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

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 extra cted from the calorimetric data. The Gibbs energies, enthalpies and en tropies of complexation of these lanthanide cations, to which are adde d the values previously found for Nd3+, Sm3+ and Eu3+, are extrapolate d to infinite dilution using the Fitter interaction model and the ioni c strength effect previously observed with neodymium. An attempt is al so made to extract the enthalpies of complexation from the available h eats of dilution by comparing the nitrate and perchlorate heat data; t he results agree quite well with the values found by microcalorimetry. The trends in the thermodynamic properties of association with the ca tion radius are examined. Some of the properties show breakpoints that delimit four subfamilies, a pattern that may possibly be attributed t o a tetrad effect.