The Influence of Ionic Strength on Yttrium and Rare Earth Element Complexation by Fluoride Ions in NaClOsub4/sub, NaNOsub3/sub and NaCl Solutions at 25 °C

Stability constants of the form _F β ₁ (M)=[MF ²⁺ ][M ³⁺ ] ⁻¹ [F ⁻ ] ⁻¹ (where [MF ²⁺ ] represents the concentration of a yttrium or a rare earth element (YREE) complex, [M ³⁺ ] is the free YREE ion concentration, and [F ⁻ ] is the free fluoride ion concentration) were determined by direct potentiometry in NaNO ₃ and NaCl solutions. The patterns of log _{10   F} β ₁ (M) in NaNO ₃ and NaCl solutions very closely resemble stability constant patterns obtained previously in NaClO ₄ . For a given YREE, stability constants obtained in NaClO ₄ were similar to, but consistently larger than _F β ₁ (M) values obtained in NaNO ₃ which, in turn, were larger than formation constants obtained in NaCl. Stability constants for formation of nitrate and chloride complexes ( NO3β1(M)=[MNO32+][M3+]−1[NO3−]−1">NO3β1(M)=[MNO2+3][M3+]−1[NO−3]−1NO3β1(M)=[MNO32+][M3+]−1[NO3−]−1 and _Cl β ₁ (M)=[MCl ²⁺ ][M ³⁺ ] ⁻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 NO3β1(M)">NO3β1(M)NO3β1(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 log ₁₀   NO3β1o(M)=0.65±0.06">NO3βo1(M)=0.65±0.06NO3β1o(M)=0.65±0.06 and log _{10   Cl} β ^o ₁ (M)=0.71±0.05. Although these constants are identical within experimental uncertainty, the distinct ionic strength dependencies of NO3β1(M)">NO3β1(M)NO3β1(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 NO3β1(M)">NO3β1(M)NO3β1(M) .