T H E SORPTIOS OF ORGANIC VAPORS
BY -4CTIVATED S r G d R CHARCOAL’ BY J.
Tv. McB.UX,
D. N. JACKXAX’, A . 11. BAKR A S D H. G . SMITH
Introduction There is great need of a body of data t o exhibit the typical behavior of sorption by charcoal, more especially with a view of determining to what extent this is affected when differently activated samples of the same pure charcoal are employed. Also, it will be of the greatest interest to compare the sorptions of the vapors of very different liquids in their dependence upon the different activations and experimental procedures. We find, for example, that the form and position of a sorption curve for a particular vapor with a particular specimen of charcoal depends to a surprising degree upon the extent to which the charcoal has previously been freed from other sorbed inipurities. Reference to the diagrams will show that some specimens of actiw t e d sugar charcoal sorb almost as much at very low pressures as they do \Then the vapor is nearing saturation. This, for example, excludes the hypothesis of capillary condensation. The better and more prolonged the pre1iriiinary evacuation of the charcoal, the more striking is this behavior. Experimental Kahlbauni’s pure sugar charcoal was used in all experiments, but three difTerent methods of activntion were employed as follows : I. “T7aczizciri A,” This charcoal was that prepared and used in 1921 by I?akr and IIcRain? by heating in a vacuum in a silica flask for thrpe hours in n blowpipe flame and cooling in vacuo. “Air and T-cicirrcnr B.” This saniple vias activated by -4.11.B. in 2. October 1923 by heating in a continuously evacuated silica flask in a n open oven at the tip of a blowpipe flame for j 6 hours. Air was admitted during the first day. 3 . “ A i r C.” This sample of charcoal was air activated by H . G. S. in 192 j by heating in a Heraeus platinum-strip-wound electric furnace at a temperature of II~OOC. for 1 2 hours. During this tinie a slow stream of air was passed over the charcoal at intervals, so that about 20 per cent of the original charcoal was oxidized. Miss 31. C . Rattue, in our laboratory, found that the unactivated charcoal was completely volatilized when ignited and hence was free from iron. These experiments were carried out at the University of Bristol, England, in 1923-25 inclusive. For continuation see J. W. McBain, H. P. Lucas and P. F. Chapman: J. Am. Chem. Soc., 52, (1930) and J. W. McBainand G. T. Briton: J. Am. Chem. Soc., May, 1930. * A . M. Bakr and J. W. McBain: J. Am. Chem. Soc., 46, 2719 (1924).
I440
J. W. MCBAIN, D. N. J A C K M A N , A . M. BAKR AND H. G. SMITH
The aqueous extract, obtained by boiling 2 gm. of charcoal with jo cc. of boiled out conductivity water, allowing the mixture to stand over night and then filtering, was not found to be appreciably acid. The sample of n-hexane used by one of us (H. G. S.) was obtained from the Eastman Kodak Co. All the other liquids were Kahlbaum’s best and were redistilled before use a i t h the exception of the methyl alcohol used by H. G. S. which was a fresh sample.
