STUDIES ON THE SORPTION OF THE HALOGENS BY SILICA GEL AND CHARCOAL1 L. H. REYERSON AND ANGUS E. CAMERON School of Chemistry, University of Minnesota, Minneapolis, Minnesota Received J u n e 14, 1034
There are recorded in the literature several instances of adsorption studies in which one of the halogens was adsorbed by either silica gel or charcoal. Probably the best work to date is that of Magnus and Muller (4), who studied the sorption of chlorine by silica gel in an all-glass system a t O", 20" and 40°C. Bosshard (2) determined the retention of bromine by various silica gels when the bromine vapor was carried over the gel by an inert gas. Qualitative measurements on sorptions measured dynamically in the presence of foreign gases constitute most of the other recorded measurements. The great difficulties involved in the sorption of halogen vapors have no doubt hindered successful study in this most important field. Some time ago it was decided to study rather completely the sorption of the halogens in an all-glass-quartz system. Experimental technique in the handling of gases had advanced to such a point that success seemed possible. I n this investigation the helical quartz spring sorption balance of McBain and Bakr ( 5 ) was combined with the quartz spiral manometer described by Bodenstein and Katayama (1). The design and operation of the apparatus is fully described by the authors in another publication. I n brief, the apparatus of all-glass and quartz was sealed from the vacuum pumps during measurements. The vapor pressure of the halogen was maintained by condensing it into a tube immersed in an automatic cryostat as described by Cameron (3). By proper control of the temperature of the bath, the vapor pressure of the halogen could be varied throughout wide limits. The sorption balance and quartz manometer were maintained a t constant temperature during measurements in a specially constructed air thermostat. This thermostat was capable of close temperature regulation up to 220°C. The silica gel used in this investigation was of the glassy type. It was kindly furnished by the Silica Gel Corporation. This gel was digested with nitric acid and then thoroughly washed and finally electrodialyzed for several days in an electrodialysis cell. Following the dialysis it was Presented before the Eleventh Colloid Symposium, held at Madison, Wisconsin, June 14-16, 1934. 181
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L. H. REYERSON AND A . E . CAMERON
dried and air-activated a t 600°C. Granules which passed a standard 60mesh and were retained by a 100-mesh screen were used in this investigation. The charcoal was prepared from cocoanut shells which had been cleaned and extracted with ether, alcohol, and water in the order named. The shells were coked by bringing the temperature slowly to 550-575°C. and holding it there for half an hour after the last flammable vapors had been generated. The charcoal was crushed and then activated by passing superheated steam over it in a silica tube heated in an electric tube furnace to 850°C. On analysis this charcoal was found to have an ash content of 0.185 per cent. The bromine and iodine were purified chemically by the best recommended procedure. They were then further purified by successive distillations or sublimations in specially constructed vacuum systems. The middle portions only of the final vacuum purifications were used in this investigation. Sorption-desorption isotherms were obtained over a wide pressure range for bromine on silica gel a t 58", 79", 99.9", 117.5", and 337.7"C. Figure 1 presents the results graphically. Except for the highest desorption readings a t 79"C., the sorption and desorption values seem to be completely reversible. Throughout the temperature and pressure range studied there is no hysteresis evident. Slight irregularities may be noted in the curves, and these may be due to errors in the determination. However they seem to be slightly larger than experimental errors and may be evidence for the type of discontinuity reported recently by Allmand and other investigators. The curves obtained for the two highest temperatures indicate that the sorption of bromine is directly proportional to the pressure a t these temperatures. The results for the sorption-desorption of iodine at 98.2", 337.6", 158.3", 178.4", and 198.5"C. are shown graphically in figure 2. The ordinates of this graph are magnified tenfold as to millimoles adsorbed over figure 1. This makes the curves appear to rise more abruptly in the case of iodine sorption. However the millimoles of bromine sorbed under comparable temperature and pressure conditions were on the average tenfold greater than in the case of iodine. Except a t the highest pressures, equilibrium between the silica gel and the halogen vapor was established rather rapidly. There was no evidence for a slow sorption of a different type from the initial rapid sorption. The isotherms for iodine a t the two lowest temperatures indicate a slight break in the lowest pressure range. This may indicate a different type of sorption a t low pressures. I n the sorption studies on charcoal a very different behavior was found. The rate of sorption was slower than in the case of silica gel, about 24 hours being needed to obtain reasonably constant results. As a result it was decided to determine the rates of sorption and desorption on charcoal. Measurements of this rate were made at constant vapor pressure of the
SORPTION OF HALOGENS BY SILICA GEL AND CHARCOAL
S O L I D POINTS I N D I C A T E
DESORPTION
READINGS
FIQ.2. SORPTION OF IODINEBY SILICA GEL TEE JOURNAL OF PHYSICAL CHEMISTRY, YOL.
