Kinetics of the Oxidation of Uranium (Iv) by Iron (Iii) in Aqueous Solutions of Perchloric Acid

Abstract

The kinetics of oxidation of uranium (IV) b y iron (111) in aqueous solutions of perchloric acid have been investigated a t four temperatures between 3.1°C. and 24.8"C. The reaction was followed by measurement of the amount of ferrous ion formed. For the conditions (Il+) = 0.1-1.0 iM, ionic strength = 1.02, (FelII) = 10-'-10-5 hf, and (UIV) = lO-'-lO-6 ill, the observed rate law is d(FeZ+)/dt = -2d(UIV) /dt -K1 and Kn are the first hydrolysis constants for Fe3+ and U4+, respectively, and K'and K" are pseudo rate constants. At 24.8"C., K' = 2.98 sec.-I, and K" = 10.6 mole liter-' sec-'. The corresponding temperature coefficients are AH' = 22.5 kcal./mole and AH" = 24.2 kcal./mole. The kinetics of the process are consistent with a mechanism which involves, as a rate-controlling step, electron transfer between hydrolyzed ions. INTRODUCTIONOxidation-reduction reactions between cations in aqueous solution have been thought until recently t o proceed very rapidly, and the term "instantaneous" has frequently been used to characterize their rates. However recent studies with radioactive tracers have shown that even such elementary reactions as electron transfer between iron (11) and iron (I 11), or between Ce (I 11) and Ce (IV), do not proceed instantaneously (18,8). Such results have reawakened interest in the whole subject of both isotopic and ilo~lisotopic electron transfer reactioils between cations in solution. Some kinetic information is now available relating to the reactioils between the nonisotopic pairs Sn (11) -Fe (111) (5), T1 (111) -Fe (11) (2), and N p (IV) -Fe (111) (10).Several review articles have recently appeared in which the current status of theory and experiment have been summarized for both isotopic and nonisotopic clcctron transfer reactioils (1,14,19). The proceedings of a symposium held in 1954 on this subject have also been published (12).As a further example of this type of reaction, the present paper describes experiments relating to the kinetics of the oxidatioil of uranium (IV) by iron (111) in aqueous acidic media. Perchloric acid and sodium perchlorate were chosen as the principal electrolytes, since amongst the common inorganic anions, perchlorate has the least tendency to form ion-pairs with cations. T o bring the rate of reaction into a range suitable for study, it has been necessary to work with solutions to 10-5 M in uranium (IV) and iron (111). T h e course of the reaction has been followecl by measurement of the rate of formatioil of iron (11). Such measurements, in conju~lction with a knowledge of the stoichiometry of the reaction, were sufficient to enable a kinetic analysis of the results to be made. I t would have been useful, as confirmatory evidence, lMa?zz~script

Kinetic Investigations on the Reduction of Plutonium (VII) in Alkaline Solutions

“…7. Arrhenius plot of data shown in Table 3 δ kcal/mol) has also been observed by BETTS [12] in the oxidation of U(IV) in HC10 4 by Fe 3+ involving hydrolysis of U(IV) and Fe (III) where both the hydrolyzed species are positively charged, unlike Pu(VII).…”

“…5, which turn out to be of the order of 8.3 χ 10" 4 s -1 and 0.036 respectively. At the lower hydroxide ion concentration (below 0.52 M), the rate controlling reactions (4) and (5) lead to modification of the rate law as shown in equation (11 Since both the terms in the numerator of equation (12) are assumed to be comparable and the order of k\ Κ λ value has already been obtained, equation (12) can be modified to equation (13) However, the formation of OH or O -has not been verified experimentally. Benzene is known to act as a scavenger for OH and O -radicals.…”

Reaction between U(IV) and H2O2 in hydrochloric acid medium

“…A controversy exists in the literature about the second hydrolyzed species of U(1V) [5,. Sobkowski [5] and Betts [12], while explaining their results on the kinetics of U(1V) oxidation by 0 2 and Fe(III), respectively, in HC104 medium, considered the following reaction: U4+ + H20 e U(OH)3+ + H+ a t p = 2M in Cloy medium. In view of the extensive complex formation a reaction similar to eq.…”

Kinetics of reaction of uranium(IV) and hexachloroiridate(IV) in acid perchlorate solutions. Evidence for a binuclear intermediate

“…It was determined from the empirical equation In k' = ln[H+]" (ref. 26) where n was found to be intermediate between -1 and -2, suggesting a mixed-order reaction.…”