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

580 A. Discussion of selected referencesvalue is in excellent agreement with the value E ◦′ = (0.982 ± 0.002) V determined byConnick and McVey [51CON/MCV].Also, from measurements of the redox potential as a function of the acid concentrationat an ionic strength of 1 M, the first hydrolysis constant for Pu 4+ , β 1 ,wasdetermined. No time dependence of the results was mentioned, although this mighthave been expected, particularly in the less acidic mixtures. Therefore, the uncertaintyin log ∗ 10 β 1 was estimated in the present review as ±0.2.Comments on the use of this work to determine formation constants for chlorocomplexes of Pu(IV) (and for correction for Pu(III) chloride complexes) are providedbelow in the discussion of the paper by Rabideau and Cowan [55RAB/COW].The authors explained the shift of the redox potential in nitrate media to more positivevalues (as compared to the value in perchlorate media) as evidence for the formationof PuNO 3+3 , and calculated the overall formation constant log 10 β 1 = (0.54 ± 0.01).The possible formation of nitrate complexes of Pu(III) and activity coefficient changeswhile substituting perchlorate ions for nitrate were neglected. Even though the changesin the medium are small, so are the changes in the potential. In the course of the presentreview an attempt was made to account for the measured potential changes consideringonly activity coefficient changes. The potential differences can be expressed as alinear function of the nitrate concentration with slope 59.2 ε mV·mol −1 . In spite ofa very good fit, a very large interaction coefficient difference (ε = 1.39 kg·mol −1 )was obtained. This may partly be the result of the contribution of mixing terms in theε value in addition to contributions from complexation. More data would be neededto separate the two effects, and no recalculated value of the constant could be obtainedfrom these data.[52COH/HIN]a) NpO 2+2 /NpO+ 2 systemCohen and Hindman determined the potential in 1.027 molal HClO 4 at 25 ◦ CforthereactionNpO 2+2+ 1 2 H 2(g) Å NpO + 2 + H+as E ◦′ = (1.1373 ± 0.0008) V (for comments see [61SUL/HIN]). From potentialmeasurements as a function of the neptunium concentration, they estimated the junctionpotential of their cell to 0.0002 V. The investigators did, however, not consider thisin further correction because it was relatively small compared to the uncertainty in theredox potential. From potential measurements at 15 ◦ C and at 35 ◦ C, two isothermaltemperature coefficients (dE/dT ) isotherm = −0.00030 and −0.00023 V·K −1 were determined.This yields an average value (dE/dT )=−(0.00027 ± 0.00005) V ·K −1 .UsingE ◦′ and this value, this review determines r S m =−(26.1 ± 4.8) J·K −1·mol −1 , r G m =−(109.73 ± 0.08) kJ·mol −1 ,and r H m =−(117.5 ± 1.4) kJ·mol −1 for theabove noted reaction at standard temperature.