Thermochemical parameters for solution of lanthanide(III) ethylsulphate and trichloride hydrate series: Tetrad effects and hydration change in aqua Ln(3+) ion series

Thermochemical data for solution at 25 degrees C and 1 atm (Delta G(s)degre es, Delta H(s)degrees and Delta S(s)degrees) of fully isomorphous LnES(3). 9H(2)O (ES = C2H5SO4-) and partly isomorphous LnCl(3). nH(2)O (n = 7 for La -Pr and n = 6 for Nd-Lu) are discussed, because they provide important clue s to understand the tetrad effects in REE patterns of geochemical samples. All the differences in Delta H(s)degrees, Delta G(s)degrees and Delta S(s)d egrees between LnCl(3). 6H(2)O and LnES(3). 9H(2)O show convex tetrad effec ts, and they correspond to Delta H-r, Delta G(r) and Delta S-r for the reac tion series: LnCl(3). 6H(2)O (c) + 3(ES-)(aq) + 3H(2)O(1) = LnES(3). 9H(2)O (c) + 3Cl(-)(aq). The convex tetrad effects are explained by the refined sp in-pairing energy theory (RSPET) and thermodynamic principles: LnES(3). 9H( 2)O have larger Racah (E-1 and E-3) parameters than LnCl(3). 6H(2)O by abou t 0.5% and 1%, respectively. The differences in E-1 and E-3 relate to minut e but significant differences in dissociation energies for bondings of Ln(3 +) ions with ligands, and then to Delta H-r and the vibrational entropy dif ferences of Delta S-r(vib). They emerge as convex tetrad effects in Delta H -r and Delta S-r. A similar tetrad effect is seen in Delta G(r), because De lta H-r dominates in Delta G(r) = Delta H-r - T Delta S-r for the low tempe rature reactions. Each series variation of Delta H(s)degrees, Delta G(s)deg rees or Delta S(s)degrees for LnES(3). 9H(2)O or LnCl(3). 6H(2)O consists o f (i) a tetrad effect due to the differences in Racah parameters between Ln (3+)(aq) and each isomorphous Ln(III) hydrate series, (ii) an irregularity caused by the hydration change of light Ln(3+)(aq) from nonahydrate to octa hydrate with going from La to Tb, and (iii) the smooth residual variation. Concave tetrad effects are seen in Delta H(s)degrees, Delta G(s)degrees and Delta S(s)degrees for heavy LnES(3). 9H(2)O, but no such variations in tho se for heavy LnCl(3). 6H(2)O. This means the Racah parameters decreasing in the order: LnES(3). 9H(2)O > LnCl(3). 6H(2)O approximate to Ln(3+)(aq, oct ahydrate). The RSPET makes it possible to determine the irregularity due to the hydration change of light Ln(3+)(aq) from Delta H(s)degrees, Delta G(s )degrees or Delta S(s)degrees for the two isomorphous Ln(III) hydrate serie s. The irregularity is the thermodynamic parameter (Delta H-h*, Delta S-h* or Delta G(h)*) for the stabilization of real light Ln(3+)(aq) relative to octahydrate Ln(3+)(aq).