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The equilibrium pressures of oxygen on activated charcoal Waddington, Guy 1929-12-31

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U.B.C. U B R A R Y CAT MO LSi <67. ACC. MO.  <( ^  THE E Q U I L I B R I U MPRESSURESOFOXYGEN ON A C T I V A T E D  CHARCOAL  by Guy WadDington.  A Thesis submitted for the Degree,of' M A S T E R OF A R T S in the Department of CHEMISTRY  The University  of British Columbia  APRIL, 1929.  ^^^ 6 /^/  TABLh O F 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, m u c n work nas been done upon the various pnenomena attending the adsorption by activated charcoal, of various ^ases, such as, oxygen, nitrogen, hydrogen, helium ana many  others.  M o s t 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 ^  ^p  A . - A,<*-  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 h i g h value of the heat of adsorption drops off rapidly as the concentration increases, finally a constant value of about adsorbed oxygen.  reaching  calories per mole of  Keyes and Marshall have also snown that,  this high initial heat is m u c h 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 m a n n e r from that which is subsequently added.  In fact, it  seems that the reaction m a y be more a chemical  combination  (3)  than an adsorption, or it m i g h t be that there are certain 7  "active centers" on the charcoal w h i c h 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 w h i c h 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 m i g h t 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, a n a to determine  tne  composition of the residual gases. Apparatus. The principle features 01 tne apparatus usea are shown in the figure.  The tube A c o n t a m e o  chbm.icaj.iy pure  potassium chlorate to whicn hao Leen aaaeo a trace 01 manganese dioxide.  This was carefully m e l t e a 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 m e r c u r y .  The connection between L ana the mercury  resevoir F was all of glas^, tne level of tne mercury controlled  being  by means of a water pump ana by admitting air  through the stop cock as sho^n. in a quartz tube E .  The charcoal was container  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 w i t h hyorochloric acid and afterwards treatea w i t h hydrofluoric acid, the final a s h content being .267%. hours at 9bO*C.  Before use it was outgassed for  After o u t g a s ^ m g it weighed  grams.  A platinum wound resistance lurnace was used i o r heating.  A n 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 w i t h considerable accuracy.  The  apparatus was exhausted w i t h a mercury difiusion pump by a Cenco-Hyvac oil pump.  backea  With this arrangement it was  possible to obtain a vacuum w h i c h would register no pressure whatever on the sensitive M c L e o a guage employed, below 10**mm.  certainly  (6)  Experimental. After the apparatus had been set up, all accessible parts were baked a n d exhausted simultaneously in order to eliminate, as far as possible, gas adsorbed on the walls. This process w a s repeated until no appreciable  pressure  developed in the apparatus after standing for 24 hours. It was found that m u c h gas could be removed from the walls by m e a n s of a h i g h frequency discharge, this m e t h o d 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 m e r c u r y .  usually required from 6 to 8 hours. to decrease the pressure further.  This  Prolonged heating  failed  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 m e a s u r e the equilibrium pressure was performed as follows.  Oxygen was admitted,  to produce a concentration of about per gram of charcoal.  sufficient  .6'x 10*'moles of oxygen  This was given about o m i n u t e s to  attain equilibrium and the pressure was read off on the guage.  This value was apparently steady and showed  little drop w i t h time.  but  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 w i t h 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 a n d even w i t n the concentrations used, it is believed that the values are not strictly accurate, owing, to the fact that n o t sufficient time was given for a true equilibrium to occur. A t the higher concentrations useu it was  tnought  w o r t h while to m a k e a series of measurements on the rates of adsorption.  This w a s 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 w a s made to obtain an equilibrium pressure in the following m a n n e r .  As before, a  small amount of oxygen was added a n d the pressure  observed,  G was closed, the system evacuated, G was openeu a^ain ana the system allowed to come to equilibrium, taken again.  the pressure  being  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 a t 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 m o l e s 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 M o l e s per gram. .613 X io-'  1.  P x 10 mm.  *  P x 10 mm.  2.19  1.5  1.31  X 10"  3.21  1.94  1.9o  X 10"  3.76  2.^3  2.57  X 10-'  3.89  2.4o  3.19  X 10"  3.8o  2.26  4.95  8.57  4.02  5.87  X 10-' -t X 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 m i n u t e s . Table II. P x lO**mm.  Time in m i n s .  Time in m i n s .  P x 10**mm  40  242  100  34  45  18o  no  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, w h i c h  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 a n d then  oxygen at a pressure of 2.o x 10*cm. w a s 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 m m . 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.  A t 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 m m . 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 m o l e c u l e s 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 w h i c h 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 f^orsha^ 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 m o l e s of oxygen per gram of charcoal, adsorption is almost  2.  instantaneous.  At higher concentrations a large fraction of the oxygen goes on quickly, tne remainder  being aasorbea  slowly. 3.  At small concentrations, after attaining a slow but regular increase in pressure  4.  equilibrium,  occurs.  A value of the heat of adsorption wa^ oLtained of 3,990 calories per m o l e .  (13)  Bibliography. 1.  Proceedings of the Royal Society. 74, 122 ana 127, 1904. Nature, July lo, 187o.  2.  Compte R e n d u  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.  8.  (36) 91, 1919.  Journal of the American Chemical Society. 42, 1401, 1920.  


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