NOTES
962
are 9.31 f 0.01, 11.14 f 0.04, and 1.83 f 0.02 kcal. mole -I, respectively. The data permit calculation of the entropies of the solids. Starting from Kelley and King's value for the standard entropy of SiC1, vapor (79.1 f 1.0 e.u.),13 one obtains the entropy a t the melting point and 760 mm. by subtracting 7.75 e.u. (from Kelley's specific heat equation),14 and the entropy of the vapor a t the saturated vapor pressure a t the melting point by subsequently adding a pressure correction term (to 0.617 mm.) of 14.14 e.u. By subtraction of the entropy of sublimation: 10350/204.3 = 50.65 e.u. (present data), a value of 34.8 f 1.3 e.u. is found for the entropy of solid SiC1, a t the melting point. This value is close to that obtained in ref. 13 from a recalculation of Latimer's datal2 (35.05 e.u.), and it is still compatible with Latimer's original value (33.85 e.u.).12 The entropy of GeC14 vapor a t the melting point and saturated vapor pressure (0.590 mm.) is obtained similarly by subtracting 6.53 e.u.I6 from the standard entropy of the vapor (83.0 f 0.8 e.u.)16 and next adding a pressure term of 14.22 e.u. A value of 40.4 f 1.2 e.u. for solid GeC14 follows after subtraction of 11,140/221.6 = 50.30 e.u. for the entropy of sublimation (present data). No value for solid GeC14has been listed in the literature. Vapor pressure equations for the liquid tetrachlorides also follow from Kelley's free energy equations." They are of the form logp
=
-A/T
+ B + Clog T
(2)
where C is the difference between the specific heats of vapor and liquid, divided by the gas constant R, and the constants A and B were apparently chosen to fit the experimental data above 10 mm. The present data for undercooled Sic&are about 5% higher than the values obtained from Kelley's expression. Inspection shows that a satisfactory fit to the present data and the higher pressure data can be obtained by a slight adjustment in the values of A and B. For undercooled GeCl,, the data reported in this note are 20% higher. It is not unlikely that this discrepancy is caused by the larger scatter in the experimental points from which the constants of eq. 2 were derived, and by the rather high estimate of the value for the difference in the specific heats. (12) W.M. Latimer, J. Am. Chem. SOC.,44, 90 (1922). (13) K. K . Kelley and E. G. King, Bureau of Mines Bulletin 592, U. S. Govt. Printing Office, Washington, D. C., 1961, p. 83. (14) K. K. Kelley. Bureau of Mines Bulletin 584, U. S. Govt. Printing Office, Washington, D. C., 1960, p. 163. (15) See ref. 14, p. 72. (16) See ref. 13, p. 42. (17) K. K. Kelley, Bureau of Mines Bulletin 383, U. S. Govt. Printing Office, Washington, D. C., 1935, pp. 47, 91.
T h e Journal of Physical Chemistry
Electrolytic Separation and Purification
of Oxygen from a Gas Mixture by Stanley H. Langer" and Robert G. Haldeman Central Research Division, American Cyanamid Company, Stamford, Connecticut (Received September 9, 1963)
Oxygen is usually prepared by fractional distillation of air or electrolysis of water. The latter has the disadvantages of overvoltage, resulting in greater than theoretical power requirements, and the need to separate and dispose of simultaneously evolved hydrogen. An alternate convenient electrolytic technique would be separation of oxygen from air or other gaseous mixtures using porous catalytic oxygen electrodes working in opposite directions with an electrolyte transport and barrier phase between them. Oxygen is selectively removed, transported, and regenerated according to the following equations in basic aqueous solution.
O2
+ 2H20 + 4e +40H-
(at the cathode) (1)
40H- transported through the 40H- +O2
electrolyte to the anode
(2)
+ 2H20 + 4e (at the anode)
(3)
Thus, oxygen is separated from other less electroactive materials which are relatively insoluble in the electrolyte barrier phase. The applied external voltage must be sufficient to overcome the characteristic overvoltage loss for reactions 1 and 3 as well as concentration and ohmic polarization. This may still be considerably less than the 1.5 v. or more generally necessary for the electrolysis of water ; furthermore, no hydrogen is evolved. I n acid solution, reaction may be represented as
+ 4H+ + 4e 2Hz0 (cathode) 2H20 -+ O2 + 4Hf + 4e (anode)
O2
---t
(4) (5)
Using the schemes above and molded catalystpolyethylene or catalyst-polytetrafluoroethylene1v2gas permeable electrodes, we have obtained substantially pure oxygen from air (purity > 97%, mass spectrometrically) with acid, base, and ion-exchange membrane
* Chemical Engineering Dept., University of Wisconsin, Madison 6, Wisc. (1) (a) G. V. Elmore and H. A. Tanner, J.Electrochem. SOC.,108, 669 (1961); (b) W. T. Grubb, "Proceedings of the 16th Annual Power Sources Conference," PSC Publications Committee, Red Bank, N. J., pp. 31-34. (2) S. H. Langer and R. G. Haldeman, Science, 142, 225 (1963).