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L. H. REYERSON AND A. E. CAMERON
halogen for 24 hours. The vapor pressure was then increased and the rate of sorption determined for this higher pressure. After sorption at the highest vapor pressure, similar rate studies were made on desorption by successively dropping the vapor pressure of the halogen to the same pressures as were maintained during sorptions. Figure 3 shows graphically the results obtained at 137.6"C. It should be noted that the desorption points reached a t the end of the 24-hour period always lie slightly above the corresponding sorption points. It is possible that these points would coincide after a very long time, but the sorption-desorption curves came so close to one another and were so nearly parallel that it was not felt necessary t o
0 - SORPTION 0 - DUORPSION #-TEMPERATURE RAISED FROM 98.Io TO 137.6'
5.5
K >
! i w
3
2 5.0 P
w K
I
b
LL
0
d
45
1 1
B a
3-I
f 4.0
0
zj 3.5
-
z
2.5 0
200
400
600 BOO IO00 TIME IN MINUTES
1200
1400
I600
FIG.3. RATESOF SORPTIONOF BROMINEBY CHARCOAL AT 137.G"C.
prolong the study. Rate curves for other temperatures than the one given in figure 3 are much the same. However, the higher the pressure and the higher the temperature the closer to the sorption points lie the desorption points. It should also be noted that more than 2.5 millimoles of bromine remain adsorbed when desorption is carried out a t 0.03 mm. pressure. I n fact it is impossible to remove all the bromine from the charcoal by thorough evacuation. When the charcoal containing the halogen is heated to about 500°C. and the tube containing the liquid bromine is frozen by immersion in liquid oxygen, equilibrium is reached when slightly more than 2 millimoles of bromine remain sorbed by a gram of charcoal. Under similar heat treatment of the charcoal containing bromine, evacuation of the system was carried out a t 10-6 mm. pressure. After 24 hours about 0.7
SORPTION OF HALOGENS BY SILICA GEL AND CHARCOAL
SOLID
P O I N T S INDICATE
DESORPTION READINGS
FIQ. 4. SORPTION OF BROMINE BY CHARCOAL
SOLID POINTS INDICATE DESORPTION READINGS
FIQ. 5. SORPTION OF IODINE B Y CHARCOAL
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millimole of bromine remained per gram of charcoal, and a t the end of 72 hours this quantity had only been reduced to 0.6 millimole. These results are in agreement with those of Ruff, Rimrott, and Zeumer (6), who found that charcoal must be heated to 1100°C. before the last of the bromine was removed. The products then obtained were hydrogen bromide, bromine, and some carbon-containing bromine compounds. The isotherms for bromine and iodine sorbed by the charcoal are given in figures 4 and 5. It should be noted that the amount of iodine taken up by the charcoal is much less than the quantity of bromine. However the types of isotherms obtained are similar. The differences in amount of iodine sorbed by the charcoa€ as compared to bromine are very similar to the differences obtained on silica gel. DISCUSSION
I n considering the character of the sorption of the halogens by two such different porous sorbents, it is important t o know the surface area covered by the sorbed gases a t various temperatures and pressures. From viscosity measurements Rankine gives the molecular diameters of bromine and cm., respectively. If we asiodine as 3.42 X lo-* cm. and 3.76 X sume that the molecules cover an area equal to the square of the diameters, then 1 millimole of bromine would cover 7.1 X lo5 cmS2and 1 millimole of iodine would cover 8.58 X IO6 cm2. Multiplying the number of millimoles sorbed a t varying pressures by these areas would give the approximate area covered under the different conditions. The data obtained for the sorptions by silica gel do not fit the classical sorption isothermal, neither can they be fitted to the Langmuir expressions. The shape of the curves seems to indicate a rather loose type of physical binding of the van der Waals type, or they may represent capillary condensation. I n fact the sorption values fit the expression of Patrick,
for capillary condensation better than any other, and even here the agreement is not good a t either very high or very low pressure values 'for bromine. The sorption of iodine shows still less agreement with this expression. I n order t o test this equation it is necessary to know the volume, V , of the liquid sorbed, the surface tension, y, of the liquid a t the temperature of the isothermal, the equilibrium pressure, P , and the saturation pressure, Ps, a t the temperature of the isothermal. The data for all the isothermals should fall on the same straight line, where log V is plotted against log
py. - The values used in checking the Patrick expression were obtained as
PS
follows, The values of Ps were read from a plot of log P against 1/T.