Experiments (by D . N . J . ) , using the Method of Bakr and King’and “Vucuum A” Activated Sugar Charcoal. The method was possibly the most accurate previous to the introduction of the sorption balances of hlcBain and Bakr2 and RlcBain and Tanner.3 The apparatus consisted of a glass tube, closed by a bulb at both ends, the tube being bent into a U-shape near one end. The upper bulb contained the charcoal and was surrounded by a hot thermostat. The lower bulb was in an adjustable thermostat of lower temperature. The liquid was placed in the lower bulb, and it’svapor, saturated at the lower temperature, cont’rolled the pressure throughout the system. In a “direct” experiment the charcoal was initially free from sorbed material, and the vapor was sorbed by it. I n a “reverse” experiment the liquid was first poured upon the charcoal and then distilled into the lower bulb, thus approaching equilibrium from the opposite side. If sufficient time was allowed, direct and reverse experiments gave the same result. h single tube produced only one experiment, because the tube had to be sealed off and the two parts weighed to ascertain the amount of sorbed material in the charcoai. I t was necessary to know the initial weight of charcoal, to allow for the amount of vapor in the upper sealed-off portion, and to make appropriate vacuum corrections. It was found advisable to place t’he charcoal in the bulb before sealing it to the rest of the tubing, then to bend the tubing and insert t,he liquid through a side tube near the lower bulb. Before sealing off, evacuation was carried out through the side tube, meanwhile cooling the liquid in the lower bulb with carbon dioxide snow or liquid air. I n these experiments evacuation was carried only to a fairly constant pressure of a few hundredths of a millimeter, during v-hich the loss in weight of the charcoal was 1.1. per cent. Much higher results would undoubtedly have been obtained with pentane and part,icularly with water if the evacuation had been more complete. Each set of esperiment,s included direct and reverse experiments. Care was taken to avoid condensation in the middle portion of the tube while sealing off. This method is probably still the most accurate for the study of mixed vapors, but it ought to be combined with the great improvements in evacuation and introduction of the liquid developed in our later work with the sorption balance. ‘ A . M. Bakr and J. E. King: J. Chem. Soc., 119, 454 (1921). J. W. McBain and A. M. Bakr: J. Am. Chem. SOC., 48, 690 (1926). 3 J. W. McBain and H. G. Tanner: Proc. Roy. Soc., 125.4, 579 (1929).
SORPTIOX O F ORGANIC VAPORS BY S r G A R CHARCOAL
1441
The results are recorded in Table I, where TI is the temperature of the upper bulb in “C., T2is the temperature of the lower bulb, p and pa are the vapor pressures of the liquid at Tz and TI, respectively, and x/m is the sorption in grams per gram of charcoal. The time given in the fifth column is the total duration of exposure of charcoal to vapor.
TABLE I Sorption of Vapors by “Vacuum A” Charcoal Substance
Toluene f
J
JJ-ater
, ,,
)f
Acetic Acid Butyric Acid
>. f,
, AIrt hyl Alcohol ,)
>. ,, 1
Octane
,, ,
No, of Expts.
Temperature
TI
T2
7
12j
25
4
125
75
3
125
IO0
I
126
77
x/m
0.0257
0.116
50 560
0.238
0.130
0,533
o
313
O.Ij4
0 021
2
I20
72
251
0.1;o
0.008
125
78
40
327
0.188
0.012
I
I20
90
45
525
0
4
124
84
92
238
0.1;4
4
I90 125
25
0.123
0.1
128
45 90 94
2 70
4 4 4
‘33 80
0.264 o 135
2 jo
0,532
0.135
127
I20
220
0.14
50 63 60 63 30
20
382
0.688 0.746 0.446
4 2
3 3 1 3 4
148 I37 126 8; i8 73 12 I21
Ethyl Alcohol
2
( ’hloroform
-i
Hexane
2
13
11
PIP,
3
3
3
Pentane
mm.
D.
40 45 54 40
96 111 I08 8j i8
,,
Time
1 2
2
_-
94 38 29
IO1
--
I / IO0 IO1
63 58 62 82
123 i2
49 96 I 60 65 I12
89 123
94
2
6j6 5 80 656 Ij o
365 187 353 365 40 i 710
50
610 1140
30
89 93
21
117
611 joo
352
0.503
132
0.116 0.110
0.081
0.129 0.131
0.682 0.560
0,132
0.553
0.088
0.623 0.654 0.794
0.1
0.40; 0,534
0 . 2
0.083 0.086 0.12.; 0.191
c.663 0.731 0.736
0.0ii
0.820
0.021
0.1 0.021
The largest value for x m is 0.191 for chloroform, followed by acetic acid 0.174,butyric acid 0.13j , tolucne 0.132,methyl alcohol 0.131, ethyl alcohol 0.125,octane 0.087,hexane 0 . 0 7 7 , pentane 0.021, down to water 0.02. These values are not all strictly comparable amongst themselves, because some of them refer t o different temperatures, but the data arc useful for comparison with the results of later experiments, using highly evacuated charcoal. The values for toluene and acetic acid agree with those preriously observcd by Rakr and l\lcRain’ and arr in the proportion of one molecular weight of A. 11.Bakr a n d J. K. bIcBain: J. Am. Chem. SOC., 46, 2718 (1924:.
