T H E SORPTIOS OF SULPHUR DIOXIDE, CARBON DIOXIDE, AKD NITROUS OXIDE BY ACTIVATED CARBON BY D. 0. SHIELS
Introduction In view of the probability that adsorption or sorption of a gas by a solid adsorbent depends very largely on the stray field of force about the molecule, or in other words on secondary valence forces of the molecile of gas, it was considered advisable to investigate the adsorption or sorption of gases which have a very similar electronic configuration and in which the secondary valence forces, as judged by critical temperature, surface tension in liquid state, etc., are very similar. The gases selected were carbon dioxide and nitrous oxide. In addition the sorption of sulphur dioxide was measured, as this gas has a much higher critical temperature, and it was thought that a comparison of the sorption isotherm of this gas with those of the first two gases mentioned would be interesting. Table I shows some of the physical constants of the gases concerned-
TABLE I C.P.
K20
co2 so*
75
77 78.9
C.T. 3 5 .O°C 31 .z°C
Viscosity zo°C
148 X IO-^ 148 X IO-^
Ijj.4OC
When this work was commenced (November, 1926) no other work had been published in which the adsorption of carbon dioxide and nitrous oxide by activated charcoal had been compared, although this had been done for the adsorption of these gases by silica gel (see Patrick, Preston and Owens.)l Subsequently to the carrying out of the experimental part of this investigation Gregg2 has published an extensive investigation into the heats of adsorption of carbon dioxide, nitrous oxide, sulphur dioxide and other gases by activated charcoal in which the isotherms have been determined as well as the heats of adsorption.
The Apparatus and Method of Experiment These were very similar to those used for the sorption of sulphur dioxide by platinised asbestos. The activated carbon was contained in the glass container C (Fig. I ) , the volume of which was determined by weighing the amount of water it contained. J. Phys. Chern., 29, 421 ( 1 9 2 j ) . J. Chem. SOC., 1927, 1494.
SORPTION OF SCLPHUR DIOXIDE BY ACTIVATED CARBON
I387
This was connected by means of the ground joint and tap TI with the reservoir R, the volume of which between Tz, T1was also determined as above. To R was attached a mercury manometer Mi and McLeod gauge G. Between this part of the apparatus and the part which supplied the respective gases were two drying tubes D1 and Ds in series containing Merck’s phosphorus pentoxide.
FIG.I
The whole of the apparatus could be evacuated by the mercury vapor diffusion pump backed by water pump. The sulphur dioxide was prepared in same way as in the case of the platinised asbestos experiments. Carbon dioxide was prepared in a similar way but by using Merck’s pure sodium carbonate instead of sodium sulphite. Nitrous oxide was prepared in the following way. Crystals of the pure salt were placed in bulb Bq which was then fused to the bulb Bg. The whole apparatus was then evacuated to low pressure. .A strong solution of pure ferrous sulphate from which the oxygen and nitrogen had been boiled out under vacuum was introduced through Tlz, allowing some of the solution
1388
D. 0 . SHIELS
to remain above the tap, so as to prevent the egress of any air to the apparatus. The apparatus was further evacuated to very low pressure. The pump was then shut off and the ammonium nitrate gently heated to decomposition. The ferrous sulphate absorbed any nitric oxide and a supply of pure nitrous oxide was obtained in the reservoir S. This was in connection with a drying tube D3 containing Merck's phosphorus pentoxide. The tube L was packed with soda lime and was used to absorb sulphur dioxide as much as possible before using the pump, so as to prevent as far as possible corrosion of the iron mercury vapour pump. Activated Carbon This was obtained from a German gas mask and was graded to 12-14 mesh. It was extracted by repeated boiling with concentrated hydrochloric acid and subsequent washing in boiling distilled water until the washings showed no trace of chloride when tested with silver nitrate. The ash content was reduced by this means from 1 2 . 9 to ~ ~less than o.I?. The volume of free space in the container when the activated charcoal was present was determined in the following nay. The density of the evacuated charcoal was determined by taking a known weight and evacuating it a t 2 7 0 O C : to low pressure and then allowing water a t known temperature to enter the container and completely fill the free space. Froni the weight of water its volume could be obtained and, knowing the total internal volume of the container, the volume of the evacuated charcoal was obtained and hence its density. From a knowledge of the weight of the evacuated charcoal used in an adsorption experiment and its density its volume could be calculated and hence the volume of the free space with the charcoal preqent in the container. Method of Determining Adsorption The charcoal was first of all evacuated, and heated, the details of the process being given later. After evacuation of the charcoal the furnace was removed and a large beaker placed around C and the rezervoir R to act a5 a thermostat. The temperature of the water in the thermostat n a s niaintained constant to k0.03"C by a gas thermo-regulator and sinal1 stirrer driven by electric motor. K i t h tap TI shut, the gas for which the isotherm n a s being determined was allowed to enter R to a convenient presbure and then TBwas ?hut. After allolying time for the gac to come to temperature equilibrium the pressure was lead to 0.01 m.m. by means of a travelling microscope mounted on a thick glass plate resting on a solid wooden foundation. TI was then opened and the charcoal allowed to adsorb the gas. The pressure was read at intervals until it remained constant to Tithin 0.02 m.m. for 20 minute period. From a knowledge of the volumes of the vessel C and the connecting tube up to Tap TI, of the reservoir R from T, to a mark on the manometer tube, and of the manometer tube per em. length, the volume of gas adsorbed could be readily calculated.
