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

728 A. Discussion of selected referencessolutions was estimated to be less than 0.01 mM but there is not much informationin the experimental procedure discussing how this was achieved or how the calculationsto do this estimation were carried out (the redox potential measurements indicatethat solutions were equilibrated with the air, and this is not consistent with low CO 2concentrations in the solutions). Experimental solubility values, when plotted againstpH (7 to 12.4), are scattered and there is practically no significant pH dependency at8 < pH < 11.5. Most of the results are not in agreement with previous solubility measurements[76SEV/KHA, 78MUS, 85LIE/TRE]. The erratic results are likely the resultof carbonate complexation, and gradual ripening or dissolution of colloidal material.[88ULL/SCH]The sections of this publication dealing with the uranium system were discussed ina previous volume of the NEA review [92GRE/FUG]. The paper reports results ofcalorimetric experiments for the heat of reaction of Np(VI) (0.0289 to 0.0447 M) andPu(VI) (0.010 to 0.016 M), perchlorate salts in aqueous Na 2 SO 4 solutions (0.15 M,for the Np(VI) solutions, 0.07 M, for the Pu(VI) solutions) with aliquots of Na 2 CO 3(0.1 to 1 M). The actinides were initially present in solution as AnO 2+2 ,AnO 2SO 4 (aq)and AnO 2 (SO 4 ) 2−2. By the end of each titration, the limiting carbonato complex,AnO 2 (CO 3 ) 4−3, had formed. The distributions of the SO2−4complexes were calculatedfrom equilibrium constants (and enthalpies) previously determined using the same apparatus,[86ULL/SCH]. The calculation procedure for the evaluation of all equilibriumconstants and the enthalpy of the non-limiting complex is different from that of thisreview. The enthalpy of the limiting complexes is essentially model independent. Itwas not possible to re-evaluate this data set with the appropriate assignment of activitycoefficients. The errors assigned to the equilibrium constants and the enthalpiesof the intermediate carbonato complexes must be expanded to reflect the non-standardprocedure.The authors [88ULL/SCH] concluded that only the limiting carbonate complex,NpO 2 (CO 3 ) 4−3, was formed when there was an excess of carbonate. The mean ofthe experimental heat of reaction values for the neptunium system, for experimentsin which the final CO 2−3 :Np ratio is > 3, is H =−(64.5 ± 2.3) kJ·mol−1 . Thismolar enthalpy of reaction value is more negative than the value previously reportedby Schreiner et al. [85SCH/FRI], probably because the initial sulphate concentrationswere not the same. Correction of the value for sulphate complexation as done in the originalpaper [88ULL/SCH] by addition of 22.5 kJ·mol −1 (with an uncertainty assumedequal to that found for the enthalpy of solution reported in this paper (±2.3 kJ·mol −1 ))results in the value r H m = −(42.0 ± 3.2) kJ·mol −1forNpO 2+2+ 3CO 2−3Å NpO 2 (CO 3 ) 4−3This values applies in an ill-defined aqueous medium that was different for differentstages of the experiment, ranging in ionic strength from 0.3 to 1.1 M.