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

A. Discussion of selected references 703The chemical conditions for some of the measurements of the potential of theNp(VI)/Np(V) couple were similar to those of the potentiometric titrations. Nevertheless,the results cannot be plotted on a single curve as a function of [CO 2−3]. As thetotal concentration of Np(VI) was approximately three orders of magnitude greater inthe titration experiments, the two sets of experiments, taken together, provide evidenceof formation of at least one polymeric Np(VI) complex containing hydroxide anions.Curve-fitting indicates that none of the known hydroxide Np(VI) complexes can beidentified as the intermediate complex(es). Thus, this work provides the first (and essentiallythe only) experimental evidence of the formation of (at least) one polymericNp(VI)-OH complex that also contains carbonate as a ligand.The author fitted the overall formation constants, β a,b,x for the equilibria aNpO 2+2 +bCO 2 (g) + (b + x)H 2 O Å (NpO 2 ) a (CO 3 ) b (OH) x2a−2b−x + (2b + x)H + which (asexplained above) refer to the free species, NpO 2+2. This is always a minor species[98VIT/CAP], and the complexation reactions involving this species are probably notdirectly measurable. For this reason the equilibrium constants (βa,b,x ′ = β a,b,x/β1,3,0 a )for the dissociation of the limiting complex aNpO 2 (CO 3 ) 4−3+ (b − 3a)CO 2 (g) +(b + x − 3a)H 2 O Å (NpO 2 ) a (CO 3 ) b (OH) 2a−2b−xx +(2b + x − 6a)H + should be used.Graphical analysis [98VIT/CAP] indicates that many possible complexes couldform on hydrolysis of NpO 2 (CO 3 ) 4−3at lower partial pressures of CO 2 (g). Among others,these include the species (NpO 2 ) 2 CO 3 (OH) − 3 , (NpO 2) 3 (OH) − 7 or (NpO 2) 3 (OH) 2−8(analogous to species proposed for uranium [92GRE/FUG, 95PAL/NGU]). Curvefittingresults (many different approaches have been used [98VIT/CAP] in preparingthis section of the present review) lead to similar conclusions if we accept not only thebest fit but also those that are within reasonable confidence limits (±1.96 σ min )ofthe“best fit” set. The fit proposed by Maya is only one among the many possible that givequite reasonable results. The values calculated for the formation constants for severalof the more likely species are highly correlated. This review accepts the numbers heproposed; but only as possible maximum values, and then only for the equilibria correspondingto βa,b,x ′ and not β a,b,x (NpO 2+2was always a minor species). To limitthe discussion, this review considered only the species analogous to those proposedin the TDB uranium review [92GRE/FUG] or those suggested recently by Palmer andNguyen-Trung [95PAL/NGU]).There is evidence [86GRE/RIG, 90RIG] that for conditions similar to those used byMaya, (NpO 2 ) 3 (CO 3 ) 6−6, could not be neglected [98VIT/CAP] in the buffering regionduring the titrations at the highest carbon dioxide gas partial pressure. A formationconstant for this species is proposed, and incorporated in the other recalculations. For(NpO 2 ) 2 CO 3 (OH) − 3, the value proposed by the author is marginally consistent withlater work [86GRE/RIG, 90RIG]. By analogy with uranium, Maya’s value for theformation constant is accepted in the present review with an uncertainty of ±1.0 inlog 10 K for2NpO 2 (CO 3 ) 4−3+ 7H + Å (NpO 2 ) 2 CO 3 (OH) − 3 + 5CO 2(g) + 2H 2 O(l)The simple dicarbonato complex, NpO 2 (CO 3 ) 2−2, was undoubtedly only a minorspecies in Maya’s solutions, and the formation constant he proposed is clearly an over-