THE HEAT OF ADSORPTIOX OF CERTAIN ORGhXIC TAPORS BY CHARCOAL AT 25' AXD 50" J. N . PEARCE AND G . H. REED
Although the thermal effect accompanying the adsorption of gases by charcoal was first noted by Rlitscherlich' in 1843, it was not until 1874 that Favre? first attempted to determine the effect quantitatively by means of a mercury calorimeter. Of the numerous invesrigations which have been made since that time we shall cite only the more recent. Lamb and Coolidge3 have measured the heats of adsorption of eleven organic vapors on charcoal. Beyes and Marshall4 determined the heats of adsorption of oxygen, chlorine, carbon dioxide, ammonia and ether on gas-mask charcoal. Greggj has made similar measurements for eight gases on birchwood charcoal. The ice calorimeter was used in all of these investigations. Gregg also made a few measurements with a phenol calorimeter at 40.35'. lysing a potentiometric method Pearce and McKinley6 have measured the heats of adsorption of nine organic vapors on an acid-washed, ash-free, steam-activated coconut charcoal at h comprehensive study of the heat of adsorption of oxygen on coals and 25'. charcoals at temperatures ranging from 18' to 450' has been made by a number of English investigators.' Their data indicate that the heat of adsorption of oxygen increases with rise in temperature. Owing to the ease of combination of oxygen with the adsorbent, and the accompanying heat effect, their results do not suffice for the calculation of the temperature coefficients of the heat of adsorption. The heat of adsorption of hydrogen on copper catalysts, poisoned and unpoisoned, was determined by Kistiakowski, Flosdorf and Taylor.a Patrick and Greiderg measured the heats of adsorption of ammonia and sulfur dioxide on silica gel. Beebe and Taylor'" made similar measurements for hydrogen on nickel and copper catalysts, 3.nd Beebe" has determined the heat effect acconlpanying the adsorption of carbon monoxide by copper. I n all of the experirnents with metallic catalysts the catalyst tube itself served as the calorimeter.
-
Mitscherlich: Ann. Chim. Phys., (3) 7, I j (1843). Favre: Ann. Chim. Phys., ( j ) 1, 209 (1874). Lamb and Coolidge: J. Am. Chem. Soc., 42, 1146 (1920). Keyes and Marshall: J. Am. Chem. SOC., 49, 156 (1927). Gregg: J. Chem. Soc.. 130, 1494 (1927). Pearce and McKinley: J. Phys. Chem., 32, 360 (1928). 'Ward and Ridertl: J. Chem. SOC., 130, 3 1 1 7 (1927); Blench and Garner: 125, 1288 (1924); Garner and McKie: 130, 3451 (1927); McKie: 131, 2870 (1928). * Kistiakowski, Flosdorf and Taylor: J. Am. Chem. Yoc., 49, 2200 (192j ) . Patrick and Greider: J. Phys. Chem., 29, 1031 (192j). l o Beebe and Taylor: J. Am. Chem. Soc.! 26, 43 ( 1 ~ 2 4 ) . Beehe: J. Phys. Chem., 30, I 538 (1926).
J. N. PEARCE AND G. H. REED
906
I n many of the previous investigations heats of adsorption have been measured at the temperature of melting ice. Since most vapors may exist as liquids a t oo, the measured heat effect may consist of at least two heat effects, a heat of adsorption and a heat of condensation. The object of the present work was to study the effect of temperature upon the heat of adsorption. To this end we have worked at Z j ” and joo, temperatures which are near, or above, the boiling points of the vapors employed. In this way it is possible to eliminate to a considerable extent at least’ the heat of condensation. The procedure makes possible also the calculation of the temperature coefficient of the heat of adsorption. The apparatus used is shown in Fig. I . It consists entirely of Pyrex tubing and flasks with no stopcocks or rubber tubing above the mercury levels. A detailed
a”
FIG.I
V
FIG.2
description of the apparatus is omitted since it is essentially the same as that first used by Coolidge,’2 and later by Pearce and McKinley.6 The entire apparatus is inclosed in a large, double-walled air-bath provided with adequate means for the rapid circulation of the air. The temperature of the bath is electrically controlled by means of a four-foot mercury thermoregulator and a Bunnell relay to within *o.ogo. The adsorption bulb, shown in Fig. 2 , is made of ‘‘7ozP” glass into which is sealed a spiral of tungsten wire. The spiral is so arranged that the ends protrude through the bottom of the bulb, thus facilitating the conduction of heat from the charcoal to the calorimeter liquid. When filled the charcoal is a t no point more than three millimeters from the wire. Coolidge: J. Am. Chem. SOC., 46, 596 (1924).
