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

690 A. Discussion of selected references[82SUL/WOO]Spectrophotometric and calorimetric data were used to calculate the stability constantsfor complexes with PuO 2+2 :CO2− 3ratios of 1:1 and 1:2. On the basis of the observationthat initial and final pH values of the solutions were the same and the initial form ofPu(VI) was PuO 2 (OH) 2 (aq) at pH = 8.3, the 1:1 complex was assumed to be of theform PuO 2 OHCO − 3 . This is not a necessary conclusion. Sufficient HCO− 3was addedto convert all of the PuO 2 (OH) 2 (aq) to PuO 2 (CO 3 ) 2−2with no impact on the final pH.By analogy to the stoichiometry for the corresponding carbonate complexes ofUO 2+2and NpO 2+2 , the reported equilibrium constant (β 1(I = 0.1) = 463 ± 60,(log 10 β 1 = 2.67 ± 0.60)) was determined spectrophotometrically and is assumed torefer to the following reactionPuO 2 (OH) 2 (aq) + HCO − 3Å PuO 2 CO 3 (aq) + H 2 O(l) + OH − (A.58)As there is no change in the net charge for this reaction, the extrapolation procedureyields a standard state formation constant identical to that reported for 0.1 M.At 0.1 M ionic strength and pH = 8.3 the equilibrium constant for the reactionPuO 2 (OH) 2 (aq) + 2HCO − 3Å PuO 2 (CO 3 ) 2−2+ 2H 2 O(l) (A.59)is reported as log 10 β 2 = 4.36 ± 1.00.The enthalpy of the reaction is taken: (1) from the temperature dependenceof the stability constant measured spectrophotometrically, −(6.3 ± 2.4) kJ·mol −1(data re-evaluated by the reviewers) and (2) from the calorimetric results,−(1.46 ± 1.0) kJ·mol −1 .The mean of these values with the uncertainties expanded to include the range fromboth values, −(3.9 ± 4.8) kJ·mol −1 ,isusedbythisreview.[82WES/MUL]The paper describes the preparation, X-ray powder pattern and visible and near-IRspectra of Np(OH) 2 SO 4 . Although the compound is well characterised structurally,there are no thermodynamic data useful for this review.[83HAL/OLI]This describes a thorough extraction study of sulphate complexation of Np(V) usingHDNNS in n-heptane. Experiments were performed at I = 0.5, 1.0 and 2.0 M and t =5, 25 and 45 ◦ C. Insufficient data have been provided for the results to be recalculated,and the reported values are used here. It is unclear whether the reported uncertaintiesreflect 1σ or 2σ . We assume 1σ . Only monosulphate complexes were observed. All[SO 4 ] T was assumed to be [SO 2−4]. This is a reasonable assumption over the pH rangeof the experiment (4 to 7). The data at 25 ◦ C can be used in the assessment of the bestvalues, and enthalpies of reaction can be calculated using the remaining results.