NOTES
2260
pentanone as compared to that of 2-pentanone4,5,5-d3. Since Z-pentanone has three H atoms in the y-position, while 2-pentanone-4,5,5-d3 has only one H atom and two more strongly bonded D atoms in this position, 2-pentanone would have a relatively higher probability of undergoing a molecular elimination than 2-pentanone-4,5,5-d8. TABLE I11 VAPORPHASEPHOTOLYSIS OF CH3COCH2CH2CH8. THE EFFECT OF OXYGENAT 3130 1. Ketone pressure = 26.5 mm.
la
1.57 X IO" quanta/cc./sec.
Poi, mm.
T , OK.
*ethylem
1.4 6.4 56.5
305 306 303 305
0.30 .25 .20 -13
...
Vol. 65
from which decomposition occurs does, however, depend on the equilibrium temperature. It is ipteresting t o note that in the vapor phase at 3130 A. the effect of temperature on the ratio C2D3H/ CaHzDs is of the same order of magnitude. This is not surprising in view of the fact, as was pointed out before, that the ratios of the ethylenes in the two phases are comparable. TABLE IV LIQUIDPHASE PHOTOLYSIS OF CH2COCH2CHDCD2H A,
A.
3130 3130 3130 2537 3130
Liquid Phase.-In the liquid phase (Table IV) the ratio CzD3H/C2H2D2decreases with an increase in temperature. A plot of log CZD3H/ C2HsDz against 1/T yields a difference of 1.15 0.15 kcal./mole in the activation energy for the transfer of a D atom a2d an H atom. Since the point obtained a t 2537 A. lies on the same line of the Arrhenius plot, the ratio CzDsH/CzH2Dz is independent of wave length. It may indeed be expected that in the liquid phase, collisional deactivation is important. The mean energy level
*
T,OK.
198 214 273 296 343
Rethyleas X IO' (co./min.)
5.40 6.55 27.8
...
26.2
CzDaH/ChHzDi
6.40 5.37 3.44 2.76 2.12
The relative quantum yield of ethylene is within experimental error constant from 343 to 273OK. At temperatures below 273'K., however, there is a pronounced decrease in the yield of ethylene. Because ethylene can be formed only by a molecular elimination process, cage recombination cannot be invoked to explain the reduction in quantum yield. It is more likely that an activation energy of a few kcal./mole is required for the decomposition process.
NOTES THE ELECTRODE POTENTIALS OF GERMANIUM: SOME COMMENTS ON THE INTERPRETATION BY LOVRECEK AND BOCKRIS BY J. I. CARASSO, M. M. FAKTOR AND H. HOLLOWAY
atm. for brown GeO and about atm. for yellow GeO.) Such low hydrogen pressures might well have been attained in the solutions which LovreEek and Bockris swept with helium. However, replacement of the atmosphere of helium by one of hydrogen would render the postulated corrosion reaction 3 impossible. Yet the authors have rePost Ofice Research Stataon, DoEZzs H i l l , London N.W. 8, England ported that the measured potentials were unaffected Received March 80,1961 by sweeping with hydrogen. The only possible I n a recent paper' Lovrerek and Bockris have conclusion appears to be that at least one of the described measurements of the electrode potentials two postulated electrode reactions 1, 2 does not of germanium over a range of pH in deoxygenat,ed contribute to the measured Potentials, and that, solutions. These potentials were interpreted as if corrosion does occur in deoxygenated solutions, mixed potentials arising from the simultaneous it does so by a process other than the postulated occurrence of the two electrode reactions corrosion reaction 3. A further objection to the interpretation by Ge + HzO +GeO + 2H+ + 2e(1) LovreEek and Bockris concerns their claim that 2Hf + 2e- +Hz (2) the difference between the measured mixed poThese authors showed that the resultant over- tentials and the reversible potentials for the anodic all corrosion reaction reaction is not more than 20 mv. This result was derived from the assumption that the corrosion Ge + H20 ---+ GeO + H2 (3) current in deoxygenated solutions and the exchange would be thermodynamically feasible only in solu- current for the anodic reaction are both about 2 X tions which are in equilibrium with a low pressure amp. cm.-2. of hydrogen. (The maximum hydrogen pressure The value assumed for the corrosion current for which the postulated process is possible depends was derived from the results of Brattain and Garupon the form of GeO involved, being about rett? who do not appear to have deoxygenated (1) B. LovreEek and J. O ' M . Bockris, J (1959).
Phya. Chem.. 68, 1368
(2) W. H. Brattain and C. G. B. Garrett, Phys. Rev., 94, 750 (1954): BelZ System Tech. J . , Sl. 129 (1956).
