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The equilibrium pressures of oxygen on activated charcoal 1929

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U.B.C. UBRARY CAT MO ACC. MO. LSi <67. ^^^ 6 <( ^ / ̂  / THE EQUILIBRIUM PRESSURES OF OXYGEN ON ACTIVATED CHARCOAL by Guy WadDington. A Thesis submitted for the Degree,of' MASTER OF ARTS in the Department of CHEMISTRY The University of British Columbia APRIL, 1929. TABLh OF CONTENTS Pa^e Introduction 1 Object of Investigation 3 Apparatus 4 Experimental Details 6 Results & Discussion of Results 10 Summary Bibliography 13 TRE E^UILIRnlUin uF uXYGEiJ un ACTIVATED CnA^COA^. Introouction. 1 Since the time of Sir James Dewar, mucn work nas been done upon the various pnenomena attending the adsorp- tion by activated charcoal, of various ^ases, such as, oxygen, nitrogen, hydrogen, helium ana many others. Most of the investigators in tnis field nave measured tne absorption at pressures greater than one millimetre of mercury, ^na there are few published aata on the ausorptive powers oi activateu cnarcoai at t^e very low pressures which are of interest tu tnuse en^agea in high vacuum work. G. Clauae has measured the adsorption of hyarogen, helium, neon ana nitrugen at ^ow tempeiatures ana aown to pressures of .00o millimetres of mercury. b Miss. Iaa F. Homfray has investigated in some detail the amounts of various gases adsorbed at aiiferent temperatures ana pressures, but nere a ^ a m we f m a that the range of pressures investigateu did not extend down to the lower limit. 4 More recently, Rowe has measured the equilibrium of oxygen, nitrogen, carbon monoxide and carbon dioxiae over a pressure range of 10**mm^ to 10***mm. at room (a) temperatures. He arrives at the conclusion that for oxygen the equilibrium pressure, at pressures below k.d x 10 mm. is a linear function of the quantity of gas aasorbea, while at higher pressures the equilibrium is better representea by an equation of the type ^ A. - A,<*-^ p where A. and A. are constants ana<*. is the quantity adsorbed This equation is more satisfactory than the familiar Freundlich aasorption isotherm Turning from pressures to heats of adsoiption we 5 find that Keyes and Marshall have shown that the initial heat of aasorption of oxygen on activated charcoal reacnes the high value of 72,000 aalories per mole, while Marshall 6 and Bramston-Cook nave obtainea a value of 89,6<JU calories per mole for the heat of adsorption at zero concentration. This high value of the heat of adsorption drops off rapidly as the concentration increases, finally reaching a constant value of about calories per mole of adsorbed oxygen. Keyes and Marshall have also snown that, this high initial heat is much greater in the case of oxygen other gases not exhibiting such a high initial value. This suggests that the first oxygen adoed to a freshly "outgassed" charcoal surface is-held in a different manner from that which is subsequently added. In fact, it seems that the reaction may be more a chemical combination (3) than an adsorption, or it might be that there are certain 7 "active centers" on the charcoal which hold the adsorbed gas more tenaciously than does the remainder of the surface. 8 These views are supported by the work of Lowry and Hulett who found that only one half of the adsorbed oxygen could be recovered as such, the remainder coming off as carbon dioxide and carbon monoxide when outgassed at hign temperatures. Object. The fact tnat little work nas been done.upon equilibrium pressures at very low pressures, and that the heats of adsorption of oxygen do not vary linearly at low pressures, suggested that interesting results might possibly be obtained by measuring the equilibrium pressures of oxygen upon activatea charcoal at veiy low pressures. Ana it was also thought, that by measuring the equilibrium pressures over a range of temperatures, that data would be obtained which would enable the heats of adsorption to be calculateo by means of the Clausius Clapeyron equation, which could be applied in this case since any slight change in the pressure would have but little effect on the actual concentration of gas on the charcoal. At the low pressures it was proposed to work with, any slight traces of impurity in the oxygen present would invalidate the results. These impurities might be derived from the admitted oxygen, or might possibly be displaceo from the charcoal surface by the adsorbed oxygen. A third (4) possibility presents itseii; that carbon monoxioe ana caibon dioxide are formea as the oxygen aasorbs on tne surface. Hence it was proposed to analyse the residual ^ases over the surface of the charcoal. To sum up; in this investigation it is proposeo to measure the equilibrium pressures of oxygen on activatec charcoal over a range of temperatures, ana to determine tne composition of the residual gases. Apparatus. The principle features 01 tne apparatus usea are shown in the figure. The tube A contameo chbm.icaj.iy pure potassium chlorate to whicn hao Leen aaaeo a trace 01 manganese dioxide. This was carefully meltea ana exhaustea several times in order to remove water vapor ano otner gasc,s which might possibly be present. The bulb B, of about 2o0 c.c. capacity, was used to store tne oxygen. A ooo c.u. I^cLeoa guage (D) was used to measure the resiu^ai pressures ol oxygen over the charcoal. This gua^e wa^ carefully standardized and was sensitive down to pressures of lo'^u... of mercury. The connection between L ana the mercury resevoir F was all of glas^, tne level of tne mercury being controlled by means of a water pump ana by admitting air through the stop cock as sho^n. The charcoal was container in a quartz tube E. The quartz tube was lined with platinum in order to prevent any possible interaction between tne charcoal ana silica wnile outclassing.  (5) The charcoal used was a sample 01 couoanut charcoal obtained from the National Carbon Co. It was washed in a Soxhlet extractor with hyorochloric acid and afterwards treatea with hydrofluoric acid, the final ash content being .267%. Before use it was outgassed for hours at 9bO*C. After outgas^mg it weighed grams. A platinum wound resistance lurnace was used ior heating. An electrical pyrometer was used to checK tne resistance furnace. Plotting amperes against temperature we obtain a straight line over the range 900*to i,0u0*C. The electrical pyrometer had previously been checkeu against a constant boiling point sulphur bath and nad been founo. accurate to within one aegree, hence tne outgassin^ temperature was known with considerable accuracy. The apparatus was exhausted with a mercury difiusion pump backea by a Cenco-Hyvac oil pump. With this arrangement it was possible to obtain a vacuum which would register no pressure whatever on the sensitive McLeoa guage employed, certainly below 10**mm. (6) Experimental. After the apparatus had been set up, all accessible parts were baked and exhausted simultaneously in order to eliminate, as far as possible, gas adsorbed on the walls. This process was repeated until no appreciable pressure developed in the apparatus after standing for 24 hours. It was found that much gas could be removed from the walls by means of a high frequency discharge, this method being particularly applicable where a flame could not be used. The charcoal was out^assed at 9bo*C until the pressure fell to approximately 6 x 10'mm. of mercury. This usually required from 6 to 8 hours. Prolonged heating failed to decrease the pressure further. In oraer to be sure that all undesirable gases were removed from the charcoal surface, it was saturateo with oxygen several times, outgass&ng alter each addition of oxygen. The first attempt to measure the equilibrium pressure was performed as follows. Oxygen was admitted, sufficient to produce a concentration of about .6'x 10*'moles of oxygen per gram of charcoal. This was given about o minutes to attain equilibrium and the pressure was read off on the guage. This value was apparently steady and showed but little drop with time. The stop-cock G was then closed and the system evacuateo completely. G was then opened and after o minutes the pressure was read off. This value increases slightly with time but never reached the pressure value ob- ( ? ) tained immediately after admitting the oxygen. It is assumed that the true equilibrium lies between tnese two pressures. A series of readings were made in this fashion from concentrations of .6 x 10'"to 8 x lu^moles per g. of charcoal. Above this concentration equilibrium took longer to occur and even witn the concentrations used, it is believed that the values are not strictly accurate, owing, to the fact that not sufficient time was given for a true equilibrium to occur. At the higher concentrations useu it was tnought worth while to make a series of measurements on the rates of adsorption. This was done merely by taking pressure measurements at regular time intervals until the pressure showed some signs of reaching a constant value. A further careful attempt was made to obtain an equilibrium pressure in the following manner. As before, a small amount of oxygen was added and the pressure observed, G was closed, the system evacuated, G was openeu a^ain ana the system allowed to come to equilibrium, the pressure being taken again. Next G was closed again, the system evacuated and then oxygen was admitted to the system at a pressure somewhat greater than the approximate equilibrium pressure. The stop-cock G was then opened, and the pressure re^d after 5 minutes. In this fashion fairly consistent and reproducible results were obtained. the last small addition of oxygen would have a negligible effect on the concentration of oxygen (8) already on the charcoal. These readings were repeated at 0°C. and at room temperature which was C. Results. The first series of equilibrium pressures obtained are given in Table 1 and are plottea on Graph 1. Concentrations are expressed in moles of oxygen per gram of charcoal. P is the pressure after the oxygen has stooo over the charcoal for 5 minutes, and P is the pressure ob- tained after closing G, evacuating tne system, tnen opening G and reading the pressure after equilibrium has been reached. As was statea before, the true equilibrium probably lies between these two \alues. Table 1. * Moles per gram. P x 10 mm. P x 10 mm. .613 X io-' 2.19 1.5 1.31 X 10" 3.21 1.94 1.9o X 1 0 " 3.76 2.