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

264 12. Neptunium group 14 compounds and complexesThis is accepted as the best estimate of log 10 K ◦ 5 (12.25) at25◦ ; the uncertainty inlog 10 K ◦ 5(12.25) has been increased from 0.14 (= 1.96 σ ) to 0.30 because the qualityof the spectrophotometric and pH measurements is not as good as for the similar studyfrom the same laboratory on the corresponding plutonium system [96CAP/VIT]. Veryrecently Rai et al. [99RAI/HES] found a dramatic increase in Np(IV) solubility withincreasing KHCO 3 molality. Recalculations (Appendix A) indicate a slope of +4 forthe log-log plot, and the data can be interpreted in terms of the equilibriumNpO 2 (am, hyd) + 4CO 2−3+ 2H 2 O(l) Å Np(CO 3 ) 4−4+ 4OH − (12.26)Using the low ionic strength experimental data (molal units) reported in[99RAI/HES], the molal constant, log 10 K s,4 (12.26, 0.033 to 0.11 m KHCO 3 ), has avalue of −(17.36 ± 0.12), from which the selected standard valueslog 10 Ks,4 ◦ (12.26, 298.15 K) = −(17.79 ± 0.22) r G ◦ m (12.26, 298.15 K) = (101.55 ± 1.26) kJ·mol−1are obtained. The selected value f G ◦ m (Np(CO 3) 4−4, 298.15 K) =−(2812.78 ± 8.24) kJ·mol−1is obtained from this Ks,4 ◦ value, log 10 ∗ K s,0 (8.13, 298.15 K) = (1.53 ± 1.00), f G ◦ m (Np4+ , 298.15 K) =−(491.77 ± 5.59) kJ·mol −1 selected in Sections 8.2.5.2and 7.4, and f G ◦ m (CO2− 3 , 298.15 K) =−(527.9 ± 0.39) kJ·mol−1 (Table 5.1) andlog 10 β4 ◦ (12.27) = (36.69 ± 1.03) is calculated for the equilibriumNp 4+ + 4CO 2−3Å Np(CO 3 ) 4−4(12.27)Neglecting possible formation of mixed hydroxide-carbonate complexes, and systematicerrors in selection of ε(12.26), a relatively unimportant quantity for these lowionic strengths, might result in overestimation of the stability of Np(CO 3 ) 4−4 .Thisinturn would result in overestimation of the stability of Np(CO 3 ) 6−5, and of the standardpotential E ◦ (12.21) for the Np(V)/Np(IV) redox couple.12.1.2.1.4.c Other Np(IV) carbonate or mixed hydroxide-carbonate complexesRai and Ryan [85RAI/RYA] measured Np(IV) solubility in neutral media and concludedthat no detectable anionic complex was formed in their conditions. This observationis consistent with the values for the stability constants of Np(CO 3 ) 4−4andNp(CO 3 ) 6−5selected above (see also the discussion of [85RAI/RYA] in Appendix A).Rai and Ryan also excluded the soluble complexes of Np(IV) proposed by Moskvin[71MOS2] to interpret his solubility study. Pratopo and Moriyama have tried to reproduceRai and Ryan’s measurements under exactly the same conditions [89MOR/PRA]or similar conditions [90PRA/MOR]. They found higher solubilities and interpretedthis as evidence of new carbonate or mixed hydroxide carbonate soluble complexes.As explained in Appendix A the results were too scattered and cannot reasonably be