The Influence of Ionic Strength on Yttrium and Rare Earth Element Complexation by Fluoride Ions in NaClO₄, NaNO₃ and NaCl Solutions at 25 °C

Abstract

Stability constants of the form _F β ₁(M)=[MF²⁺][M³⁺]⁻¹[F⁻]⁻¹ (where [MF²⁺] represents the concentration of a yttrium or a rare earth element (YREE) complex, [M³⁺] is the free YREE ion concentration, and [F⁻] is the free fluoride ion concentration) were determined by direct potentiometry in NaNO₃ and NaCl solutions. The patterns of log_{10  F} β ₁(M) in NaNO₃ and NaCl solutions very closely resemble stability constant patterns obtained previously in NaClO₄. For a given YREE, stability constants obtained in NaClO₄ were similar to, but consistently larger than _F β ₁(M) values obtained in NaNO₃ which, in turn, were larger than formation constants obtained in NaCl. Stability constants for formation of nitrate and chloride complexes ( $_{\mathrm{NO}_{3}}\beta_{1}(\mathrm{M})=[\mathrm{MNO}_{3}^{2+}][\mathrm{M}^{3+}]^{-1}[\mathrm{NO}_{3}^{-}]^{-1}$ and _Cl β ₁(M)=[MCl²⁺][M³⁺]⁻¹[Cl⁻]⁻¹) derived from _F β ₁(M) data exhibited ionic strength dependencies generally similar to those of _F β ₁(M). However, in contrast to the somewhat complex pattern obtained for _F β ₁(M) across the fifteen member YREE series, no patterns were observed for nitrate and chloride complexation constants: neither $_{\mathrm{NO}_{3}}\beta_{1}(\mathrm{M})$ nor _Cl β ₁(M) showed discernable variations across the suite of YREEs. Nitrate and chloride formation constants at 25 °C and zero ionic strength were estimated as log₁₀  $_{\mathrm{NO}_{3}}\beta_{1}^{\mathrm{o}}(\mathrm{M})=0.65\pm 0.06$ and log_{10  Cl} β ^o₁ (M)=0.71±0.05. Although these constants are identical within experimental uncertainty, the distinct ionic strength dependencies of $_{\mathrm{NO}_{3}}\beta_{1}(\mathrm{M})$ and _Cl β ₁(M) produced larger differences in the two stability constants with increasing ionic strength whereby _Cl β ₁(M) was uniformly larger than $_{\mathrm{NO}_{3}}\beta_{1}(\mathrm{M})$ .