T H E SORPTIOX O F WATER VAPOUR BY ACTIVATED CHARCOALS PART 111. ISOTHERnIALS I N P R E S E S C E OF X I T R O G E S BY A . J. ALLhlASD A N D P. G . T. H A S D
1. Introductory \Then considering the data set out in Part 11.' of this series, it appeared possible that the oxygen present in the air stream might be exerting a specific effect on the results. For example, oxygen is slowly taken up by charcoal at room temperature in such a way that subsequent removal is only possible in the form of oxides of carbon. It was reasonable t'o suppose that the surface of the charcoal might thereby undergo such an alteration as to change its sorptive properties. Recent work of Rideal and Kright? and of Garner and 1\IcKie3would support this conclusion, as far as a true adsorption process is concerned, whilst Coolidge4 definitely states that the presence of oxygen appears to alter the sorptive properties of charcoal. Against this is to be set the recent statement of Magnus and Rothj that two per cent of oxygen in the hydrogen used in investigating the sorption of hydrogen-carbon dioxide mixtures by charcoal out-gassed a t 450' has no appreciable effect on the result. A second possibility was that oxygen has a specific effect in retarding the setting-up of sorption equilibrium, as Harned6 had found the rate of sorption of chloropicrin to be retarded if the charcoal used contained oxygen. Such an effect, of course, would be anticipated to be small in comparison with the great retardation in sorption velocity naturally caused by the mere presence of a n y admixed gas. To test these possibilities, we therefore carried out experiments on Charcoals d and C by a dynamic method in which the air stream was replaced by one of purified nitrogen (for experimental details, see Part I.' ) I t may be mentioned that, in both the experiments with Charcoal C, as also in that with Charcoal A, outgassed a t gooo, the total time of passage of the moist nitrogen stream was between 1400-1joo hours for the complete sorption-desorption cycle. In the case of Charcoal -4,outgassed at z;oo, where the maximum water vapour pressure used was only 20.06 mm. (saturation value 23.;6 mm.), this duration was only 430 hours. Assuming (see Part I.) that the proportion of oxygen by volume did not exceed I :1'4 X IO?, and that the rate of passage of gas was 7 5-80 C.C. per minute, the maximum quantity of oxygen passed into the charcoal tube in the course of a complete run would be about 12 mg., i.e. 8-9 mg. per gram of charcoal, of which only a fraction would be absorbed.
*
J. Phys. Chem., 33, 1151 (1929). Soc., 127, 134; (1925). J. Chem. Soc.. 1927,2 4 j 1 . J. Am. Chem. Soc., 46, 596 (1924). Z. anorg. Chem., 150,311 (1926). J. Am. Chem. SOC.,42, 372 (1920). J. Phys. Chem., 32, 452 (1928).
* J. Chem.
'
A . J. ALLYAPiD .4SD P. G . T. H A S D
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It should be added that, for the experiment with Charcoal C previously outgassed at 2 i o o , a specimen was used which had been extracted by concentrated hydrochloric acid and in which the ash content had been reduced to some sixty per cent of that of the unextracted material.' 2. Rates of
Sorption and Desorption
The actual lengths of time of passage of the moist nitrogen stream found necessary for the readjustnient of equilibrium when passing from one point
R PRESSNRC
I."
FIG.I
to another on the isothermals were much as in the air-stream experiments (extreme limits 8. j and 241 hours). Expressing, as before, the rate of sorption or desorption by AlT, W? x A t x A p where Awl is the loss or gain in water by a weight w2 of charcoal in time A t , and A p is the difference between the initial and final equilibrium aqueous vapour pressures, precisely the same conclusions are drawn as from theair experiments, viz. (i) Other conditions being similar, there is little difference between the charcoals employed, as also between charcoals outgassed a t 2 i o o and a t 800". Charcoal .A ( 2 70') however came to equilibrium more quickly than Charcoal h (800~). (iij the sorption process is more rapid than the desorption process at higher, and less rapid a t lower, pressures. (iii) there is a marked increase in the rate of sorption or desorption in the intermediate pressure range, as compared with the rates at immediately higher and lower pressures. In addition, it was definitely shown that, in cases where the isothermal exhibits an inflexion a t high pressures t,hereby tending to approach the saturation pressure asympotically) the velocity of sorption (or desorption) increases again as the pressures approach saturation. This had already been noticed in the air stream experiments in the only case in a-hich data were available with such an isothermal. (Charcoal A outgassed a t goo"). The ~
1
Part
I,p. 453.
