1780
SOTES
the rotational transitions. Data on the widths of the individual lines, even at very low pressures, are not available. However, by comparison with other gases where the long-range dipoledipole interactions are also weak, one might expect line widths of the order of 0.1 to 0.2 cm.-l atm.-'. Furthermore, in other gases exhibiting both resonant and non-resonant absorption, it has been generally observed that the resonant line widths are several times larger than the non-resonant widths, or relaxation frequencies. Actually, calculations based on the Van Vleck-Weisskopf line shape indicate that the deviation between curve 3 and the experimental data can be accounted for with an average width for the rotational lines of 0.12 cm.-' atm.-'. The loss arising from the rotational transitions would then be insignificant for pressures below 10 atm. and comprise about 25y0of the total loss at the highest pressure. Discussion It is apparent, by analogy with the liquid state, that pronounced changes in dielectric behavior must occur when the gas is sufficiently compressed. Polar liquids show no rotational quantization and the entire dipolar loss is non-resonant, or Debye type. Such a transition is not yet apparent in CClF3. The loss observed at the highest pressures is consistent with predictions based on the behavior a t low pressures. If the entire rotational spectrum had undergone a transition from resonant to non-resonant, the totalloss would have been much greater. The relaxation frequency for the non-resonant loss continues to increase with increasing density throughout the range of this investigation. Below 10 atm. the variation is linear. At higher pressures interference from the rotational lines precludes an exact determiuation of the functional dependence. However, if a reasonable allowance is made for the effect of the rotational lines, the relaxation frequency appears to vary more or less linearly with the density over the entire range. At the highest density ( p / Z = 45 atm.), the relaxation frequency of CC1F3 is of about the same magnitude as found for similar substances in the liquid state. I n liquids, however, the trend with density is reversed, increasing density causing a decrease in relaxation frequency. The details of the relaxation mechanism are therefore quite different in the two cases. Molecules of the liquid are in almost constant collision, but many collisions are needed to cause random reorientation because of the restraining effect of neighboring molecules. In gases, collisions are less frequent, being directly proportional to the density a t lower densities, but their effect on molecular reorientation is much greater. Thus, the relaxation frequency of the gas may approach or exceed that of the liquid. It probably is fortuitous that the relaxation frequency of CClF3 varies more or less in proportion to the density over such a wide range. The effective collision diameter for molecular reorientation is about 5 A., or about the same as the molecular diameter. At the maximum density, the mean free path is of the same order. Therefore, the collision frequency will increase faster than the
Vol. 64
density, since the volume occupied by the molecules is an appreciable part of the total volume. The probability of multiple interactions, which should reduce the effectiveness of collisions, also increases. It would appear that these effects largely compensate each other. At still higher densities the relaxation frequency should reach a maximum and then decrease as the latter effect becomes the determining factor. Data over a similar range of pressures have been reported by Phillips* for chlorodifluoromethane. These results were interpreted in a manner that required the entire loss to be non-resonant for pressures above several atmospheres and led to the conclusion that the relaxation frequency was independent of the pressure. This interpretation is not in accord with the present study. Although CHCIFz is a slightly asymmetric top, its spectrum, from the present standpoint, is qualitatively analogous to that of the symmetric top. The +K, -K degeneracy is removed, but the direct transitions between these levels are of such low frequency that the resulting absorption has the character of a non-resonant ~ p e c t r u m . ~At the comparatively high frequency (ca. 25 kMc.) employed by Phillips, the rotational and nonresonant spectra would overlap strongly and make difficult any detailed analysis. (8) C. S. G. Phillips, J . Chem. Phys., 88, 2388 (1955). 69, 1339 (1959). (9) E. B. Wilson. Jr., THISJOURNAL,
NOTE ON THE SOLUTION OF HYDROGEN IN PALLADIUM WIRES BYJAMESP. HOARE* Scient~ficLaboratory, Ford Motor Company. Dearborn, Mcchruan Receaved June 13, 1960
If the open-circuit potential of a clean palladium wire which previously has been anodized in pure hydrogen-stirred 2 N H2S04 solution is followed as a function of time, one observes a curve having a plateau at 50 mv. more noble than a Pt/H2 electrode in the same solution.' It was suggested that Frumkin and Aladjalova2 did not observe this plateau because they mounted their palladium electrodes in platinum holders sealed in glass.3 The local cell action due to the contact of two dissimilar metals in acid solution charged the Pd to @-Pdand the potential fell rapidly to a value of zero volt. It was suggested recently4 that the platinum holder did not have any effect on the rate of solution of hydrogen and that metastable states causing plateaus in the potential-time curves were due to the extreme inactivity of the palladium surface of the electrodes used by Hoare and Schuldiner. Evidence is presented here to show a very marked * General Motors Research Laboratories, General Motor6 Grporation, Warren, Michigan. (1) J. P. Hoare and S. Schuldiner, THISJOURNAL, 61, 399 (1957): J. P. Hoare, J . Electrochem. Soc., 106, 640 (1959). (2) A. Frumkin and N. Aladjalova, Acta Physicochim. U.R.S.S., 19, 111844).
