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Low pressure equilibrium of oxygen on charcoal Findlay, Robert Artemas 1935

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Low Pressure E q u i l i b r i u m of Oxygen on Oharooal by Robert Artemas Fin&lay A Thesis submitted f o r the Degree of MASTER OF ARTS i n the Department of CHEMISTRY INDEX. (1) I n t r o d u c t i o n • - page 1 (2) Object of Research — .__— - page 6 (3) Apparatus — -------- page 6 (4) Experimental Method - - page 8 (5) Results . - page 11 (6) Table 1 — — — - page 11 (7) Table 2 - —. - page 12 (8) D i s c u s s i o n of Results — - page 13 (9) Results I I — ,— • page 15 (10) D i s c u s s i o n of Results I I • page 19 (11) Results I I I • page 20 (IE) Descussion of Results I I I — page 21 (13) General D i s c u s s i o n page 21 (14) Summary of Results —.__ page 23 LOW PRESSURE SqUlLIBRIIffi OF OXYGEN GK CHARCOAL INTRODUCTION; Adsorption of oxygen on charcoal has long - been known to he unusual. As f a r baok as 1814 De Saussure"1" n o t i c e d that charcoal continued to take up oxygen over a p e r i o d of months. H.A. Smith i n 1863 8.130 n o t i c e d t h i s , though he found that most of the oxygen went on i n a few hours. He also n o t i c e d the second p e c u l i a r i t y , that only part of the oxygen can he pumped o f f at room temperature, the r e s t only corning o f f at higher temperatures and then i n the form of carbon dioxide and carbon monoxide, This i s s t r i k i n g l y d i f f e r e n t from the behavior of ordinary gases, n i t r o g e n , argon, etc. which reach e q u i l i b r i u m i n a few minutes a f t e r being placed i n contact w i t h charcoal and oan e a s i l y be completely pumped o f f below 100°C i n an unchanged c o n d i t i o n , Lowry and H u l e t t 0 thoroughly i n v e s t i g a t e d t h i s subject at high pressures. They i n d i c a t e d the presence of two kinds of adsorption, - ordinary p h y s i c a l adsorption, the part that; can be; pumped .off;., and. chemical adsorption, 1) De Saussure, G i l b . Asm.. ; 47, 115, (1814) 2) R.A. Smith, Proc. Roy. Soo.; 12, 424, (1865) 3) H.H. Lowry and G.A. K u l e t t , J. A.G.S. ; 42, 1408, 1,1920) the part which comes o f f only as oxides of carbon at high temperatures. The slow adsorption of oxygen i s due to increase of chemically adsorbed oxygen, the p h y s i c a l l y adsorbed part reaching e q u i l i b r i u m i n a few minutes. At l i q u i d a i r temperature, the adsorption of oxygen i s completely p h y s i c a l . I t i s r e v e r s i b l e , i n other words, the oxygen can be pumped o f f , and. i s j u s t l i k e hydro-4 gen or n i t r o g e n . Work has been done by Dewar and l a t e r by Shah^ on t h i s s u bject. At room temperature part of the oxygen can be pumped o f f , and on r a i s i n g the temperature only traces of carbon dioxide come o f f t i l l about 280° C when the rush of oxides of carbon begins according to Shah; Some carbon dio x i d e i s obtained as low as 75°. N i t r i c oxide is s i m i l a r t o oxygen i n behavior. Lo?