Oct., 1952
FLUORIDE CATAT~YSIS OF CEROUS-CERIC ELECTRON EXCHANGE
869
CATALYSIS OF THE CEROUS-CERIC ELECTRON EXCHAKGE REACTION BY FLUORIDE BYH. C. HORNIG AND W. F. LIBBY Department of Chemistry and Institute for Nuclear Studies, University of Chicago, Chicago 97, Illinois Received March 15. I068
The electron exchange reaction between cerous and ceric ions in 6 M nitric acid solution, discovered by Gryder and Dodson, has been studied and its rates a t room temperature, 0" and -14.5' observed. The half-time a t -14.5' was found to be nearly 4 hours. Working at this low temperature the authors have studied the effects of the addition of small amounts of fluoride and have found a large catalytic action. As little as 10-4 mole of K F er liter of solution doubles the exchange rate. This corresponds to 10-6 M fluoride, indicating a very efficient catalytic mecIanism exists involving fluoride, possibly one in which a single fluoride ion can form symmetrical transition com lexes with the fluoride ion being shared by the exchanging cerium ions. It seems possible that a complex involving a single iuoride may be one of the collision partners generating the symmetrical transition complex.
I. Introduction The electron exchange reaction between cerous and ceric ions in nitric and perchloric acid solution was discovered by Gryder and D0dson.l The separation was accomplished by extraction with diethyl ether. The reaction was found to be first order in cerous ion, but of an order between zero and one in ceric ion depending on the concentration of ceric cerium and the nature of the medium. The exchange rate was found to decrease inversely with the square of the Hf concentration in the HNOa medium and with the first power in the perchloric solutions. The activation energy found for the nitric acid solutions was 7.7 kcal./mole. Evidence has been accumulating for the occurrence of a first order catalysis of electron exchange reactions by simple ions such as chloride, the most marked effects having been found in the case of europous and europic2 and ferrous and ferric3 Since some theoretical basis for this observation has existed4based on the general ideas of Franck on the application of the Franck-Condon principle to electron transfer reactions in solution, the present authors have investigated the effect of fluoride in the cerous-ceric system. 11. Experimental The reagents were made from ceric ammonium nitrate, reagent grade, and hydrated cerous sulfate, both purchased from G . Frederick Smith Company. The radioactive cerium was Celr4,275 day half-life, obtained from t.he Isotopes Division, Oak Ridge, Tennessee. The radiocerium was incorporated in the cerous valence state and measurements made on the transfer of radioactivity to the tetravalent state. The solutions were counted through a thin glass wall solution counter, the 3 MeV. @-radiation of the 17.5 minute Pr144 daughter presumably being the principal radiation measured. Care was taken to allow the solutions to sit for at least one hour before measurement. Measurements were made at room temperature, 0" and -14.5'. The lowest temperature bath consisted either of elcoho!-water slush or ice-salt water mixtu!e. It is est.imated the temperatures were a x z r e t e to 1.0 . Some difficulty with the reduction of the ceric by the eilii: WRS experienced even at the lowest temperature. Furthermore, at the lowest temperature the formation of two separate layers was barely possible. It appbars therefore that this (1) J. W. Gryder and R. W. Dodson, J . Am. Chem. SOC.,71, 1894 (1949); T8, 2890 (1951). (2) D. J. Meier and C. 8. Garner, ibid., 71, 1894 (1951). (3) J. Silverman and R. W. Dodaon, Brookhaven Quarterly Report BNL-93,p. 65. Oat.-Dec. 1980; Tau JOURNAL, I C , 846 (1962). (4) W. F. Libby, Abstracb, Physical end Inorganic Seation, 116th Mwtinr Am. Chem. Soo.. Sen Francboo, Calif., Maroh 27-April 1, 1040; TBIO JOURNAL,16, wa (ieaz).
range of temperatures is about the maximum obtainable with this separation method. The diethyl ether used was Mallinckrodt analytical ether. The nitric acid was made by dilut,ion of Baker and Adamson C.P. Reagent Grade concentrated nitric acid. The solutions were 6 ill nitric acid in all cases, and the cerous and ceric ion concentrations were maintained at 2.0 X 10-3 M . After separation the total cerium content of the ether layer was determined by titration and the total cerium content of the a ueous layer similarly deterrmned. I t was assumed on thegoasis of control experiments that cerous cerium was not extracted into the ether layer. The potassium fluoride solution used for the study of the fluoride catalysis was prepared by heating KHF2 in a.plat,inum vessel until a constant weight was obtained. Thls was stored in a desiccator and used to prepare the aqueous KF solutions which were kept in plastic bottles.
