June, 1956
NOTES
unable to observe directly the size of the reaction zone. However, the fact that some ammonia was always found in the trap, even in the presence of a considerable excess of atomic oxygen, suggests that the reaction is a fairly slow one. (b) NzH4 0.-The observed emission spectrum of the NzH4 0 atomic flame consisted of bands of NH2, NH, OH and NO. These bands were all at least two orders of magnitude more intense than their counterparts in the NHI 0 atomic flame in the same apparatus and under the same experimental conditions. Ten minute exposures with a 100 p slit, width gave well exposed plates for all these species. The luminous zone of the NzH4 0 atomic flame is very small compared with that of other atomic flames a t the same pressure. The reaction volume is, for example, less than one tenth of the size of that for a comparable C2H2 0 atomic flame. This shows that the reaction between NzH4 and 0 is a very fast one. When a small amount of NZH4 is added to a large excess of atomic oxygen, the air afterglow appears very strongly below the mixing zone. This indicates that the oxidation of hydrazine by atomic oxygen involves the formation of NO, a fact which is also borne out by the emission of the 7-bands of NO in the reaction zone. The trap analysis showed that all the original hydrazine was destroyed in the reaction vessel. Since the thermal decomposition of hydrazine yields NH3'3 and since no NH3 was found in the trap, thermal decomposition of the N2H4 would seem to be ruled out. Qualitative analysis of the trap contents by the brown ring test indicated the presence of oxides of nitrogen. Reactions with Atomic Hydrogen. (a) NH3 H.-In agreement with Dixong no evidence was found which would indicate that NHI reacts with atomic hydrogen. No light emission was observed under our experimental conditions and with exposure times of -3 hr., and the ammonia was recovered quantitatively in the trap. Our techniques could not, however, be expected to provide any direct evidence for possible exchange reactions or stripping-reforming reactions which may proceed without the formation of electronically excited intermediates. It was found that the energy released by hydrogen atom recombination to a platinum filament inserted into the reaction chamber decreased with increasing partial pressure of ammonia under constant discharge conditions. This would seem to indicate that the presence of ammonia in the reaction system reduces the steady-state coricentration of atomic hydrogen. Our observations can readily be accounted for on the basis that the NHI reacts with the H atoms only by an exchange reaction of the type proposed by Farkasand Melville'o
+ +
+
+
+
+
NHgH'
+ H +NH3 + H'
(2)
(7) R. C. hlurry and A. R . Hall, Trans. Faraday Soc., 47, 742 (1951). ( 8 ) G . K. Adanis and G. W. Stocks, "Fourth Syiriposiiiin on Conibustion." The Willinnis and Wilkins C o . , Baitinlore, 1952. (9) .J. K . Dixon, J . Am. Chern. Sor., 54, 4 Z i 2 (1932). ( I O ) A. Barkas and 11. W. RIrirille, Pror. Rou. S o r . ( / , o n d o , , ) . 1578, 625 (193G).
815
and that the decrease in the steady state concentration of H atoms is due to a recombination with NH3 as the third body. (b) NzH4 H.-The reaction between hydrazine and atomic hydrogen took place with the emission of moderately strong NH2 bands and relatively weak N H bands. Trap analyses showed, in agreement with Dixon's resultslg that ammonia was a major reaction product. The ratio p = (moles of NH3 formed/moles of N2H4 decomposed) was observed to depend on the partial pressure of the reactants and was found to vary from about 0.45 to about 1.14 f 0.05. In general a higher ratio of hydrazine to atomic hydrogen a t a given pressure favored the production of ammonia. The luminous reaction volume, although considerably larger than that for the reaction N2H4 0 at the same pressure, did not fill the reaction chamber. Order of magnitude calculations show that the observed reaction volume is compatible with the rate constant for the hydrazine-atomic hydrogen reaction obtained by Birse and Melville." The dependence of p on atomic hydrogen concentration suggests that the reaction between H atoms and N2H4 proceeds by at least two competing mechanisms of different order in H atoms.
+
+
(11) E. A. B. Birae and H. W. Melville, ibid., 175A, 1G4 (1940).
