Radiochimica Acta 77, 245-246 (1997) © R. Oldenbourg Verlag, München 1997 Letter to the Editors Reply to the Letter of G. L. Silver Commenting on: "Stability of Pentavalent Plutonium" By H. Capdevila and P. Vitorge Centre d'Etudes de Saclay, DCC/DESD/SESD, F-91191 Gif sur Yvette, France (Received June 20, 1997; accepted July 2, 1997) Silver pointed out that, in contradiction to our previous conclusion [1], modestly increasing temperature in- creases the stability of Pu(V). He is certainly right. His letter is then useful. Our error was to use a wrong numerical value for the potential of the following redox equilibrium, £ 6/4 : PuOr + H 2 + 2 H + o Pu 4+ + 2 H 2 0 (1) at 45 °C and zero ionic strength. But there is no error in our calculations and Eq. (23) in [1] is valid. We here use the correctwvogVKGEA E 6/4 redox potential and our calculation leads to the same qualitative conclusion as Silver according to the estimated temperature influence on Pu(V) stability. However, we do not completely agree with his discussion; we explain below with more de- tails our calculation for entropy estimate. Silver reported in acid solution, the equilibrium fractions for Pu 3+ , Pu 4+ , Pu0 2 + and PuOf to be 0.020, 0.47, 0.002 and 0.51, respectively. As he said, the values in the reference [4] page 823/824 are based on measurements performed at 1 M HCl [6], while our study [1] was in non complexing media. As Silver said, PuOÍ mole fraction increases from 0.002 to 0.004 when temperature is increased from 25 to 35.24° C. This variation is certainly less than the un- certainty. It is then still needed to produce thermody- namic data that confirms more clearly Silver's remark, specially for non complexing aqueous solutions of moderate acidity, where PuOJ is more stable. For this, we will merely give the correct numbers which are calculated from erroneous E 6/4 value in our earlier pub- lication [1], In solutions of moderate acidity, Pu 4+ dis- proportionates nearly completely, and Pu0 2 + stability mostly depends on the numerical value of K v , the con- stant of the following equilibrium 3 PuOÍ + 4H + o Pu 3+ + 2 PuO! + + 2 H 2 0 (2) We measured K v by using a spectrophotometry tech- nique to observe PuOJ partial disproportionation at 25° C and at different ionic strength [1], We then checked that this ionic strength influence was consist- ent with the "SIT" formula and we used it to extra- polate K v to zero ionic strength. We found log 10 #v = 2.6 ± 0.5 at I = 0 and then deduced AG V = - 1 5 ± 3 kJ · mol" 1 . Using the formation Gibb's energy for the species involved in the equilibrium (2), taken from the reference [3] as suggested by Silver, we obtain AG V = - 1 7 ± 27 kJ · mol" 1 (then \og w K v = 3.0 ± 4.7). From the reference [4] (Table 17.14 and page 824 of this reference) also suggested by Silver, we obtain AG V = - 1 5 ± 28 kJ · m o r 1 (then log 10 Ä\, = 2.7 ± 4.9). The results of both methods are in reasonable agreement; but the accuracy of the data deduced from thermodynamic cycles is certainly not enough to ob- tain a clear conclusion. We perform the same type of calculation from indi- vidual entropies that are tabulated in the same refer- ences [3, 4], These individual entropies were originally obtained from thermodynamic cycles using numerous auxiliary data, analogy between Pu and Sm for the en- tropy value of the salt PuCl 3 , and classical approxi- mations typically no ionic strength correction of Pu 3+ enthalpy obtained from data measured in concentrated HCl aqueous solution. Concerning this last point, ex- perimental data have proven [5] that the ionic strength influence onS Δ SyxwvutsrponmlkihgfedcbaTSPMIHFCA or Δ Η is not negligible even from 0 to 1 molai. For these reasons, we decided to use [1] another thermodynamic cycle to estimate the entropy change of the reaction (2), taking into account only redox measurements corrected at / = 0 with the same theory, namely S.I.T. ASv = - 3 S 6/5  + S 4/3  + AS 6/4  (3)  Table 1. pH (at / = 0) or -log,„[H + ] (in 1 M C10 4 ", acidic  media) where [Pu0 2 + ] = [PuO¡ + ] = [Pu 3+ ], (log m ^ v )/4 values are tabulated. They are calculated from log,oAf v value measured at 25°C and the corresponding entropy change, AS V (Table 2). The first column of this table is the correction of the pH l/2 values given in the reference [1]. We add values at / = 1 M calculated in the same way, and (last column) from the plutonium mole fractions given by Silver [from 6] Γ(° C) 7 = 0 IM 1M  (this work) (this work) [Silver]  0 0.99 ±0.16 1.46 ±0.18  25 0.65 ±0.13 1.21 ±0.13 1.45  35.24 1.12 ± 0.13 1.32  50 0.36 ±0.13 1.00 ± 0.15  75 0.12 0.82 ± 0.22  100 -0.10 ±0.21 0.66 ± 0.28  Brought to you by | provisional account Unauthenticated Download Date | 6/11/15 1:12 PM