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

A. Discussion of selected references 751of the data in the NpO 2 CO − 3predominance domain, but less than reported by Vitorge[84VIT] and Lemire, Boyer and Campbell [93LEM/BOY], while no scattering at allwas reported for the conditions used to determine the solubility product, K s,1 ,andthesecond complexation constant, β 2 .Earlier work from the same laboratory required correction for liquid junction effectson the calibration of their glass electrodes (cf. p. 281 in [95SIL/BID]). In thiswork a different method was used for pH calibration, but there are also problems withthe revised method. An incorrect definition of pH was used involving the mean activitycoefficient of HClO 4 (instead of the H + activity coefficient in NaClO 4 aqueous solutions).Under other circumstances this might be a useful approximation, but when theSIT is applied the ε (H + ,ClO − 4 ) and ε (Na + ,ClO − 4 )values are not the same, nor is this typeof approximation valid when using a third order virial expansion [95NEC/RUN]. It isalso not clear which molality to molarity conversion factor they used. The calibrationprocedure used may have been self-consistent in that the measured values of the waterionic product are in accord with published data, and with the SIT as used by this review.However, the same was not true for the values of the carbonic acid deprotonationconstants. This is apparently not the result of experimental problems in controllingcarbonate speciation, as the error increased with ionic strength (the deviation in pKchanges from 0.03 to 0.38). To allow for this, the uncertainty assigned in this reviewto systematic error in log 10 [CO 2−3] for these results has been increased. Informationgiven in a later paper [94NEC/RUN] indicates the junction potential corrections couldbe quite important. There is no way to predict these since they strongly depend on theexperimental set up. In the course of the present review, an attempt was made to derivejunction potentials from the pH values in Table 1 of Neck et al.[94NEC/RUN], usingSIT values for the activity coefficients of H + and OH − . However, this led to differentresults for acidic and basic media. Further, as a 3 M NaCl liquid junction was usedthe junction potential correction might be expected to be at a minimum for solutionshaving an ionic strength near 3 M, which was not the case. Thus, there seems to havebeen a problem, but is not clear whether its source was chemical (carbonate equilibration),the junction potential corrections or the incorrect definition of pH. However,most of these problems would remain the same during solubility measurements and becancelled out. For this reason, this review accepts and uses the measurements from thispaper as extrapolated to zero ionic strength.To recalculate the stability constants proposed by Kim et al. [91KIM/KLE] andNeck et al. [94NEC/RUN, 95NEC/FAN, 95NEC/RUN], this review used the experimentalsolubility measurements tabulated in the reports [91KIM/KLE, 94NEC/KIM]and scanned those for 5 M NaCl aqueous solutions as reported in Figure 3d of Necket al. [95NEC/RUN]. The possibility of different solid phases was also consideredin the reanalysis of the data. Only the set of solubility measurements extrapolated toI = 0 was used to select thermodynamic data. It was also found that ionic strengthcorrections for the solubility products were not consistent in the original papers (see thediscussion of [94NEC/RUN] in this appendix). Therefore, independently determinedparameters were used to estimate the corresponding ionic strength corrections. Thereported parameters [91KIM/KLE, 94NEC/RUN] were also not consistent with thosein a later paper [95FAN/NEC] from this group.