/ry and H u l e t t found l i t t l e carbon monoxide and d i o x i d e corning o f f above 1090°. How i s t h i s oxygen attached to the charcoal so that i t can not be pumped o f f and drags carbon away rat h e r 6 than leave alone? Several s o l i d oxides e x i s t but they 7 probably have no connection. Langmuir In working on a carbon filament found a s t a b l e oxide coating formed which 4) Dewar, Proc. Roy. S o c ; 74, 127", (1924) 5) Shah, J . Ohem. Soc. ; 2670, (1929) 6) IC.A. ICobe, J . Ghem. Ed.; 8, 222, (1928) 7) Rhead and Wheeler, J . Oh em. S o c ; 2958, (1927) -3-d i d not break up t i l l 1900°0 or so. Rhead and h i s oo-workers conducted a long s e r i e s of i n v e s t i g a t i o n s of the mechanism of combustion of carbon extending from 1912 to 1928. I t was found that the f i r s t step i n combustion i s the formation of a "phjrsi c o-oh em i c a l " complex, C^Oy. The second i s the breaking up of ih.is complex to g i v e carbon monoxide and d i o x i d e . This adsorbed oxygen i s quite a c t i v e . Calvert showed that i t would o x i d i z e e t h y l a l c o h o l to a c e t i c a c i d and Alexander King e x t r a c t i n g charcoal which had been exposed to a i r found o x a l i c a c i d . He l a t e r farnd t h i s d i d not work i f the charcoal was thoroughly d r i e d f i r s t . The process c o n s i s t e d of o x i d a t i o n of the water. Hydrogen peroxide i s obtained on e x t r a c t i o n w i t h d i l u t e s u l p h u r i c a o i d . Heats of adsorption of oxygen on charcoal are unusual. At l i q u i d a i r temperature the heat of adsorption of oxygen, n i t r o g e n , argon and carbon monoxide are almost the same according to Dewar"P Eeyes and Marshall"5""1" u s i n g an i c e c a l o r i m e t e r found that the heat at 0°C s t a r t e d at 72,000 c a l o r i e s f o r i n i t i a l concentrations and f e l l o f f 8 C a l v e r t , J . Ghent. S o c ; 20, 293, (1867) 9) A. Z i n g , J . Ghem. S o c ; 842-6 (1933) J . Chem. S o c ; 22-6 (1934) 10) Dewar, Compt. Rend.; 139, 261, (1904) 11) J.Am. Ghem. S o c ; 49, 156,(1927) _4-rapidly to a constant value of 4,000 calories as the 1' concentration on the charcoal increased. Garner and McEie ' found a lower i n i t i a l value with, a maximum, at a higher concentration (1x10 moles per gm.) Marshall and Bramston-Cook dispute this and Bull, Hall and Garner using a different method of introducing the oxygen, as suggested "by Dr. Marshall, find Marshall and Bramston-Cook correct. The i n i t i a l heat of about 90,000 calories is approximately equal to the heat of formation of carbon dioxide. (94,000) Carbon monoxide is 26,000 calories. Garner"^ finds heats higher at 100°-450°C rising to 224,000 calories at 450°C. (1927) In view of his recent work these values are doubtful. Very l i t t l e work has been done on adsorption 15 in general at low pressures, and less on oxygen. Rwe worked on carbon dioxide, nitrogen and oxygen using a -5 Mcjueod guage down to 1x10 mm. He fo:und oxygen very similar to carbon dioxide, the pressure-concentration curves being parallel. For oxygen the pressure varied directly -3 as the concentration at pressures below 2.5x10 mm. This 5 line cut the pressure axis at 8 x 10 mm. He found the 12) Garner and McKie, J. Chem. Soo.; 2451, (1927) 13) Marshall and Bramston-Cook, J. A.G.S.; 51, 2019, (1929) 14) Bull, Hall and Garner, J. Chem. Soo,; 837, (1931) 15) Howe, Phil . Mag. (7); 1. 109-31, (1926) -5-rate of adsorption quite fast u n t i l higher concentrations. L.A. Swain in a thesis at this University in 1932 measured pressures for oxygen on charcoal at liquid air and room temperature using a 500 oo. Mcleod. He found a high pressure at zero concentration followed "by a slight drop, then a linear rise as the concentration increased. For liquid air temperatures his pressures were of the same order, for similar concentrations, as the room temperature ones. At both temperatures the pressures were .irreversible. If he allowed the gas to expand off from the charcoal the pressure was not "the same as i f he added oxygen and allowed the pressure to f a l l to a steady value* It was lower. Work conducted by myself"*"''' on the process of outgassing of charcoal indicated that i f pumping were not continued while the furnace was cooling, some gases might be l e f t adsorbed on the charcoal and be driven off on adding a new gas, thus giving a false pressure. The removal of this gas might explain Swain's irreversible pressures. 