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

A. Discussion of selected references 615for 14 days. All three compounds were reported to be stable with respect to changesin the oxidation state of plutonium over a period of several months.[67ERM/STA]Although this paper does not give any hydrolysis constants for Np(III) or Pu(III), theextraction constants K ex with HTTA in benzene are determined for Am(III), Cm(III)and Fm(III). That system has been used by several authors for obtaining actinide hydrolysisconstants. This paper therefore serves as a useful reference, as the experimentsand results seem reliable.[67GEL/MOS]The data in this paper may have predated the data in [62GEL/MOS]. The experimentis identical but the results are reported in terms of a single complex, PuO 2 (CO 3 ) 2−2with a stability of log 10 β 2 = 15. No hydroxy-complexes appear in this interpretation.No description is given of the solid phases used in these solubility experiments andtherefore no reliance can be placed on these results.[67MOS/ESS]The authors measured the solubility of Th(HPO 4 ) 2·4H 2 O(s) at 0.35 M HClO 4 whilevarying the concentration of H 3 PO 4 (aq) between 10 −4 and 1.5 M. The solubility datafor the thorium system were explained in terms of the formation of aqueous complexesof the general composition Th(HPO 4 ) n4−n with n = 1 to 3. Based on these data, thedata on the analogous Pu(IV) system [60DEN/MOS2, 60DEN/MOS] and the resultsof the analysis of the solubility data on the U(IV) system [64MAR/VER], the authorsproposed interpolated values for the stability constants of four aqueous complexes ofthe general composition Np(HPO 4 ) 4−nn with n = 1to4Np 4+ + nHPO 2−4Å Np(HPO 4 ) 4−2nn(A.28)log 10 β n (A.28, n = 1, unreported temperature, I → 0) = 12.4log 10 β n (A.28, n = 2, unreported temperature, I → 0) = 23.1log 10 β n (A.28, n = 3, unreported temperature, I → 0) = 32.0log 10 β n (A.28, n = 4, unreported temperature, I → 0) = 41.0and a value for the solubility product of the corresponding solid.Since the chemical model used by the authors in the corresponding Pu(IV) system(see comments for [60DEN/MOS2]and[60DEN/MOS] in this appendix) has not beenaccepted, the Np(IV) constants estimated in this work are also rejected in the presentreview.