THE THERMODYNAMICS OF AQUEOUS TRIVALENT RARE-EARTH ELEMENTS - APPARENT MOLAR HEAT-CAPACITIES AND VOLUMES OF ND(CLO4)(3)(AQ), EU(CLO4)(3)(AQ), ER(CLO4)(3)(AQ), AND YB(CLO4)(3)(AQ) FROM THE TEMPERATURES 283 K TO328 K

The apparent molar heat capacities and apparent molar volumes of Nd(Cl O4)(3)(aq), Eu(ClO4)(3)(aq), Er(ClO4)(3)(aq), and Yb(ClO4)(3)(aq) have been measured at the temperatures T = (283.2, 298.2, 313.2, and 328.2 ) K and molalities from approximate to 0.05 mol.kg(-1) to approximate to 0.7 mol.kg(-1) in a Picker flow microcalorimeter and vibrating tube densimeter. The results were analysed by means of the Pitzer equation s to derive standard partial molar heat capacities C(p,2)degrees, stan dard partial molar Volumes V-2 degrees and expressions for the excess properties. The revised Helgeson-Kirkham-Flowers (HFK) model has been used to represent the temperature dependence of the standard partial m olar properties within the experimental uncertainty. Plots of C(p,2)de grees and V-2 degrees at T = 298.15 K against the ionic radii of La3+, Nd3+, Eu3+, Er3+, Gd3+, and Yb3+ clearly display the discontinuous be haviour known as the ''gadolinum break''. The ionic-radius dependence of the standard partial molar volumes is consistent with changes in th e primary hydration number. The effect of temperature on the behaviour of the standard partial molar heat capacities across the series sugge sts that secondary sphere hydration has a major effect on the values o bserved for C(p,2)degrees. (C) 1997 Academic Press Limited.