1442
J . W. MCBAIN, D . N . JACKMAN, A . M. BAKR AND H. G. SMITH
toluene to two of acetic acid. The values of x / m rise only slightly over the upper range of relative pressures (relative humidities), p/p.. The time required to reach equilibrium depends greatly upon the temperature and may be several days more a t room temperature than a t IOOOC.Butyric acid and, to a lesser extent, octane are particularly slow at arriving a t equilibrium even a t a very high temperature. Further experiments were carried out with five other samples (not listed above) of air-activated sugar carbon of varying degrees of activity. The ratios between the amounts of chloroform and methyl alcohol sorbed varied by 50 per cent. The most active sample of charcoal was prepared by heating in a sili,ca flask in a furnace over a Meker burner for 57 hours in a gentle stream of air. It sorbed twice as much toluene, methyl alcohol, ethyl alcohol and acetic acid as did the ‘‘Vacuum A” specimen; and it sorbed ten times as much pentane, bringing the value of x/m to 0 . 2 1 . Some further preliminary experiments were carried out with an improvised balance of the beam type, placed in a hot thermostat, the temperature being regulated by keeping the liquid a t a lower temperature. The most active specimen of charcoal, just mentioned, was used. In many cases equilibrium was rapidly attained. For example, wit’h chloroform the value of x i m was 0.352 at the end of one minute, 0.404after t’wominutes and had attained the nearly constant value of 0.490 in seventeen minutes, rising only to 0,494 after sixteen hours. Toluene was slower, and the more difficultly sorbable vapors, or those more affected by poor evacuation, were extremely slow in approaching any constant value. For example, the value of x/m for water was 0.024 at, one minute, 0.034 at forty-eight minutes and 0.06 a t 135 minutes. It is interest,ing to note that the value of n in the empirical sorption isotherm increased greatly with time, I/n being in the case of chloroform 0.21 a t two minutes and 0.14at four minutes. The change was even more marked with the more slowly sorbed vapors, giving the appearance that for extremely short-time-experiments there might be a n approsiniation to Henry’s law. For example, with acetone, I / n was 0.59 at two minutes and required 46 minutes to reach the value 0.18, when x/m was beginning to seem constant. Experiments by one of u s ( A . M . I?.), using the JfcHain-Bnl;r ,Eo,ption Balaiice and “Air and T7acitum B” dctivated Sugar Charcoal. This was the first series of measurements carried out with the 3IcBainBakr sorption balance’ in which the charcoal and the sorbed vapor are continuously weighed in a platinum vessel hanging from a calibrated, coiled spring of fused silica, all enclosed in the hotter thermostat a t a temperature, TI, the lower portion being kept in the cooler thermostat at a temperature, TB. The liquids were introduced into the sorption apparatus in small, sealed bulbs which were filled with air-free liquid in the following manner. h store of liquid was placed in a large bulb at the end of a tube to which were sealed side tubes, each leading to a small bulb. The connections were drawn down ready for sealing off. The main bulb \vas surrounded by solid carbon dioxide
J. W. IIlcBain and A. M. Bakr: J. Am. Chem. SOC.,48, 690 (1926).
SORPTION O F ORGANIC VAPORS B Y SUGAR CHARCOAL
I443
and ether during evacuation with a good mercury pump. The top of the tube was then sealed. The liquid was nest distilled until it filled the small bulbs, each of which was subsequently warmed and sealed off whilst the liquid was distilling from it. The tip of each bulb was scratched with a file and had a piece of iron attached to it, enclosed, if necessary, in a sealed glass tube tied to the bulb with platinum wire. The sorption tube was evacuated t o 0.005 mm. of mercury before breaking the bulb of liquid within it. The initial evacuation has to be slow to avoid blowing out the charcoal. During evacuation the tube was tilted to allow the platinum bucket to rest against the side of the tube which was heated to
--TUBED-- 176‘ --TUBEI1--256’ 0 --TUBE II --302’
A
0
o
0.01 0.02 0.03 OM
a05 P ps
a06 a07 008
3
FIG.I The sorption of toluene by “Air and Vacuum B” charcoal. (Data of d.hl.B.). Esperiments a t ,302’ were after z j days, a t 2j6‘ between I 5-16 days. those a t 176” lew than 1.1 days, esplaining why the 176” curve does not lie higher than the others.
