il. A. ORNINGA X D E. STERLING
1014 TABLE Io E.tn.f. obs. cor. to one a t m . Hydrogen press. (ref. cell A) (17.)
HCI/EtOH conon.. M
M e a n activity coeff. of HCI
f i.e., in molar scale
0.000084
0.50820 0.9890 ,000149 ,47955 .9541 .000771 ,39620 .8890 .001030 ,38175 .8751 .001290 ,37100 ,8539 .001780 ,35700 .8061 .003530 .32500 .7431 .006154 ,29920 .6946 .009070 .27940 .6878 .011834 .27035 ,6194 .015130 .26590 .5316 ,021790 ,24395 .5520 ,037241 .22160 .4975 ,037400 .22105 .4958 ,065090 .19555 ,4607 .130180 .I6320 .4234 E" in molar scale (ref. cell A ) = fO.00977 volt; E o in molar scale (ref. cell B; Bee fig. 1) = +0.21245 volt. (I
violet spectrophotometry which was adopted throughout as a routine.6 It is not unlikely that the reported discrepancies might be attributable t o alcohol whose quality was not rigorously defined by the previous workers. The influence of traces of water in alcohol often referred to by some workers as the cause of the observed discrepancies was also looked into. It has been shown that about 0.02
Yol. 58
molar solution of HCl in ethanol could retain up to about 1.3% by vol. of water without any noticeable change in the observed e.m.f.; solutions of lower concentrations are, however, much more sensitive to addition of water. Conclusion. 1. The standard reduction potential of silver-silver chloride in ethanol is found to be +0.00977 volt in molar scale (Le., +0.02190 volt in molal scale), the nature of the electrode process, as suggested thereby, is in agreement with that in aqueous medium, vix., AgCl e --t Ag CI-. 2. The experimental value of mean activity co-efficients of HC1 in alcoholic solutions have been correlated with the Debye-Huckel equation taking into account the extended terms introduced in it. 3. Reproducibility and steadiness of the e.m.f. values of such systems seem to depend primarily on the state of purity and dryness of the medium which must be rigorously defined. Acknowledgment.-My grateful thanks are due to Dr. S. K. Mukherjee, D.Sc., of the department^ of Applied Chemistry, University College of Science and Technology, Calcutta, and to Dr. A. K. Ganguly, D.Sc., a t present a Post-Doctorate Fellow a t the University of Notre Dame, Indiana, U.S.A., for their helpful suggestions and keen interest in the work. I also wish to express my sincere thanks to Prof. B. N. Ghosh, D.Sc., F.N.I., Palit Professor of Chemistry, University of Calcutta, for giving laboratory facilities.
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OXYGEN TRANSFER BETWEEN CARBON DIOXIDE AND CARBON MONOXIDE I N THE PRESENCE OF CARBON
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BY A. A. ORKIKGAND E. STERLING' Coal Research Laboratory, Carnegie Institute of Technology, Pittsburgh, Pa. Received April 90, is64
Radioactive carbon, '214, was used as a tracer to study oxygen transfer reactions between COn and CO in the presence of different carbons in the temperature range from 650 to 900". The reaction was dependent as much upon the physical stat? of the "carbon" as upon composition and the presence of catalytic agents. Oxygen transfer, bptween carbon and oxidizing and reducing gases, is a probable reaction step in the gasification of carbon that explains many observations of the kinetics of the gasification reaction, The tracer technique is discussed as a tool for testing theories on the mechanism of these reactions between gases and solids.
Adsorption and degassing experiments2 show that carbon adsorbs oxygen in chemically bonded structures that decompose upon heating to liberate oxides of carbon. It has been found that a degassed sugar carbon reduces carbon dioxide t o carbon monoxide a t 600°, the extra oxygen of the dioxide remaining on the ~ a r b o n . Experiments ~ in flowing systems have shown that carbons, previously reduced in a stream of hydrogen, have a limited capacity a t about 600' for reducing carbon dioxide to carbon monoxide and that the oxygen deposited on the carbon can be removed by a stream of hydrogen to form water.4 Radioactive (1) Explosives Depart,tnent, E. I. d u Pont de Neinours & Co., Cibhstown, N. J.
(2) H. 15. Cowry and G . A. IIulett, J. A m . Cheni. Soe., 42, 1408 (1920). (3) A. F. Seineclikova and 11. A. ~I.sirk-ICaiiienetRky,Acta Phusicohim., (I.R.S.S., 12, 879 (1940). (4) J . I).P. kIars11, Inst. GasEngra., Coruniiiii. No. 393, 1!>51.
carbon has been used as a trac.er to show that, when carbon dioxide is reduced by carbon, the carbon of the dioside remains in carbon monoxide in the gas phase.j These observations show that carbons r e a d with osidizing and reducing gases to form, or to remove, chemically bonded oxygen on the carbon surface. These reaction& occur a t measurable rates at temperatures below those required for the breakage of carbon to carbon bonds in the gasification reaction so that the oxygen transfer reactions involved in the oxidation and reduction of the carbon surface may be studied under such conditions that there is negligible complication resultiiig from simultaneous gasification reactions. Avtdal)ility of a radioadve tracer, C1* in COZ, enabled measurement of rates of oxygen transfer l)et,ween CO, atid CO over carbon in a system at, ( 5 ) I?. Bouuer aud
J. Turkevicll. 1.Ani. Chem. S o n . , 7 8 , 51'11
(1051).
