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Henry’s law constants for gases are known to be roughly proportional to molecular weight a t a given t e m p e r a t ~ r e . ~There is also a correlation between molecular weight and boiling point provided the interactions in the pure liquid are similar, as should be the case in this series of alcohols. These monomeric alcohols in carbon tetrachloride are in no way different from a dissolved gas. Thus the Henry’s law constants for them would show similar temperature dependences, and if extrapolated to their respective boiling points would lead to a common value for it. This means that the concentration of all the alcohols in solution at their respective boiling points is similar and thus would lead to the observed common value of E”. VOLUME
Figure 2. Temperature dependence of the 0-H stretching vibration of 0.005 M methanol in CCId as a function of the vapor volume above the solution in a sealed cell. The ordinate is expressed as a percentage change, (ASP - AS~)/AS X 100, where A is the absorbance a t the indicated temperature.
volume (head space) above the solution in the sealed cell. In view of our interpretation, an increase in the free volume above the solution would require that a larger fraction of the solution molecules enter the vapor phase to establish equilibrium at a new temperature. These resulk are presented in Figure 2. The temperature dependence is seen to increase as the vapor volume increases. I t is apparent from an extrapolation of the data that if there were no free volume above the cell there would be no temperature dependence. This variable is not discussed in the published paper., Experiments using a cell with zero vapor volume are difficult to perform since liquids are incompressible and an increase in 1 emperature would fracture a normal infrared cell. In conjunction with this experiment, we also studied the effect of concentration on the temperature dependence and found that the temperature dependence is decreased as the concentration is increased, as would be expected in view of our interpretation. These experimental observations support our conclusion that the apparent temperature dependence of E” as observed by hlotoyama and Jarboe is due to a decrease in the solution concentration of the alcohol and not to any true temperature dependence of the 0-H intensity. Their use of very dilute solutions (0.0020.004 X ) to alleviate any problems due to hydrogen bonding magnified the effect described here. The observation that a plot of E” us. T leads to common value of E” for all the alcohols when extrapolated to their respective boiling points can also be explained. The Journal of Physical Chemktry
Acknowledgment. The author wishes to thank Mrs. Mary Clemmer for her assistance with the experiments and in the preparation of the manuscript. (4) G. W. Castellan, “Physical Chemistry,” Addison-Wesley Publishing Co., Reading, Mass., 1964.
DEPARTMENT OF BIOCHEMISTRY UNIVERSITY OF IOWA Iowa CITY,IOWA
CHARLES
A. SWENSON
RECEIVED JUNE 2. 1967
The Mechaidsm for the Isotopic Exchange between Deuterium and Acidic Solutions on Palladium Surfaces
It is well known that if H, is bubbled through HzO solutions containing catalytically active palladium metal, absorption will occur spontaneously until H/Pd = 0.69 (1 atm of Hz, 2 5 O ) . ’ It has been shown that the slow step in this type of experiment is diffusion of dissolved Hz through the Brunner-Nernst diffusion layer up to the surface of the palladium.2a3 Analogous experiments have been performed in which Dz is absorbed by palladium from solutions of Dz0.4 I n the research reported here, we have bubbled Dz through H 2 0 solutions and have determined the isotopic content of the gas absorbed by the palladium wire specimens by mass spectrometric analysis of the gases evolved from the specimen following heating in vucuo. Surprisingly, when specimens are employed which are unpalladized
Sir:
(1) E.Q.,see T. B. Flanagan and F. A. Lewis, J . Chem. Phys., 29, 1417 (1958). (2) R. J. Fallon and G. W. Castellan, J. Phgs. Chem., 64,4 (1960). (3) A. Carson, T. B. Flanagan, and F. A. Lewis, Trans. Faraday Sac., 56, 1311, 1324 (1960). (4) T.B. Flanagan, J . Phgs. Chem., 65,280 (1961).
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but active toward Dzuptake, e.g., anodized prior to the absorption run, exchange is relatively small. For example, approximately 90% D is found when Dz is bubbled through H 2 0 or approximately 92% H when H2 is bubbled through DzO. I n the former case, HzO, 1 N HC1, and 0.1 N NaOH were used as solutions. On the other hand, appreciable exchange takes place with specimens which have been activated prior to an absorption run by palladization. The total rate of hydroD) uptake is generally reproducible under gen (H similar conditions of stirring. (The rate of uptake is comparable for both the palladized and unpalladized specimens). By contrast, the amount of exchange is markedly dependent upon the experimental conditions employed, e.g., thickness of palladized layer. The exchange reaction was investigated in some detail on palladized specimens. The exchange is markedly reduced in the presence of specifically adsorbed species, e.g., I-, and again the rate of hydrogen uptake (H D) is not appreciably affected. I n dilute H2S04 solutions, if the palladium was made anodic with respect to an inert electrode during the absorption run exchange was diminished. This suggested that exchange in acidic solutions may proceed by an electrochemical mechanism such as
+
+
H++e-+H
D +D+
*
*
+ e-
(la)
(1b)
where the asterisks represents adsorbed H or D. If such an electrochemical mechanism is the predominant mode of exchange, then the corresponding exchange current can be calculated from the isotopic content of the specimen; for example, if the total rate of absorption is 1.0 X 10l6 molecules of Dz cmF2 sec-’, then 50% H gives io = 3.2 ma/cm2; this value of io is not unreasonable for an active metal, e.g., Pt. The exchange current determined by isotopic analysis was compared to the exchange current measured during the same absorption run by an electrochemical method. The exchange cur-
isotopic exchange occurs predominantly by an electrochemical mechanism, e . g . , reactions 1. Therefore, mech-
anisms involving either dissociatively adsorbed HzO or a switch of an adsorbed D with an H from an adjacent HzO are not important under these experimental conditions. However, at higher pH values this simple picture of the exchange mechanism is no longer valid because the “exchange current’’ determined from the isotopic analysis exceeds that measured electrochemically, e.g., in 1 N NaOH io(isotopic analysis) = 4.9 ma/cm2 and &,(electrochemical) = 0.2 ma/cm2. This indicates that exchange mechanisms other than electrochemical must come into play as the pH is increased. The only related research appears to be that of Trusov and Aladzha1ovaj6 who have used a quite different experimental approach to this problem. The results found by these workers, which differ from those of the present research, will be compared elsewhere where further details of the present research will be given. Acknowledgment. The authors are grateful to Dr.
J. W. Simons for making the initial observation in this laboratory that Dz does not exchange appreciably with H20 during absorption by unpalladized palladium. The authors are also grateful to the U. S. Atomic Energy Commission for financial support of this research. (5) K. Vetter, “Elektrochemische Kinetik,” Springer-Verlag, Berlin, 1961. (6) G . N. Trusov and N. A. Aladzhalova, Dokl. Akad. Xauk S S S R , 130, 370 (1960).
CHEMISTRY DEPARTMENT G . L. HOLLECE OF VERMONT TEDB. FLANAGAN UNIVERSITY BURLINGTON, VERMONT05401 RECEIVED JUNE9, 1967
Confirmation of the Predicted Hartree-Fock Limit in BeHz
(1) R. C. C. Lao and J. R. Riter, Jr., J. Phys. Chem., 71, 2737 (1967).
Volume 71, Number 9 August 1967