NOTES
963
-
Table I : O2 Purification Cell Performance D a t a (Cell Initially a t 25’)
Electrodes
Electrolyte
Pt (11.2 mg./i:m.2) on stainless
5 disks of filter paper saturated with 237, KOH
0.44
5 disks of filter paper saturated with 6 N H2SOa
0.23
bteel screen
Pt (9 mg./cm.z) on tantalum screen N i ( 1 . 6 mg./cm.2) deposited o n graphite (14.4 mg./cm.2)
3 disks of filter paper saturated with 23% KOH
electrolyte phases. The purified oxygen is saturated with water vapor as is the inlet air (except when water is added directly to the electrolyte phase). Platinum, silver-carbon, and nickel-carbon have all been found to be effective catalysts for oxygen purification. For the work described here, we have used a purification cell (electrode area 4.9 ems2)aind ancillary apparatus similar to tha,t which we have described previously for hydrogen purification. 2 , 3 The arrangement differs in that ithe impure gas stream impinges on the cathode while purified oxygen emerges from the anode compartment. Voltage and current, coulombicly related to the amount of oxygen purified, are monitored in the same fashion as previously.2 Some operating data are given in Table I. Performance can be improved by varying catalyst, electrolyte, and pore distribution, to minimize gas diffusion polarization. A polarization curve for a high performance cell with platinum black catalyst (9 mg./om.2) is shown in 1.2
y
Initial cell resistance, ohms
1.0
r
t
0.43
----Voltage drop at-given current Volts Amp.
0.62 0.95 1.00 0.78 1.05 1 .os 1.19 1.24
0.05 0.2 0.2 0.1 0.5 0.3 0.3 0.15
Inlet gas 0 2
02 Air 0 2 0 2
Air 0 2
Air
Fig. 1. We have compared the rate of evolved oxygen with current supplied to the cell with both acid and base electrolyte and found the over-all four-electron mechanism to be operative. On this basis, 1 amp.min. is equivalent to about 4 ml. of O2 a t our operating conditions. Where power requirements are a consideration, it would be desirable to have the oxygen react through a two-electron mechanism5 if it were possible. The cell described is useful for pumping and metering oxygen in addition to providing a convenient source of the pure gas. It also provides a tool for studying catalytic materials and mechanisms a t the oxygen electrode and makes possible simultaneous studies of polarization a t both the oxygen-reducing and oxygenevolving electrode. (3) See also J. E. McEvoy, R. A. Hess, G. A. Mills, and H. A. Shalit, Petroleum Preprints, Symposium on Production of Hydrogen, 145th National Meeting of the American Chemical Society, New York, N. Y., September, 1963, p. B61. (4) H . B. Urbach, “Fuel Cells,” Vol. 11, E. G. Young, Ed., Reinhold Publishing Gorp., New York, N. Y., 1963, Chapter 7 ; L. G. Austin, ibid., Chapter 8. (5) W. G. Berl, T r a n s . Electrochem. Soc., 83, 253 (1943); M . 0. Davies, M. Clark, E. Yeager, and E’. Hovarka, J . Electrochem. Soc., 106, 56 (1959), and references therein.
4 I-
J
0
>
A n Effusion Study of the Decomposition
0.8
of Iron(II1) Bromide
-I
n n
4
by R. R. Hammer and N. W. Gregory
0.6
Department of Chemistry, University of Washington, Seattle, Washington (Received October $1, 1968)
0
1.0 CURRENT (AMPERES)
2.0
Figure 1. Performance of 02 purification cell: Pt blackpolytetrafluoroethylene elertrodes; electrolyte, glass fiber disk saturated with 2 ’2 HClOa; internal resistance, 0.07 ohm.
The kinetic behavior of vaporization-decomposition processes appears quite varied and unpredictable. We have been studying a number of reactions of the type Volume 68, Number 4
A p r i l , 1964
