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 t he very weak complexation of the trivalent neodymium cation by the nit rate anion in water at 298.15 K. The apparent association constant and enthalpy of reaction at a given ionic strength are both extracted fro m the calorimetric data. The ionic strength, which is controlled by ad dition of sodium perchlorate, is varied from 2.0 to 0.5 mol kg(-1). Fo r comparison, the association constant is also determined by visible s pectroscopy under similar conditions. The thermodynamic properties of complexation are extrapolated to finite dilution using the Fitter inte raction 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.