Am Aqueous Thermodynamic Model for the Complexation of Sodium and Strontium with Organic Chelates Valid to High Ionic Strength. I. Ethylenedinitrilotetraacetic Acid (EDTA)

摘要

An aqueous thermodynamic model is developed, which accurately describes the effects of Na~+ complexation, ionic strength, carbonate concentration, and temperature on the complexation of Sr~(2+) by ethylenedinitrilotetraacetic acid (EDTA) under basic conditions. The model is developed from the analysis of literature data on apparent equilibrium constants, enthalpies, and heat capacities, as well as on an extensive set of solubility data on SrCO_3(c) in the p0resentce of EDTA obtained as part of this study. The solubility data for SrCO_3(c) in the presence of EDTA obtained as part of this study. The solubility data for SrCO_3(c) were obtained in solutions ranging in Na_2CO_3 concentration from 0.01 to 1.8 m, in NaNO_3 concentration from 0 to 5 m, and at temperatures extending to 75 deg C. The final aqueous thermodynamic model is based upon the equations of Pitzer and requires the inclusion of a NaEDTA~(3-) species. An accurate model for the ionic strength dependence of the ion-interaction coefficients for the SrEDTA~(3-) and NaEDTA~(3-) aqueous species allows the extrapolation of standard state equilibrium constants for these species, which are significantly different form there 0.1 m reference state values available in the literature. The final model is tested by application to chemical systems containing competing metal ions (i.e., Ca~(2+)) to further verify the proposed model and indicate the applicability of the model and indicate the applicability of the model parameters to chemical systems containing other divalent metal-EDTA complexes.