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

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Rare earth elements, Fluoride complexation, Nitrate complexation, Chloride complexation, Stability constants, Ionic strength, Lanthanide, Yttrium

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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 log10F 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 ( 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)=[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 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 log10  NO3β1o(M)=0.65±0.06">NO3βo1(M)=0.65±0.06NO3β1o(M)=0.65±0.06 and log10Cl β o1 (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) .

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Journal of Solution Chemistry, v. 36, p. 673-689