111. Results Table I presents part of the experimental data. The actual points are shown on Figs. 1 and 2, in which the logarithm of the fraction of the exchange remaining to occur is plotted against the time for TABLE I SUMMARY OF ELECTRON EXCHANGE RATESFOR CEROUSA N D CERICIONSAS CATALYZED BY FLUORIDE Run 1
K F Added, M
Temp., OC.
Half-time
for exchange
(minuteda
None 25 11 * 2 2 None 0 63 8 None 0 64 6 8.4 X IO-' 0 -2 8.0 x 10-4 0 2.6 7 9 8.0 x 10-4 0 3.1 11 8.0 x 10-4 - 14 4.4 -13.5 5.8 12 6.3 x 10-4 13 4.7 x 10-4 -14.5 6.8 14 2.5 x -14.5 12.5 19 1.26x 10-4 -14.7 26 15 None -14.6 224 16 None -14.5 220 Correction for zero time exchange was made graphically. This usually amounted to about 7%.
the various temperatures and fluoride concentrations used. Figure 8 is a plot of the logarithm of the reciprocal of the half times, ie., the exchange rate vs. the reciprocal of the absolute temperature. As is seen, the slope of this line corresponds to an activation energy of 11.7 0.8 kcal. Figure 4 presents the fluoride data for - 14.5'. It is readily seen that a rapid rise in rate proportional to the total fluoride added is observed. The highest
H. C. HORNIG AND W. F. LIBBY
870
3.3
3.4
Vol. 56
3.5
3.6
I XI03 (de;'
f
3.7
3.0
3.9
4.0
Id,
Fig. 3.-Effect of temp. on Ce III)/Ce(IV) exchange rate: 6 M " 0 8 , ether extraction, [ e(III)] = [Ce(IV)] = 2 X 10-*M; AHsct = 11.7 kcal. mole-1.
b
CONTACT TIME (mid. Fig. 1.-Ce(III)/Ce(IV) exchange, temp. effect: 6 M "Os, 2.0 X 10-8 M , Ce(1II) = Ce(1V). No salt added.
12 CONTACT llME(minJ.
Fig. 2.-Effect
of KF on Ce(III)/Ce(IV) exchange rate: - 14.5', 6 M "0,.
-
Fig, 4.-Effect of KF concn. on Ce(III)/Ce(IV exchange rate: -14.5', 6 M "01, ether extraction, [c?e(III)] [Ce(IV)] = 2 X 10-8 M .
IV. Discussion fluoride concentrations 'may show a slight deviaThe dissociation constant for HF is given as 7.2 X tion from linearity. In general it is clear that the 10-4 at 2 5 O . 5 Assuming the ionic entropy of fluooriginal data of Gryder and Dodson have been con-. (b) "The Oxidation Statea of the Elements and Their Potentiale in firmed and that an additional effect, the CrttalydiEt Aqueouo Eolutlon," W , M. Latimer, Prentior-Hall Publishem, Inc., by fluoride, has been found, 1988.
Oct., 1952 -_
FLUORIDE CATALYSIS OF CEROUS-CERIC ELECTRON EXCHANGE
ride t o be -2.3,6and the entropy of aqueous HF to be +41.5,b and neglecting the undoubtedly appreciable effect of the 6 M nitric acid, one calculates the dissociation constant for -14' to be 2 X 10-8. On this basis the F- concentration in these solutions can hardly exceed 6 X (HF) or 6 X 10-8 times the number of moles of KF added per liter of reacting solution. The proportionality constant is such that the exchange rate appears to mole of KF added per liter of double for 3.4 X solution a t -14.5'. I n other words, 2 X lo4 M fluoride appears to double the rate. The existence of a first power negative ion catalysis of electron exchange reactions appears to be borne out by experiment. It is difficult to be cer-
871
tain that the magnitudes of the catalytic effects obtained are in agreement with the theoretical4 model of a transitory collision complex consisting of the two positive ions between which the small negative ion lies for a short time. It seems possible, however, that one of the two reacting ions may form a relatively stable bond with the catalyzing negative ion, in this case F-, which then is available on collision with the uncomplexed ion of different valence to form the desired symmetrical complex with the negative ion lying on the line joining the two tenters of positive charge and bisecting it. It is to be noted that the chloride catalytic effects are indeed of much smaller magnitude. In general one might expect the larger ions to be less effective.