APPEARANCE POTENTIAL STUDIES. I' BY STEPHEN S. FRIED LAND^ A N D ROBERT E. STRAKNA Department of Physics, The Uniuersity of Connecticut, Storre, Connecticut Received December 88, 1966
The measurement of appearance potentials of molecular fragments leads to some understanding of molecular structure (for example, see ref. 3). I n addition, the magnitude of the appearance potential of polyatomic molecules, such as the esters and alcohols, may be used elsewhere. The proper combination of a polyatomic gas with an inert gas in a self-quenching Geiger-Muller counter is determined by the appearance potential of the polyatomic molecule. This is possible since the quenching process of the gas discharge in the counter depends upon the probability of electron transfer from the inert gas to the polyatomic m ~ l e c u l e . ~That is, the ionization potential of the inert gas has to be higher than the ionization potential of the polyatomic molecule or of the appearance potential of any of its fragments (if the fragment is t o participate in the quenching action) ; the closer the two values, the higher the probability of electron transfer and thus the more effective quenching action. Further, the value of the appearance potential of the fragments may be utilized to determine whether the fragments formed in the discharge5 will also act as quenching agents. (1) W o r k supported by the Research Corporation, New York City. (2) On leave with the Nuclear Development Corporation of America, White Plains, New York. (3) (a) H. D . Hagstrum, Rev. Modern Phys., 13, 185 (1951); ( b €1. B. Rosenstock, H. B. Wallenstein. A . I.. Wahvhaftig and, H. Eyrina, Pror. Nall. Acad. Sci., 38, (107 (1952). (4) S. A . Korff and R . D. Present, Phi/.*. Rei)..65, 275 (1944). (5) R. S. Friedland, ibid., 74, 898 (1948).
816
NOTES
With the above two points as a motivation, a systematic study of gases of interest has been undertaken. Methyl and ethyl alcohol and methyl and ethyl acetate are herein reported. Methyl and ethyl alcohol were reported on by Cummings and Bleakneye many years ago, but it was felt that the new techniques developed since their study warranted a re-investigation of these molecules, Experimental All measurements were made on a 90" sector type mass spectrometer. The electronics were of conventional design except for the ion gun control unit, which included a built-in potentiometric circuit for the measurement of the electron energy. The electron accelerating potential was obtained across a Leeds and Northrup precision resistance box of 0 to 10K ohms through which a constant current of 10 milliamperes was maintained. This current was continuously stabilized by use of a Weston standard cell as a reference. Simultaneously, the highly stabilized current wa8 used to produce the potentials necessary to operate the other elements in the electron gun. The standardization circuit functioned by comparing the voltage drop across a resistor adjusted to 101.86 ohms with 1.0186 volt output, which is the same as the standard cell. If 10 milliamperes are flowing through the resistance box, the sum of the voltage drops across the 101.86 ohm resistor and the standard cell will be zero. However, if the current should depart slightly from its proper value, a resultant error voltage appears across the contacts of a Brown Converter and is chopped into a 60 cycle square wave by the motion of the armature contacts. The phase of the resulting voltage depends upon the polarity of the error. The error signal was amplified by n narrow band 60 cycle amplifier and was then fed into a phase sensitive detector which produced a d.c. output of similar polarity but of much greater size than the error signal. The output thus derived was used to correct the grid bias of a tube placed in series with the current to be regulated. The action was degenerative and hence tended to reduce the error to zero. A precaution taken to ensure stability of the system waR that a d.c. feedback path was provided which wm capable of correcting the system in the event of a major disturbance. This was necessary as the high gain of the ax. amplifier tended to produce saturation and loss of phase sense when a sudden large signal was applied. A phase correction network was included to fulfill the Nyquist criterion for the frequency phase relations in any feedback circuit. The unknown gas and the calibrating gas were admitted into the gas inlet simultaneously. The ion peak to be studied was adjusted in height to equal the ion peak of the calibrating gas with 50 