STATIC SORPTION ISOTHERMALS. ADSORPTION OF CARBON DIOXIDE BY CHARCOAL
BY
L.J. BURRAGE
1. Introduction As the result of previous work on the discontinuous nature of sorption isothermais,' in themselves largely the outcome of a new method for the determination of sorption isothermais,' it was decided to carry out a further series of experiments using the static technique. This has the obvious advantage that the discontinuity cannot in any measure be due to the method employed. The criticism which has been levelled at all previous work is that in no cases have points been taken closely enough together, in many inst,ances a dozen points or so being deemed sufficient to cover the complete range of the isothermal. In the present work it was decided to start from a pressure of about 80 mm. and obtain points as close together as was possible, the main object, in view being to determine, if possible, the exact number and position of the breaks. 2. Experimental The technique employed was similar to that described in a previous paper.3 This was adopted after careful consideration of the only other method which was suitable for this type of work-the McBain Sorption Balance. I t is not considered that the latter has any advantages over the method employed in the present work. In the sorption balance the quartz spring has first to be calibrated. This is achieved by noting the elongation of the spring on the addition of known weights to the cup, which is att,ached to the spring. When set up for use, charcoal is placed in the cup and bhe apparatus evacuated. -4 certain charge of vapour is admitted to the system, which is then allowed t o come to equilibrium. One notes the position of the spring with a cathetometer to give the weight of substance adsorbed and reads off the vapour pressure on some type of manometer. This latter involves two readings on the cathetometer, if some type of U-tube manometer is employed. In the method employed in the present, work, it was necessary to know the volume of the apparatus accurately and then for each point on the isothermal four cathetometer readings were necessary, two for the initial pressure and two for the final pressure. I t would appear that the accuracy of the latter method was not in any way inferior to that of the sorption balance, judging by t,he degree of reversibility which has been obtained. 'Alimand and Burrage: Proc. Roy. Soc., 130A (1931); J. Phys. Chem., 35, 1692 (1931). Burrage: J. Phys. Chem., 34, 2202 (1930). 3 Proc. Roy. Soc., loc. cit.
ADSORPTION OF CARBON DIOXIDE B Y CHARCOAL
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Several refinements have been instituted in the method described in a previous paper, and it would be of advantage to describe the method in detail. The complete train 6Fig. I ) consists of a Leybold mercury condensation pump backed by a Hyvac Oil Pump, a McLeod Gauge, a small section of narrow tubing about z-!j inches long with taps a t either end (A and B, the volume between these points being approximately 1.5 cos.), and a freezer C, followed by a mercury manometer E. This is fitted with a tap in the ‘closed’ limb which
A
B
FIG.I is connected to the vacuum line, so that, if necesssary, the manometer may be pumped out on the closed side. This is useful in freeing the mercury from the last traces of air, when the apparatus is first set up. F is a two-way tap, one limb being connected to the vacuum line and one to the air. Attached to this latter limb are a phosphorus pentoxide tube, calcium chloride tube, and a soda lime tube. The container for the charcoal was attached to the common limb of this tap. This container G, made in silica, was exactly similar to that described by Chaplin.1 Between the mercury pump and the tap A, a T-piece was inserted leading to a bulb which was shut off from the apparatus by a tap. This constituted the supply bulb. It has been suggested that interference with the position of the breaks might be caused by the COZon the surface which is constantly being driven Roc. Roy. Soc., 121A, 34.4 (1928).
