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

Chapter 3Selected neptunium dataThis chapter presents the chemical thermodynamic data set for neptunium specieswhich has been selected in this review. Table 3.1 contains the recommended thermodynamicdata of the neptunium compounds and complexes, Table 3.2 the recommendedthermodynamic data of chemical equilibrium reactions by which the neptunium compoundsand complexes are formed, and Table 3.3 the temperature coefficients of theheat capacity data of Table 3.1 where available.The species and reactions in Tables 3.1, 3.2 and 3.3 appear in standard order ofarrangement (cf. Figure 2.1). Table 3.2 contains information only on those reactions forwhich primary data selections are made in this review. These selected reaction data areused, together with data for key neptunium species (for example Np 4+ ) and auxiliarydata listed in Table 5.1, to derive the corresponding formation quantities in Table 3.1.The uncertainties associated with values for the key neptunium species and for someof the auxiliary data are substantial, leading to comparatively large uncertainties in theformation quantities derived in this manner.However, then the fact remains that for many reactions uncertainties in quantitiessuch as r G ◦ m are known more accurately than would be calculated directly from theuncertainties of the f G ◦ m values for species listed in Table 3.1 and the auxiliary datain Table 5.1. The uncertainties in, for example, the value of f G ◦ m (NpO 2SO − 4 ) incorporatesnot only the uncertainty in r G ◦ m for the complexation reaction from which itis derived,NpO + 2 + SO2− 4Å NpO 2 SO − 4but also the uncertainties in f G ◦ m (NpO+ 2 , aq, 298.15 K) and fG ◦ m (SO2− 4 , aq,298.15 K). Thus, back-calculation from the uncertainties of the separate speciesoverestimates the uncertainty in the Gibbs energy of reaction and the uncertaintyin the corresponding equilibrium constant. The inclusion of a table for reactiondata (Table 3.2) in this report allows the use of equilibrium constants with totaluncertainties that are directly based on the experimental accuracies. This is the mainreason for including both the table for reaction data (Table 3.2) and the table of f G ◦ m , fHm ◦ , S◦ m and C◦ p,m values (Table 3.1). In a few cases, correlation of smalluncertainties in values for ligands has been neglected in calculations of uncertaintyvalues for species in Table 3.1 from uncertainty values in Table 3.2. However, forthose species the effects are less than 2% of the stated uncertainties.Furthermore, for some reactions that involve concurrent ligand protonation or deprotonation(e.g., in certain neptunium-carbonate, -sulphate or -phosphate systems) theuncertainties in equilibrium constants are less than would have been calculated directly39