16) L.A. Swain, Thesis, U.B.C. (1933) IV) Pi.A. Findlay, Thesis, U.B.C. (1934) -6-OBJECT OF RESEARCH: In view of Swain's anomolous results i t was t decided, to analyze, by micro-gas-analysis methods, the gases causing the pressure over the charcoal. APPARATUS: The apparatus is similar to that used by Swain. The charcoal is contained in a clear fused quartz tube. Pressures are measured with a 500 oc. MeLeocL guage sensitive to 0.1 x 10" cm (with luck)-. The oxygen added is measured in a gas pipette. The micro-analysis apparatus is shown in diagram. The gas to be analyzed is pumped off the charcoal with a diffusion pump Djbacked by a Toepler pump T. It is collected in the analysis apparatus AA. This apparatus works on the principle of the MeLeod guage. The gas can be compressed to a small volume and its pressure measured. 5 The stop cock^oan be ppened and the gas admitted to various small chambers -where constituents are removed. Carbon dioxide is removed with liquid a i r , oxygen with a copper sp±ral heated electrically, etc. When any constituent has been removed the mercury in the Mcleod is lowered drawing a l l the gas into its large volume. S can then be closed and the gas. again compressed and its pressure measured. A Glass Tub in 2 TuUj} con-tec/^/no M -7-The v/hole analysis apparatus can be pumped out including the diffusion pump D 1 , by means of a second diffusion pump Dg backed by a Hyvac pump. Volume of pipette — 7.121 cc. Volume of MeLeod • • 550.0 cc. Volume of system — - • --—. 733.0 cc. Weight of Charcoal — . 30.71 gnu Volume of capillary 0 nC o ) in MeLeod of analysis apparatus) )-- 0.072 oo, 1 mm.reading in Analysis ) apparatus is equivalent to) ) —.„__.„ 0.000095 cc, The method of determining these last two values w i l l be described later. A pressure,when compressed, of, say, 50 mm. gives accurate enough readings to have good enough analyses. This corresponds to about 0.00047 cc. -8-EXPERIMEIfTAL- METHOD: The method of pumping gas off the charcoal was open to some doubt as i t had never been used before. It was possible that the diffusion pump might i t s e l f give off adsorbed gases while hot thus contaminating the sample. To test this i t was "thoroughly outgassed with the second diffusion pump D^. Diffusion pump D^ was kept hot during the process. D^  was then shut off and D^ was run with meroury cutoff C up and the gases produced pumped by the Toepler pumpT into.the analysis apparatus and their quantity measured. This gave an idea how much gas came: from the Diffusion Pump It s e l f . Sery l i t t l e was produced, the amount being negligible for i t would be almost undetec-table on analysis. 'TIME 03? P U M P I N G P R E S S U R E I E " 0^2 (mm.) • Q U A N T I T Y 70 min. 2*0 mm. ... 