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

A. Discussion of selected references 627extraction mechanism is complicated ([78PAT/RAM] state that perchlorate is extracted),and no data on distribution coefficients are reported to enable a reviewer to carryout a recalculation. The values of the constants together with semilogarithmic fractiondiagrams calculated with these constants are the only data reported in this work. Becauseof the high and variable ionic strength, the complicated extraction mechanismand the lack of experimental details, the constants reported in this work are rejected inthe present review.[70MOS]This paper describes a spectrophotometric study at I = 0.3 M HNO 3 . Temperatureand experimental details are not provided. At the low acidities of this experimentwe anticipate problems with the formation of hydrolysis products and air oxidation ofPu(IV). The results of these experiments are not included in the recommended valuedeterminations.[70PAU]In this report, values of −1.90 and −1.91 were given for log 10 “β Hydr. ” based on spectrophotometricexperiments using two different cuvettes. The values for this mixedconstant rest on at least two assumptions, (a) that the molar absorptivity for the hydrolysedspecies at 960.4 nm is very small compared to the molar absorptivity for Np 4+ ,(b) that no hydrolysis beyond the first hydrolysis step occurs for pH values below 2.Assumption (b) is inconsistent with the results of Sullivan and Hindman[59SUL/HIN] who found evidence for polymerization at acid concentrations less than0.02 M. Furthermore, examination of the raw data in Table 6 of [70PAU] indicatesthe absorption does not change smoothly with pH, for pH values above 1.65. Thus,the absorbances at higher pH are probably influenced by the formation of hydrolysisspecies other than NpOH 3+ , and these measurements should not be used in the dataanalysis. This leaves, in essence, duplicate measurements at only five concentrations,from which at least two constants ( ∗ β 1 and the molar absorptivity of NpOH 3+ )mustbedetermined. In this review the data were fitted assuming uncertainties of 0.005 in theabsorbances in the 1.0 cm cell and 0.10 in the absorbances in the 0.5 cm cells. Variousschemes were tried, and it was found that a significantly better fit was obtained ifboth the molar absorptivity of Np 4+ and NpOH 3+ at 960.4 nm were allowed to vary.Similar fitting parameters were obtained whether the data from the two cuvettes wereanalysed separately or together, nor was any statistically significant difference foundin the ratio of the cuvette path lengths from that calculated using their nominal values.The fitted value of the molar absorptivity of Np 4+ was approximately 139 cm −1 ,verysimilar to the maximum value reported in Table 3 of [70PAU] forNp 4+ in 1.0 Maqueous HClO 4 . The value of the molar absorptivity of NpOH 3+ was calculated tobe 67 cm −1 (inconsistent with assumption (b), above), and thus the ratio of the molarabsorptivities is 0.48. Sullivan and Hindman [59SUL/HIN] reported the ratio of themolar absorptivity of NpOH 3+ to that of Np 4+ to be 0.14. The fitted value of ∗ β 1 is0.0605 (molal units).