Calorimetric Studies of NpO⁺ ₂ Hydrolysis

Abstract

The enthalpy change for the reaction NpO^a) + OH" = Np0 2 0H (a) was found by calorimetrie titration to have a value of -22.1 ±0.4 kJ mol" 1 using a log0 1O i value of 4.68 ±0.06. This gave an entropy change of 16± 5 J K" 1 mol" The solutions had 1.0 M [(CH 3 ) 4 NC1] ionic strength at 298 K. These values are used to estimate the hydrolysis constant at elevated temperatures expected in the near field of nuclear waste repositories.

Report on neptunium speciation by NMR and optical spectroscopies

“…This value is significantly greater than the value of 35.8 kJ/mol found calorimetrically (Sullivan et al 1991) Of the 72.5 kJ/mol, 57.9 kT/mol comes from the enthalpy of the dissociation of water (Sullivan et al 1991). Therefore, the standard enthalpy change for the reaction in equation 24 is only 14.6 kJ/mol.…”

“…This rather small value for the neptunium contribution to AH is not surprising, as it represents, at least to a first approximation, the change in bond energy between Np02-OH2 and NpO2-OH. This value indicates an endothermic reaction, while the previous calorimetric titration study reported a small value of -22.1 kJ/mol for this reaction, indicating an exothermic reaction (Sullivan, Choppin et al 1991). There is no a priori reason to reject either number, as hydrolysis can result in both positive (e.g.,…”

“…The hydrolysis constants for U022+, Am3+, Pu022+ and effective charges were tabulated in Sullivan et al (Sullivan et al 1991), the Pu02+ hydrolysis constant is from Bennett et al. (Bennett et al 1992), the hydrolysis constants for U4+, Th4+, and P u b from Clark et al (Clark, Hobart, et al, in press), and the Np02+ hydrolysis value is from this work.…”

“…To better assure the constancy of AH with temperature, the hydrolysis equation should be re-cast in its isocoulombic form (Sullivan, Choppin, et al 1991) as shown in equation 24. Table 5 and Fiawe 18 shows the results of our experiments for the temperature dependence of the Np02+ hydrolysis reaction written in the isocoulombic form.…”

“…In addition, modeling will require defensible constants at higher temperatures to make predictions that account for the near field. Extrapolation of the calorimetric data (Sullivan et al 1991) gives an estimate of an increase of an order of magnitude in the hydrolysis constant at higher temperature, but more direct evidence is warranted. (Varlashkin et al 1984).…”