Thermochemical parameters for solution of lanthanide(III) ethylsulphate and trichloride hydrate series: Tetrad effects and hydration change in aqua Ln3+ ion series.

Abstract

Thermochemical data for solution at 25°C and 1 atm (ΔGs°, ΔHs° and ΔSs°) of fully isomorphous LnES3·9H2O (ES = C2H5SO4-) and partly isomorphous LnCl3·nH2O (n = 7 for La-Pr and n = 6 for Nd-Lu) are discussed, because they provide important clues to understand the tetrad effects in REE patterns of geochemical samples. All the differences in ΔHs°, ΔGs° and ΔSs° between LnCl3·6H2O and LnES3·9H2O show convex tetrad effects, and they correspond to ΔHr, ΔGr and ΔSr for the reaction series: LnCl3·6H2O (c) + 3(ES-)(aq) + 3H2O(l) = LnES3·9H2O(c) + 3Cl-(aq). The convex tetrad effects are explained by the refined spin-pairing energy theory (RSPET) and thermodynamic principles: LnES3·9H2O have larger Racah (E1 and E3) parameters than LnCl3·6H2O by about 0.5% and 1%, respectively. The differences in E1 and E3 relate to minute but significant differences in dissociation energies for bondings of Ln3+ ions with ligands, and then to ΔHr and the vibrational entropy differences of ΔSr(vib). They emerge as convex tetrad effects in ΔHr and ΔSr. A similar tetrad effect is seen in ΔGr, because ΔHr dominates in ΔGr = ΔHr - TΔSr for the low temperature reactions. Each series variation of ΔHs°, ΔGs° or ΔSs° for LnES3·9H2O or LnCl3·6H2O consists of (i) a tetrad effect due to the differences in Racah parameters between Ln3+(aq) and each isomorphous Ln(III) hydrate series, (ii) an irregularity caused by the hydration change of light Ln3+(aq) from nonahydrate to octahydrate with going from La to Tb, and (iii) the smooth residual variation. Concave tetrad effects are seen in ΔHs°, ΔGs° and ΔSs° for heavy LnES3·9H2O, but no such variations in those for heavy LnCl3·6H2O. This means the Racah parameters decreasing in the order: LnES3·9H2O > LnCl3·6H2O ≈ Ln3+(aq, octahydrate). The RSPET makes it possible to determine the irregularity due to the hydration change of light Ln3+(aq) from ΔHs°, ΔGs° or ΔSs° for the two isomorphous Ln(III) hydrate series. The irregularity is the thermodynamic parameter (ΔHh∗, ΔSh∗ or ΔGh∗) for the stabilization of real light Ln3+(aq) relative to octahydrate Ln3+(aq).