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°C to 225°C

The concentration quotients for the following reactions with acetate have been measured at temperatures from 25°C to 225°C, at saturated water vapor pressure, and in 0.1 molal NaCl medium, using a potentiometric method:Nd3++Ac−=NdAc2+Nd3++2Ac−=NdAc2+The concentration quotients for both of these reactions increase strongly with increasing temperature. The values of these constants obtained at 25°C are in good agreement with those reported in the literature for both NaCl and NaClO4 media, indicating that complexation of Nd3+ by chloride is weak at room temperature. The concentration quotients were corrected for complexation by chloride using experimentally determined stability constants from the literature; these calculations confirmed the lack of a significant effect due to chloride complexation at 25°C, but showed that the effect becomes more important with increasing temperature. Extrapolation of the chloride-corrected concentration quotients to zero ionic strength using an extended Debye–Hückel expression yielded values of the stability constants for the first and second complexation steps that are significantly higher than theoretical estimates reported in the literature. Our data show that Nd-acetate complexes are considerably more stable than Nd-chloride complexes, but that the stability constants for both systems exhibit very similar temperature dependences. In sedimentary basinal brines REE (rare earth elements)-acetate complexes may predominate over REE-chloride complexes, even where chloride is present in greater concentrations than acetate. However, oxalate, fluoride, or carbonate complexes could be more important than either acetate or chloride if the concentrations of the former ligands are relatively high.