NOTES
Dee., 1961 their solutions. Harvey and Gatosa have found that the rate of dissolution of germanium in oxygenated aqueous solutions at 35’ is about 1 pg. cm.-%r.- which correspondsto a corrosion current of about amp. More recently an investigation of the polarized germanium electrode by Paleolog, Tomashov and Fedotova4 has shown directly that the corrosion current in air saturated solutions is about 10-6 amp. at 25’. Harvey and Gatos3 found that in deoxygenated solutions the corrosion rate was much less than 1 pg. cm.-2 hr.-l and this implies that the value of the corrosion current (2 X amp. assumed by LovreEek and Bockris is much too high. The value of the exchange current for germanium oxidation quoted by LovreEek and Bockris was stated to be “extrapolated from Turner.”6 In fact, the reference quoted does not imply a value for this exchange current. Turner’s paper gives the slope of the Tafel line for germanium dissolution and, less accurately, its position. In order to derive a value for the exchange current one also must know either the position and slope of the Tafel line for the reverse reaction (deposition of germanium from solution) or the reversible potential for the anodic reaction. Neither of these pieces of information is given by LovreEek and Bockris so that the source of their value for the exchange current is a matter for conjecture. There is the further point that Turner’s Tafel slope applies a t current densities where germanium has been shown to be oxidized to the quadrivalent state6 and a different Tafel slope might apply to the postulated oxidation to the divalent state. In the absence of any information about the relative magnitudes of the corrosion current and the exchange current, consideration must be given to the possibility that the mixed potential which is measured is very different from the reversible anodic potential. Thus, if the corrosion current is one or two orders of magnitude greater than the anodic exchange current, the reversible anodic potential may be 120 to 240 mv. more negative than the mixed potential. This would admit the possibility that the anodic reaction is oxidation of germanium to the quadrivalent state, for example Ge -+- 2H20 -+ GeOz + 4H+ 1- 4e- or Ge
+ ~ H z O+ HnGeOa + 4H+ + 4e-
Our conclusions are: 1. the potential of the germanium electrode in deoxygenated solutions is not determined by the corrosion reaction which has been postulated by LovreEek and Bockris. 2 . LovreEek and Bockris’ statement that the corrosion current and the exchange current for the anodic reaction are of comparable magnitude appears to be quite unjustified. Therefore the proximity of the ~ ~ e a s u r epotentials d to reversible potentials calculated for the reaction. .( :e
+ H,O
GeO
+ 2H+ + 2e-
(3) W. W. Harvey and H. C. Gatos, J. Electrochem. SOC.,106, 654 (1958). (4) E. N. Paleolog, N. D. Tomoshov and A. 2. Fedotova, Zhur. K z . Khim., 34, 1027 (1960). (5) D. R. Turner, J . Electrochem. Sac., 103, 252 (1956). (6) F. Jirsa, L.anorg u allgem Chem., 568, 84 (1952).
2261
is not evidence that the latter reaction has any potential determining significance. Acknowledgment is due to the Engineer-in-Chief of the British Post Office for permission to publish. THE REACTIVITY OF HYDROGEN ATOMS I N T H E LIQUID PHASE: T H E LACK OF EFFECT OF LINEAR ENERGY TRANSFER IN THE RADIOLYSIS OF HYDROCARBONS BY W. G.BURNS Chemzstry Dimsion, Atomic Energy Reaeareh Eetablishment, Harwell, Didcot, Berkahire, England Received M a y 29, 1961
An encouraging trend in the radiation chemistry of alkane hydrocarbons has been the explanation of some features of the product yields in terms of the reaction behavior of atoms and radicals determined in the gas phase when generated by means other than radiation.lv2 Hardwick3 has recently attempted to show that the collision yields of many reactions of the type H. paraffin, which occur in the radiolysis of the liquids, are roughly the same as found in the gas phase (Le., -lo-’), the reason for high rate constants in the liquid phase being the large frequencies (-10’4 see.-’ molecule-’) for solute-solvent collision^.^ The atoms considered are scavengeable, and not “hot.” If the kinetic interpretation 5 v 6 of the decrease in radiolytic hydrogen yield with scavengers present is correct, there are two characteristics of the hydrogen yields from pure cyclohexane and pure n-hexane which seem to require explanation. They are: 1. The apparent lack of contribution to the H. -+ HZfor hydrogen yield by the reaction H. radiation of low LET3; 2. The invariance of the yields with changing LET of the In the reaction scheme
+
+
+
H. He +Hz ki (1) H. GH12 +HP CSHii. ICs (2) we have kz = 6.6 X lo9 cc. mole-’ see.-‘ (ref. 3), and if we take kl = 6 X 10l2 cc. mole-’ set.-',
+
+
the value used for H atoms in v ~ a t e r ,reaction ~.~~ 1 can provide effective competition for reaction 2 if [H.] approaches (k&) [C~HIZ], i e . , -lo-’ M . Such concentrations of hydrogen atoms might be exceeded in the center of the track left by a densely ionizing particle if they were formed very near the center of the track. For example, in a column of 10 A. radius in which G(H,) -4 (the value for cyclohexane)6r11for radiation of LET 5 e.v./,&., the concentration of H atoms is 1.1M . To reduce the recombination to negligible proportions this concen(1) J. H. Futrell, J. A m Chem. Soc., 81, 5921 (1959). (2) T. J. Hardwick, J . Phys. Chem.. 64, 1623 (1960). (3) T. J. Hardwick. zbzd., 61, 101 (1961). (4) E. -4. Moelwyn-Hughes, J . Chem. Sac., 95 (1932). (5) G E. Adams, J. H. Baxendale and R. D. Sedgwiek, J . Phys. Chem., 63, 854 (1959). (6) J. G. Burr and J. D. Strong, Abstracts of the 137th National Meeting of the A.C.S. p. 43-R. (7) R. H. Schuler and .4.0. Allen, J . Am. Chem. Soc., 7 7 , 507 (1955). (8) H. A. Dewhurst and R. H. Schiiler, zbid., Si, 3210 (1959). (9) A. K. Gangulyand J. L. Magee, J . Chem. Phys., 25, 129 (1996). (10) P. J. Dyne and J. M. Kennedy, Can. J . Chem., 38, 61 (1960). (11) P. J. Dyne and W. M. Jenkinson, zbid., 38, 539 (1960).