volts supplied to the electron beam. The ionization efficiency curves for both gases were obtained rapidly and the method of initial breaks was used to determine the appearance potential. Reproducibility from day t o day was better than 0.05 volt and all standards checked against each other were accurate within the same range. The deviations indicated for the results are indicative of the reproducibility of the ionization efficiency curves from day to day. The accuracy of the ionization potentials, in terms of an absolute number, was approximately 0.2 of a volt for all values given. If the ionization efficiency curve of the unknown turns out to be parallel to that of the calibrating gas and the points of initial break fall within 0.1 of a volt of each other, then it is possible to cite an accuracy less than the above value. However, in most cases, the two curves are not parallel, and it is difficult to find calibrating gases that have the proper appearance potential and therefore a value of 0.2 is most realistic. The results of the measurements of the appearance potentials are as follows: Methyl Alcohol.-Mass number 32, 11.36 f 0.08 volt; 31, 12.26 f 0.10 volt; 29, 14.26 f 0.10 volt; 28, 14.31 f 0.05 volt; 15, 14.96 f 0.10 volt. Ethyl Alcohol.-Mass number 46, 10.88 f 0.15 volt; 45, 11.23 f 0.08; 31, 12.28 f 0.15; 29, 13.91 f 0.15; 27, 15.31 f 0.15. Methyl Acetate.-Mass number 74, 10.95 f 0.10 volt; 59, 12.31 f 0.15; 43, 11.86 f 0.10; 42, 11.81 f 0.15; 31, 12.65 f 0.20; 15, 14.26 f 0.10. (6) C. 8. Cummings, 11, and W. Bleakney, Phpe. Rsv., 18,787 (1940).
Vol. 60
Ethyl Acetate.-Mass number 88, 10.67 f 0.05 volt; 61, 11.24 f 0.10; 45, 11.44 f 0.10; 43, 12.31 f 0.20; 29,12.47 f 0.08; 27,15.32 f 0.20. If we compare the values with the appearance potential of argon (15.77), krypton (14.01), xenon (12.14) and other inert gases, we see that all of the gases studied may be used as quenching vapor in conjunction with inert gases. However, as noted in the introduction, the smaller the difference of the potential, but with the polyatomic vapor having the lower value, the more positive will be the quenching action when used in a Geiger-Muller counter. It will be noted that the criterion for good quenching i s more closely satisfied by the fragments of the polyatomic molecules. Thus in the high counting rates where the fragment may be utilized in the quenching action, this factor should be considered. Cummings and Bleakneys have analyzed the data of ethyl and methyl alcohol in terms of the struchre of the molecule. The discrepancies introduced by their data, that is, the appearance potential of mass 45 in ethyl alcohol is lower than mass 46, is removed by these results. Similar analyses may be made for methyl and ethyl acetate and these will be the subject of a future publication.
LONGITUDINAL DIFFUSION IN ION EXCHANGE AND CHROMATOGRAPHIC COLUMNS. FINITE COLUMN BY W. C. BASTIAN AND L. LAPIDUS Contribution from the Department of Chemical Engineen'ng, Princeton University, Princeton, N . J . Received December 8. 1866
I n a recent communication' the effect of longitudinal diffusion on the effluent concentrations from an ion-exchange column was considered. The equations developed, however, were only applicable to a column infinite in length. I n the present note the authors present the equations for a column of finite length operating under equilibrium conditions. Consider a column of unit cross-sectional are and let c = concn. of adsorbate in the fluid stream, moles/ unit vol. of soh. n = amount of adsorbate on the adsorbent, moles/ unit vol. of packed bed V = velocity of fluid through interstices of the bed z = distance variable along the bed D = diffusion coefficient of the adsorbate in soln. in the bed eo = concn. of soln., admitted to the bed CY = fractional void vol. in the bed kl,k2 = constants in equilibrium relation L = length of bed
A material balance on an elemental section of the bed produces The column is assumed t o be operating under equilibrium conditions and t o initially be free of adsorbate. This can be described by the equations4
+
n = klc kr n = e = 0,t
=O
(2) (3)
I n addition the behavior of the fluid phase must be described at the inlet, z = 0, and at the outlet, z = L, of the bed. One may postulate a number of different representations at these two points but the recent and detailed work of Wehner and Wilhelm2 has shown that those described by Dank(1) L. Lapidus and N. R . Amundaon, THIBJOURNAL, 56,984 (1952). (21, J. F. Wehner and R. H. Wilhelm, personal communication.
