Selectivity variation with organic solvents was shown to be supplemented in the presence of a mineral acid.l0J1 This is because the dissociation of the resin -cation complex is facilitated by formation of other complexes, including anionic complexes. Such complex formation is frequently enhanced by the dehydrating action of the organic solvent.In 1959, Korkisch and co-workers12 9 1 3 started an intensive study1 9 1 4 , 1 5 of anion-exchange patterns with complexing eluants involving aqueousorganic solvents. Adding ethanol to thorium( IV) nitrate and uranium(V1) chloride complex systems procured the preferential sorption of the thorium and uranium, respectively, on the anion exchanger, Dowex 1. The general increase in sorption frequently observed with increased alcohol content in the solvent mixture soon became an accepted characteristic phenomenon.14Rigorous theoretical treatment of ion-exchange equilibria in mixed solvent systems is difficult. Nevertheless, experiment has established useful general features for this purpose. Thus, in addition to the general increase in selectivity coefficients noted above, there is a linear relation between log selectivity coefficient and the reciprocal of the solvent dielectric c ~n s t a n t . ~~? ~~ Explanations of these phenomena range from the hypothetical de-solvation . energy involved in ion exchangel8J9 to ion-pair formation,20,21 and to changes in the solution and resin-phase activity coefficient^.^^)^^ Allowance must also be made for other factors, such as the dependence of separation factors on differences in the solvent composition of the resin phase and the external solution. This, it is claimed, causes a kind of partition chromatography to be superimposed on the ion-exchange c h r o r n a t ~g r a p h y . ~, ~~ Related to this is the view that the distribution of an uncharged species between two phases without further interaction is the main reaction and not simple ion exchange.26In recent years, considerable effort has been given to extending the above studies1 ,I4 9 1 5 v2' *2* ,29,30 y 3 1 and in the present review the ion exchange of inorganic materials is considered on the separate premises of cation and anion exchange. As a preface to such a discussion, some of the basic principles of ion-exchange selectivities, thermodynamics and kinetics, and the behaviour of ion-exchange resins, are outlined... .. .. where V , is the effluent volume corresponding to the peak maximum, Vi the free volume of the resin bed, V , the volume of the mixing chamber, C, the concentration in the mixing chamber and m the weight of resin in the column. The constant, a, is characteristic of individual lanthanides, dependent on both the distribution coefficient, K,, and the complexing species (nitrate ion) concentration Kd = a[NO,] . . .. .. .. . . (7).Apart from predicting the positions of elution peaks, equation (6) can also be used for determining Kd.
