Interactions between lanthanide cations and nitrate anions in water Part 2 Microcalorimetric determination of the Gibbs energies, enthalpies and entropies of complexation of Y3+ and trivalent lanthanide cations

Abstract

Microcalorimetry is applied to the study of the very weak complexation of Y³⁺ and trivalent lanthanide cations (La³⁺, Ce³⁺, Pr³⁺, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺ and Lu³⁺) by nitrate anion in water at 298.15 K. For each cation, both the apparent association constant and enthalpy of reaction at an ionic strength of 2.0 mol kg^-1 are extracted from the calorimetric data. The Gibbs energies, enthalpies and entropies of complexation of these lanthanide cations, to which are added the values previously found for Nd³⁺, Sm³⁺ and Eu³⁺, are extrapolated to infinite dilution using the Pitzer interaction model and the ionic strength effect previously observed with neodymium. An attempt is also made to extract the enthalpies of complexation from the available heats of dilution by comparing the nitrate and perchlorate heat data; the results agree quite well with the values found by microcalorimetry. The trends in the thermodynamic properties of association with the cation radius are examined. Some of the properties show breakpoints that delimit four subfamilies, a pattern that may possibly be attributed to a tetrad effect.