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

A. Discussion of selected references 785equilibrium constant for the stepwise addition of CO 2−3to the penultimate complexis given as log 10 K 5 (3M) = (2.25 ± 0.29) and log 10 K5◦ = −(1.36 ± 0.09) withε =−(0.11 ± 0.08) kg·mol −1 .Thelog 10 β 5 (3M) was found to be (35.8 ± 1.3),butinsufficient independent evidence is available to extrapolate this value to the standardstate. The constants in this paper supersede constants reported in [92CAP].[96CLA/CON]This is mostly a review paper that was based primarily on experimental observationsof the authors and a literature comparison that they published previously. Nevertheless,Clark et al. reported new results from their experimental study of the temperatureinfluence on the NpO 2 CO − 3 and NpO 2(CO 3 ) 3−2formation constants. Values of β 1 andβ 2 were measured in 0.1 M tetrabutylammonium nitrate aqueous solution by spectrophotometry,possibly using the same methods described by Neck et al.[94NEC/RUN].(However, the spectrophotometric results of that publication were not particularly accurateand were not used in the final data assessment in the present review. See thediscussions of [91KIM/KLE] and[94NEC/RUN] in this appendix). Clark et al. foundthat log 10 β 1 and log 10 β 2 increased by about 0.4 units from 30 to 70 ◦ C, while they estimatedan uncertainty of ±0.2inthelog 10 β 1 value. The influence of temperature wastherefore not much greater than the experimental accuracy. Values of log 10 β 1 ,log 10 β 2and log 10 K 2 (K 2 = β 2 /β 1 ) were plotted as a function of 1/T and straight lines werefound with slopes 829±27, 811 and 18 respectively, from which H 1 =−(15.9±0.5),H 2 =−(15.5 ± 0.5) and thus H 2 − H 1 =−(0.3 ± 0.5) kJ·mol −1 respectively.These results were not accepted in the present review since the information needed tocheck them (e.g., method of pH electrode calibration at temperature, values used forcarbonate protonation constants at temperatures other than 25 ◦ C, spectral changes withtemperature for the key species) was not reported in this publication [96CLA/CON].An as yet unpublished later paper, cited in this reference [96CLA/CON] as “submitted”,may contain details that will allow later reviewers to make good use of theseresults.[96CLA/CON2]This is an interesting paper that proposes values for bond lengths in NpO 2 (CO 3 ) (1−2i)iaqueous complexes from EXAFS studies, assuming reasonable geometry of these species.Tetrabutylammonium cations instead of the more usual alkali metal cations, wereused in the supporting electrolyte when fixing the aqueous speciation (a classical wayto avoid precipitation of the Np(V) solids containing alkali metal cations). No informationfrom this publication was used in assessments in the present review.[96DEL/VIT]a) Preparation of Np(IV) in carbonate mediaNp(IV) was prepared in 0.3, 0.6, 1.0 and 1.5 M Na 2 CO 3 aqueous solutions, by reductionof Np(V) at −1.6 V/SHE. During this process water was reduced, and probably