SORPTIOS O F SULPHUR DIOXIDE B Y ACTIVATED CARBOS
I389
A similar process was gone through for the desorption experiments, but in these cases the vessel C containing a known pressure of gas was exposed to the completely t ;acuated reservoir R and then the equilibrium pressure in C and R measured. Preliminary Evacuation of Charcoal The sorption isotherm of SO, was determined first. The charcoal was evacuated to 4 X IO-^ m.m. pressure in the cold, and then heating was commenced. The evacuation a t 2j0-3oo0C was continued for one hour, a t
1
P
the end of which the pressure was 1.6 X IO-? m.m. The pump was left running during the cooling, the final pressure a t the end of a further half hour being I . j X I O + m.m. The charcoal was then allowed to adsorb SOs, but there was apparently a leak, and so the evacuation was repeated. After evacuation for I hour 1 2 mins. at 3 0 0 T the pressure was z X 10-~m.m. Adsorption experiments were carried out, and then the desorption. C was then evacuated at 25' for I hour. In another hour the amount of gas evolved was 0.084 ccs. The charcoal was then heated to 270°C for an hour and evacuated to 1.3 x IO-^ m.m. The second adsorption experiments were then carried out. (Table IIb). The charcoal waS then accidentally exposed for a few seconds to a pressure of about I O cms. of air. It was then evacuated a t 300'C for I: hours and for I z hour during cooling the final pressure being 1.2 X IO-^ m.m.
1390
D.0.SHIELS
The experiments with C o n were then carried out. The charcoal was then evacuated a t 3 0 0 T for 13hours, the pressure finally being 1.3 x 10-3 m.m. while charcoal was still hot. The experiments with KrO were then carried out. The heating was done by means of a small electric furnace placed around the container C. The results are tabulated in Table 11.
FIG.3
TABLEI1 Temperature 2 5 T . Pressure in m.m. Hg = p. ccs. a t K.T.P. per gram of charcoal = q
coz
so2
(4
(b)
N-20
q
P
q
P
q
2.39 4.12 7.035 10.75
4.467 7.36 12.79 17.30
8.86
1.33 3.32 5.15 2.93
3.26 I O .58 28.62 44.80
0.52 I .76 4.17
16.20
22.81
I .29
20.01
22.67 13.06 9.03 7.50 6.32 4.00 2.93
26.50 24.44
0.64
IO .oo
P
0.385 2.67 4.90
26.04 46.43 19.57 8.86 4.39
6.02
3.23 1.75
4.95
I .02
22.92
2.82
21.60
1.12
0.58 0.14
20.47 16.16 12.98 1.349 4.171 7.181
SORPTION O F SCLPHUR DIOXIDE BY ACTIVATED CARBON
1391
Discussion From the isotherms plotted in Fig. z it will be observed that the amounts adsorbed a t any given pressure are in the order of the respective Critical Temperatures] and that in the case of COz there is practically no hysteresis, the sorption and desorption curves practically coinciding. I n the case of K\;2Othere is definite though small hysteresis, whereas in the case of SO2 the hysteresis is very marked.
2
V'%103 8
IO
I2
FIG.4
Furthermore, the isotherms for COZand N20 very nearly coincide, as had been anticipated. If instead of plotting p against volume of gas adsorbed the expression p / P is plotted against V, the volume of liquid adsorbed, where p is the observed equilibrium pressure during adsorption, and P is the saturation pressure a t 2 5 O C , the curves for COz and N20 coincide. That for SOZhowever does not come near the common curve for the other two gases.
