Qi^Y '.^^^/ ^-^4- 92^i '\2^ UNITED STATES ATOMIC ENERGY COMMISSION AECU-924 THE AUTOREDUCTION OF AM(VI) AND AM(V) IN DILUTE ACID By L. B. Asprey S. E. Stephanou November 10, 1950 [TIS Issuance Date] Los Alamos Scientific Laboratory Technical Information Division, ORE, Oak Ridge, Tennessee styled, retyped, and reproduced from copy as submitted to this office . Work performed \jnder Contract Wo. W-7ij-05 eng 36 PRINTED IN U.S.A. PRICE 5 CENTS AEC, Oak Ridge, Tenn., 1 1-10-S0-675-A26227 THE AUTOREDUCTION OF AM(VI) AND AM(V) IN DILIITE ACID* By L. B. Asprey and S. E. Stephanou In various studies on the higher oxidation states of americium, it was always observed that reduction of Am(Vl) to Am(V) followed by the re- duction of Am(V) to Aiii(lll) occurred. This phenomenon was investigated in order to determine the cause of this reduction and to establish a quantitative expression for the rate. Bate studies were made^ varying such factors as the hydrogen ion concentration, americium concentration, the ratio of the various ionic species of americiiim, and the ionic strength. The concentration of the different ionic species of americium was determined "spectrophotometrically with the Model DU Beckman spectrophotometer and quartz microcells, using volumes of the order of 100 111. Perchloric acid solutions were used in the range 0,035 to l.OM in all but one experiment to avoid complex ion formation. The americium used in the studies was greater than 98 pei" cent pure. Concentrations of americium varied from O.OO8M to 0.033M. The ionic strength was adjusted by means of sodium perchlorate. The preparation of Am(Vl) was achieved by means of peroxydisulfate oxidation at about 85°C . Preparation of Am(V) was carried out using alkaline hypochlorite at ~75°C according to the method of Werner and Perlman . When the concentration of Am(VI), Am(V), or Am(lII) was plotted against time, a straight line was obtained. This indicates that the re- duction is a zero order reaction with respect to the particular ionic species. Figure 1 shows that Am(Vl) in IM perchloric acid reduces to Am(V) with only a very small growth of Am(lll). Am(V) increased linearly, reaching a maximum concentration when the Am(VI) was completely reduced. Then Am(V) reduced linearly to Am(lII) at approximately half the Am(Vl) reduction rate. In Table 1, rates of reduction occurring at a number of different concentrations of americium are tabulated. ^Manuscript for a 15-minute talk given by L. B. Asprey at the Chicago meeting of the American Chemical Society on September 5^ 1950, based on work performed at the Los Alamos Scientific Laboratory of the University of California. AECU-92l^■ AECU-92i+ Table 1- -Experimental Resiolta; Postulated Rate Law: -d Ain(V)/dt = k (Am) Rate of change, (M x 10^) (liter houi (H) M Acid Total Am, Moles/liter -d(Am(V dt R +d(Am(lII)) dt 1.1 1.1 1.0 1.0 1.1 1.1 1.0 1.0 HClOi^. HClOr HCIO, HCIO, 0.009 0.025 0.033 0.016 0.20 0.62 0.70 0.39 0.21 0.66 0.73 O.il-2 0.5 1.1 HC10ij_ 0.3 1.0 HClOij. 0.1 0.1 HClOij. 0.035 1.1 HClOij^ 0.1 0.3 HgSOjj^ 0.023 0.011 0.008 0.009 0.016 0.83 0.22 O.llj- 0.15 0.25 0.83 0.25 0.15 0.15 0.26 k, (Moles Am(V) Reduced) (Moles Am hour) Am(V) 0.022 0.025 0.021 0.02l<- Average . 