using a cation-exchanger and a liquid extractant (di(2-ethyIhexyl)phosphoric acid [HY](_2), in toluene) as the second phase were developed and measurements carried out with La, Ce, Eu, Gd, Am and Cm with one or more of the ligands, a-hydroxy-isohutyrate, fluoride, sulphate and oxalate. The isotopes La(^140), Ce(^144)/Pr(^144), Eui(^152), Gd(^153) and Am(^241) were used in conjunction with conventional methods of measurement of β- and ϒ-emitters. Liquid-scintillation counting procedures were developed for the measurement of the α-activity from Cm(^244). It was found that in the presence of a constant and representative concentration of each of the ligands used only M(HY(_2))(_3); extracts into the toluene phase and metal ion complexed by the ligands concerned does not partition. A cation-exchange resin on the other hand is known to take up partially complexed species in addition to 'free' metal ions. A method was evolved whereby data obtained separately by the ion-exchange and solvent- extraction methods could be used to calculate distribution coefficients of partially complexed species partitioning into the resin phase. The results showed that residual cation charge as well as the size of the exchanging species are important in determining the extent to which ion-exchange partition occurs. It was also found that the Pronaeus' method of treatment of ion-exchange data where more than two ligands are attached to a metal ion may be Inadequate and that assumptions frequently necessary regarding the distribution coefficient of partially complexed species may be considerably in error depending on the ligand. The investigations carried out in this work have indicated the application of HDEHP to the determination of equilibrium constants particularly of α-emitting higher actinides and in studies where more than two complexes are formed. Equilibrium constants for the complexes formed were computed and various tests applied to the method of computation from ion-exchange distribution data. Equilibrium constants obtained are generally in good agreement with relevant published data.