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 to 328 K

Abstract

The apparent molar heat capacities and apparent molar volumes of Nd(ClO4)3(aq), Eu(ClO4)3(aq), Er(ClO4)3(aq), and Yb(ClO4)3(aq) have been measured at the temperaturesT=(283.2, 298.2, 313.2, and 328.2) K and molalities from ≈0.05 mol·kg−1to ≈0.7 mol·kg−1in a Picker flow microcalorimeter and vibrating tube densimeter. The results were analysed by means of the Pitzer equations to derive standard partial molar heat capacitiesCp,2°, standard partial molar volumesV2° and expressions of the excess properties. The revised Helgeson–Kirkham–Flowers (HFK) model has been used to represent the temperature dependence of the standard partial molar properties within the experiemntal uncertainty. Plots ofCp,2° andV2° andT=298.15 K against the ionic radii of La3+, Nd3+, Eu3+, Er3+, Gd3+, and Yb3+clearly display the discontinuous behaviour known as the “gadolinum break”. The ionic-radius dependence of the standard partial molar volumes is consistent with changes in the primary hydration number. The effect of temperature on the behaviour of the standard partial molar heat capacities across the series suggests that secondary sphere hydration has a major effect on the values observed forCp,2°.