Apparent molar heat capacities and apparent molar volumes of Pr(ClO 4) 3(aq), Gd(ClO 4) 3(aq), Ho(ClO 4) 3(aq), and Tm(ClO 4) 3(aq) at T=(288.15, 298.15, 313.15, and 328.15) K and p=0.1 MPa

Abstract

Acidified aqueous solutions of Pr(ClO 4) 3(aq), Gd(ClO 4) 3(aq), Ho(ClO 4) 3(aq), and Tm(ClO 4) 3(aq) were prepared from the corresponding oxides by dissolution in dilute perchloric acid. Once characterized with respect to trivalent metal cation and acid content, the relative densities of the solutions were measured at T=(288.15, 298.15, 313.15, and 328.15) K and p=0.1 MPa using a Sodev O2D vibrating tube densimeter. The relative massic heat capacities of the aqueous systems were also determined, under the same temperature and pressure conditions, using a Picker Flow Microcalorimeter. All measurements were made on solutions containing rare earth salt in the concentration range 0.01 ⩽ m/(mol · kg −1) ⩽ 0.2. Relative densities and relative massic heat capacities were used to calculate the apparent molar volumes and apparent molar heat capacities of the acidified salt solutions from which the apparent molar properties of the aqueous salt solutions were extracted by the application of Young's Rule. The concentration dependences of the isothermal apparent molar volumes and heat capacities of each aqueous salt solution were modelled using Pitzer ion-interaction equations. These models produced estimates of apparent molar volumes and apparent molar heat capacities at infinite dilution for each set of isothermal V φ,2 and C p φ,2 values. In addition, the temperature and concentration dependences of the apparent molar volumes and apparent molar heat capacities of the aqueous rare earth perchlorate salt solutions were modelled using modified Pitzer ion-interaction equations. The latter equations utilized the Helgeson, Kirkham, and Flowers equations of state to model the temperature dependences (at p=0.1 MPa) of apparent molar volumes and apparent molar heat capacities at infinite dilution. The results of the latter models were compared to those previously published in the literature. Apparent molar volumes and apparent heat capacities at infinite dilution for the trivalent metal cations Pr 3+(aq), Gd 3+(aq), Ho 3+(aq), and Tm 3+(aq) were calculated using the conventions V 2 ∘(H +(aq)) ≡ 0 and C p2 ∘(H +(aq)) ≡ 0 and have been compared to other values reported in the literature.