450°C. by means of a heating coil slipped around it. After sealing the sorption tube and reading the initial length of the spring in the sorption apparatus, the bulb of liquid was broken by raising it with an electromagnet and allowing it to fall. It was necessary to cool the bulb first to a very low temperature in order to avoid a sudden up-rush of vapor. Tihen the vapor is thus being sorbed by the charcoal, it is called a “direct” experiment, as for example, whenever the two temperatures, TI and T1, are being brought closer together. .1 “reverse’! experiment is produced by lowering the lower temperature, raising the upper temperature, or even by niomentarily raising T? above T, and condensing liquid upon the charcoal and then restoring Tz to the lower value. Under the conditions of these esperiments, which were prolonged, the results of “reverse” and “direct” esperiments were indistinguishable. The sorption noticeably increased until about the third or fourth day but thereafter did not increase by more than one or two per cent during the next few weeks. The whole series of experiments with any one sample of liquid and charcoal was carried out without opening the tube. Each series took from one to one and a half months for completion. Pressures throughout the apparatus were
1444
J. W. M c B A I S , D. N. JACKMAN, A. XI. BAKR AND H. G. SMITH
always those of the vapor tension of the liquid a t TSand were taken from standard tables where available; otherwise they were calculated by using Diihring’s rule, comparing them with the vapor pressures of water as a reference substance. p,, the saturation pressure, is the vapor pressure of the . . liquid a t TI, p/pBbeing the relative humidity.
0
0.1
0.2
03
04
-0.5
0.6 0.7
0.8 0.9
ps
FIG. 2
The sorption of toluene and of octane by “Air and Vacuum B” charcoal. (Data of A.M.B.). Readings for Tube VI11 a t I O O O were taken after one month. -.xxx-
0--
*0
0.1
TUBE IF--IOO, TUBEIP--1850
Q2
0.3 0.4
05
-k
0.6
0.7
OB
09
FIG. 3 The sorption of benzene by “Air and Vacuum B” charcoal, (Data of A.M.B.,”.
Some of the data are presented in Tables 11-VI, while the remaining results, as well as a few of those printed in the tables, are graphed in Figs. 1-5. The time given in the second column is the interval between the breaking of the bulb (exposure of the vapor to the charcoal) and the reading of T2.
SORPTION O F ORGANIC VAPORS BY SUGAR CHARCOAL
I445
CJ-TUBEP--~~~ A --TUBE P-120. A --TU BE XI- 120. 0 --TUBEXI--21 I 0
0.1
a2
03
0.4
0.5
P -
06
0.1 0.8 0.9
PS
FIG.4 The sorption of acetic acid by ‘,.4ir and Vacuum B” charcoal. (Data of A M.B.)
- - p E T O N E - - T U B E I X - - 40: --ACETONE--TUBEIX--103 0 - HEXANE --TUBEIZII--IOO~ A - - HEXANE - - T U B E X U - - 165 x
O
l0 FIG.5
The sorption of acetone and of hexane by “Air and Vacuum B” charcoal. (Data of A.M.B.)
1446
J . W. MCBAIN, D. X. JACKMAN, A. M. BAKR A S D H. G. SMITH
TABLE I1 Sorption of Toluene by “Air and Vacuum B” Charcoal*
T?
Time in days
15 I7
28
I8
29 3 16
21.
17 I
“C.
I9 19 19 22
p in mm. Hg
Isotherm a t TI = 3 4 T . 17.5
Tube I. 0,135 0 . ‘34 0.113
2 1 . j
0.455 0.482 0.482 0.482
24.9
o.jj8
20.3 2s
Isotherm at
x/m
0.392 0,435
19.4
2
PIPS
.5 j
TI=
-8t ot
29 29
251.
