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

772 A. Discussion of selected referencesthe slight changes in absorption near 851 nm in that figure. Giffaut [94GIF] also studiedthe chloride complexes of Pu 4+ by spectrophotometry. He observed the formationof PuCl 3+ ,PuCl 2+2 ,PuCl+ 3 and PuCl 4(aq). For the stepwise constants, Giffaut obtainedlog 10 K 1 (I = 3M) = (0.20 ± 0.08), log 10 K 2 (I = 3M) =−(0.56 ± 0.16),log 10 K 1 (I = 4M) = (0.25 ± 0.08), log 10 K 2 (I = 4M) =−(0.61 ± 0.18), and,at variable ionic strength, the approximative values of −0.7 ≤ log 10 K 3 ≤−1andlog 10 K 4 < −1. Large discrepancies are observed among different literature values forthe second formation constant, cf. Table18.4. We use the first constants reported forI = 3MandI = 4 M in our regression analysis and assign them an uncertainty of±0.2.As to the chloride complexes of neptunium, Giffaut [94GIF] observed practicallyno changes in the absorption spectrum of Np(V) in 4 M NaCl solution with respectto 1 M HClO 4 solution. For the chloride complexation of Np 4+ and NpO 2+2 ,Giffaut evaluated the available literature and corrected the measured constants to I =0 by using ion interaction coefficients that he had compiled or estimated. He obtainedlog 10 β1 ◦(NpCl3+ ) = (1.45 ± 0.25), log 10 β2 ◦(NpCl2+2) = (2.4 ± 0.3), andlog 10 β1 ◦(NpO 2Cl + ) = (0.7 ± 0.4). However, his recalculations of these values toionic strengths of 2, 3 and 4 M ([94GIF, p. 126], Table III/III) in many cases resultedin log 10 β values that are higher than the available experimental determinations.[94GIF/VIT]In this paper Giffaut, Vitorge and Capdevila derived formulas for calculating the temperaturedependence of the enthalpy, entropy and heat capacity of a reaction in anaqueous ionic medium, when using the SIT (Appendix B) to account for activity coefficientand ionic strength effects.[94HOB/PAL]The results obtained in a series of spectrophotometric Pu(IV)/carbonate complexationstudies over wavelengths from 400 to 700 nm were summarized. The pH was varied bycontrolling p CO2 (pH = 7.17 to 9.67; p CO2 = 0.3 to 100%). The authors concluded thatthe results indicated there are three important species over this range of conditions. Theresults were found to be consistent with two independent but simultaneous equilibria,the addition of one CO 2−3and the addition of one OH − and perhaps an additional carbonateto an initial species in neutral solution. The results of a computer analysis listedequilibrium constants for a number of possible stoichiometric models. However, as theinitial species in near-neutral solution remain unidentified, no definitive assignment ofequilibrium constants can be made.[94MAT/CHO]Mathur and Choppin studied phosphate complexes of dioxoneptunium(VI) at1 M NaClO 4 and 25 ◦ C by solvent extraction using either HDNNS (dinonylnaphthalenesulphonicacid) or a mixture of DBM (dibenzoylmethane) and