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

2.7 Presentation of the selected data 35be needed in order to ascertain that proper chemical models are used in the modellingof hydrothermal systems. For many species, experimental thermodynamic data are notavailable to allow a selection of parameters describing the temperature dependence ofequilibrium constants and Gibbs energies of formation. The user may find informationon various procedures to estimate the temperature dependence of these thermodynamicparameters in [97PUI/RAR]. The thermodynamic data in the selected set refer to infinitedilution for soluble species. Extrapolation of an equilibrium constant K, usuallymeasured at high ionic strength, to K ◦ at I = 0 using activity coefficients γ ,isexplainedin Appendix B. The corresponding Gibbs energy of dilution is dil G m = r G ◦ m − rG m (2.55)= −RT r ln γ ± (2.56)Similarly dil S m can be calculated from ln γ ± and its variations with T, while dil H m = RT 2 ∂∂T ( r ln γ ± ) p (2.57)depends only on the variation of γ with T, which are neglected in this review, when nodata on the temperature dependence of γ ’s are available. In this case the Gibbs energyof dilution dil G m is entirely assigned to entropy difference. This entropy of reactionis calculated using the Gibbs-Helmholtz equation, the above assumption dil H m =0,and dil G m .