0.00019 cc. 145 min. 2.5 mm. 0.00023 oo. 100 min 0.8 mm. 0.000075 cc. min. 0.5 mm. 0.000047 cc. 360 (6 hrs.) 1.2 mm» 0.00011 cc. As the apparatus was used, less gas came off. The adsorbed gas on the walls of the diffusion pump was diminishing. The capillary tube C^Cg was standardized as to volume by getting a measured volume of gas in the large Mcleod connected with the charcoal apparatus (which is known to have a volume of ,550 cc.) and pumping this gas into the analysis apparatus. It was then compressed up into the capillary C^ Og and its pressure measured. VOLUME- ; PRESSURE OM VOLUME EQUIVALEM! of 0^2 of 1 mm. 0.0008 cc. 9.3 mm. 0.07 cc. 0.00009 cc. 0.0044 cc. 45.0 mm. 0.074 cc. 0.000097 oo. The second determination is more accurate, as i t was made with a larger quantity of gas. The accuracy of actual analyses made with this apparatus has been thoroughly checked by Dr. M.J. Marshall. The charcoal was now outgassed for 10 hours at 1000°0. 'it-.-.was...allowed to cool slowly with the pump s t i l l running. The pressure on cooling completely was A 0.15 2 10 cm. by an estimate on an approximate mark on the tip of the MeLeod guage. Fifteen hours later the pressiire over the charcoal was 0.27 x 10~6 cm. i -10-The charcoal was now pumped with the diffusion pump-Toepler pump combination to see i f any gas could be obtained. TIME OF PUMPING PRESSURE IN 0-,C? 1 QUANTITY 100 min. .0.9 8.5 x 10-5 This was l i t t l e more than would have been obtained from the diffusion pump alone. There.was not therefore any gas s t i l l on the charcoal. A measured quantity of oxygen was no?/ added and when the pressure had stopped f a l l i n g this was recorded as the equilibrium pressure. After the f i r s t addition* when the pressure was unreadable on the 10 ratio... i t was necessary to find an approximate mark for the 10 ratio by measuring a pressure on the Id ratio:-: and compressing i t t i l l i t read 10 times as much. After a steady pressure.was reached the diffusion pump-Toepler .pump was used to try to. get some gas for analysis. The amount of gas pumped off in various times was measured. For instance,for the f i r s t 10 minutes, the next ten, etc. -11-RESULTS: p a is the pressure after addition of gas has stopped, p is the pressure after some gas had been pumped off. TABLE 1. • CONCENTRATION Moles/gin x 10 4 Pa 6 om. x 10 P om.. x 10^ i TIME TO REACH EQUILIBRIUM 0.0070 0.1 <10 min. 0.0E44 0.7 0.2 0.0494 0.8 . 0.1 12 min. 0.0730 12.0 0.3 0,0873 2.3* 0.2 0.105 1.1 0,2 0.13 7 2.0 0.2 0.163 7.6 0.1 0.175 1.6 0.2 40-60 min. 0.190 2.8 0.2 >45 min. 0.259 5.3 0.295 16.0 0.2 0.361 4.6 0,400 2,1* 60-90 min. ' 0.458 0.6 >90 min. 0,521 2.0 0,2 >7-|- hrs • 0.586 4.8 0.3 * New oxygen was used. I I -12-TABLE 2, Amounts which could he pumped off.. GOKGEETEATIOF Mol/gm. x 10 4 •TIME OP ptaceiiG AMT, PUMPED 0FP P Vol. (cc. x 10 0.0070 fl) 175 min. (2) 50 min. 1.9 mm. 0.3 mm. 18 2.8 0.0244 15 min. 0.6 ram. 5.6 0.0494 (lj 35 min. (2) 27 min. (3) 30 min. 0.6 mm. 0.1 mm. " 0.0 mm. 5.6 0.9 0.0 0.0730 (1) 11 min. (2) 20 min. (3) 35 min. 1.0 mm. 0.3 mm. 0.1 mm. 9.5 2.8 0.9 0.0873 45 min. 2.0 mm. 19.0 0.105 48 min. 1.3 mm. 12.0 0.137 43 min. 6.0 mm. 56.0 0.175 100 min. 2.2 mm. 20.0 0.190 25 min. 2.8 mm. 26.0 0.521 45 min. 2.8 mm. 26.0 0.586 (1) 60 min. (2) 120 min. 7.5 mm. 4.0 mm 70.0 38.0 -15-DISCUS3I0U OF RESULTS: Iflxen oxygen ia added the pressure f a l l s to a very low value, much lower than had been ever suspected before, being immeasurable on the 10 ratio. The higher pressures which- were obtained by Rowe and by Swain were probably due to the presence of impurities. The attainment of the lower pressures was due to oareful outgassing and purer oxygen. (S) The amount of oxygen which can be pumped off at these concentrations can be assumed to be zero. The amount of gas obtained is very l i t t l e more than would be obtained from the diffusion pump alone. Any extra can be assumed to be impurities in the source