IO
29.0
29 1 4 and 29 3
30.2 46.8
IOO’C.
+61
7.57
0.108
0.128 0.128 0.103
Tube 11. 0.00832 0.013: o .0;23 O.O.i4j o ,0845
0.12j
0.129
150
0.2iI
I2
IjO
0.2jI
64t 67
14 3
1;o
0.271
0.131 0.132 0.132 0.137 0’139 0.139
I74
0.314
0 , 136
80
27
2
88
0 j 2 0
328 3 30 3 40 3 40 377 48i
0.592 0.614 0.614 0.614 0.680
0,139 0’137 0.138
t
26
35 64t 64
84 85 8jt 85
t
88
t t
1 . j
9 14
28 I
0.140
0.139 0.132
0.140 0.879 14 ;22 0.942 0.139 0 . 143 11 .i36t I .o** 9 .9 . * The charcoals used in Tubes I and I1 e e r e two different portions; other experiments with Tube I1 are graphed only. t Desorption experiment. * * Condensation occurred in the upper part of the tube at this point.
96 98
14
TABLE I11 Sorption of Benzene by “Air and Vacuum B” Charcoal* Tt
“C.
Time in days
p in mm. Hg
Isotherm a t TI 16
I
16 16 16
6 2. i 27
= 40T.
61.5 61.5 61.5 61.5
P/P.
x:’m
Tube 111. 0,340 0,340 0.340 0.340
0.120 0 . I49
0,150 0 . I j I
SORPTION OF O R G A S I C VAPORS BY SUGAR CHARCOAL
I447
TABLE111 ( C o n t ' d ) Tz
Time in days
p in mm. Hg
PIP8
16
".
32
61.5
0.340
18
3
18
I1
67.6 67.6
I9
33
TO
0,373 0..373 0.389
Isotherm
.4
at T1 = IOOOC.
Tube
0.1j0
111.
T i t
6
64.8
o 0482
0.129
2ot 26i
32
74.5
0 . I34
5 5
98.2
0.0554 0.0730
181
0.135
0.135
4 4
269
0.200
0.137
822
0.611
0.141
26
897
0.667
0.142
1 O i
jot 83 86i
0.132
* This is not the same portion of charcoal as that used in the experiments which are
graphed in Fig. 3. These experiments are included here to exhibit the effects of timr. t Desorption experiment.
TABLE IV S o r p t i o n of Acetic Acid b y "Air and V a c u u m B'' C h a r c o a l * Time P 1n PIPS in days mm Hg I s o t h e r m at T, = 3 1 T . T u b e V .
9 03 9 52 9 52 9 52 IC 7
46 I
3 I9
1 41 Isotherm a t
T, = 3 6 16 26
15
15 14
14 '3
o 41~
0.221
433 433 433
0.186
0 0 0
o 488 o 488
IO j 2 I IO('.
Tube
0.194 0.217
0.196 0.220
\-I.
46
0.000442
j4 7
0 . 0 0 0 8 3j 0.00213
9 89 3
x;'m
0.00343
0.026 0.031 0.045 0.0j3
0.0114
0.083
2j8
0.0329
0.11;
'3
5;s
o.oj3i
0.14;
I3
I353
0,173
0,168
I3
2274
0.290
0.182
I2
3292
0.420
0.181
I2
7082
0.904
0.182
14
* The charcoals used in Tubes V
and VI were two different portions, and the two ex-
periments are independent; both are graphed in Fig. 4. t Desorption experiment.
1448
J. W . McBAIN, D . N. JACKMAN, A. M. BAKR A N D H. G. SMITH TABLE
5’
Sorption of Hexane by “Air and Vacuum B” Charcoal T2
Time in days
p in mm Hg. *Isotherm at TI = 4ooC.
