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

32 2. Standards, Conventions, and Contents of the Tablesthe data are also stored in the data base.The following thermodynamic parameters, valid at the reference temperature of298.15 K and at the standard pressure of 1 bar, are stored in the data base: f G ◦ m the standard molar Gibbs energy of formation from (kJ · mol −1 )the elements in their reference state f Hm◦ the standard molar enthalpy of formation from the elements(kJ · mol −1 )in their reference stateSm ◦ the standard molar entropy (J · K −1 · mol −1 )Cp,m ◦ the standard molar heat capacity (J · K −1 · mol −1 )and C◦ p,mFor aqueous neutral species and ions, the values of f G ◦ m , fHm ◦ , S◦ mcorrespond to the standard partial molar quantities, and for individual aqueous ions theyare relative quantities, defined with respect to the aqueous hydrogen ion, according tothe convention [89COX/WAG] that f Hm ◦ (H+ , aq,T ) = 0, and that Sm ◦ (H+ , aq,T ) =0. Furthermore, for an ionised solute B containing any number of different cations andanions: f H ◦ m (B ±, aq) = ∑ +ν + f H ◦ m (cation, aq) + ∑ −ν − f Hm ◦ (anion, aq)S ◦ m (B ±, aq) = ∑ +ν + S ◦ m (cation, aq) + ∑ −ν − Sm ◦ (anion, aq).As the thermodynamic parameters vary as a function of temperature, provision is madefor including the compilation of the coefficients of empirical temperature functions forthese data, as well as the temperature ranges over which they are valid. In many casesthe thermodynamic data measured or calculated at several temperatures were publishedfor a particular species, rather than the deduced temperature functions. In these cases,a non-linear regression method is used in this review to obtain the most significantcoefficients of the following empirical function:F(T ) = a + b × T + c × T 2 + d × T −1 + e × T −2 + f × ln T + g × T ln T+ h × √ T + i √T+ j × T 3 + k × T −3 . (2.52)Most temperature variations can be described with three or four parameters, a, b ande being the ones most frequently used. In the present review, only Cp,m ◦ (T ), i.e., thethermal functions of the heat capacities of individual species, are considered and storedin the data base. They refer to the relationC ◦ p,m (T ) = a + b × T + c × T 2 + d × T −1 + e × T −2and are listed in Tables 3.3 and 4.3.The pressure dependence of thermodynamic data has not been the subjectof critical analysis in the present compilation. The reader interested in highertemperatures and pressures, or the pressure dependency of thermodynamic functionsfor geochemical applications, is referred to the specialised literature in this area, e.g.,