023 0.036 0.020 0.018 0.017 0.016 Am(lII) 0.023 0.026 0.022 0.026 0.024 0.036 0.023 0.019 0.017 0.016 It can be seen that the reduction is first order with respect to the total concentration of americium for the reduction of Am(V) to (ill) . At a particular hydrogen ion concentration, the values of k obtained are in fair agreement. In the one experiment in O.5M HC10|^ an abnormally high value was obtained. In the experiments on the reduction of Am(VI) to Am(V), values of k were found to be 0.046 and 0.054 in 1 and O.5M HClOi^., respectively. As would be expected, these values are about twice those for the reduction of Am(V) to Am(lII), since only one ec^uivalent is in- volved . These results suggest that the reduction of Am(VI) to Am(V) fol- lowed by the reduction of Am(V) to Am(lII) is caused by the Am24l alphas reacting with the water. Probable reductants formed by the reaction of alphas with water are H2O2 and H atoms but other fragments may be in- volved. Throughout the reduction, gas was evolved from the americium solutions but no determination of the amount or composition of the gas has been made in the present study. The variation in the rate constant may be due in part to a small effect of the hydrogen ion concentration. Probably a more important factor is the variation between solutions due to small amounts of adventitious impurities which decompose the reducing agent or change the degree of recombination of the water fragments. If the products of the alpha radiation of water cause this reduction, then the rate of reduction should be a function of the americium half -life and the energy of its alpha radiation. A calculation showing the number of electron volts available from alpha radiation in terms of one equivalent of Am(V) reduced to Am(lII) is given herewith. AECU-92ij- ^ 3 Calculation of the number of ev/molecule of Am(V) reduced by IM HCIOl At. Wt. ev/alpha d/m/g of Am of Am d/m/to d/hr from Am^^l ev/mole Am/hr 7 X 10l2 241 60 5A7 X 10^ = 5.55 X 10^3 ev 5.5^ X 19^3 mole Am - hr = 33 ev/molecule O.O2I+* z 6.02 X 1023 molecules Am(Y) reduced ^^(^^^ reduced to mole Am - hr Am(lll) or 19 ev/per equivalent *Average value of k from four experiments in IM HClOj^ A summary of the results appears in Table 2, The value of 19 ev obtained in IM HCIO^. agrees well with the value of 20 ev found by Duane and Scheuer in experiments using radon as the alpha source. However, the values found by a n\jmber of investigators cover the range from 20 to 80 ev. Table 2 — Electron Volts per Equivalent of Am(V) Reduced ev/equivalent (H+) /i Acid Am(V) reduced Am(Vl) reduced 20 1.1 1.1 1.0 1.1 1.0 1.0 1.0 1.0 0.5 1.1 0.3 1.0 0.1 0.1 0.035 1.1 0.1 0.3 Acid ev/equivalent Am(V) reduced HCIO]^ HCIO, 20 18 HCIO, HC107 HC10ij_ HCIO, 21 18 13 21 HCIO, HCIOk HgSOj^ 25 27 29 17 The reduction of Am(Vl) and Am(V) is zero order with respect to the species being reduced and first order with respect to the total americium concentration. These results can be satisfactorily explained as due to the reaction of the americium alpha particles with water. Since other workers have found H2O2 to be a principle product of the reaction between alpha particles and water, this offers a reasonable explanation for the reduction observed. No direct experiments have been made to determine whether a hydrogen peroxide mechanism is involved. AECU-92J4- tH O o CM T3 g d -H cd -P o •s 3 ^-^ -a _> (D ^1 2 o ■=^ ^ »» cd . V M Q) (£ > X! -P ■ CO ft^^ w ^( a > o < ^ o 1 (0 ri cd ^^ rH H M M > ^ w •r-l g"a f>4 < < o AllSN3a -IVOIldO END OF DOCUMENT Digitized by tine Internet Archive in 2011 with funding from University of Florida, George A. Smathers Libraries with support from LYRASIS and the Sloan Foundation http://www.archive.org/details/autoreductionofaOOusat UNIVERSITY OF FLORIDA 3 1262 08917 1085 i