“C
1st
I3
20
20
21
3 26
21 22
I
22
18
23
2
24
I IO
-187
34 34 34 34 33 33 33 33 33 33 33
1st 39t 42
t
57t 77 90
108 I45 205
t
x/m
Tube VII. 0 0 0 0
41: 435 454 454
o 065 0.072
0.065 0.0jj
0,474 0 474
0.069
495 o j16 o 670
0.062
0
30
ot
.
P /P.
0.072
0.060 0 075
0.025
0.031 0.03j 0.041
0.043 0.047
0,049 0.049 0.049 0.019 0.049
t Desorption experiment. * In connection with the results obtained for this isotherm, it should benoted that time
is more important than pressure.
T A B L E 5’1 Sorption of Octane by “Air and Vacuum B” Charcoal*
“C
Time in days
p in mm. Hg Isotherm at TI = qo°C.
20
IO
10.4
20
20
10.4
21
3
10.8
22
18
11.5
15
9
T2
Isotherm at Ti 24 t
27
29
I
35
8
= 45’C. 7.3.7
13 .o 17.1 23.8
*On the 12th day TI was maintained a t 160°C.
t Desorption experiment.
P/Pa
x/m
Tube VIII. 0.339 0,339 0.350 0.371
0.07j 0.077
0.068 0,075
Tube 5’111. 0 . I93 0,330 0.436 0.60;
0,075
0.076 0.066 0.068
S O R P T I O S O F O R G A S I C VAPORS BY SUGAR CHARCOAL
I449
Experi?nents by one oj u s ( H . G. S.), using a .Modification of the M c B a i n B a k r Sorption Balance and " A i r C" Acticated Sugar Charcoal. A new arrangement of the RlcBain-Bakr sorption balance was used in order t o make direct readings of the pressure of the vapor in contact with the charcoal over a considerable range, especially at low pressures. By means of an inverted \--piece of glass tubing the lower end of the sorption tube was sealed on to a small NcLeod gauge and a direct manometer andtoalarge, wellground, two-way tap (mercury sealed) which made connection with a reservoir containing a glass bulb filled with the liquid to be sorbed. The whole was permanently sealed to the vacuum main, connection being made when desired by a mercury sealed tap. The mercury in the YIcLeod gauge was raised and lowered by manipulation of the pressure over the mercury reservoir which was attached to the gauge and the rest of the apparatus by an all-glass join, eliminating possibility of leakage. .is a further safeguard, an air trap was placed in the path of the mercury intake of the gauge. For greater accuracy, the manometer was constructed of glass tubing 0.6 em. in internal diameter, and the mercury levels for both direct and JIcLeod gauges were read on a vertical wooden scale placed in actual contact with the manometer, using a telescope with cross-wires in the eye-piece at a distance of about four feet. Khen using the JlcLeod gauge, a small correction was made for the pressure of gas over the "evacuated" side of the gauge. The sorption tube was maintained at the temperature of the desired isotherm by use of a nichrome-vvound tube resistance furnace, while the liquid reservoir was surrounded by a bath at, a suitable temperature to give the vapor pressure desired. The glass tubing which connected the sorption tube with the reservoir of liquid and with the McLeod gauge was heated by nichronie wire coils to about jo"C. during the measurement of the 3ooC. isotherms. The charcoal was evacuated at 48o-joo"C., and evacuation was not considered complete until a pressure of the order of IO& mm. had been niaintained for two to twelve hours. Even after evacuation for some hours, the charcoal still contained a fair amount of impurities as was: shown by the fact that the pressure in the sorption tube rose to IO-^ nini. after disconnecting the Pump. Thin glass bulbs of about 1.j-2.0 cc. capacity were filled with the liquid to be sorbed by bending the fine capillary necks of t,he bulbs so that, the ends could be placed below the surface of the pure liquid in a corked tube, Each bulb was then slightly warmed, air was expelled and, on cooling, liquid came in to take its place. The liquid was boiled vigorously, and the bulb practically filled with liquid; this was boiled again until the bulb was only about threequarters full. The bulb then filled entirely on cooling. Finally, the liquid was boiled for a few seconds once more, and the temperature was not allowed t o fall below 40°C. as the very small bubble of vapor was replaced by liquid. Khile still hot, the bulb was sealed off and, after cooling, was placed in the reservoir. After evacuation of the apparatus, the zero of the sorption balance was taken, the tap to the reservoir closed and the bulb of liquid broken. The
I450
J. W. MCBAIN, D. N. JACKMAN, A . M. BAKR A N D H. G. SMITH