(3) S. Schuldiner and J. P. Hoare, J . Chem. Phys ,89, 1551 (1955): S. Schuldiner, G. W. Castellan and J. P. Hoare, %bid., 88, 16 11958). (4) J. O'M. Bockris, Chern. Revs., 49, 525 (1948); J. P. Hoare and
S. Schuldiner, J . Electrochem. SOC.,109, 237 (1956); J . Chem. Phut., '26, 1771 (1957).
Nov., 1960 effect on the rate of solution of hydrogen in Pd by the electrical contact of Pt with Pd in acid solution. A palladium wire 10.5 cm. long and 0.38 cm. in diameter was mounted in a Teflon cell filled with 2 N H2S04 solution. All purification, electrode preparation and measurement procedures were identical to those found in the literature.lq3 Both the resistance of the wire and the potential against a Pt/H2 electrode in the same solution were recorded as a function of time while the Pd wire absorbed hydrogen from the acid solution. The solution is stirred with hydrogen flowing a t a rate of 230 10 cc./min. Zero time is taken when the hydrogen flow is first started and all potentials are measured against a Pt/H2 electrode in the same solution. The A-curves in Fig. 1 are plots of the data taken on the wire described above. The filled symbols are the potential-time points and the open symbols are the resistance-time points. The symbols represent points taken directly from the meters while the smooth curves were obtained from the recorder chart. After these data were collected, the wire was removed from the cell and annealed a t red heat in a flame for five minutes to remove the dissolved hydrogen. A piece of platinum gauze about 0.5 in area was spotwelded to the center of the palladium wire. After this wire had been mounted in the cell, the same procedures which had been used in obtaining the data for the Acurves were repeated. The B-curves in Fig. 1 are a plot of these data. The C-curves were obtained in the same way except that a platinum gauze 7 cm. long and 3 mm. wide was spot-welded a t about 10 points along the center of the Pd wire. From Fig. 1 it is seen that, as the contact with platinum is increased, the length of the 50-mv. plateau is shortened and the steady-state value is lowered for the potential-time curves. The length of the a-Pd-plateau' on the resistance-time curves is reduced and the slopes of these curves are increased as contact with platinum is increased. This may be explained by an increased rate of solution of hydrogen due to local cell action at the points of contact with platinum as suggested before.2 The reason why the points on the resistancetime curve in the case of the C-curve are lower than what might be expected may be traced to the parallel resistance of the platinum gauze. These data support the contention that conditions of high purity with regard to electrode materials, cell construction materials, solutions and gases are of the utmost importance4 in investigations of this kind. It is suggested that these data show why a 50 mv. plateau was not observed2 and why Frumkin and co-workers6 seem to have more active electrodes than Hoare and Schuldiner since the criterion for surface activity may be taken as the rate a t which a Pd/H2 electrode reaches the equilibrium value of zero volt os. Pt/H2 or the rate a t which palladium dissolves hydrogen. An interesting feature of the potential-time curves in Fig. 1 is the increase in the amount of initial overshoot of the plateau value with increas-
NOTES
1781
*
(5)
Private communication: A. Fedorova and A. Frumkin, Zhur. U.S.S.R.,t o be publlshed.
F i z . Khrm.
Fig. 1.-Potential us. time (filled symbols) and relative resistance, R/Ro, us. time (open symbols) curves for Pd wires in HI-stirred 2 N H2804 solution. Smooth potential-time curves are taken from recorder chart but broken resistancetime curves were not recorded. T = 24 i lo, H ~ f l o w= 230 i 10 cc./min. A curves for pure Pd wire; B curves for same Pd wire with one contact to P t ; C curves same Pd wire with about ten contacts to Pt (eee text). The insets show curves on expanded time scale.
ing rate of the solution of hydrogen due to contact with platinum. These data may provide supporting evidence for the existence of a supersaturated a-Pd-H phase.a (6) R. J. Ratchford and G. W. Castellan, THISJOURNAL, 61. 1123 (1958).
MECHANISM OF THE ISOTHERMAL DECOMPOSITION OF POTASSIUM PERCHLORATE' BY KURTH. STERN^ AND MARIJONBUFALINI Department of Chemistry, University of Arkansas, Fayetleville, Arkansas Received June 1.9, 1960
Recently considerable evidence has been presenteds that both the liquid and solid phase decomposition of KCIOl proceeds according to the mechanism
+ + +
KC104 +KClOs '/202 KCIOa --3 KC1 s/z02 KClOa +a/&C104 '/41