S O R P T I O S OF WBTER TAPOUR BY ACTIVATED CHARCOhLS
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same behaviour was now found in the nitrogen stream experiments with both A and C charcoals outgassed at 800". The point' is one of interest, in that, like the regularit'y noted under (iii), it has a probable bearing on the structure of the charcoals. It may be added that Charcoal A outgassed at 2 j o D also gives isothernials of this type, but that, in neither the air nor the nitrogen stream experiments, are the data available for seeing whether the velocity relations correspond. Diagrammatically, these velocity relations can be represented as in Fig. I . Finally, the velccities found in t'he nitrogen stream were quite similar to those obtained in the air stream-no systematic difference could be detected. Under the conditions of experiment, then, oxygen has no retarding effect on the processes concerned in the sorption of water vapour by charcoal, a result which accords with anticipation. 3. Results An example of the results of a complete experiment using an air stream has been given in Part 11. The course of an experiment using a nitrogen stream was very similar and there is no need to reproduce one i n extenso. As in part 11, our data will be presented in the form of isothermal diagrams (v), together with information on (i) the outgassing process; (ii) the sorptive power of the charcoal for dry nitrogen prior to the passage of the moist nitrogen stream; (iii) the increase in iveight after the final passage of the dry nitrogen stream (referred to the weight of the original evacuated charcoal); (iv) the results of the subsequent evacuation of the charcoal.
(a) Charcoal A-wacztated at 270OC. (i) A preliminary outgassing at room temperature, followed by three hours at 26oo-2jo0. (ii) 11.06 mg. gram. (iii) 12.64 mg.,'gram. (iv) j . 9 mg. gram of water recovered by evacuation a t z j o ' . The evolved gases amounted to S? 9 . 3 nig.,'gram 0 2
0 . j
c024.5
CO 0 . 0 7 (Y)See Fig. P A . It will be noticed that the aqueous vapour pressure was not pushed beyond 20.06 mm. (23.76 mm. is saturation pressure). (b) Charcoal A-ei,acunted at 800°C. (i) A preliminary outgassing at room temperature, followed by 3: hours a 780"-800°. (ii) 16.65 mg.,'grani. (iii) 43.53 mg., gram. (iv) 9.7 mg.,.'gram of water recovered by evacuation at
270'.
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A. J. ALLYAND A S D P . G . T. H A S D
The evolved gases amounted to
li? 0 2
COz CO
10.1
mg./gram
0.07
Ij.5
8.3
FIG. 2 2376
20
15
IO
A
5
0
FIG.3
The final evacuation temperature was 800". (v) See Fig. zB. (c)
Charcoal C-evacuated
at 270°C.
(i) Preliminary outgassing at room temperature, followed by five hours at 260'-2 70'. (ii) 10.46 mg./gram.
(iii)
I j.j~
mg./gram.
SORPTION O F T A T E R VAPOCR BY ACTIVATED CHARCOALS
(iv) 4.7mg./gram of water recovered by evacuation at
1 16 j
270".
The evolved gases amounted to N? 9.8mg./gram Con 3.5 CO 0.4 (v) See Fig. 3A. (d) Charcoal C-evacuated at 800°C. (i) Preliminary outgassing a t room temperature, followed by 41 hours a t 780"-800".
(ii) 22.68 mg./gram. (iii) 40.02 mg./gram. (iv) 9.2 mg./gram of water recovered by evacuation (90% of it at 2 7 0 ' ~ the residue at Soo", the final evacuation temperature). The evolved gases amounted to Nz 9.9mg./gram CO' 2 1 . 0 CO 9.j (v) See Fig. 3B. 4. Discussion
I n Table I, certain data obtained in the above experiments are collected together, all figures quoted representing mg. of sorbate per gram of sorbent.
TABLE I Charcoal d Outgassed Outgassed at 270" a t 800'
Kitrogen sorption Saturation figure Final dry nitrogen figure
Charcoal C Outgassed Outgassed at 270" at 800"
11.06 -
16.65 419 3
10.46 394.5
22.68 483.6
12.64
43.53
13.71
40.02
The isothermals, on the whole, resemble those described in Part 11, although there are differences in detail. There is again considerable hysteresis, of about the same magnitude as observed previously. (Charcoal B, which in the air-stream experiments, showed this phenomenon to the greatest extent, was not worked on in nitrogen.) The air and nitrogen curves for Charcoal A are very similar both for " 2 7 0 ~ C." and for rr8000C." charcoals, and it is reasonable to imagine that if the full pressure range had been worked over with the "270' C." charcoal in nitrogen, the similarity would have been still more marked, particularly in the low-pressure regions of the desorption curves. The increased amount of water held a t "zero" water vapour pressure by the r r 8 0 ~ o C . charcoal " in the nitrogen stream as compared with the air-stream experiment is undoubtedly closely connected with the increased saturation figure.
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I n the case of Charcoal C, the “27ooC.” isothermals in air and in nitrogen are again very similar. But this is not so with the “ 8 0 0 ~ C . charcoals, ~’ except over the range of intermediate pressures. Both at lower and at higher pressures, considerably more water vapour has been taken up in the nitrogen experiment; and indeed, to such an extent that, at pressures near saturation, the isothermal is convex to the pressure axis, as is always the case for Charcoal d. The general effects of a higher outgassing temperature are, in all respects, as found for the air-stream isothermals and, in fact, are even more marked. A fuller discussion is reserved for a later paper. Cnic3ersity o j London, Kang’s College, April 17, 1929.