liquid was cooled to -78'C. and the t a p to the sorption chamber and to the vacuum pump opened. After ten minutes pumping to remove any gas liberated, the connection with the pump was closed, the space over the mano-
0.1
0.0 o --METHYL ALCOHOL--TUBEX
--Joe
-METHYL ALCOHOL--TUBE= --30* #--METHYL ALCOHOL--TUBEX- -3O'--DESORPTION A-HEXANE --TUBEXII--30° 0
0
0.1
0.2 03
0.4
0.5 0.6
P -
0.7 0.8 0.9
ps
FIG.6 The sorption of methyl alcohol and of hexane by "Air C" charcoal.* (Data of H. G. S . ) *The slightly lower values obtained for the sorption of methyl alcohol in Tube XI are probably due t o the presence of a very small fragment of broken glass which was found in the platinum bucket and which would play the part of a non-adsorbent relative to the charcoal.
meter evacuated to IO+ mm., and the measurements were made with the sorption balance. The data are graphed in Fig. 6.
Discussion The method employed in most of the experiments here recorded is unique in that throughout the whole series of readings at varying temperatures,
SORPTIOS OF ORGANIC VAPORS BY SUGAR CHARCOAL
1451
times and pressures, the sorption tube remains sealed so that all such results are strictly comparable amongst' themselves. When the evacuation has imperfectly removed extraneous impurities, these remain constant within the tube and reveal their presence by a surprising influence upon the time required for sorption, the minimum pressures at which sorption is nearly completed, and even the magnitude of the sorption observed. The effects of time are most prominent in the experiments of Jackman and in Bakr's Tables 11-VI, where frequently time is of more importance in determining the amount of sorption than is the pressure of the vapor. So much is this the case that in the graphs care had to be taken to compare only such results as had been carried out in comparable periods involving at least five days. The instances observable are too numerous for individual mention. They depend upon temperature, prexlous evacuation, and the nature of the vapor itself. One example map suffice, for which the data are not presented elsewhere. Bakr found the sorption of octane by charcoal very slow at 4 5 O , and even at 100' when p;'p. = 0 .j35, x j m = only 0.059 for one day, rising to 0.066 at three days, to 0.068 a t six days, and to 0,075 at 29 days, in spite of the fact that, the values of p,!ps had been lowered to 0.362, 0.111and only 0.082, respectively. Sorption is much quicker at very high temperatures. As evacuation is improved, whether in degree, duration or increased temperature of evacuation, the effects become more marked. (We recommend the highest temperature which the glass can stand and a vacuum of the order of IO-^ mm. maintained for many days.) I t would almost appear as if perfect, evacuation would lead to sorption going to completion at very low pressures, thus producing a saturation value of x/m which would be independent of further increase of pressure. Even in Fig. I at, 30z0, it is seen that the charcoal is half saturated with toluene at a pressure only I / I O , O O O of the vapor pressure of liquid toluene at that temperature; a t 100' (Fig. 2 ) the sorption is, within the experimental error, constant from p/p. = 0 . 2 j up to saturated vapor where p/ps = 1.0. This behavior was exceptionally developed with this particular charcoal, which may have been due to the very prolonged heating with the blowpipe in vacuo after the air activation. Other air activated charcoals, with which we have had experience and in which the charcoal has been freely oxidized throughout the activation, have not shown nearly the same tendency to exhibit saturat,ion values. As to magnitude of sorption, better evacuation raises the horizontal part of the sorption curve from x,'m = 0.20 to 0.2 j for acetic acid. I
The ordinary empirical sorption formula, x,!m = kp;, fails completely. At the lowest pressures it gives a flat curve instead of the required straight line on the logarithmic diagram, changing suddenly into a nearly horizontal line for all the higher range of pressure. No such low values of I/n have hitherto been recorded for the sorption of vapors, for this becomes zero for a saturation value. Even for the lower range of pressure, n may be greater than 30, especially at the lower temperatures; a t the highest temperatures n is many times less, as usual.