of oxygen. The -4 amount of gas obtained is of the order of 10 oc. whereas the amount of oxygen on the charcoal is of the order of 50 cc. This would require a very small amount of impurity to give this amount off. (5) The amount coming off f e l l off rapidly with successive periods of pumping so the smallness of i t can not be due to a slowness of desorption. (4) 'Thus we may say that the equilibrium pressure of oxygen on charcoal is zero at these concentrations, (otherwise a diffusion pump could pump off something.) The measurable pressure on adding oxygen is due to some impurity in the gas,- a vey small actual percentage of -14-impurity in the gas which could no l i k e l y he eliminated. This also explains why Swain's pressures were irreversible -5 and Rowe's curve cut the pressure axis at 8 x 10 mm. (6) The oxygen tended to "become more impure on standing in the generator, possibly due to adsorbed gases coming off the walls. Then new oxygen was manu-factured the pressure was much lower. (7) The low pressures (p) after pumping on the charcoal are about as low as any readings can be obtained on the guage even by pumping direct. (8) Ho tendency was observed for a pressure to accumulate over the oharcoal on standing. (9) The rate of adsorption was extremely rapid at f i r s t , equilibrium' being attained in less than ten minutes. The rate gradually becomes slo?/er and at'about 0.5 x 10~ 4 moles per gm, i t becomes very slow. Soon after this the rate became so slow that i t was too long to wait for the pressure to f a l l to zero. To.hasten i t oxygen was pumped off. Measurements of rate were also made. -15-RE3ULTS I I ; INITIAL GONG. Mols/gra. x 10 4 QUANT. ADDED •t P p. em. x 106 0.0244 1.70 cc. 4 min. 900 6 230 7 110 8 34 43 0.8 0.361 2.64 oo. 45 min. 4.3 60 «3 © X 90 2.1 110 2«1 0.400 3.99 co. 45 min. 6 60 4,8 75 4.1 90 3.4 0.6 0.458 4.37 oo, 120 min. 8.6 180 5.6 270 4.5 450 ,3.0 2.0 0.521 4.43co. 45 min. 1140 165 109 290 48 345 35 405 20 21 h r s . 45 min. •5.8 -16-IITITIALl GOTO. moIs/gml x 1 0 4 ' QUANT. ADDED 0,586 1.72 oo. 1 nr. 2 nr. 4 nr. 5 hr. 15 min. 40 min. 40 min. 5 min. 45 min. AJfALY; 0,611 0,11 GO pump 20 min. pump 25 min. 22 hr. 40 min. pump 75 min. AMT. OFF 0.0038 oo, 0.0047 co, 0.0014 oo. PIS: Carbon dioxide — Oxygen •-—-— none - 99.2$ 10 min. 1 hr. 20 min. 1 i l l * o 40 min. 2 hr. § min. 3 hr. 15 min. 3 hr. 40 min. 4 hr. 30 min. 6 hr. 45 min. 8 nr. 5 min. 23 hr. 55 mm. 25 hr. 30 min. om. x 10 1900 1030 470 260 146 62 4:4: 5,4 450 88 78 69 45 43 35 26 19 4 3 -17-Sorne gas was pumped o f f to analyze. Amt. taken was 0.0019 eo. The pressure a f t e r t h i s was 1.1 x 10" om. ANALYSIS: Garbon dioxide --- none Oxygen • 100$ A f t e r 16 hours the pressure was 0.6 x 10 cm. Pumping o f f anouther 0.002 oe. reduced the pressure to 0.2 x 10"* Virhich i s equivalent to •sero pressure. INITIAL GONG. Mo Is /gift, x 1 0 4 QUANT. ADDED '% cm. x 1 0 5 0.613 1.30 cc • 1 Ill* © 27 min. 3140 1 nr. 47 min. 1730 2 hr . 45 min. 810 3 hr . 20 min. 620 3 hr . 50 min. 38 5 q 4 hr . 20 min. 320 .632 2.84 co. 5 hr. 30 min. 2980 5 55 min. 2160 8 in*« 45 min. 840 9 hr . 45 min. 650 11 hr „ 20 min. 480 21 h r . 45 min. 120 23 h r . 15 min. 119 0.012 cc* of gas were pumped o f f to analyze. ANALYSIS: Carbon dioxide 0.44$ Oxygen 98.00$ Nitrogen 1.55$ Removal of t h i s much gas lowered" the pressure to 4.6 x 10" 5 cm, 18 hours l a t e r the pressure had f a l l e n to 0.7 x 10"^ cm. -18-INITIAL GOTO, molts/gm, x 1 0 4 0.672 teUANT. ADDED 3.48 cc. 8 h r . 24 h r . 26 h r . 26 h r , 27 h r . 28 h r . 30 hr. 31 h r . 