Ionic strength dependency of stability constants of uranyl and thorium chloro-complexes

Stability constants, Kst, of uranyl and thorium chloro-complexes decrease rapidly with increasing ionic strength. The ionic complexes are typical outer-sphere complexes, as the Fuoss formula is applicable to this relationship and the heat of formation is remarkably small. The inter-ionic distance of 1.9 x 10-8 and 4.0 x 10-8 cm were obtained for the uranyl and thorium chloro-complexes. The thermodynamic equilibrium constants of 33 and 212 mol-1 1 were also determined for these chloro-complexes. The relatively low Kst values, which have been found for higher ionic strengths by previous investigators, are explained by the same outer-sphere complex model. The upper limit of the complexation was estimated as a function of ionic strength on the basis of the above model. (author)