l*
HEBT OF ADSORPTION BY CHARCOAL
907
A diagram of the calorimeter and fixtures is also shown in Fig. 2 . The calorimeter is a Dewar flask fitted to accommodate the adsorption bulb, the stirrer and the heater. The water equivalent of the calorimeter and fixtures was accurately determined according to the method described by Pearce and McKinley.6 A 24-junction copper-constantan thermocouple, made according to the specifications of White,13 was used in conjunction with a Leeds and Xorthrup, Type K, potentiometer to measure the heat effects. The thermocouple was previously standardized by Dr. McKinley at the temperatures of liquid air, melting ice and the transition points of Na2S04.~oH20and MnC12 zH20. The thermal capacity of the calorimeter and fixtures when in use amounts to approximately 170 cals.; the thermocouple is sensitive to O . O O I ~ . Hence, our calorimeter system will respond to a heat transfer of 0.1 j cal. The calorimeter liquid is "Finol," a light oil whose specific heats at 2 5 ' and 50' were found to be 0.4528 and 0.4674, respectively. The charcoal used as the adsorbent was taken from a large supply obtained from the Xational Carbon and Carbide Corporation for an exhaustive study of the adsorption of gases and vapors carried on in this laboratory. I t is a coconut charcoal, steam-activated and acid-washed until the ash content is reduced to 0.28 percent. Employing the method of Cude and Hulett," KnudsonIj found the density of the charcoal to be 1.80; its loss in weight on outgassing is 2 .5 percent. The liquids whose vapors were studied were purified by the generally accepted methods.I6 Only the constant boiling middle fractions were used. In all cases the purification was done immediately before the liquid was used. Carbon tetrachloride, chloroform, methylene chloride and methyl chloride were chosen with the hope of observing a possible effect of the number of substituted chlorine atoms on the molecular heat of adsorption. The method of experimentation, as well as the method of calculating the heats of adsorption, was the same as that described in the previous paper.6 The data collected in the experimental work are given in Tables I to VIII. I n the first column are listed the final equilibrium pressures. The second column gives the number of cc. of vapor adsorbed by one gram of charcoal. In the third column are the observed heat effects, hobs; while the fourth column contains the values of h calculated by means of the formula given below the Table. The deviations between the observed and calculated heat effects are given in the last column. The data given in these Tables has been obtained with various weights of charcoal, some fresh and others previously used, and with variations in the rate of admission of vapor to the charcoal. The agreement between runs, as well as that within individual runs, indicates that the heat of adsorption is definite and reproducible. Lamb and Coolidgea find that chlorine-containing molecules poison charcoal in such a way that the heat of adsorption decreases in subsequent runs on White: J. Am. Chem. Soc. 36, 2292 (1914). Cude and Hulett: J. Am. bhem. Soc., 42, 391 (1920). 15 Pearce and Knudson: Proc. Iowa Acad. Sci., (1927). 18 Mathews: J. Am. Chem. Soc., 48, 562 (1926). I3
I*
908
J. N, PEARCE AND G . H. REED
the same sample of charcoal. This poisoning effect is not evident from our data; the agreement between runs on new and previously used charcoal is entirely acceptable. The heat of adsorption is independent of the previous use of the charroal provided that it has been outgassed a t 550' to a pressure of 0.0001mni. Fig. 3 shows the results obtained when the logarithm? of the heats of adsorption are plotted againat the number of cc. of vaDor adsorbed per ~- gram of charcoal. To avoid overcrowding these curves have been displaced upward by definite increments. The curves are represented mathematically by the expression, log h = log m
+ log XI