Nov., 1954
OXYGEN TRANSFER BETWEEN
co AND co:,I N THE PRESENCE O F CsRBON
dynamic equilibrium so that interpretation of data would not be complicated by effects of heat release or of changes in gas composition. Study of these oxygen transfer reactions was considered important because of their relation to the gasification of solid fuels, a process of increasing importance for the production of synthesis and fuel gases. Apparatus and Procedure Apparatus consisted of equipmcnt for flowing various proportions of carbon dioxide and carbon monoxide through a bed of granular carbon and for estimating the proportions of radioactive carbon in the components of effluent gas. Radioactivity measurements were made on the gas samples by use of a mica window Geiger-Mueller counter, Victoreen V.G. 10A, with a 2.5 mg. per window. The gas flowed through a cylindrical cell, 13/8 in. in diameter and 11/8 in. deep, under the counter window. Radioactive carbon dioxide was made by decomposition of barium carbonate with lead chloride.E The barium carbonate, containing (314, was obtained from the Isotopes Division, U. S. Atomic Encrgy Commission. One millicurie of the carbonate was sufficient,, diluting with nonactive carbon dioxide, to produce 250 1. of carbon dioxide giving about 3000 counts per minute. The sample of carbon, 7 g. nominal weight and sized between 8 and 20 mesh U. S. Standard sieves, was held in a 1/2" i.d. Vycor tube set vertically in an electrically-heated furnace. A packing of crushed porcelain and quartz wool served as a sample support and to preheat the entering gas. Temperature was measured by a thermocouple set in the middle of the bed. Calibration indicated constant temperature within 4 ~ 2 'throughout the bed. The sample was preconditioned in non-radioactive gas for three hours under the temperature, gas composition and flow conditions of the experiment. A gas flow of 34 cc. per minute, N.T.P., was used except as otherwise indicated. After reconditioning, gas containing carbon dioxide tagged with El4 was substituted for the non-active gas. When radioactivity measurements on the entering and effluent gas streams indicated complete flushing of the system, counting rates on effluent gas, before and after removal of C02, were made to determine the distribution of radioactive carbon.
Analysis of Data.-Analysis of the data required quantitative relations between counts per unit time and the specific radioactivity and concentration of tagged components. Calibration experiments showed that, under the conditions of the present investigation, dilution a t constant pressure with inactive COz, CO or Na reduced the number of counts per minute in proportion to the ratio of the original to the new volume. Some deviation from this relation might have been expected15*'but none was observed. Specific radioactivities of individual gas components were calculated from observed numbers of counts per minute, on the basis of inverse proportionality with the volume of diluted gas, as the number of counts per minute above background that would have been observed had the counting cell been filled a t normal temperature and pressure with the given component alone. The gas metered into the reaction system had radioactive carbon in the carbon dioxide only. No significant amount of radioactivity was found on the solid carbon following an experiment and the specific radioactivity of the efluent gas was the same as that of the entering gas as long as the temperature was low enough so that gasification was negligible. Exit gases were found to have radioactivity in both the carbon dioxide and the (6) N. Zwiebel, 71, 376 (1949).
J. Turkevich and W.W.Miller, J . Am.
(7) J. T. Kummer, Nucleonics, 3, 27 (1948).
Chem. SOC.,
1045
carbon monoxide, the relative amounts depending on the carbon sample and experimental conditions. These findings were consistent with an assumption that reaction occurred a t the carbon surface as 1
coz Ir ( 0 )+ co 2
The symbol (0) is used to represent an atom of oxygen held on the carbon surface in a chemically bonded structure. For a given state of the solid surface the rates of the forward and reverse reactions are proportional to the partial pressures of the gases entering the reactions. These rates will be represented by hpco, and k,pco, respectively. Since the samples were brought to equilibrium with respect to oxygen transfer in the gas mixture to be used in each experiment, these rates were equal. klpcor =
~ P C O
(1)
The radioactive carbon remained in the gas phase either in the carbon dioxide or the carbon monoxide, The sum of the partial pressures of these gases, each multiplied by its specific radioactivity, was the same for entering and effluent gases. The partial pressure of each gas component, multiplied by the ratio of specific radioactivities of the given component and the original carbon dioxide, was treated as the equivalent partial pressure of radioactive gas and will be designated here by an appropriate symbol with an asterisk. The conservation of radioactivity in the gas phase can be expressed with such symbols as P60,
+ P 8 0 = PO*
(2)
The transfer of radioactivity from the COZ is a first-order process.8 It satisfies a rate equation of the form
where the ratio, W/F, of sample weight to gas flow rate a t the temperature and pressure of the experiment, is used as a measure of contact time. Equations 2 and 3, when all of the radioactivity is initially in the COz, lead to the following solution
The numerator of the last term in the logarithmic factor is the fraction of the total radioactivity found in the CO, while the denominator is the fraction of CO in the mixture of CO COZ, and hence the value toward which the numerator approaches with increasing time. Equations 1 and 4, giving the ratio and the sum of rate constants, were used to calculate the forward and reverse constants, k~ and kz, respectively, from the gas composition and observed change in distribution of radioactivity. Table I gives experimental data for various materials. Figure 1 shows the effect of gas composition and temperature upon the reaction rate for the high temperature coke listed in Table I. Figure 2 gives similar data for Graphite 11.
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(8) R. (1946).
B. Duffield and M. Calvin, J . Am. Chem. Xoc.,
6 $ , 5.57
A. A. ORNINGAND E. STERLING
1046
Vol. 58
TABLD I DATA ON OXYGEN TRANSFDR Material
Porcelain Porcelain Graphite I Activated coconut char Activated coconut char Coke Leached coke Coke 6% K&Oa 6% K&Os Porcelain 6% K&Os Porcelain
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Ash, %
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