The aqueous geochemistry of the rare earth elements IX. A potentiometric study of Nd3+ complexation with acetate in 0.1 molal NaCl solution from 25 degrees C to 225 degrees C

The concentration quotients for the following reactions with acetate have b een measured at temperatures from 25 degrees C to 225 degrees C, at saturat ed water vapor pressure, and in 0.1 molal NaCl medium, using a potentiometr ic method: Nd3+ + Ac- = NdAc2Nd3+ + 2Ac(-) = NdAc2The concentration quotients for both of these reactions increase strongly w ith increasing temperature. The values of these constants obtained at 25 de grees C are in good agreement with those reported in the literature for bot h NaCl and NaClO4 media, indicating that complexation of Nd3+ by chloride i s weak at room temperature. The concentration quotients were corrected for complexation by chloride using experimentally determined stability constant s from the literature; these calculations confirmed the lack of a significa nt effect due to chloride complexation at 25 degrees C, but showed that the effect becomes more important with increasing temperature. Extrapolation o f the chloride-corrected concentration quotients to zero ionic strength usi ng an extended Debye-Huckel expression yielded values of the stability cons tants for the first and second complexation steps that are significantly hi gher than theoretical estimates reported in the literature. Our data show t hat Nd-acetate complexes are considerably more stable than Nd-chloride comp lexes, but that the stability constants for both systems exhibit very simil ar temperature dependences. In sedimentary basinal brines REE (rare earth e lements)-acetate complexes may predominate over REE-chloride complexes, eve n where chloride is present in greater concentrations than acetate. However , oxalate, fluoride, or carbonate complexes could be more important than ei ther acetate or chloride if the concentrations of the former ligands are re latively high. (C) 2000 Elsevier Science B.V. All rights reserved.