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

688 A. Discussion of selected references[81VOL/VIS4]This is a general discussion of the work about actinide(V) carbonate solidcompounds in the nine previous publications of this series [74SIM/VOL,74VIS/VOL, 74VOL/KAP, 74VOL/KAP2, 77VOL/VIS, 79VOL/TOM, 79VOL/VIS2,81VOL/VIS2, 81VOL/VIS3]. NaNpO 2 CO 3 (s) is no longer proposed as isostructuralto rutherfordine [79VOL/VIS2]; but there is no additional experimental evidencefor the new interpretation. The general explanation of the structure changes seemsreasonable; but some of the results were not confirmed. Some of the proposednon-stoichiometric solid compounds may have been kinetic artefacts. Only onepossible non-stoichiometric compound was proposed [79VOL/VIS2], but thisstudy was not completely convincing ([84VIT, 98VIT/CAP] and discussion of[79VOL/VIS2, 84VIT] in this appendix). Later Np(V) solubility studies in sodiumcarbonate/bicarbonate media showed no evidence of non-stoichiometric compounds.[81WES/SUL]Np(V) was electrolysed in 1 M carbonate and in 1 M bicarbonate solutions by controlledpotential electrolysis at −1.8 V to produce Np(IV). Water was also partiallyreduced. Only the use of a platinum electrode was reported, but almost certainly amercury electrode was used. The same methodology was used later [84VAR/HOB]with some difficulties. One Np(IV) absorption spectrum in 1 M Na 2 CO 3 or NaHCO 3solution is reported. The spectrum does not vary from pH 8.3 to 11.2 and is thesame as has been reported elsewhere [77SAI/UEN, 84VAR/HOB, 93LI/KAT, 95VIT,96DEL/VIT] for Np(IV) in concentrated carbonate media. This is evidence, that a limitingcomplex is formed under these conditions. The measurements by Fedosseev et al.[79FED/PER] were carried out under similar conditions. No thermodynamic data andno stoichiometry can be deduced from this work.The redox potential of the Np(VI)/Np(V) couple was measured in 1 M Na 2 CO 3 and1 M NaHCO 3 solutions. The authors stated that their measurement, 0.221 V/S.C.E.,was in very good agreement with previous work [74SIM/VOL, 77SIM]. Only the valuereported for a 1 M Na 2 CO 3 medium, is consistent with the value proposed in the presentreview (also see [98VIT/CAP]). The Na + concentration and ionic strength were differentin the different buffer media, and therefore a difference would be expected betweenpotentials measured in the different media. The authors did not report nor discuss suchdifferences, and also may not have taken the junction potentials into account. For thesereasons, the value from this paper was not used in the present review.[82ALC/ROB]This paper does not contain any thermodynamic data for review. It reports excellentstructural information concerning the solid phase 2NpO 2 SO 4·H 2 SO 4·4H 2 O(s).