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 log10 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 log10 Cl β 1 o (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 $_{\mathrm{NO}_{3}}\beta_{1}(\mathrm{M})$ .