2274
L. J. BURRAGE
off during the course of the experiment to a greater or lesser degree. Since the chief object in view in this investigation was the establishment of the exact pressures at which breaks occur, and also the quantity increment per break, when these are unaffected by disturbing conditions, it appeared that carbon dioxide might be a suitable substance to employ. Since the carbon complex, CXOY,which covers the surface of the charcoal, exerts a pressure of carbon dioxide at ordinary temperatures, it seemed as though there was a probability that an isothermal with carbon dioxide might be somewhat less disturbed by those complications which have been present heretofore. I t was decided to work with a charcoal which was known to have a large capacity for carbon dioxide' and, D I , a steam-activated coconut charcoal, (packing density = 0,575) was selected. About 2 gms. of this was sieved (10-12 mesh) and heated in an air oven at I 50' to remove most of the adsorbed water to facilitate the evacuation. This is an important point in the evacuation of charcoals, which has been previously overlooked, as it lowers the time necessary for evacuation down to IO-* mm. by at least 7 5%. This half-dried charcoal was packed into the silica container and attached to the evacuation apparatus, and pumped down to IO-* mm. at room temperature. The temperature was then raised to I I O O C . and the charcoal evacuated to the same pressure as before. The container was detached, cleaned with alcohol and ether, and weighed, and finally attached to the apparatus proper, the joint being made vacuum-tight with Everett's Wax. The supply bulb on the apparatus was filled with sodium bicarbonate and the whole apparatus evacuated to a pressure of IO-* mm., the tap on the container being kept shut during this operation. Between the supply bulb and the main apparatus a small U-tube, filled with phosphorus pentoxide, was inserted to free the carbon dioxide from water as it passed into the apparatus. The taps J, leading to the pumping system, D, forming the closed limb of the manometer, and A, leading to the main apparatus were closed. Tap I was opened and the supply bulb H warmed until gas was evolved. A Dewar flask containing melting methylcyclohexane was put on the freezer C and the taps A and B opened to admit a charge into the apparatus, the two-way tap F being open, connecting with G. The freezing bath removes any traces of water which may have passed the phosphorus pentoxide. Tap B was now shut and the pressure read off on the manometer E. Then tap G was opened and when the pressure was constant the manometer was read again. Knowing the initial pressure, the final pressure after sorption, and the volume of the apparatus, the weight of carbon dioxide sorbed was found. This was repeated several times until sufficient carbon dioxide had been adsorbed to give a pressure of about IOO mm. Tap G was then shut, and liquid air placed around the freezer C, the carbon dioxide between B and G being frozen out. The two-way tap F was then turned to admit air and the container removed for weighing. The air that is Allmand and Burrage: R o c . Roy. Soc., 130A, 610(1931).
ADSORPTION OF CARBON DIOXIDE BY CHARCOAL
2275
let into this portion of the apparatus is freed from water and carbon dioxide by passing over soda lime, calcium chloride and phosphorus pentoxide. Immediately the container has been removed, the open end of the tube K, where the container had been attached, was closed by a waxed glass stopper. In this way the container can be removed a t any time during the experiment without interfering with the pressure of carbon dioxide in the apparatus. The container was next reattached, the air being removed from the side arm system by means of a hand pump attached a t L, the liquid air removed and the whole apparatus evacuated down to IO-^ mm. Taps A and B were closed and the container tap opened. When equilibrium had been obtained (which was almost instantaneous) the pressure was read off. Knowing the volume of the apparatus and the pressure, the weight of carbon dioxide lost by the charcoal can be calculated and hence the quantity corresponding to the new pressure. The container tap was now shut and tap B opened and shut immediately, the time interval being approximately 3th second. By this means the pressure in the main apparatus was lowered to a very slight degree. The pressure was read off and the container tap opened, causing the pressure to rise slightly. The carbon dioxide lost by the charcoal was again calculated. Tap A was opened to the pumps and the carbon dioxide between A and B removed. Tap B was then shut and the apparatus was now ready for the next point. I n this manner points may be obtained as closely as one may desire. When the pressure was low it was necessary to remove gas several times before a sufficient drop in pressure was obtained in the main apparatus. If it was desired to remove the container during the course of the run to check the weight, the carbon dioxide in the apparatus between B and G was first frozen out in the manner which has been previously described. The volume between B and G was determined by a sharing method and checked by a second process. A bulb N, provided with a tap, was joined to the apparatus by means of a ground glass joint M, sealed with Everett’s wax. The volume of the bulb below the tap was accurately determined both by evacuation and filling with water-96 ccs. The bulb was filled with nitrogen at atmospheric pressure and attached to the main apparatus which was then evacuated. On opening M, a pressure developed in the apparatus. Knowing the initial and final pressures and the volume of the bulb, the volume of the main apparatus was calculated. The calibration was carried out with the container G in position. This determination was repeated several times, close agreement being obtained. The volume was checked by filling G with charcoal and charging with vapour. In this way the loss in weight of the container gave rise to a certain pressure in a similar fashion to the first calibration. Good agreement was again obtained, the average value being 359 ccs. A correction was applied for the change in volume with change of level in the manometer tube but this was almost negligible, except at the higher pressures. The change in volume does not affect the pressure, since the latter is in equilibrium with the adsorbed carbon dioxide. It affects, however, the calculated quantity of carbon dioxide
L. J. BURRACE
2276
adsorbed, which is in equilibrium with that pressure, but it was found that the error introduced in the calculated weight was exceedingly small. The container was surrounded by a thermostat and the rest of the apparatus enclosed in a constant temperature box by which means an accurate temperature control was exercised. The dead-space volume of the container
73
65
57
49
41
32
was very small and could be neglected since the variation in the weight of carbon dioxide occupying this area was negligible in comparison with the weight adsorbed. In weighing the container a counterpoise of the same material and of approximately the same displacement and surface was used. It should be mentioned that the frozen carbon dioxide, after its liberation from the bicarbonate was exhausted before measuring its pressure at 25°C. in
ADSORPTION OF CARBON DIOXIDE BY CHARCOAL
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the apparatus, to remove any traces of permanent gas, this process being repeated several times. Tests for permanent gas were also made a t intervals during the experiment but the results were always negative. 3. Results Approximately 3 j o points have been obtained on the isothermal within the pressure range 8 I mm. - IO-^ mm. a t 2 jo. This comprises a desorption curve, a resorption, then a redesorption, followed by a second resorption and a further redesorption.
