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

A. Discussion of selected references 761with thermodynamic conventions (and standard states) for the electron “e − ” (as usedby electrochemists in half-cell reactions).[93LEM/BOY]Lemire, Boyer and Campbell studied NaNpO 2 CO 3 (s) and Na 3 NpO 2 (CO 3 ) 2 (s) equilibrationwith HCO − 3 /CO2− 3 ,1MNa+ (NaClO 4 ) solutions at 30, 50, and 75 ◦ C, and 0.5 MNa + aqueous solutions at 75 ◦ C, and KNpO 2 CO 3 (s) equilibration with KCl solutionsat 30 and 75 ◦ C. The glass electrode was calibrated in concentration (− log 10 [H + ])and not activity (pH) units at each temperature. Measurements of − log 10 [H + ] at differenttemperatures were used to calculate [CO 2−3] from the known total carbonatemolarities. Uncertainties mainly came from solid phase transformations and lack ofbuffering effect at low [CO 2−3]. Previous work [74VIS/VOL, 77SIM, 77VOL/VIS,79VOL/VIS2, 81VOL/VIS2, 83MAY, 84VIT] on Np(V) solubility and soluble complexes[85BID/TAN, 85INO/TOC] in carbonate media were well documented and usedto guide and interpret the experiments. It was assumed that the reactionsor the equilibriaMNpO 2 CO 3 (s) + (i − 1)CO 2−3Å NpO 2 (CO 3 ) (1−2i)i+ M +M 3 NpO 2 (CO 3 ) 2 (s) + (i − 2)CO 2−3Å NpO 2 (CO 3 ) (1−2i)i+ 3M +controlled the solubility. As discussed elsewhere in this appendix [90RIG], it is clearthat hydrated NaNpO 2 CO 3 (s) is not very well crystallised at low [CO 2−3 ]andtemperature.Its solubility was not very reproducible despite the 2-4 week equilibration timesused in this work [93LEM/BOY] (or longer — successive equilibration periods withthe same solid samples meant some solid samples were in contact with solutions forover two months). Grenthe, Robouch and Vitorge [86GRE/ROB] had predicted that thehydrated NaNpO 2 CO 3 (s) compound would be transformed into Na 3 NpO 2 (CO 3 ) 2 (s),when [CO 2−3 ] became greater than 1 mM in [Na+ ]=3 M solution (and thus if greaterthan 9 and 18 mM in [Na + ] = 1.0 and 0.5 M solutions, respectively), and this was confirmedby later work [91KIM/KLE, 95NEC/RUN]. The kinetics of this transformationare very slow at 25 ◦ C and were found here to be somewhat more rapid at higher temperatures.It was difficult to deduce solubility products and formation constants fromcurve fitting, because NpO 2 CO − 3 was transformed to NpO 2(CO 3 ) 3−2in the [CO 2−3 ] domainwhere a solid phase transformation occurs, and this leads to strongly correlatedfitting parameters.To estimate equilibrium constants, this review accepted the qualitative interpretationin[93LEM/BOY]but used a classical graphical procedure to reanalyse the data.The results are shown in Table A.28. At30 ◦ C, Lemire, Boyer and Campbell found approximatelythe same solubility as Maya [83MAY] had at 25 ◦ Cfor[CO 2−3 ] < 1mM.Values of K s,1 and β i were estimated graphically for i = 0, 1 and 2, assuming thatsome of the data shown of the Figure 1a ([93LEM/BOY]) corresponded to supersaturatedsolutions. For this reason, the values estimated in the present review were smaller