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

12.1 Neptunium carbon compounds and complexes 20312.1.2 Neptunium carbonates12.1.2.1 The aqueous neptunium carbonatesStudies of the carbonate complexes are not straightforward since the carbonate ion,CO 2−3, is a basic anion. Therefore hydrolysis, including formation of mixed CO2−3 –OH − complexes, may occur together with carbonate complexation. In acidic media,the carbonate ion is converted to carbon dioxide gas, CO 2 (g). Therefore, when alkalimetal or ammonium carbonate aqueous solutions (M 2 CO 3 ) are in contact with the air,log 10 p CO2 can increase from −7 or−6 to−3.5(i.e., the pH of the solution is not buffered).This can introduce a shift of more than six orders of magnitude in the calculatedformation constant for the Np(IV) carbonate complex as deduced from redox measurementsof the Np(V)/Np(IV) couple. One way to avoid this problem is to work in closedcells containing carbonate-bicarbonate buffers. To control (and study) the effects oflower concentrations of carbonate ions, solutions buffered by bicarbonate and carbondioxide are also extensively used. Carbon dioxide gas is continually bubbled throughthe open system (this type of buffering cannot be done in a closed system), and themajor anion of the carbonate system is then the bicarbonate anion, HCO − 3. The carbonateaqueous system is a two-component system, and it can be formally describedusing only two species (if OH − and CO 2 (g) are used to describe the system, HCO − 3and CO 2−3are often written as OHCO − 2 and (OH) 2CO 2−2, respectively). Consequently,when experiments are carried out in equilibrium with air, the effects of CO 2−3are equivalentto those of 2OH − , and no unique interpretation can be proposed from suchexperimental work (typically such methodology cannot distinguish between reactionof NpO + 2 to form NpO 2CO − 3 or to form NpO 2(OH) − 2). This problem was discussedin detail in the earlier review of the uranium-carbonate system [92GRE/FUG] (pages308 and 312). The same observations apply when the neptunium-carbonate aqueoussystem is studied, and the same methodology is used in the present review for treatingmeasurements reported in the literature.Some published measurements were performed in concentrated aqueous M 2 CO 3 orMHCO 3 media (M = Na, K or NH 4 ) without addition of an inert salt. The ionic strengthwas then dependent on the concentration of the complexing species (typically CO 2−3 ).The junction potentials also varied during these pH or potential measurements. It isdifficult to develop a quantitative interpretation from such work, or to deduce accuratethermodynamic values from it. Nevertheless, these measurements are considered in thepresent review, as there are too few other experimental results.Parallel species (having the same stoichiometry) with similar formation constant (orsolubility product) values are expected within the actinide series: U(V), Np(V), Pu(V)and Am(V) (but not Pa(V)). Neptunium is the most stable actinide element in the +5oxidation state. Hence there are more published works on complexes of Np(V) than onthe complexes of the other elements of this actinide(V) series, and the stoichiometriesof these actinide(V) species were usually determined from experimental informationconcerning Np(V). The analysis of the Pu(V) system for which there is less data wasguided by analogy to the Np(V) system as was previously the case for U(V) species[92GRE/FUG]. Conversely, Np(IV) and Np(VI) species are expected to be the same