chemical thermodynamics of neptunium and plutonium - U.S. ...

Appendix BIonic strength corrections †Thermodynamic data always refer to a selected standard state. The definition given byIUPAC [82LAF] is adopted in this review as outlined in Section 2.3.1. According tothis definition, the standard state for a solute B in a solution is a hypothetical solution,at the standard state pressure, in which m B = m ◦ = 1mol· kg −1 , and in which theactivity coefficient γ B is unity. However, for many reactions, measurements cannot bemade accurately (or at all) in dilute solutions from which the necessary extrapolation tothe standard state would be simple. This is invariably the case for reactions involvingions of high charge. Precise thermodynamic information for these systems can only beobtained in the presence of an inert electrolyte of sufficiently high concentration, ensuringthat activity factors are reasonably constant throughout the measurements. Thisappendix describes and illustrates the method used in this review for the extrapolationof experimental equilibrium data to zero ionic strength.The activity factors of all the species participating in reactions in high ionic strengthmedia must be estimated in order to reduce the thermodynamic data obtained from theexperiment to the state I = 0. Two alternative methods can be used to describe theionic medium dependence of equilibrium constants:• One method takes into account the individual characteristics of the ionic mediaby using a medium dependent expression for the activity coefficients of the speciesinvolved in the equilibrium reactions. The medium dependence is describedby virial or ion interaction coefficients as used in the Pitzer equations [73PIT]and in the specific ion interaction theory.• The other method uses an extended Debye-Hückel expression in which the activitycoefficients of reactants and products depend only on the ionic charge and theionic strength, but it accounts for the medium specific properties by introducingionic pairing between the medium ions and the species involved in the equilibriumreactions. Earlier, this approach has been used extensively in marinechemistry, cf. Refs.[79JOH/PYT, 79MIL, 79PYT, 79WHI2].The activity factor estimates are thus based on the use of Debye-Hückel type equations.The “extended” Debye-Hückel equations are either in the form of specific ion† This Appendix contains essentially the text written by Grenthe and Wanner [92GRE/WAN] whichwasalso printed in the uranium NEA–TDB review as Appendix B [92GRE/FUG]. The equations presentedhere are an essential part to the review procedure and are required to use the selected thermodynamicvalues. The main differences between this Appendix and the one in Grenthe et al. [92GRE/FUG] are:Table B.1, Eq.(B.11), Example B.3 and Sections B.1.2 and B.1.4. The contents of Tables B.3 and B.4have also been revised.799