4.0
3.5
?. 0
FIG.3
Finally sorption was carried out to the initial pressure and then desorption to a considerable distance along the isothermal. The pressure measurements were obtained with a cathetometer reading to 0.01mm. and the pressure readings could always be reproduced to this figure. This was demonstrated by freezing out the carbon dioxide and then allowing it to vaporise and also by a redetermination of the meniscus after resetting the cathetometer. For the sake of brevity all tables of figures have been omitted and the complete results are expressed graphically in Figs. 2-4.
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L. J. BURRAGE
One is justified in drawing the curves through each point and not taking a mean, since the experimental error of each point is very low, as is instanced by the reproducibility. If there were any serious error this reproducibility could not be obtained in an experiment employing this technique. It should be noted, in passing, that the breaks are in no way due to drift as there was no change in the pressure o.66-(Point 217) after the charcoal had
FIQ.4
been allowed to stand in contact with the vapour for 76 hours; similarly there was no change in the pressure 0.20 mm (Point 223) after 168 hours, while at 2.48 mm. (Point 304) on the next desorption series thare was no change on standing for 5 months. Once or twice during the course of the experiment the container was detached and weighed and the quantity so obtained compared against that obtained by the ‘pressure change’ method employed in this work. At 0.04 mm. (Point 2 29)
Quantity = 1.23 mgs/gm (Pressure Change) = 1.5 mgs/gm (Weight)
ADSORPTION OF CARBON DIOXIDE B Y CHARCOAL
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At 1.6 mm. Quantity = 2.33 mgs/gm (Pressure Change) (Point 3 19) = 2.6 mgs/gm (Weight)
It was not possible to compare the weights a t the end of the experiment as, owing to an accident, air was allowed to enter the container.
The cause of the discrepancy between the weighed and calculated figures is not known and in any case is very small since a weight error of 0.5 mgm would account for the difference.
L. J. BURRAGE
2280
I n Fig. 5 a selection of points from Figs. 2-4 have been plotted giving the course of the isothermal throughout the entire range investigated. As will be seen from the diagram a smooth curve has been drawn through the points, giving a normal type of isothermal. If this isothermal were plotted on the same scale, which is normally employed for vapours on charcoal ( I cm. = I mm. pressure or I O mgs/gm quantity), a straight line could be drawn through practically all the points; this is in agreement with other results obtained in this laboratory.'
N
Discussion From Figs. 2-4, it will be seen that excellent reversibility has been obtained. The structure of the isothermal was found to be very complicated and a verification has been obtained of the discontinuities which have been found in other cases using different methods. The discontinuities previously noted, have only been observed up to a pressure of approximately 30 mm. In the present work this range has been practically trebled and still the breaks persist up to this pressure. From this it would appear reasonable to predict that they would be found at whatsoever pressure the experiment was carried out, provided that the pressure exceeded 0.1mm. and that the surface of the charcoal was not com1
Allmand and Chaplin: Proc. Roy. Soo., 132A, 460 (1931).
