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

A. Discussion of selected references 697ported uncertainties by a factor of 2.5 to 3 to take account of the unknown protonationconstant used for fluoride ion and the fact that small proportions of 1:2 complexes ofactinide(VI) may have been present at higher fluoride concentrations.[84KAS/JOS]In this study the potential difference (with respect to the calomel electrode) required toreduce Np(VI) to Np(V) was recorded as a function of HF(aq) or HSO − 4 concentration.The study was carried out at 23 ◦ C (“room temperature”) in 0.5 M HClO 4 (I = 0.5M)(and also in 0.5 M HClO 4 +0.5MNaClO 4 (I = 1 M) for the fluoride complexationmeasurements). The authors assumed that no Np(V) complexes were formed in thecourse of the experiments. It should be mentioned that precipitation of KClO 4 (s) mightoccur at the liquid junction between the saturated calomel electrode and the HClO 4solution, and that such a precipitation will influence the measured potentials. However,this systematic error cancels to some extent since no absolute values were used but onlypotential changes resulting from different ligand concentrations.The HF(aq) concentrations used were 2.5 × 10 −2 M, 5 × 10 −2 M, 0.1 M and0.15 M, and [Np] t = 5 × 10 −2 M. In addition, Np concentrations of 3 × 10 −4 Mand1.2 × 10 −3 M were used at the two lowest HF(aq) concentrations. The procedure issatisfactory. The proportion of the proposed 1:2 complex, NpO 2 F 2 (aq), reached 18%at most, and the values reported for ∗ β 2 are thus less certain than those for ∗ β 1 .Weusethese values in our evaluation in Part III, Section 9.2.1.4, and we assign uncertaintiesof ±0.2inlog 10 ∗ β 1 ,and±0.4inlog 10 ∗ β 2 .Our recalculated association quotient for formation of NpO 2 SO 4 (aq) is based on alinear least squares model. Given the range of reported experimental errors a non-linearapproach does no better. There is no evidence for the disulphato complex. Recalculatederrors have been increased slightly to account for possible errors in the estimation ofthe protonation constant for SO 2−4 .[84KIM/BER]This paper contains a set of solubility data for a solid initially identified asPuO 2 (OH) 2 (s), although the nature of the solid was not determined after the equilibrationperiod of “several months”. The experiments were done using 238 PuO 2 (OH) 2 (s)in 0.1 M NaClO 4 . The isotope 238 Pu has a half life shorter than that of 239 Pu by afactor of approximately 270, and both isotopes decay primarily through the productionof 5 MeV α-particles. Thus, autoradiolysis would be expected to be much greaterfor 238 Pu than for 239 Pu. It would be expected the oxidation state of the solutionspecies, especially for solutions having relatively high 238 Pu concentrations, would nolonger have been primarily Pu(VI) after the long equilibration period. The authors’Figure 4 refutes this. Apparently trace quantities of chloride (and hence hypochlorite[93PAS/KIM]) were present as an impurity in the solutions, and this helped maintainthe plutonium as Pu(VI), at all values of pH, despite the short half-life 238 Pu isotopehaving been used [94KIM]. Rai and Ryan [82RAI/RYA] also reported Pu(VI) as