The Ionic Strength Dependence of Rare Earth and Yttrium Fluoride Complexation at 25°C

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Rare earth elements, fluoride complexation, stability constants, sodium perchlorate, ionic strength, lanthanide, yttrium

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Formation constants for the complexation of yttrium and rare earth elements(YREE) by fluoride ions have been measured at 25°C. The ionic strength (μ)dependence of YREE formation constants in perchlorate solution for ionicstrengths between 0 and 6 molar can be expressed aslogFβ1 (M, μ) =logFβ1 o (M) −3.066 μ0.5/(1 + 1.769 μ0.5)+ 0.1645 μwhere logFβ1 o(M) represents MF2+formation constants at zero ionic strength.The logFβ1 o(M) results obtained inthis work are: Y(4.46), La(3.62), Ce(3.86),Pr(3.84), Nd(3.82), Sm(4.15), Eu(4.27), Gd(4.24), Tb(4.37), Dy(4.39), Ho(4.28),Er(4.27), Tm(4.29), Yb(4.39), and Lu(4.25). The relative magnitudes of YREEformation constants are independent of ionic strength. The pattern oflogFβ1(M,μ),formation constants obtained in this work [relative magnitudes oflogFβ1 o (M)],exhibits a shallow minimum between Dy and Yb. In contrast to the smoothpattern of stability constants expected if fluoride were to interact with bare ions(with monotonically decreasing crystal radii between La and Lu), theinteractionof F with YREEs, which have extensive hydration spheres[M(H2O)8–9 3+] resultsin a relatively complex pattern of lanthanide stability constants. The fluoridecomplexation behavior of yttrium differs distinctly from the behavior of any rareearth. Although the crystal radius of Y3;pl is approximately equalto that of Ho3+,differences in the covalence/ionicity of Y3+ relative to therare earths leads to aYF2+ stability constant that exceeds that of any rare earthelement (REE).

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Journal of Solution Chemistry, v. 29, p. 1089-1099