where X is the number of cc. of vapor adsorbed by one gram of charcoal, n is the slope of the curve and m is the value of h when S is unity. From the slope of the curve and its intercept on the h-axis we obtained the constants for the equations given beneath the Tables. The values of the heat of adsorption calculated from these empirical equations are in good agreement with the observed values as shown in the last column of each Table. For the sake of comparison, we have followed the method of Lamb and Coolidge3 in calculating the molecular heats of adsorption, h,. Table IX shows the values calculated for the heat of adsorption of a gram molecular weight of the vapor on 500 grams of charcoal. This Table shows also the values of the constants, m and n, of the separate vapors. I n all cases the values of n deviate but little from one and are always less than unity. This implies that successive equal increments of a given vapor adsorbed liberate practically the same amount of heat. A comparison of the molecular heats of adsorption of the four vapors at the three temperatures, Table X, indicates that the heat of adsorption is practically constant over the whole range of temperature from on to 50'. These results can only lead to the definite conclusion that the temperature coefficient of the heat of adsorption, if there is one, is very, very small. There is at present no satisfactory explanation for the apparent independence of heat of adsorption and temperature, unless it be that the field forces operative in adsorption processes are too strong to be influenced by the tempera-
HEAT O F ADSORPTION BY CHARCOAL
909
ture. Lorentz and Landel’ believe that temperature should have some influence, while Eucken,I6 on the other hand, holds to the view that the heat of adsorption is independent of the temperature. Lamb and Coolidge3 consider that the observed heat effect is the sum of two effects,-the heat of vaporization of the substance adsorbed and the net heat of adsorption. If the influence of temperature on these two heat effects were equal and opposite in direction, then no observable effect of temperature should be expected. The net heats of adsorption have been calculated, but, owing to the lack of sufficiently reliable density and heat of vaporization data, they are not included in this paper. In general, the heat of vaporization decreases with increasing temperature while the net heat of adsorption increases. The slopes of the adsorption isosteres have frequently been used in calculating heats of adsorption. I t has been shown that these (log p- I/T) plots yield, in general, straight and parallel lines for a given vapor. If, therefore, the Clausius-Clapeyron equation is applicable to adsorption, no change in the heat of adsorption with rise in temperature should be expected. However, the values obtained by this method are lower than those obtained experimentally. I n the study of the adsorption of chlorine-containing molecules it has been generally assumed that the molecules are oriented with the chlorine atoms toward the surface of the adsorbent. The chlorine atoms possess seven valence electrons in their outer orbits, and the force fields about them should be large. If we may assume that the heat effect observed in adsorption is a result of the neutralization or saturation of the powerful force fields about the surface atoms of the adsorbent, then carbon tetrachloride with its greater number of chlorine atoms should exhibit the greatest heat of adsorption. This is exactly what we do find. From Table X we see at once that for each temperature the molecular heat of adsorption increases with the number of chlorine atoms in the molecule.
TABLE I The Heat of Adsorption of Methyl Chloride Vapor a t 2 5’ X h(0bs) h(Ca1c)
P
cc/g.
c4g.
cal/g.
0.433
11.17
1.021
21 .OI
4.95 8.91
4.89 8.94
I . j48
29.11 41.34
cm.
3,202
I . I20
8.57
2.475 3.864 5,955 9.312
15.79 21.79 29.66 39.08
I 2 .OI
12.20
16.70 3.94 6.80 9.37 12.69 16.34
17.06 3.80 6.81 9.26 12.43 16.17
Dev. cal/g. -0.06
+0.03
$0.19 +0.36 -0.14 +o .OI -0.11 -0
26
-0.17
Mean 0 . 1 5 17
18
h
=
0.4887 X0.9545
Lorentz and Lande: 2.anorg. Chem., 125, 47 (1922). Eucken: Ber. deutsch. physik. Ges., 12, 345 (1914).
