2021-09-16T15:43:52Zhttps://nagoya.repo.nii.ac.jp/oaioai:nagoya.repo.nii.ac.jp:000182312021-05-10T06:20:43ZThermodynamic parameters for aqueous lanthanide(III)-DTPA complex formations: Convex tetrad effects of ΔHr and ΔSr cancelled in ΔGr almost totallyKawabe, Iwao53171Thermochemical data for lanthanide(III)-DTPA (diethylenetriaminepentaacetate) complex formations at 27°C and I=0.1 M, have been examined from Jørgensen’s theory (RSPET) and the hydration change in light Ln3+ (aq) series relative to the octahydrate heavy Ln3+ (aq). The ΔHr , ΔSr and ΔGr data corrected for the hydration change of light Ln3+ (aq), can be regressed by the improved RSPET equation, with exception of ΔGr for La-DTPA and ΔHr for Lu-DTPA. The exceptional data have been corrected according to the RSPET equation, along with small experimental biases in ΔHr for Dy-DTPA and ΔGr for Er-DTPA. Racah (E1 and E3) parameters of Ln-DTPA are greater than those of octahydrate Ln3+ (aq): ΔE1=+(17±7) cm-1 and ΔE3=+(10±2) cm-1 for Ln=Nd. The corrected ΔHr and ΔSr data show similar convex tetrad effects, whereas the ΔGr data exhibit a fairly smooth variation. The tetrad effects of ΔHr and ΔSr are positively correlated: ΔSr (tetrad) ≈κ ⋅ ΔHr (tetrad) with κ(E1) = (2.9 ± 1.5) ×10−3 (1/K) and κ(E3) = (2.4 ± 0.7) ×10−3 (1/K) . The tetrad effect of ΔGr is given as ΔGr (tetrad) = ΔHr (tetrad) − TΔSr (tetrad) ≈ (1−κ ⋅ T)ΔHr (tetrad) . At T ≈ 300 K , ΔHr (tetrad) is cancelled by TΔSr (tetrad) in ΔGr (tetrad) almost totally. This is the reason why the experimental values of logK(Ln-DTPA) = −ΔGr/(2.303RT) do not exhibit an obvious tetrad effect. Combining the Ln-DTPA formation with the solution of isomorphous Ln(III) ethylsulphate nonahydrate (Ln(ES)3⋅9H2O) eliminates Ln3+ (aq), and gives the following reaction that Ln(ES)3 ⋅ 9H2O(c) + DTPA5− (aq ) = [Ln − DTPA]2− (aq ) + 3ES− (aq ) + 9H2O( l ). Thermochemical data for this reaction are useful to see structural changes and Racah parameters of Ln-DTPA relative to Ln(ES)3⋅9H2O. They show smooth variations with minor tetrad effects compatible with RSPET. Racah (E1 and E3) parameters of Ln-DTPA are fairly comparable with those of Ln(ES)3⋅9H2O，suggesting that a nine-fold coordination of Ln3+ in Ln-DTPA with the octadentate DTPA ligands plus one water molecule is analogous to Ln(ES)3⋅9H2O. This is compatible with the results by the fluorescence lifetime studies of Ln-DTPA (Ln=Eu, Tb, Dy and Yb), XAFS study of Gd-DTPA, and multinuclear NMR study of Ln-DTPA series. Since the NMR study of the lanthanide-induced 13C and water 17O shifts suggests an isostructural inner coordination of Ln3+ across the Ln-DTPA series, the exceptional thermochemical data for La-DTPA and Lu-DTPA are possibly attributable to experimental errors. The thermochemical data for Ln-DTPA complex formation provide such an example that shows similar convex tetrad effects of ΔHr and ΔSr cancelled in ΔGr almost totally. Its implications to REE solution chemistry and geochemistry have been discussed.other2014-06-12application/pdfhttp://hdl.handle.net/2237/20250https://nagoya.repo.nii.ac.jp/record/18231/files/LnDTPA_all.pdfeng