I392
D. 0 . SHIELS
In the case of the adsorption of SO?by silica gel Patrick and I\lcGavack' have shown that the plot of log (pu;'P) against log V (where u = surface tension) gives a straight line, the same curve serving for the adsorption a t all temperatures. TABLE I11 p = Equilibrium pressure in m.m. of mercury. q = ccs. of gas a t S.T.P. per gram of charcoal. P = Saturation pressure at 2 j"C. V = Volume of liquid adsorbed per gram of charcoal. u = Surface tension of adsorbate in liquid state. D = Density of liquid at z 5 O C .
cos
SzO
So2
P q 7 I 2 15 23.8 3 34.3 3
P P
Y
0.03140 0.03301
.0?280j
0.03478
0.036887
.02j614 ,028421 ,01123
I
0.03118
0.02247
0.03208
2
0.03275
0.02494
0.03484
20
3
o.oSjj7
4
0.0343 0.0360
0,02741
28
0.02988
0.02106
0.01068 0.02137 0.03205 0.042 74 0'05342
0.02166 0.02338 0.02j53 0.02811
.7 j 5.5 IO 9.0 I j 13.2 2 0 19.6 25
0.0470
0.03143 0.03234 0.03344 o.or51o
;i.1462 ;i.4788 4.6794 ;i.8381
,0424
3,3930 3.6940 3.8697 3.995
1.7280 (1920).
D = 0.796 P = 46.500m.m. u = 1 . 7 5 dynes,!cm. (above a t 25OC)
D P
= 1.3695 = 38,joo m.m. u = 23.6 dyneslcm.
(above a t 25°C)
0.0120
-5 5443 -5.8750 -4.0757
1.0287 2 .3300 2.5060
J. Am. Chem. Soc., 42, 946
0.70
49.8oom.m. o.zjdynes!cm. (above a t 2 j"C)
Log (pu/P,
-
1
=
= u =
Log v
2
4.0726 4.4397 4.6334 4.7796
D P
5.4486 3.7493 3.9255 -
Log (P/P)
5 2 0
,03119 .0317z
5.5 13
2
CO:!
pu/P
,0435 ,0475
-
'
4.2356
-4.3180 -
4,6845 4.8787 3.02jo
SORPTIOS O F SULPHTR DIOXIDE BY .iCTIVATED C B R B O S
I393
Log (p P)against log T7 gives straight lines, the curves for different temperatures converging towards the region where p ' P = I . Patrick, Preston and Owens have also shown that in the adsorption of CO,, S,O1 by silica gel log (pu,'P) plotted against log J- gives straight lines, the lines for different temperatures being parallel to each other and those for CO? being parallcl to those for S2O. If, however, the value of u was corrected for the influence of the radius of curvature on the surface tension it ~ m found s that the log \-/(pg/'P) curves for S2O at oo, 2 0 ' , 30' coincided, and those for oo and zoo for COn coincided with
FIG.
5
each other but not with those for 1 2 0 . Apparently the sample of gel used for the experiments with CO, at oo and 20' was different from that used for S , O , the water contents being 1.z8C;- and 1 . 3 8 ~ ; respectively although the authors state that the cndeavour was t o have the same samples or portions of the same samples used throughout. They do not make this point quite clear. In calculating the values of p"P, pu P the values of V have been determined from the weights of the gases adsorbed and the densities of the substances in the liquid condition at z j°C. The densities of C'O, and S?Ohave been obtained from curves drawn from the data of Patrick, Preston arid Owens. The values of P and of g have also been obtained in the same way from their data. The value of g for COe at z j"C is somewhat uncertain. The density of liquid SO? at 2 5 ' has been obtained from Lange's values in Landolt-Bornstein's Tabellen. The value of P for SO, has also been obtained from Landolt,-Bornstein's Tabellen and the value of u for SO, from a curve drawn from data of McGavack and Patrick. Table I11 gives the values of the different quantities mentioned.
