Disproportionation Equilibria and Rates in Perchloric and Hydrochloric Acid Solutions of Plutonium: Influence of α-Particles

Abstract

The data of Kasha and Sheline on rates aiid equilibria of the disproportionation reaction of plutoniuin(1V) to give plutonium(II1) and plutonium(V1) have been corrected for the reduction caused by the plutonium a-particles. I t was necessary to assume reaction paths for the a-particle-induced reduction, and one-electron reductions of Pu(V1) and Pu( IV) were chosen as most plausible. The disproportionation mechanism was found to be consistent with that previously deduced from the disproportionation of plutonium( V). The equilibrium for the disproportionation of plutonium(1V) exhibited approximately the expected fourth power dependence on hydrogen ion concentration in perchloric acid at unit ionic strength and 25'. An apparent fifth.power dependence was obtained for hydrochloric acid solutions, not at constant ionic strength. The disproportionaton rate showed an inverse third power acid dependence in perchloric acid a t constant ionic strength and an apparent-3.5 power dependence in hydrochloric acid solutions, without added salt. Approximate values of 40 kcal. and f 6 0 e x . were calculated for the heat and entropy of activation of the disproportionation reaction in onemolar hydrochloric acid. The formal potentials of the Pu(II1)-Pu(V1) and Pu(1V)-Pu(V1) couples in one molar perchloric acid a t 25" are -1.043 2~ 0.003 and -1.022 =t 0.002 volt, respectively. Thc corresponding values for one molar hydrochloric acid are -1.053 * 0.003 and -1.025 rt 0.002 volt. The discrepancy concerning chloride complexing of PuOz++ is nearly eliminated by the new values for the disproportionation equilibrium quotient. It is pointed out that the assumption of a small amount of complexing of P u f a by chloride ion would give complete agreement and evidence is advanced in support of this hypothesis.htroduction.-Attention1** has been called to a discrepancy between the observed properties of plutonium(V1) with chloride ion and that calculated from e.m.f. and disproportionation equilibrium measurements. Spectral observations show plutonyl ion, PuOz++, to be appreciably complexed in one molar hydrochloric acid.3 The e.m.f. and equilibrium results indicate the unlikely result that plutonyl ion is more strongly complexed by perchlorate than by chloride ion..The source of the discrepancy has been traced to an error in the equilibrium quotient for the reaction The average oxidation number of plutonium in a solution containing these species slowly decreases with time. The a-particles, from the disintegration of plutonium, in their passage through the solution produce species which bring about the red~c t i o n .~ When the rate of reduction by the CYparticle radiation becomes of the same order of magnitude as the rate of attainment of the disproportionation equilibrium of equation (l), then true equilibrium will not be established. The results of the calculations of this paper show that this correction is much more important than was previously suspected and that the apparent equilibrium quotients were seriously in error.Extensive measurement...

Disproportionation of Pu(IV): A reassessment of kinetic and equilibrium properties

“…(2) and K 3 for Eq. (3) and is independent of [Pu(V)] because the [PuO þ 2 ] terms in those expressions cancel [4,5]. However, as discussed previously [18], the net reaction of the two-step sequence fails to include Pu(V) as a product and fixes the equilibrium [Pu(III)]:[Pu(VI)] at 2:1.…”

“…Solutions containing Pu(IV), Pu(V), or Pu(VI) are unstable due to disproportionation of plutonium in those oxidation states. Early studies show that Pu(IV) forms Pu(III), Pu(V), and Pu (VI) in acidic media [4][5][6]. Disproportionation of Pu(V) forms Pu(III) plus Pu(VI) in some cases [1,7] and Pu(IV) plus Pu(VI) in others [1,8,9] with reaction rates varying by more than 10 6 over the pH 0-15 range [10,11].…”

“…Disproportionation of Pu(IV) is described in several studies with acidic perchlorate [4,5,12], chloride [4,[12][13][14] and nitrate [6,15,16] solutions.…”

“…The proposed reaction path (Eqs. (2) and (3)) is adopted in subsequent studies [4][5][6][12][13][14][15][16][17]. According to this mechanism, the rate-controlling step is followed by rapid reaction of Pu(V) product with Pu(IV) to establish an equilibrium state involving the four common Pu oxidation states [1,12,13].…”

Theoretical and experimental evaluation of waste transport in selected rocks: 1977 annual report of LBL Contract No. 45901AK. Waste Isolation Safety Assessment Program: collection and generation of transport data

“…Kraus andDam, 1949 a,b,c Cleveland, 1970;Katz andSeaborg, 1957 Cleveland, 1970;Ward and Welch, 1956 Degischer and Choppin, 1975;Connick and McVey, 1953 Deqischer and Choppin, 1975;Martin andWhite, 1958 Degischer andChoppin, 1975;Shilott and Marcus, 1966 Pu + HSO" ** PuHSO. Cleveland, 1970;Nair et al, 1967 Degischer and Choppin, 1975;Newton andBaker, 1956 Cleveland, 1970;Nair, 1968 Pii(OH)_,{s) *• Pu(OH)4(aq) poten.…”