J. N. PEARCE AND G . H. REED
TABLE I1 The Heat of Adsorption of Methyl Chloride Vapor a t 50' X
P
cm.
cc/g.
h(0bs) cal/g.
h(Ca1c) cal/g.
0.322
9.24 18.58
4 .OS 8.12
4.07
-0.01
7.95
-0.17
2 5 .39 32.88
10.71
10.75
13.87 16.99 4.80 8.76
13.78 16.61
.04 -0.09 -0.38
4.65 8.71
-0.05
11.97 14.43 I8 .os
11.76
-0.21
14.37 18.08
-0.06
0.932 I ,695 2.612 3.781 0.401 1,254 2.111
3 , I02 4.960
39.95 I O . 60 20.40 27.86 34.35 43.63
Dev. cal,/g.
+O
-0.2j
0 .oo
Mean 0 . 1 3
h = 0.4820 XO.gsOO
TABLE I11 The Heat of Adsorption of Methylene Chloride Vapor at P om. 0.000
0,244 0.568 I ,041 0.000 0.000
X cc/g.
h(Ca1c) cal/g.
7.89 .4I 41.30
5.23 I 5 .so 24.17
5.30
-0.07
15.37 23.91
-0.43 -0.26
57 .83 5.83 I S .68
33 ' I3 4.12
32.48 4.03
-0.65 -0.09
9.65 15.25
9.90
+0.25
1 5 .68
+0.43
23 .40 32.23
+0.62
40.75
+ o . 78
25
0.204
2 5 .97 40.37 57.35
0,433
74.19
0.050
0.075
2j o
h(0bs) cwg.
23.07 31.61 39.97
Dev. cal/g.
+o .33
Mean h = 0.8091
Xo.9100
0.39
HEAT O F ADSORPTION BY CHARCOAL
TABLEI V The Heat of Adsorption of Methylene Chloride Vapor a t 50' P cm.
0,124 0.302
0.610 I ,070 I ,660 2.602
X cc/g.
12.59 24.55 36.30 47.42 57.40 68.24
0.000
12.55
0.074 0.198
25.68 38.78 50.44 62.29
0,317
0.664 1.249
72.75
h(0bs) cal/g.
h(Ca1c) cal/g.
Dev. cal/g. +O.II
25.69 29.62 36.IO 7.28 14.39
7.36 13.79 19.92 25.61 30.65 36 .os 7.34 14.39
21.10
21.20
+o. 10
27.34 33.37 39.19
27.I4 33.09 38.79
-0.20
7.25
13.84 20.15
-0.05
-0.23
-0.08 +I
.03
-0.05
$0.06 0.00
-0.28 -0.40
Mean
0.22
h = 0.6808X0.g4w
TABLE V The Heat of Adsorption of Chloroform Vapor at P cm. 0.000 0.000 0.025
0.030
0.060 0.164 0.244 0.413 0.692
X
CC/&
9.22 18.48 28.42 40.93
h(0bs) cd/g.
25'
h(Cdc) cal/g.
Dev. cd/g.
6.34
6.33
-0.01
12.27
12.30
$0.03
18.61 26.68
-0.05
-0.37 -0.33
10.07
7.22
18.56 26.31 6.89
21.15
14.27 20.60 26.30 32.96
14.00
-0.27
20.56 26.60 33.14
+ O .SO
31.63 41.39 5 2 .IO
-0.04
+0.18 Mean
h = 0.7569X0.g560
0.17
J. N . PEARCE AND G. H . REED
912
TABLEV I The Heat of Adsorption of Chloroform T-apor at jo'
x
0.000
I1
j8
h(0bs) call g. 8.32
0.000
23.66 3 j .oo
15.72 22.96
P
cm.