I394
D. 0. BHIELS
In the present investigation the p i p , V curves for C O Pand N 2 0 coincide exactly, therefore also the log (p/P), log T' curves. The fact that the log (p,/P), log V curve is a straight line indicates that the isotherm may he represented by an equation of the form. V = K(p/P)"" which is a modified form' of Freundlich's well-known equation. If the surface tension factor be introduced as in the case of the investigations previously quoted the log V, log(pu:.'P)curves for C O z and K 2 0 are parallel straight lines and that for SO2 is, over most of the range considered, a straight line parallel to the other two curves. The exact coincidence of the log T', log (p/P) curves for carbon dioxide and nitrous oxide, and the lack of such coincidence when the log V, log (pulp) curves are plotted may indicate that the introduction of the surface tension factor u is not warranted, or t,hat the value used may be incorrect. The latter possibility is very probable, since one would anticipate t,hat these two substances would have very nearly the same values for their surface tensions, whereas the values used are quite different. (Table 111). It is interesting to note that if the gases experimented with are arranged in order according to the volumes adsorbed at any particular temperature! the order is X 2 0 , CO?, SOs for silica gelz and also for glass,3 whereas in the case of activated charcoal it is COZ,S 2 0 ,SOZ. The values of K and I , n for carbon dioxide and nitrous oxide have been determined from the curves. K is taken as the value of 1- when p u l p = I . The values for SO2 have not been calculated since t'he curve is straight over such a small range that extrapolation is not justified. These values are compared with those obtained for silica gel by Patrick, Preston and Owens.
TABLE ITGas
Silica Gel
Charcoal
CO?
0.145*
28.17
0 .j j 8 t
szo * These iThese
Silica Gel1 'n
0.866*
Charcoal
0,909
0.898t
3.98
o.891*
0.87j
values for silica gel are at 0°C. values for silica gel are at 2o°C
It appears that the value of n is characteristic of the gas whereas K depends on the nature of the adsorbent. Hysteresis It will he observed that in the case of S20and SO2 the pressure corresponding to a definite quantity of gas adsorbed is greater when the point is approached from a lower pressure than it is %Then approached from a higher pressure. This phenomenon of hysteresis has been previously observed by 11cCavack and Patrick, J. Am. Chem. Sac., 42, 946 (19zoi. Patrick, Preston and Owens: J. Phys. Chem., 29. 421 f19zj). Bangham and B u r t : Proc. Roy. Sac., 105A,481 (1924).
SORPTION O F SULPHUR DIOXIDE BY ACTIVATED CARBON
I395
a number of investigators. The explanation frequently given in the case of the adsorption of gases is that it is due to less condensible gases which are turned out by the more strongly adsorbed gas, but apparently no attempt has been made until recently to analyse the gas in contact with the solid adsorbent to see if it contains any gas other than that used as the adsorbate. In considering hysteresis it is important that particular attention be given to the conditions of the preliminary evacuation of the adsorbent and its history during experimental work.
5 '
3.5'
FIG 6
Richardson' has shown that in the adsorption of carbon dioxide by activated cocoanut charcoal hysteresis occurs, but that the effect disappears at temperatures above 55OC. McGavack and Patrick, in their work on adsorption of sulphur dioxide by silica gel, have shown that after ordinary evacuation of the gel the adsorption isotherms showed hysteresis, whereas if the gel was allowed to stand in contact with sulphur dioxide orernight, then evacuated, and the process repeated four times, the process of adsorption was reversible. Gregg2 states that in the adsorption of SO?, CO,, FJ20 and other gases by charcoal hysteresis was due to the presence of non-condensible gases in the system. When the gas to be adsorbed was liquified and then distilled in high vacuum, and when the charcoal was thoroughly evacuated the sorption and desorption curves could be made to agree. However, a careful examination of their tabulated results shows that hysteresis was present in the case of nitrous oxide and sulphur dioxide at 40.3j°C even when the gases were carefully purified as above. It appeared from the experimental history of the charcoal used in the author's work, and the incidence of hysteresis, that the phenomenon did not
*
J. Am. Chem. Soc., 42, 946 J. Chem. SOC., 1927, 1494.
(1920)
I396
D. 0 . SHIELS
depend merely on the presence of “foreign” gases in the adsorbent. Table V may indicate this more clearly.
Adsorbate
coz K,O SO?
