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

A. Discussion of selected references 605particularly true of the less acidic solutions. Thus, the values of log 10 K s,0 would bemore negative than those reported by Perez-Bustamante. This probably accounts forthe drift in the reported log 10 K s,0 values, with those from the lowest pH measurementsbeing affected the least. Colloid formation may also have led to an overestimate of theplutonium in the filtered solutions. The short equilibration period (Figure 3 in [65PER])probably introduced an error of −1 to−2 in the opposite direction. The heading inTable 1 should read “mg Pu·l” not “µ gPu·l”.Solubility data for Pu(VI) in solutions having high hydroxide concentrations indicatesamphoteric behaviour for hydrated PuO 2 (OH) 2 (s). The oxidation state of the solutionspecies at high pH was not identified, nor were ionic strength effects considered inestimating the formation constant of the “(PuO 2 ) 2 (OH) − 5 ”.[66AHR/BRA]The complexation of Np(IV) by sulphate was determined from a cation exchange distributionstudy in 4 M perchloric acid. The authors made corrections for the possibleformation of Np(V) in solution and the resulting interference in their adsorption study.The authors present reasonable, but not compelling, arguments for the corrections necessaryto account for this source of interference. The authors linearised their datafor calculations, but gave sufficient data to enable recalculation of the association quotientsand errors. In the recalculation, the data at very high SO 2−4/Np ratios were notconsidered, as the precision of these distribution measurements is very low.In the case of fluoride complexing of Np(IV), emf measurements were performedusing a Fe 3+ /Fe 2+ electrode in addition to the cation exchange measurements. Here,the emf method relies on the competition between Fe 3+ and Np 4+ for the fluoride ion– complexation of fluoride by Fe 3+ changes the cell potential and allows the stabilityof the Np(IV) fluoride complexes to be determined. The measurements were doneat a temperature of 20 ◦ C, and the ionic strength was maintained at I = 4 M withHClO 4 . The HF(aq) concentration was varied between 10 −5 and 1.4 × 10 −2 Minthedistribution measurements, and from 10 −3 to 2 × 10 −2 M in the emf measurements.From the distribution measurements the authors reported constants for the followingreactions:Np 4+ + qHF(aq) Å NpF 4−qq + qH + (A.22)log 10 ∗ β 1 (A.22, q = 1) = (4.82 ± 0.02) and log 10 ∗ β 2 (A.22, q = 2) = (7.57 ± 0.08),while from the emf measurements they reported stepwise constants according to thereactionsNpF 5−qq−1+ HF(aq) Å NpF4−q q + H + (A.23)for the formation of NpF 2+2 ,NpF+ 3 and NpF 4(aq): log 10 ∗ K 2 (A.23, q = 2) = (2.69 ±0.10), log 10 ∗ K 3 (A.23, q = 3) = (2.34 ± 0.15), andlog 10 ∗ K 4 (A.23, q = 4) ≈ 1.3.The authors were unable to determine a value for log 10 ∗ K 1 (A.23, q = 1) with the emfmethod, because they did not observe any significant potential difference at an average