0.035 0.Oj.t
I54 o 302 0
0.000 0.000
0.040
0.069
cr/g.
30.IO
45.53 54.90 64.85 12.13
36.43 42.22
7.94 I3 . a 7 22.49
23.48 35.04 47.62
0.119
58.84
0.421
69.29
30.95 37.62 4471
h(Calc) cal/g. 7.82 I j .6: 22.88
De\..
cal 19. -o..jo -0
07
-0
08
-0.57
29.53 35.41 41.62
-I
.02
-0.60
8.18
+o
15.52
+I
21 .66 31
22.90
i-0
30.84
-0.11
37.69 44.38
+0.07
-0.33
TABLE VI1 The Heat of Adsorption of Carbon Tetrachloride Vapor at P cm. 0.050 0.070 0.070 I
,071
2.370 7.678 0.000 0.010
0.086
0,494 1.915
x
h(0bs)
cal/g.
cc/g. 12.35 29.02
10.53 20.78
47.26 61.97 64.67 71 .71
31.59 41.72 44.41 48.05 6.68 16.51 22.36
8.44 22.47
34.72 50.92 55.22
34.81 38.01
h(Ca1c) cal/g. 9.48
25'
Dev. cal/g. -1 , o j
20.93 32.91 42.31 44.00 48.43 6.66 16.51
+0,15 + I .42
24.72
+I
35.26 38 . O I
50.45
t o .59 -0.41
f0.38 -0.02 0.00
.36
0.00
Mean 0.53 h
=
0.9230 X0.9270
HEAT O F ADSORPTION BY CHARCOAL
913
TABLET'III The Heat of Adsorption of Carbon Tetrachloride Vapor at. 50' X
P cm.
h(Ca1c) cal/g.
h(0hs) cal/g.
CC/&
Dev cal/g
7.68
8.12
+o
13.76
+o
21.11
14.40 20.63
.85
28.36
+O
j I
48.71 63.44
32.i7 42 .oo
+o
82
+o
0.040
12.04
9.92
95 78
0.114
23.22
17.59
33.59 42.95 9.14 16.85
0.114
33.06 42 .35
23.42 29.48 36.20
+o -0
I7
52.79
23.34 29.31 36 .oo
+o.
20
57.29
39.18
39.06
-0
I2
0.065 0,139
10.60 18.32
0.139 0.183
28.85 40.62
0.342 0.728
0.154 0.168 0.332
27
-0
-0 -0
44 64 48
74 08
Mean o 49
h
= 0.9020
X0,9310
TABLE I3 Summary of Calorimetric Data 25'
0.9230 0.i569 0.8091
n 0.9270 0.9560 0.9100
hm Cals.
CCl, CHCls CH2Clz CHsCl
0.4887
0.9545
9.2
CCl, CHC13 CH2C12 CHsCl
0.9020 0.7261 0.6808
0
0
15.4 14.5 12.1
0.4820
0
9.2
Suhstn
m
15.6 14.3 12.8
SO0
0
J. N. PEARCE AND G. H. REED
914
TABLE X Comparison of hm a t oo, Subs
2s0,
hm(Oo,L&C) hm(25', P&McK) Cals. Cals.
and 50' hm(25') Csls.
hm(50') Cals.
cc14 CHC13 CH2C12
15.3
15.4
15.6
15.4
14.5
14.5
14.3
14.5
__
-
12.8
12.1
CH,Cl
-
9.2
9.2
summary
The heats of adsorption of methyl chloride, methylene chloride, chloroform and carbon tetrachloride have been determined a t 25' and 50'. 2. The temperature coefficient of the heat of adsorption of the four vapors, if there is one, is very small. 3 . The molecular heats of adsorption of the present series increases with increase in the number of chlorine atoms in the molecule. I.
Physical Chemistry Laboratory, The State Cnivernty of Iowa.
