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

708 A. Discussion of selected references[84VAR/HOB]This work was issued in preliminary form as part of [84VAR/BEG]. A value of theNp(VI)/Np(V) redox potential in 2 M Na 2 CO 3 aqueous solution is reported. No indicationwas given about the calibration of the reference (saturated calomel) electrode andthe junction potential correction. This could explain the observed differences betweentheir potentials and those from the careful previous work of Simakin [77SIM]. Additionof NaOH to a solution of the Np(VI) carbonate limiting complex, NpO 2 (CO 3 ) 4−3 ,wasfound to lead to reduction of Np(VI) (possibly by water or hydroxide ion) to Np(V).The Np(V) absorption spectrum in (probably 2 M) Na 2 CO 3 solution showed complexationas was found previously in the work of Billon [81BIL]. The addition of0.02 to 1.3 M NaOH solutions demonstrated the formation of new complex(es), andNpO 2 (OH) − 2should not be stable enough to explain the spectral variations. The formationof mixed hydroxo-carbonate complex(es) [90RIG, 98VIT/CAP] is probable.The preparation of Np(IV) in carbonate solution, the absorption spectra and theelectrochemical measurements repeated and confirmed previous work [81WES/SUL].We carried out the same calculations discussed in this appendix for [79FED/PER].Cyclic voltammetry of Np(V) in 2 M Na 2 CO 3 solution confirmed the previous[79FED/PER, 81WES/SUL] electrochemical observations. The reduction to Np(IV) isnot reversible and is observed only at −1.7 V(atpH= 12.9), where [79FED/PER]Np(IV) is quantitatively reduced to Np(III). The reduction peak is shifted toward morenegative values (−1.8V)at[OH − ]=0.25 M, and its shape changes with a “pre-peak”at −1.63 V at pH = 10.9. This stabilisation of Np(V) can certainly be explained by theformation of a mixed OH-CO 3 complex of Np(V), for which some spectral evidence isshown in this publication, and which later was confirmed [90RIG, 98VIT/CAP].The precipitation of Np(III) is mentioned by the authors; but as is the case forAm(III), Np(III) is quite soluble in this medium. However, Np(OH) 3 (s) could precipitatedirectly on the electrode, where the electrolysis of water can locally produce a highenough quantity of OH − anion. This type of process is well known for electrodepositionon solid electrodes. This also would explain why the precipitate is re-dissolvedin the bulk solution when the electrolysis is stopped. However, this will not occur (asthe authors suggest it will) in other cases, where Np(OH) 4 (s) can also precipitate (seebelow). The changes in the shape of the electrochemical peaks could be due to someside chemical reaction such asNp(III) + Np(V) −→ 2Np(IV),precipitation of Np(IV), and/or formation of an intermediary complex. The authors haddifficulties in electrolysing the neptunium in their solution to Np(IV) and in keepingit stable in that oxidation state. As explained in the discussion on [79FED/PER] inthis appendix (see also [95VIT]), the instability of the potential can be due to ingressof carbon dioxide gas into the solution. In addition, the water was also electrolysedduring the preparation. This generated some OH − that also reacted with carbondioxide gas. A small increase of [OH − ] would be enough to dissociate the carbonatelimiting complex of Np(IV) (into Np(OH) 4 (aq) or another unidentified Np(IV)complex containing the hydroxide anion). This would result in the precipitation