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

A. Discussion of selected references 663[76PAT/RAM]This is a solvent extraction study of fluoride and sulphate complexation at 25 ◦ Cusingdinonylnaphthalenesulphonic acid in benzene as the extractant. The aqueous phasecontained 2 M HClO 4 and0.01MK 2 Cr 2 O 7 as a holding oxidant for Np(VI) andPu(VI). The procedure is satisfactory, but the authors did not mention if they hadchecked the concentration of the holding oxidant or the oxidation state of the actinidesat the end of the experiments to ensure that the actinides had not been reduced.The measurements with U(VI) were described in the uranium review of this series[92GRE/FUG].For the fluoride experiments, the authors varied the HF(aq) concentration between0.05 and 0.5 M. They evaluated the data in terms of the formation of a 1:1 complexonly. However, according to our estimates, the concentration of 1:2 complex was probablynot negligible in some solutions. We use the resulting value in our evaluation, butwith a low weight by using an uncertainty of ±0.5 inlog ∗ 10 β 1 (NpO 2 F + ). In the caseof Pu(VI), the constants obtained with the same evaluation procedure are large at lowHF(aq) concentrations and a factor of about five smaller at higher HF(aq) concentrations.The authors suspected partial reduction to Pu(IV) in spite of the holding oxidantused. The resulting constant for the Pu(VI) fluoride complex has to be discarded.The formation of sulphate complexes of Np(VI) and Pu(VI) was also studied in2MH(ClO 4 ,HSO 4 ). Cr 2 O 2−7was used as holding oxidant. The plutonium isotope(not specified) was most likely 239 Pu. The solution composition varied markedly, andactivity coefficients may have varied correspondingly. No correction was made forthat effect. There is no indication of the time required to reach equilibrium or whetherequilibrium was obtained. For Pu(VI), the results are consistent with the formationof a 1:1 complex but, because of large experimental errors, there is no evidence forthe formation of the weak 1:2 complex. A re-evaluation of the data, using a quadraticstatistical model, yields an estimate of log 10 β 1 (PuO 2 SO 4 ) = (1.08 ± 0.10),and β 2 (PuO 2 (SO 4 ) 2−2) is indistinguishable from zero. For the Np(VI) system, reanalysisleads to log 10 β 1 (NpO 2 SO 4 ) = (1.00 ± 0.05) and log 10 β 2 (NpO 2 (SO 4 ) −22 ) is= (0.87 ± 0.19).[76SEV/KHA]In this work values are reported for the first hydrolysis constant of NpO + 2, as determinedby potentiometric titration and by spectrophotometry. Despite the evidence fromthe electrophoresis experiments reported in the same paper, the formation of morehighly hydrolysed aqueous neptunium(V) species was not considered.The analysis of the potentiometric data, obtained at low ionic strength (0.02 M),relies on the rather weak assumption that, past the pH at which hydrated neptuniumhydroxide begins to precipitate (pH = 7.46 for titration of 1.068 ×10 −2 MNpO 2 NO 3at 23 ◦ C), further addition of base will not change the equilibrium concentration ofNpO 2 OH(aq) in solution. There is no reason this should be true, nor is the (assumed)rather high stability of NpO 2 OH(aq) compatible with other solubility studies[71MOS3, 85LIE/TRE] or with the authors’ own reported “limiting” solubility of