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

A. Discussion of selected references 623log 10 β(A.38, unreported temperature, I = 0.5M) = 7.18log 10 β(A.38, unreported temperature, I → 0) = 8.18PuO 2+2+ H 2 PO − 4Å PuO 2 H 2 PO + 4(A.39)log 10 β(A.39, unreported temperature, I = 0.5M) = 1.66log 10 β(A.39, unreported temperature, I → 0) = 2.33The experimental study cited here by Moskvin has not been available to this review,and probably was never published. Hence these data have been rejected.The value reported for the formation of a carbonate complex of Pu(VI)(PuO 2 CO 3 HCO − 3) is not included in the determination of best values.[70AL-/WAI]Fluoride and sulphate complexation were investigated at 25 ◦ C and five different ionicstrengths: 0.1, 0.2, 0.3, 0.4 and 0.5 M (HClO 4 ). The authors measured the redoxpotentials of mixtures of Np(V) and Np(VI) as a function of fluoride and sulphateconcentrations.In the fluoride system, no complexation of Np(V) was observed, because the acidityused in these experiments was too high to allow NpO 2 F(aq) to form. The potentialchanges measured during the titration with fluoride solution were interpreted by theauthors in terms of the formation of NpO 2 F + and NpO 2 F 2 (aq). The auxiliary data usedby them are compatible with ours for the protonation of fluoride ion, and the formationof HF − 2 is negligible in the systems investigated. The authors reported log 10 ∗ β 1 valuesbetween 1.11 (I = 0.5 M) and 1.20 (I = 0.1 M),andlog 10 ∗ β 2 values between 1.14(I = 0.5 M) and 1.20 (I = 0.3and0.4M),cf. Table9.5. The meaning of the reporteduncertainties is unknown, but they are no doubt grossly underestimated. We use thereported log 10 ∗ β 1 values in our evaluation, and we assign uncertainties of ±0.20 toeach of them. The 1:2 complex was only present in small amounts in the experimentsof [70AL-/WAI], and the error in log 10 ∗ β 2 is therefore considerable. In none of thetitration experiments was more than 10% of the Np(VI) present as NpO 2 F 2 (aq), evenat the highest fluoride concentrations employed. We assign uncertainties of ±0.40 tothese values.The Np(V)/Np(VI) redox couple was also measured as a function of SO 2−4concentrationat acid strengths from 0.1 to 0.5 M. The authors calculated a first stabilityconstant for the Np(VI)-SO 2−4complex assuming no Np(V) complexes. This assumptionappears reasonable given the relative strengths of the Np(V)- and Np(VI)-sulphatecomplexes. The disulphato complex was only observed in a single experiment. Thismay be because only a small excess of SO 2−4over NpO 2+2was used in these experiments.In most cases, the linear least squares approach and the non-linear least squaresapproach yield essentially indistinguishable results. Revised values using the nonlinearapproach were calculated and used in determining best values. No estimate ofprecision was possible for the single determination of the association quotient for the