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

702 A. Discussion of selected referencestotal Np(VI) concentration was used for all the measurements, and too narrow adomain of CO 2 (g) partial pressure was used).• Calculations using different models did not result in clear differences in the overallleast-square sum, ∑ . Therefore, it is not clear (considering only the statisticalresults) whether other species exist or not.Second, as observed by the author, the overall formation constants deduced fromthese potentiometric titrations do not agree with those deduced from the equilibriumpotential measurement data. The values for only one of the pairs of stepwise constants,β i /β 3 , are consistent.Third, there are several problems in the selection of species or chemical problemsthat could cause small systematic errors.• According to Maya, the final product of the titrations was NpO 2+2. The assumptionwas then made that NpO 2 (CO 3 ) 2−2(aq) is a major intermediate product,but, surprisingly, not NpO 2 CO 3 (aq) which is the intermediate species betweenNpO 2 (CO 3 ) 2−2and NpO 2+2. Reanalyses of the equilibrium potential measurementdata, and consideration of behaviour in the analogous uranium system indicatethat NpO 2 CO 3 (aq) is probably stable over a broader range of conditionsthan NpO 2 (CO 3 ) 2−2 .• Maya concluded (NpO 2 ) 3 (CO 3 ) 6−6was not important because slightly poorerfits to his data were obtained using models that included this species. However,based on later work, the stability of the species (NpO 2 ) 3 (CO 3 ) 6−6is now well established[86GRE/RIG, 90RIG]. The absorption spectrum of (NpO 2 ) 3 (CO 3 ) 6−6has been clearly detected [86GRE/RIG, 90RIG] in solutions similar to those usedfor Maya’s measurements [84MAY].• It has also been observed [86GRE/RIG, 90RIG] that, in contrast to the behaviourof the analogous solid in the U(VI) and Pu(VI) systems, precipitation ofNpO 2 CO 3 (s) is quite slow. Recent solubility measurements [98VIT/CAP], suggestthat some of Maya’s solutions were probably supersaturated with respect toNp(VI). This might have introduced systematic deviations in the titration data;and there are no indications that the solutions were checked for possible precipitationor colloid formation [90RIG].Any attempted reanalysis of the data is open to question because not all the datawere provided in the original publication and because there are some inconsistenciesand obvious typographical errors in the author’s Table 1 (a list of the values used inthe present review are presented in [98VIT/CAP]). Although the author used differentcarbon dioxide gas partial pressures, and varied the ratio of hydroxide to carbonate ligand,further experiments with higher carbon dioxide gas partial pressures and differenttotal Np(VI) concentrations would be required to generate data to unequivocally determinethe stoichiometries of the important species, and to obtain accurate values fortheir formation constants. This is particularly the case for any polymeric mixed-ligandspecies.