Thermodynamic property changes in lanthanide(III) cation exchange reactions with poly(styrenesulfonic acid) type cation exchangers

Abstract

Calorimetric and equilibrium distribution measurements at 298.1 K were employed to estimate standard Gibbs energy, enthalpy, and entropy changes in the ion-exchange reactions between 13 of the lanthanide(III) cations and yttrium(III) ion in dilute aqueous perchlorate solution and a strong-acid type cation exchanger. A plot of the lanthanide atomic number dependence of ..delta..G/sup 0/ for the reaction LnR/sub 3/ + Ce(III) = CeR/sub 3/ + Ln(III) showed discontinuities at Z = 64 (Gd) and between Z = 60 to 61 (Nd--Pm) and Z = 67 to 68 (Ho--Er) indicating a ''tetrad'' effect superimposed on the effect of the lanthanide contraction. No such Z dependence was found with the ..delta..H/sup 0/ values but rather a maximum exothermic effect was reached at approximately the middle of the rare earth series followed by a linear decrease in the heat evolved from Tb to Lu. The dependence of ..delta..G/sup 0/ on Z can be explained in terms of a slight modification of the effect of the lanthanide contraction by interelectronic repulsion effects in the Ln(III) cations. The unusual Z dependence of ..delta..H/sup 0/ can be rationalized by assuming that the coordination number for water in the primary hydration sphere of La(III) ions changes frommore » 9 to 8 and that strong ion-pair complexes are formed between the sulfonate groups of the ion exchanger and Ln(III) cations with only inner sphere hydration.« less