48 h r . *7 2 iix* © 50 min. 40 ruin. 30 min. 24 min. 35 min. 52 min. cm. x 10 1 unreadable 128 100 75 h r . 20 min, AMT. OFF 0.011 cc. ANALYSIS Carbon dioxide none Oxygen 98.3$ Nitrogen — — 1.7% 96 h r . 99 hr. 93.5 85.5 77.5 (5 2 o 5 56. 5 19.6 *7 e 5 2 ® X 0.0057 cc ANALYSIS: Carbon Dioxide — 0.48$ Oxygen • 91,5$' Nitrogen —•—• 0.740 r/o none 1.57 cc. ,144 h r . 35 hr, 48 hr, 1.7 0,9 0.5 unreadable 120 -19-IITITIAL GOBC.. mols/gm. x 10 0.800 ^UAIT. ADDED 0.17 cc. 0.805 2*23 oc. IB 67 Jars 90 99 115 123 142 147; 166 243 120 h r s . 240 •" ' P ( cm. x 10 144 85 70 53 46 33 30 23 12.5 267 64 DISOUSSIOIT OF RESULTS I I : ; (1) The rat e of adsorption changes very w i d e l y w i t h the concentration on the c h a r c o a l . I t also v a r i e s w i t h the amount added. (2) The tendency seems to he f o r the pressure to approach zero at a l l concentrations s t u d i e d . There i s no evidence of any steady e q u i l i b r i u m pressures. (3) Above 0.800 x 10*"4 mols/gm. the rate seems to get extremely slow, so slow that the change i n pressure i n an hour i s ve r y l i t t l e indeed,, (4) P l o t t i n g of the r e s u l t s show that a unimolecular explanation does not worfc f o r the rate although I t i s a good approximation at lower concentrations or over a short time. l o g p p l o t t e d against t does not give a s t r a i g h t l i n e . -1 f | - --•9 -- -i i 'i ! 9* l a y , 0 is. t I ' . \ ! :> 2 Cone- ( , Quant r- Cone. Added i O&tlx/O $ *vrt*//fnx l-70e:e. ; -¥ A -®d* ed o,II -• i , •• •I- I : • : ; 1 0-0 1 0 0 '4 £ / Zo z z4 -20-(5) As Lov/ry a n c l H u l e t t T s r e s u l t s i n d i c a t e d , the slow adsorption Is due to the slow increase of " f i x e d " oxygen, which has a zero pressure. The analyses show almost pure oxygen; no carbon dioxide worth speaking of was found anywhere. This eliminates, the p o s s i b i l i t y of the 0 x0y complex of Rhead breaking up t o any extant at room .temperature. (6) As the r e a c t i o n at concentrations over 0.8 x 10" 4 mols/gm. i s so slow i t should be p o s s i b l e to determine a curve o f pressure against concentration f o r the p h y s i c a l l y adsorbed oxygen, i f we assume the change i n pressure due to the r e a c t i o n producing f i x e d oxygen i s n e g l i g i b l e over the small time necessary f o r e q u i l i b r i u m i n the p h y s i c a l s t a t e , - a few minutes. RESULTS. I l l : -The pressure changes too r a p i d l y w i t h the conc-e n t r a t i o n of p h y s i c a l l y adsorbed oxygen to be able to do t h i s by adding from the p i p e t t e . To make an estimation of the change, the charcoal i s s t a r t e d w i t h a pressure of about the l i m i t of the guage. A l i t t l e oxygen i s pumped o f f and measured i n the a n a l y s i s apparatus and tire gas allowed to expand o f f the charcoal to an e q u i l i b r i u m v a l u e . This i s repeated. -21-T o t a l concentration on the charcoal equals 0.837 x 10^ moIs/ gm. Room temperature = 18.3 0. TOTAL AMOUNT PUMPED OFF PRESSURE (cm x 10 4) 0.000 cc. 2 2 © IL 0.017 oc. 15.3 0.022 cc. 9.8 0.034 cc. 5 © «3 BISQUESION OF RESULTS I I I ; (1) A l i n e a r r e l a t i o n seems t o f i t these r e s u l t s hut they are not very-good. The v a r i a t i o n w i t h temperature i s very marked. The room was kept at 18.3- 0.1 hut a thermostat might give some improvement. (2) The change i n pressure w i t h concentration i n very abrupt as can e a s i l y be seen. (3) The pressure reaches very close to i t s e q u i l i b r i u m value i n l e s s than f i v e minutes