ESTIMATION OF THE PITZER EQUATION PARAMETERS FOR AQUEOUS COMPLEXES - A CASE-STUDY FOR URANIUM AT 298.15 K AND 1 ATM

A procedure to estimate the Fitter parameters for complexes or complex formation reactions from a small number of equilibrium constants (log K vs, I) obtained in 'constant ionic media' is discussed in some deta il. It is then often impossible to determine both log K degrees, the e quilibrium constant at I=0, and all Fitter parameters. hence some esti mations or other simplifications are necessary. Solution coordination chemists have therefore preferred to use the less parametrized Bronste d-Guggenheim-Scatchard (SIT) model. By comparison of analytical statem ents for the mean activity coefficients and equilibrium constants, for the SIT and Fitter models we have shown that they an equivalent for i onic strengths less than 4 mol kg(-1), and have also established relat ionships between the Fitter parameter beta((1))and the charge type of the interacting ions, and between) Delta beta((1)) for a complex forma tion reaction and Delta Z(2) for the reaction, where Delta Z(2) is the sum of the squared charges of the reactants/products with the proper stoichiometric coefficients. The first relationship is based on data i n single electrolyte systems and the second on equilibrium-constant da ta determined in ionic media. For reactions involving species of charg es one and two in 1-1 electrolyte ionic media we recommend the use of Delta beta((1))/Delta Z(2)= 0.337 +/- 0.014 kg mol(-1) for the estimat ion of the Delta beta((1)) at 298.15 K and 1 atm. In this way we have used concentration equilibrium constant data to estimate log Kr and Fi tter parameters beta((0)) and beta((1)) for more than 30 uranium compl exes with other ions at 298.15 I; and 1 atm. The procedure makes it po ssible to use the information on complex formation, acid/base and redo x equilibria obtained in ionic media together with the Fitter paramete rs for strong electrolytes for the modeling of complex equilibrium sys tems over a broad range of ionic strength/ionic medium compositions, u p to at least 4 mol kg(-1).