Aqueous Geochemistry of Rare Earth Elements and Yttrium. XII: Potentiometric Stability Constant Determination of Bis-Tris Complexes with La, Nd, Eu, Gd, Yb, Dy, Er, Lu, and Y

Abstract

Conditional stability constants of 2-[bis(2-hydroxyethyl)amino]-2(hydroxymethyl)-1,3-propanediol (BT) complexes of trivalent rare earth element (Ln) ions (La, Nd, Eu, Gd, Yb, Dy, Er, Lu) and Y were determined potentiometrically in aqueous NaCl solutions at 30°C and 0.1 M ionic strength. Least-squares fitting shows that, at <0.04 molal BT, the complex LnBT³⁺ is dominant, with LnBT₂ ³⁺ forming a secondary complex, where:

$$\begin{gathered} { Ln}^{{3 + }} + {BT} \leftrightarrow {LnBT}^{{3 + }} { }\beta _{11} \hfill \\ {Ln}^{{3 + }} + 2{BT} \leftrightarrow {Ln(BT)}_{2}^{{3 + }} { }\beta _{21} \hfill \\ \end{gathered}$$

Conditional stability constants appear to be directly related to the ionic radius of the trivalent ion in question. The optimal ionic radius, 104–105 pm, yields values of log $\beta _{{21}}^{*} = 10.93 \pm 0.63$ (Gd) and $\beta _{{11}}^{*} = 6.83 \pm 0.14$ (Yb). Complexation drops off steeply on either side of the ideal ionic radius. In relating the stability constants to ionic radius, it is assumed that BT complexes with Gd, Dy, Er, and Lu have coordination number eight, whereas those with La, Nd, and Eu have coordination number nine. The smoothest trend of stability constants with ionic radius is obtained if Yb–BT complexes are assumed to have coordination number nine. These results may reflect the ability of BT to form an ionic radius-specific chelate structure.