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

A. Discussion of selected references 743Table A.20: Curve fitting results of solubility measurements of Np(V)in 3 M NaClO 4 carbonate/bicarbonate/carbonic acid aqueous solutions[84VIT, 85CÔM, 85KIM, 86GRE/ROB, 90RIG] (a) . K s,j =[Na + ] (2j−1) [NpO + 2 ][CO2− 3 ]j is the Na (2j−1) NpO 2 (CO 3 ) j (s) solubility product.β i =[NpO 2 (CO 3 ) (1−2i)i]/([NpO + 2 ][CO2−3 ]i ).log 10 K s,1 log 10 K s,2 log 10 β 1 log 10 β 2 log 10 β 3 1.96 Reference−10.92 4.60 8.12 10.44 [84VIT]4.12 8.60 10.76 [84VIT] (b)−10.50 −16.40 4.60 8.12 10.44 [85CÔM]−10.56 −12.44 5.09 8.15 10.46 0.269 [86GRE/ROB] (c)±0.34 ±0.57 ±0.43 ±0.38−10.63 5.33 8.13 10.56 0.114 best fit±0.38 ±0.51 ±0.84 ±0.41−10.65 5.25 8.15 10.64 0.250 this review±0.33 ±0.29 ±0.46 ±0.37(a) All values were calculated from the same set of experimental measurements, except thosefrom Vitorge [84VIT] that used preliminary results. Minimum solubility values were used todetermine K s,1 and not the best fit obtained by minimising 2 (of K s,1 and K s,1 β i ), the leastsquare sum. Uncertainties (calculated in the present review) are 1.96 standard deviations basedon a weighted mean of the corresponding equilibrium constant.(b) Assuming the formation of Na 0.72 NpO 2 (CO 3 ) 0.86 (s) solid phase.(c) Uncertainties calculated in the present review; the values were also reported elsewhere[85KIM, 90RIG].NaNpO 2 CO 3 (s). This is consistent with the X-ray diffraction results and with theseries of observations by Volkov et al. (see the discussion of [84VIT] in this appendix).Thermodynamic (solubility product) data for these non-stoichiometric solids are notaccepted in this review. If a stable Na (2x−1) NpO 2 (CO 3 ) x (s) solid phase with x < 1isformed, it would be transformed into a hydrated NaNpO 2 CO 3 (s) phase when the freecarbonate concentration is increased. One would then expect a correlation betweenthe error ([Np] measured −[Np] calculated )and[CO 2−3] which was not detected in dataanalysed in the present review. This confirmed that the scatter of the data was due tokinetic problems (slow crystallisation and possible variation of the number of watermolecules in the hydrated solid [93LEM/BOY]), and the influence of the ion exchangeequilibrium in the solid phase, if any, was less important.At higher carbonate concentrations for which batch experiments were used, fourweeks were needed to achieve equilibrium (two weeks were insufficient). This indicatesthat probably all the other previously published values based on the solubility ofhydrated NaNpO 2 CO 3 (s) at room temperature are based on experiments that had notreached equilibrium; this kinetic problem has not usually been mentioned. In 0.1 Mcarbonate (I = 3MNaClO 4 ) solution, the expected solubility was greater than wasmeasured. This was evidence of the formation of a new solid phase. The authorsidentified a hydrated Na 3 NpO 2 (CO 3 ) 2 (s) solid phase by its X-ray diffraction pattern.