1452
J. W. MCBAIN, D . N. JACKMAN,
A.
Y. B d K R A N D H. G . SMITH
The only adequate formula to express the sorption results is that of Langmuir, 5 =
m
~
, where a and b are constants. This is illustrated in ~ + a p
Fig. 7. This formula may be expressed in the form,
=
x/m
ab
+ &,b
which
is the formula for a straight line as is shown in the graphs when P/P 2is plotted x/m against p/p.. Langmuir's formula presupposes the validity of Henry's law at
"-ACETIC ACID -TUBEX-3I" '-ACETIC ACID-TUBEY-35: &-ACETIC ACID --TUBEY-- I20 4-ACETIC ACID--TUBEX-- I2< X--ACETICAClD-TUBEYI--21 Io *-HEXANE -- T U B E X I I -- 30 T
12
.
03
.
.
0.4 0.5 P -
.
.
Q6
.
0.7
.
0.0
.
09
ID
ps
FIG.7 bmption data of Figs. 4 and 6 graphed according to the Langmuir formula
sufficiently low pressures, where I / n should equal unity, as is borne out by the direct proportionality between x / m and pressure in Smith's study of the low pressures in Fig. 8. Smith's comparison, Fig. 6, of the sorption of hexane and methyl alcohol is of special interest, because one is a typical non-polar substance and the other typically polar. Hexane and methyl alcohol are very similar in such physical properties as freezing point, boiling point, critical temperature, vapor pressure, and to a less extent density, yet their sorption by the same
S O R P T I O S OF ORGANIC VAPORS B Y SCGAR CHARCOAL
I453
charcoal is very different. The non-polar hexane is the one most sorbed at the lowest pressures, and it approaches a saturation, whereas methyl alcohol does not, Smith observed no difference in the readings for fifty minutes and one week, which we ascribe to the high evacuation employed. Xevertheless, the sorption mas adjusted even more rapidly with methyl alcohol than with hexane when the pressure was changed. Comparison of the values of x/m for various solvents may advantageously be 0.06 deferred to a later communication. However, a fewrninorpointsmaybe mentioned. It d l be noted that in Table J’there is no difference in type between the sorption of o,04 hexane above and below the critical teni0.03 perature, a result which we have obtained elsewhere for other substances such as nitrous oxide. The saturation values in all cases deo,o, crease with rise of temperature (sorption being exothermic up t o the highest values), The magnitude of this effect seemed in P Bakr’s experiments to run parallel with the PS FIG.8 value of the exponent n, being greater for acetic acid and least for hexane andoctane. ~ c g , “ ~ It is a logical corollary from results pressure at very low pressures such as are here reported that heats of sorption cannot be safely or accurately calculated by use of the usual van’t Hoff isochore unless there is some certainty that in the isotherms at different temperatures the impurities have exercised a precisely similar effect.
-
Summary TTnless evacuation and previous removal of sorbed impurities from charcoal is very thorough, time is an import’ant factor in the sorption of vapors by charcoal, the less readily sorbed vapors being most affected. K i t h fairly well evacuated charcoal most of the sorption occurs at relat i d y very low pressures. Thus half the total sorpt,ion possible may occur when the vapor is at a pressure only one ten thousandth of the vapor pressure of its liquid. The ordinary empirical sorption isotherm is wholly inadequate to describe such phenomena, since the value of the exponent n rises rapidly from unity t o such values as 30 and then more or less suddenly becomes indefinitely large where the sorption is scarcely affected by increase of pressure. However, the Langmuir formula is a fair representation of the data over the whole range. Certain modes of activation are particularly effective in producing a charcoal x i t h such a “saturation value.” 1)cpnrtmoit of Cheinistry, S t n n f o r d 1-11 ivrrsziy, Cc~lifiii-nia.
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