April, 1955
NOTES
The equation for p may be written in linear form P =
a N ~ / ( 1- NB)
+
c
Least squares analysis of the data of Burton and Patrick' gives a = 0.80 and c = 3.7. The standard deviation of p is 0.6. If their first result is discarded, values of a = 0.78, c = 4.4, and u = 0.3 are obtained. Taking the latter and a value for (k6 k7)/k6 5 8.3.l
+
Gd/Ga
2 0.094
I4.0 Thus, a t least 8.6% of the excited cyclohexane molecules that decompose yield hydrogen in an elementary process, and the rate constant for H2 formation by hydrogen atom attack on cyclohexane is less than 4.0 times that for HD formation from benzene-d6. I n a communication to the author Professor Burton stated that the mechanism of this paper should be substituted for Burton and Patrick equation 15 and its discussion. k&a
EXCHANGE REACTIONS OF AMERICIUM' BYTHOMAS K. KEENAN, ROBERTA. PENNEMAN AND
ISOTOPIC
JOHNF. SUTTLE
Lou Alamos Scientific Laboratory, University of California, Los Alamou, New Mexico Received January 17, 1866
Exchange studies have been reported by others for two transuranium elements, neptunium2 and ~ r a n i u m . ~ -These ~ workers showed that exchange takes place between Np(1V)-Np (V), N p (V)-Np(VI) and U(1V)-U(V1). This note reports results of an investigation of exchange between various americium valence states. The element americium exhibits three well-defined valence states in solution; the simplest aqueous species are Am+++, ArnO2+ and AmOz++. Both Am02+ + and Am02+ are reduced in perchloric or nitric acid solution by effects of Am241a-radiation at zero-order rates of -5% and -2.5% per hour, respectively'. The ion b o z +can also disproportionate.8 However, the disappearance of Am02+ via the disproportionation path is not appreciable a t low acidities, low americium concentrations, and at room temperature. The isotope Amz4?, a 16.01 hour9 p-emitter, was used as tracer in solutions of Am241,the common aemitting isotope. Experimental
All chemicals were "Analytical Reagent" grade and (1) This work was sponsored by the Atomic Energy Commission and was carried out at the Lo8 Alamos Scientific Laboratory in oooperation with the University of New Mexico. Some of the results were presented at the 123rd meeting of the American Chemical Society, Loa Angeles, California, March 15-19, 1953. (2) (a) J. Hindman, D. Cohen and J. Sullivan, J . A m . Chem. Soc., 7 6 , 352 (1954); (b) 76, 4275 (1954). (3) E. King, MDDC-813 (1947). (4) A. Grosse, MDDC-1644 (1948). (5) R. Betts, Can. J . Research, 86B,702 (1948). ( 6 ) E. Rona, J . A m . Chem. Soc., 7 8 , 4339 (1950). (7) L. Asprey and 8. Stephanou, AECU-924 (1950). (8) L. Asprey, R. Penneman and 8. Stephanou, AECU-925 (1950). (9) T. Keenan, B. McIntuer and R . Penneman, J . Chem. Phys., 81, 1802 (1953).
381
were used without further purification. The americium was >99% pure. The most stable aaueous state of americium is the trivalent ion. In all Gxchange experiments, the p-activity was initially in the (111) oxidation state only. The 0-active isotope, penta- and hexavalent americium were prepared by techniques described e l s e ~ h e r e . ~ - ~ ~ Trivalent americium was separated from the higher valence states by precipitation of AmFa with aqueous HF and LaFa carrier. Procedure.-Solutions of pentavalent and p-active trivalent americium were combined and the time noted. Ali uots were removed at known later times for separation. Seajed tubes totally immersed in an oil-bath were used for runs at elevated temperatures. Following any separation, a sample of the fluoride supernatant liquid was taken fo? 8and a-counting to determine specific activity. Equilibrium specific activity was determined by removal and counting an aliquot of the exchanging solution without separation of the exchanging species.
Results The data obtained from 21 experiments may be summarized as follows. 1. The exchange of Am+++ and AmOz+ is negligible at temperatures 6 100" in acid concentrations 6 2.0 f a n d americium concentrations ca. 0.03 f each. The presence of incandescent or ultraviolet light or inert salt had no discernible effect. The a-reduction of AmOz+ and the 16 hour half-life of the tracer made it impossible to follow this slow exchange for more than 1-2 days. The lower limit for the exchange half-time is 200 hours. 2. Exchange half-times of ca. 15 hours were observed between 0.03 f tri- and pentavalent americium under these conditions: (a) very high temperatures (ca. 165') and acid concentrations of ca. 0.2 f; (b) moderate temperatures (ca. 90") and acid concentrations of ca. 5-10 f. Because of reduction and rapid (acid-path) disproportionation, very rapid disappearance of pentavalent americium was noted in such media. This disappearance was too rapid to allow systematic investigation of the various kinetic parameters governing the exchange. 3. The exchange of AmO2+ and Am02++ was briefly investigated and found to be complete within one minute a t 0" in 1.0 f HC104. Americium concentrations were each 10-*f. Discussion The ion Am(1V) has never been observed in solution. The quadrivalent americium ion might be expected to open a kinetic path for exchange in the system Am+++-Am02+ since the neptunium exchange-which involves Np++++ and Np02+proceeds a t a finite rate. Lacking a corresponding Am(IV) species no analogous exchange path is apparently available to Am + + + and AmOz+. It can be point'ed out that under conditions where one would expect finite concentratioiis of Am(IV), i e . , where Am02+ is disappearing very rapidly, some exchange is observed. It may be that such a system contains sufficient Am(1V) to allow exchange to take place. The rapid exchange of AmO2+ and AmOz++ is not surprising in view of the analogous results for the Np02+-NpOz++ exchange reported by Hindman, et al. 2* (10) J. Nigon, R. Penneman, E. Staritzky, T . Keenan and L. Asprey. THIS JOURNAL, 58, 403 (1954). ( 1 1 ) T. Keenan and 8. Stephanou, unpublished work described in J . Chem. Phyu.. a l , 542 (1953).
