Interactions between lanthanide cations and nitrate anions in water .1. Effect of the ionic strength on the Gibbs energy, enthalpy and entropy of complexation of the neodymium cation

Microcalorimetry has been applied for the first time to the study of the very weak complexation of the trivalent neodymium cation by the nitrate anion in water at 298.15 K. The apparent association constant and enthalpy of reaction at a given ionic strength are both extracted from the calorimetric data. The ionic strength, which is controlled by addition of sodium perchlorate, is varied from 2.0 to 0.5 mol kg(-1). For comparison, the association constant is also determined by visible spectroscopy under similar conditions. The thermodynamic properties of complexation are extrapolated to finite dilution using the Fitter interaction model: this yields K = 4.0 +/- 0.1 and Delta(r)H(circle minus) = - 1.2 +/- 0.2 kJ mol(-1). The microcalorimetric method and the data treatment are fully described.