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

224 12. Neptunium group 14 compounds and complexes12.1.2.1.3.a Np(V) carbonate complexesThree types of experimental measurements, liquid-liquid extraction, spectrophotometryand solubility, have been used to derive formation constants fordioxoneptunium(V) carbonate complexes.Using a liquid-liquid extraction technique with 0.1 or 0.2 M NaClO 4 aqueous solutions,the equilibrium constants, K 1 = β 1 ,forand K 2 = β 2 /β 1 ,forNpO + 2 + CO2− 3Å NpO 2 CO − 3(12.14)NpO 2 CO − 3 + CO2− 3Å NpO 2 (CO 3 ) 3−2(12.15)were measured by Bidoglio, Tanet and Chatt [85BID/TAN]. Their β 1 value does notcorrespond to the experimental data shown in figures of the publication, but even followingre-evaluation in the present review (see Appendix A), none of these β 1 values,nor the K 2 value are in accord with other published work and with the thermodynamicdata selected here. Inoue and Tochiyama [85INO/TOC] also used a liquid-liquid extractiontechnique to determine the values of β 1 and β 2 in1MNaClO 4 . Within thesmall uncertainties assigned by the authors, the values are not in good agreement withthose selected in the present review. Although larger uncertainties could be assumed(see Appendix A), this work was not used in the final selection of thermodynamicvalues in the present review.Values for β 1 , K 2 and the stepwise complexation constant, K 3 = β 3 /β 2 ,forNpO 2 (CO 3 ) 3−2+ CO 2−3Å NpO 2 (CO 3 ) 5−3(12.16)can be deduced from absorption spectrophotometric studies (see the discussion of[90RIG] in Appendix A); but, of these, the most difficult constant to determine isK 2 because the NpO 2 (CO 3 ) 3−2complex cannot be obtained alone to measure its molarabsorptivity (cf. Figure 12.5). This difficulty is even more important at high ionicstrengths. This in turn introduces difficulties when treating the experimental data toobtain β 1 and K 3 values. Too low a solubility can also cause problems in the determinationof values for β 1 and K 2 .Riglet’s β 1 values [90RIG] in 0, 0.5, 1 and 2 M NaClO 4 are lower than (but stillwithin the stated uncertainties of) the values selected below in the present review, andthe corresponding value of ε 1 is in fair agreement with the one selected here (seeTable 12.5). In the present review only the β 1 value in 0.5 M NaClO 4 could be redeterminedfrom the experimental information available in the thesis (see Appendix A) andwas used in the present review to calculate the selected β1 ◦ value. The extrapolation(using the SIT) to zero ionic strength using Riglet’s spectrophotometric data, gave avalue of log 10 K2 ◦ and a corresponding ε 2 value in reasonable agreement with thoseselected below, but none of Riglet’s K 2 values have been used because the experimentalinformation available is insufficient to allow K 2 to be recalculated.The spectrophotometric data used in determining K 3 were limited to those solutionsin which the extent of dissociation of the limiting complex was not greater than 25%.