TABLE CIitical Temperature 31
2OC
35 0 I55 4
5‘ Hysteresis
Sone Slight Large
Temperature of Experiment 2
5°C
>, >I
The order in which the gases were used was SO,, SO,, K 2 0 . The history of the charcoal shows that it was “washed out” once with SO2 and then the first adsorption curve and the desorption curve were determined. The second adsorption curve was then obtained. Only three points were determined on this; they lie very nearly on the first adsorption curve and show no indication of any tendency to approach closer to the desorption curve, as would be the case if less condensible gases were being removed. The charcoal was then accidentally exposed to a pressure of I O cms. of air for a few seconds. The amount of air adsorbed a t I O cms. pressure would probably be of the order of O.j-0.8ccs. per gram of charcoal. The weight of charcoal being 0.65 gram this would give about 0.5 ccs. of gas adsorbed. This would apply in the case of fresh charcoal exposed to air. I n the present case sulphur dioxide was prescnt to a pressure of several millimetres, and the quantity of air adsorbed would probably be less than t,he above estimate. The charcoal was then evacuated a t 3ooOC for 13 hours and then the carbon dioxide experiments were done. At the conclusion of the desorption experiments the charcoal was again evacuated a t 3 0 0 O C for 13 hours and then the experiments with nitrous oxide were carried out. In order to test whether the hysteresis was due to the liberation from the charcoal of less readily condensible gases owing to the adsorption of sulphur dioxide a fresh portion of the same sample of charcoal was taken and exposed to sulphur dioxide in the apparatus‘ shown in Fig. 3. The charcoal was evacuated while cold to I X IO-^ m.m. and then heated to 200°C and evacuated during one hour a t this temperature. The final pressure a t room temperature was 2 X IO-^ m.m. The temperature used was lower than that previously used and the evacuation might thus be expected to be less complete. I t was then exposed to a pressure of 4 ems. of pme dry sulphur dioxide. After standing in contact with the charcoal a t room temperature for I!Z hour a sample of the gas which then had a pressure of 0.7 cms. was pumped off into the burette B. The whole of this apparatus up to T3 had also been previously evacuated to 9 X 1 0 d m.m. The absorbing vessel C was filled with an aqueous solution of iodine in potassium iodide. 1
Low: J. Soc. Chem. Ind., 41,
I (1922).
SORPTION O F SULPHUR DIOXIDE BY ACTIVATED CARBON
I397
By raising the mercury in B the gas mas pumped via the three-way taps T2 and T3into C. Absorption of the gas was practically complete, only a minute trace of gas being unabsorbed. The liquid in C was originally in equilibrium with the oxygen and nitrogen and carbon dioxide in the air. Assuming that the oxygen and nitrogen adsorbed by the charcoal when in contact with air are adsorbed in the same proportions as they exist in air, and that the traces which might be left on the charcoal after heating and evacuation, and which might subsequently be displaced by sulphur dioxide during adsorption of the latter would also be in the same ratio, adjustment of the level of the mercury in B ensured t,hat this condition held a t the conclusion of the adsorption of the sulphur dioxide in the absorbing liquid. The result showed that in all probability if any less condensible gas or gases were turned out from the charcoal by the adsorbed sulphur dioxide neither oxygen or nitrogen were concerned. Owing to the high solubility of carbon dioxide in water the possibility remained that carbon dioxide occurred in the sulphur dioxide aft'er exposure to the charcoal, but was dissolved in the absorbing liquid. There was no opportunity to make a thorough investigation on this question of hysteresis.
Summary The adsorption of carbon dioxide, nitrous oxide, and sulphur dioxide by German activated gas-mask charcoal at 2 j°C has been determined from o t o 40 m.m. pressure for the first two gases, and from o to 2 2 m.m. pressure for the last. The adsorption process is reversible in the case of carbon dioxide. I n the cases of nitrous oxide and sulphur dioxide hysteresis occurs. The log I-,log p / P curves for carbon dioxide and nitrous oxide coincide. The log V, log pg,:P curves for the first tJTo gases are parallel straight lines, that for sulphur dioxide being straight for part of the range considered and parallel to the other two. The adsorption of nitrous oxide and carbon dioxide may be represented by Patrick's equation V = K(p/P)',". Acknowledgment The experimental part of this work, with the exception of that on the analysis of the gases, was carried out during the period November 1926 t o January 1 9 2 7 a t the University of Tasmania, and the author's thanks are due to Professor *I.L. Mchulay for facilities for the work. The author's thanks are due to Professor E. J. Hartung, D.Sc., for facilities for carrying out the gas analysis. Chemistry Department. Cnirersity of Melbourne, Feb. 12, 1929.
