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

A. Discussion of selected references 651Table A.5:[74SOL].Thermodynamic constants of M(IV) hydrolysis at ≈ 25 ◦ CasgivenbyM(IV) β ◦ 1 10−14 β ◦ 2 10−28 β ◦ 3 10−42 β ◦ 4 10−56 ReferencesZr 3.8 23.9 52.4 70 [67SOL/TSV]Pu 0.17 0.6 3.2 [74SOL]U 0.06 [74SOL]ratios (however, the author’s Table 6 does show good agreement between experimentaland calculated distribution coefficients for the U(VI) system). No information waspresented with regard to how the unstable oxidation states, U(IV) and Pu(IV), weremaintained in solution. The value of the activity product of water (K w ), as used in theauthor’s calculations, was reported as 1.27·10 −14 at all concentrations (pK w = 13.90).That value was used to analyze the experimental results (I > 0) to obtain values of β ◦ ,and that value is used in the present review to convert the β ◦ values to the values of ⋆ β ◦presented in Table 17.3. However, the approximate average value of pK w for 20 ◦ Cto25 ◦ CatI = 0 is actually 14.1.The experiments were carried out to permit semi-empirical modelling of distributionsfor separations processes, and the author made no claim the measurements weredesigned to accurately determine the hydrolysis constants. Insufficient information wasprovided to permit recalculation of the data. For this reason, and because the experimentalconditions are ill-defined, the results of this study are not used in the presentreview in selection of hydrolysis or complexation constants.[74VIS/VOL]KNpO 2 CO 3 (s) was precipitated from concentrated 0.1-0.2 M K 2 CO 3 solution. The ratioNp(V)/CO 2−3in the solid was determined by measuring the CO 2 (g) released on titrationwith acid, and coulometric determination of the Np content. A pH-metric Np(V) titrationusing 0.2 M K 2 CO 3 solution confirmed the stoichiometry of the KNpO 2 CO 3 (s)solid. X-Ray powder diffraction patterns indicated two K 3 NpO 2 CO 3 (s) phases forwhich the stoichiometry was also determined by the same techniques. One precipitatedfrom 0.8 - 1.5 M K 2 CO 3 solutions and the other from 1.8-5.6 M K 2 CO 3 solutions(see [54NIG/PEN]). It is quite surprising that two different solid phases with the samestoichiometry would form at equilibrium in these aqueous solutions as the variation ofthe water activity in these solutions was relatively small. The precipitation of the twodifferent solid phases could be due to kinetics. Despite the comments of the authors,the X-ray diffraction patterns given in this work do not appear to support the formationof additional solid phases (either with the same stoichiometry or K 5 NpO 2 (CO 3 ) 3 (s)).Potentiometric titration of a 0.06 M Np(V) aqueous solution with a 0.2 M K 2 CO 3aqueous solution showed that the precipitation started at pH = 4.6, and that the pH wasbuffered to pH = 5.6 (before the equivalence point), and the mid point for the reactionwas at pH = 5.2. Back titration of the solid compounds, with 0.1 M HCl, showedbuffering of the pH at about the same values. The mid-point of the reaction was at pH =