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

8.2 Crystalline and amorphous neptunium oxides and hydroxides 123Belyaev et al. [79BEL/DOB] have measured the heat capacity of Np 2 O 5 (cr) from350 to 750 K by drop calorimetry. This work appears to be more precise than the workon NpO 2 (cr) from the same research group [74ARK/GUT]. On conversion to a molarbasis, the equation in [79BEL/DOB] becomesCp,m ◦ (Np 2O 5 (cr)) = (99.2 + 98.6 × 10 −3 −1T ) J·K−1·moland this equation has been accepted in the present review for the temperature range298.15 K to 750 K, and from this the accepted valueCp,m ◦ (Np −12O 5 , cr, 298.15K) = (128.6 ± 5.0) J·K−1·molis calculated. The uncertainty is estimated in the present review.8.2.4.2 Dioxoneptunium(V) hydroxideExperimental values for the solubility of NpO 2 OH(am) as a function of pH, as reportedin various publications, are shown in Figure 8.3. Most reported solubility determinationsfor NpO 2 OH(am) [48KRA/NEL, 71MOS3, 76SEV/KHA, 85LIE/TRE,86EWA/HOW, 92NEC/KIM, 96ROB/SIL] are in reasonable agreement for pH valuesless than or equal to 10. Above that pH even trace quantities of dissolved CO 2 (g) willreact to form carbonato complexes and increase the solubility of the solid, and, in theabsence of CO 2 , hydrolysis reactions may be important (cf. Section 8.1.3). Hence,at high pH the solubility results would be expected to show more scatter, and this isindeed what has been found [86EWA/HOW, 85LIE/TRE, 91YAM/PRA].Neck, Kim and Kanellakopulos [92NEC/KIM] have shown that ageing a dioxoneptunium(V)hydroxide precipitate in contact with sodium perchlorate solutions havingan ionic strength greater than 0.1 m leads to a solid that is several kJ·mol −1 morestable than freshly precipitated NpO 2 OH(am). Thus, the low solubility values obtainedby Musikas [78MUS] may have been caused by the effect on the solid of the very highionic strength (I ≈ 9) medium used for his experiments. The solubilities reported byYamaguchi et al.[91YAM/PRA] are badly scattered even for experiments in which thepH value was < 11, and are generally low (see Appendix A) compared to most of thereported values. The results obtained by Itagaki et al. [92ITA/NAK] for solubilitiesin solutions of low ionic strength with pH values less than 9 show no dependence onpH. As discussed in Appendix A, this may be the result of formation of a colloidal ormetastable solid.For many of the measurements, exact temperatures were not reported (e.g.[48KRA/NEL, 91YAM/PRA]), nor were details provided concerning the calibrationof the glass electrodes (including junction potentials). In most cases, the experimentalsolubilities were reported only in graphical form. Thus, additional uncertainties areintroduced in recovering the values from the original reports and journal articles.Furthermore, in most cases, the nature of the solid in equilibrium with the aqueousphase was not established. Most of the equilibration times ranged from a few hoursto two weeks, although the experiments of Roberts et al. [96ROB/SIL] lasted for 37days, and some of those of Nakayama et al.[88NAK/ARI] for two months.