ADSORPTION OF CARBON DIOXIDE BY CHARCOAL
2281
pletely covered by the adsorbed molecules. I n this connection it should be noted that in some high pressure adsorption measurements carried out by McBain and Britton' marked breaks were obtained. The authors, however, do not comment on this. At higher pressures the breaks are almost rectangular, but this form tends to disappear somewhat a t lower pressures when the pressure and quantity increments per break become very small. I n Figs. 6 and 7, respectively, the pressures a t which the breaks occur, and the pressure increments for each break, have been plotted against the sequence of the breaks, commencing a t a pressure of 2 . 7 2 mm., as the pressures and more especially the quantity values of the breaks are too small to allow of accurate definition below that point. One interesting detail, which must be noted in passing, is that there is a very definite break at 0.06 mm., whereas in other cases there are no breaks below 0.1 mm. I n a previous paper2mention is made of a break a t 0.6 mm. in a water isothermal and this was considered to be due to carbon dioxide on the surface which had not been cleaned-up by the water vapour. Since carbon dioxide itself has a break in this region and vapours like carbon tetrachloride, which remove carbon dioxide with, comparative ease, have not, providing that the surface has been sufficiently cleaned, this would appear to be a correct assumption. Allmand and Putticka found a break in this region using carbon tetrachloride, but a desorption isothermal carried out after further resorption to a much higher pressure gave a break a t approximately 0.1 mm., showing that the surface could not have been completely cleaned-up in the first case. I n Fig. 6 , the log of the pressure of the break has been plotted against the sequence of the breaks, calling the first break plotted No. I , etc. and in Fig. 7 the quantity increment per break has been plotted against the sequence of the breaks. Both of these plots show a marked tendency to regularity, especially that representing log P/n. This was only to be expected since the pressure is probably less affected by foreign influences than is the quantity. Assuming that each break marks the completion of a ring of molecules round all the activated points, then if foreign molecules are already present in the space where a given ring will form, the number of adsorbed molecules necessary to complete that ring will be fewer than would be expected and hence the quantity value for a given break is too small. There must, of necessity, be some effect on the pressure, but this will not be very marked unless a very large number of foreign molecules are present. Under these conditions, when the pressure reaches a value a t which the ring should form, it is insufficient to anchor the molecules concerned and a greater force is needed, so as to expel the foreign molecules and thus allow the ring to be formed. Hence the pressure for a given break would be too high. J. Am. Chem. SOC.,52, 2198(1930). Allmand and Burrage: J. Phys. Chem., 35, 1692 (1931). * Proc. Roy. SOC., 13OA, 197 (1930).
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L. J. BURRAGE
Although the diagram representing the curve in which q is plotted against N may not appear to be very regular, yet when it is noted that I cm. on the vertical scale = 0.1mgs/gm, it will be seen that experimental errors will be greatly magnified. In Fig. 3 the curve in the region of 9 mm. pressure is seen to be non-reversible over a range of approximately 0.7 mm. The second desorption cycle is to the right of the first' pointing to a definite cleaning of the surface between the carrying out of these two cycles. Since the points coincide below this portion of the isothermal, it would appear to be due to a purely local disturbance. It cannot be ordinary experimental error as the figures would not have overlapped below this point. I t is not proposed to go int,o any theoretical discussion in detail as that will be reserved until furt,her work, which is now in progress, has been completed. One thing, however, would appear to arise from this investigation, namely, that instead of carbon dioxide on charcoal giving rise to the simplest' conditions, in point of fact it gives rise to the most complicated. It would appear that the carbon dioxide, which is adsorbed, changes over continuously into the complex CXOy, (a kind of surface compound) dependent on the pressure. As the pressure of carbon dioxide is increased more CxOy will form and viceuecsa. This means that with increase of pressure the complex builds itself up outwards from the surface of the charcoal, forming a spongy mass, of which the molecules nearest the charcoal actually form the complex, wit'h a continuous gradation to ordinary carbon dioxide molecules on the surface. This hypothesis tends to shed light on the anomalous results previously obtained with carbon dioxide on charcoal' DI. I t follows from the hypothesis that the amount of carbon dioxide adsorbed depends very largely on t'he degree of evacuation and this, in turn, depends on t,he ease with which carbon dioxide can diffuse away from the charcoal surface. Again, from the hypot'hesis, it follows that the higher the initial charging pressure the more carbon dioxide may be more or less permanently transformed into the carbon complex. Since, in the previous work, the initial charging pressure a t 25' was 4. j 8 mm. and a t 40°, 8.04 mm., the statement that it' would appear that less carbon dioxide was taken up at, 25' than at 40°, could be easily explained. The portion of the isothermal which will be principally affected by these conditions is the section which runs along the quant,ity axis, in other words, that quantity adsorbed, which is so firmly held, that it gives rise to practically zero pressure. If this assumption be correct, it is obvious why such a complex curve should be obtained since one is dealing with two simultaneous effects. ( I ) The sorption of carbon dioxide and ( 2 ) the conversion of some of the previously adsorbed carbon dioxide to the complex CXOy. I t is these two processes occurring together which probably prevents the rectangular steps being observed. -4llmand and Burrage: Proc. Roy. Soc., loc. cit.
ADSORPTION OF CARBON DIOXIDE BY CHARCOAL
2283
Summary A careful study has been made of the carbon dioxide isothermal a t 2.5' over a pressure range 81 mm. - 0.04 mm., some 350 points being determined. 2. The pressure and quantity increments of the breaks have been observed. 3. A hypothesis has been suggested for the complex nature of the isothermal The author wishes to express his thanks to Professor A. J. Allmand for the interest he has taken in this work. I.
.
King's College, London. February 29,1952.