though there seems to be a slow r i s e a f t e r t h a t . GENERAL DISCUSSION: The explanation o f t h i s phenomenon i s somewhat more d i f f i c u l t . I t would seem that on exposed charcoal surface oxygen combines very r a p i d l y to form a s t a b l e complex. The heat of formation of t h i s compound i s very h i g h , - around 90,000 c a l o r i e s . -22-As the carbon surface becomes p a r t i a l l y covered w i t h t h i s complex, however, the oxygen f i n d s i t much harder to combine. This might be explained i n s e v e r a l ways. It may be that the exposed and e a s i l y a c c e s s i b l e carbon atoms are covered and the oxygen has. to d i f f u s e i n t o s m all pores* This woiild h a r dly e x p l a i n the extreme slowness, however, and a b e t t e r idea i s that of d i f f u s i o n Into the ch a r c o a l grains themselves, - a process s i m i l a r to s o l i d s o l u t i o n . This would assume that as soon as the oxygen f i n d s an|exposed carbon atom i t forms a complex w i t h zero e q u i l i b r i u m pressure. The p h y s i c a l l y adsorbed oxygen might c o n s i s t of oxygen atoms h e l d on the carbon-oxygen complex. I t i s obviouly h e l d i n a .very feeble, way and i t doesn't take much of a concentration of i t to give a high pressure. I t i s c o n t i n u a l l y going over to the complex obviously as the l a t t e r has no pressure of oxygen, and so no true equilibrium- pressure f o r oxygen on charcoal could ever be obtained, at l e a s t as long as there were any carbon atoms w i t h f r e e valences to form complex. The heat of adsorption of p h y s i c a l l y adsorbed oxygen i s around 4000 c a l o r i e s , - the f l a t part on Mar s h a l l and Bramston-Cook'.s curve as above 0.8 x 10" 5 mols/gm. ,-the rate of formation of f i x e d oxygen i s so slow that no i c e c a l o r i m e t e r could measure i t s heat e f f e c t , - the r e s u l t i s i t only measures the other. 23-ibo.oth.er explanation, might "be Taylor's a c t i v a t e d adsorption theory. The more exposed oarDon atoms react more e a s i l y producing a higher heat e f f e c t . They react f i r s t . The l e s s exposed produce l e s s heat and do not overcome the energy of a c t i v a t i o n as e a s i l y . The p h y s i c a l l y adsorbed i s i n a h i g h f u g a c i t y s t a t e and i s s l o w l y going over to chemically adsorbed, As to the r a t e of r e a c t i o n measurements they are over only a l i m i t e d range of pressure due -to the l i m i t a t i o n of the McLeod guage used. Temperature c o n t r o l might have been b e t t e r . SUMMARY OF RESULTS: < (1) Oxygen forms a complex w i t h charcoal of zero pressure r a p i d l y on an exposed surface and slowly on a p a r t i a l l y . c o v e r e d one. (2) There i s no true e q u i l i b r i u m pressure of oxygen on charcoal as the p h y s i c a l l y adsorbed i s always going s l o w l y over t o form the complex. (3) There i s no evidence of carbon dioxide being formed at room temperature. (4) The p h y s i c a l l y adsorbed oxygen i s present only In small q u a n t i t i e s as i t has a very high pressure. (5) I t i s shown that extremely small traces of i m p u r i t i e s produce a f a i r l y high pressure. This i n d i c a t e s the d i f f i c u l t y of low pressure work. -24-In conclusion I would, l i k e to express my g r a t i t u d e to Dr. M.J. M a r s h a l l whose assistance has been i n v a l u a b l e during the course of t h i s research. 


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