^3 2.57 X 10-' 3.89 2.4o 3.19 X 10" 3.8o 2.26 4.95 X 10-' 8.57 4.02 5.87 X -t 10 2<,.l 10 6.48 X 10" 21.3 13.1 7.9 X 10-' 32.3 lo.o 8.03 X 10" 38.4 17.5 The results of the study of rates of adsorption are given in Table II and are plotted on Graph II. The (9) t concentration of oxygen on the charcoal before measurements were commenced was 37.42 x lO^moles per gram wnile the fraction admitted was 4.94 x lo'mole. The range of tne McLeod guage was such that measurements could only be commenced after 40 minutes. Table II. Time in mins. P x lO**mm. Time in mins. P x 10**mm 40 242 100 34 45 18o n o bO 50 146 120 24.0 00 116 130 23 60 95 140 20 6o 85.0 loO lo.3 70 70.o 160 17.3 75 63 170 15.8 80 53 180 14.1 85 46.5 190 13.1 90 42.5 200 12.1 95 39.5 <=<o 0.95 The following are the results obtained from an attempt to find the equilibrium pressure at two temperatures. The concentration of oxygen on the charcoal was 2.04 x 10 ** moles per ^ram. It was allowed to stand for 24 hours, which at that concentration would be ample time ior equilibrium to be attained. The stop-cock G was closed and the system exhausted, G was then opened, a pressure of 1.88 x lO^mm. (10) was observed. The system was exhausted again and then oxygen at a pressure of 2.o x 10*cm. was added, the pressure observed was 2 x io'mm. Further addition of two small quantities of oxygen both gave pressures of 2.U4 x io'mm. The mean pressure of the four measurements being 2.04 x 10 mm. at 2<?C. The same procedure was repeatea at 0*C, a mean pressure of 1.178 x io'mm. being obtained. Applying these results to the integrated form oi tne Clausius Clapeyron equation ) f- . Q f i . -L \ we obtain for the heat of adsorption 3,990 calories per mole. Discussion oi Results. From Graph I we see that at very low concentrations the equilibrium pressure is a linear function oi tne amount of gas adsorbed, while at higher concentrations it oeparts from this linear relationship. It is possible tnat if more time had been allowed for the gases to come to equilibrium, that a linear relationship would have been Observed even at the higher concentrations. It was observed that only at concentrations below 2.5 x 10'moles of oxygen, per gram of charcoal, was equilibrium attaineo almost instantaneously. At low con- centrations however, activated charcoal appears to a very efficient means of creating high vacuum. In one particular (11) case the pressure was decreased from .478 mm. to .0000219mm. in one minute, i.e. the fraction of oxygen not adsorbeu amounted to only .0000458 of the whole. At higher concentrations of oxygen such as is shown in Table II, although the greater part of the gas goes on very quickly there is always a certain fraction which adds on slowly in a regular manner. Tne rate of aecrease is not a linear function of pressure but will very likely be a function both of pressure and of the condition of the charcoal surface. These observations suggest that the rapid adsorption is due to condensation taxing place on the more active carbon centers first, this being followed by a surface rearrangement of the molecules or by a diffusion into the interior of the charcoal particles. Another interesting observation made was that, when the concentration of oxygen on the charcoal was small, there was a rapid drop to the equilibrium value, this being followed by a slow increase in pressure which occurred invariably when the sample was allowed to stand for any length of time. For example, after standing for 12 hours the pressure increased from 2.67 x 10*mm. to o x 10*mm. It seems probable that this is caused by the formation of carbon monoxide or carbon dioxide, although this can not be stated definitely until an analysis of the gases has been carried out. The heat of adsorption determined by means of the (15) Clausius Clapeyron equation do@JS not agree witn the hi^n s initial heat found by Keyes and Marshall and by f̂ orshâ and Bramston-Cook, but does a^ree quite closely witn tne final heat of 4,000 calories per mole iounu by these investigators. Summary. 1. At concentrations below 2.b x 10 moles of oxygen per gram of charcoal, adsorption is almost instantaneous. 2. At higher concentrations a large fraction of the oxygen goes on quickly, tne remainder being aasorbea slowly. 3. At small concentrations, after attaining equilibrium, a slow but regular increase in pressure occurs. 4. A value of the heat of adsorption wa^ oLtained of 3,990 calories per mole. (13) Bibliography. 1. Proceedings of the Royal Society. 74, 122 ana 127, 1904. Nature, July lo, 187o. 2. Compte Rendu lob, 861, 1914. 3. Zeitschrift fur Pnysikolische Chemie. 74, 129, 1910. 4. Philosophical Magazine. (7) 1 (1926) 109 5. Journal of the American Chemical Society. 49, loo, 1927. 6. Journal of the American Chemical Society. June, 1929. 7. N. K. Chaney, Transactions of the American Electrochemical Society. (36) 91, 1919. 8. Journal of the American Chemical Society. 42, 1401, 1920.  


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