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

780 A. Discussion of selected references[95NEC/FAN]The investigation of the interaction between NpO + 2 and Cl− by absorption spectroscopyand solvent extraction, and the derivation of Pitzer coefficients for NpO + 2in chlorideand perchlorate media were reported in this paper. Much of the same material wasprovided in an earlier report [94NEC/KIM]. Also see the discussions of [94NEC/KIM,96RUN/NEU] in this appendix.[95NEC/RUN]In this publication Neck et al. reported some additional solubility measurements (in5 M NaCl-carbonate aqueous solutions). The solubility measurements were carriedout in a similar fashion to those that had been described in previous reports fromthe same group of authors, and that are examined elsewhere (see the discussions of[91KIM/KLE, 94NEC/KIM] in this appendix). The presentation of the new experimentalresults also included figures showing the transformation of the initial hydratedphase, NaNpO 2 CO 3 (s) into a Na 3 NpO 2 (CO 3 ) 2 (s) phase. Np(V) concentrations weremeasured during this solid phase transformation. The shape and the scatter about theexperimental curves in 5 M NaCl media might suggest solid phase transformation possiblybetween different hydrated forms of Na 3 NpO 2 (CO 3 ) 2 (s) as previously suggestedfor 3 M NaClO 4 media [93LEM/BOY], or to a new solid phase with intermediate stoichiometry(see the discussions of [74VIS/VOL, 90RIG] in this appendix). However,the authors also suggest that trace activities of anions do not depend simply on thenature of the primary cation in the solution. Anion-anion interactions, not consideredin the current version of the SIT, may be important for carbonate and for the Np(V)carbonate complexes. This is correct, but there seems to be direct experimental evidenceof this effect only for the first Np(V) carbonate complex (and surprisingly not forthe other two carbonate complexes). Another possible (but no more probable) interpretationcould involve formation of a mixed carbonatochloro Np(V) complex such asNpO 2 ClCO 2−3 .Pitzer coefficients for the various species were proposed to extend modelling of theneptunium complexation and hydrolysis reactions to 5 M NaClO 4 and NaCl aqueousmedia.[95OFF/CAP]This was the publication of Offerlé’s pre-thesis experimental work as an open literaturereport. The original experimental results were tabulated and discussed. Using cyclicvoltammetry, the redox potential of the Np(VI)/Np(V) couple versus an Ag/AgClreference electrode was measured. Typically the initial Np(V) concentrations were0.6mMin0.22to2MNa 2 CO 3 aqueous solutions at 5 to 60 ◦ C. For each Na 2 CO 3concentration, a new working solution and a new reference electrode were prepared.Measurements were usually made at several temperatures in a single day. Therefore,in most cases, there are several measurements for each set of experimental conditions([Na 2 CO 3 ], t) corresponding to measurements following increase and decrease of temperature.The reference electrode was checked twice daily. A remark in the original