SORPTION OF HALOGENES BY SILICA GEL AND CHARCOAL
387
The values of the vapor pressure used in the graphs and of the surface tension and density a t 20°C. of bromine were obtained from the International Critical Tables. The constant of the Ramsay-Eotvos-Shields equation was calculated for bromine and found to be 2.074. Using this constant and the values of the surface tension, y , obtained by straight line extrapolation between 20°C. and tc+, the density of bromine a t the temperatures of the various isothermals was calculated. The various values so obtained for bromine are given in table 1. The values of V a n d 5 can then be obtained and the logarithms of
Ps
these values plotted against each other. Figure 6 shows the results so obtained. The points all lie well along a straight line except a t the two extremes, where considerable deviation is to be noted. I n view of the many assumptions made use of in obtaining the necessary quantities for the Patrick equation, the agreement seems rather good. I n addition, if one reads the equilibrium pressures for the same value of X / M for the various iso-
yhl*=
TABLE 1 41.5 dynes per centimeter; d , ~= 3.119; t , = 302°C. Y
dt
36.0 32.6 29.4 27.0 23.8
3.14 3.04 3.11 3.148 3.12
PS
mm.
58.0 79.0 99.9 117.5 137.7
740 1340 2631 3881 5650
thermals, then it is possible to calculate the differential heats of sorption from the Clausius-Clapeyron equation.
The average of eleven such calculations gives the differential heat of sorption as 7719 cal. per mole. The latent heat of vaporization of bromine a t the boiling point is given in the International Critical Tables as 7167 cal. per mole. Thus the differential heat of sorption is only slightly larger than the latent heat of vaporization. This difference may be significant, but the sorption forces are apparently not much different from those involved in the formation of the liquid from the vapor phase. Finally the question should be raised as to whether it is necessary to have capillary condensation or not. I n these experiments the maximum sorption measured for bromine on silica gel was about 4.5 millimoles a t 79°C. 4.5 X 7.1 X 3 Os gives 31.95 X lo6 em2. as probably the area covered by bromine. This
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L. H. REYERSON A N D A. E. C A M E R O N
is not as great as the total surface area per gram of silica gel as reported by many investigators. However, the shape of the isothermals indicates that the gel will continue to take up more of the halogen as its pressure is increased. It is therefore possible that we have capillary condensation superimposed on loosely acting surface forces. Calculations similar to those given above were made for iodine, but the results did not agree so well with the Patrick expression as did those for bromine. The average of four calculations of the differential heat of sorption gave a value of 11,340 cal. per mole. The latent heat of vaporization at the boiling point is given in the International Critical Tables as 10,516
FIG.6. LOGARITHMIC PLOTOF BROMINE SORPTION BASEDON PATRICK’S EQUATION
cal. per mole. However, the heat of sublimation is of the order of 14,520 cal. The differential heat of sorption is thus only slightly greater than the latent heat of vaporization. Similar physical forces seem to be acting in the case of iodine sorption as was found for bromine. As already noted, the character of the sorption of the halogens on charcoal is quite different from that on silica gel. By far the largest part of the sorption occurs in the lowest pressure ranges. There is a relatively small difference between the amounts sorbed at the lowest and highest temperatures a t the same vapor pressure of the halogen. This is not true in the case of silica gel. The type of isotherm obtained indicates the formation of a monomolecular layer of sorbed molecules. This appears to be followed a t higher pressures by slow diffusion into the charcoal.
SORPTION OF HALOGENS BY SILICA GEL AND CHARCOAL
189
The average of eleven values of the differential heat of sorption calculated as before was 11,420 cal. per mole for bromine on charcoal. The average of four such calculated values for iodine was found to be 13,500 cal. per mole. The value for bromine is some 4000 cal. per mole higher than the latent heat of vaporization. I n the case of iodine it is about 2500 cal. higher. These significantly higher values indicate that some form of activated sorption is taking place. The simplest form of.the Langmuir sorption expression is
X
abp ii?=lfbp
0
FIG.7 .
100
200
300 400 500 800 PRESSURE IN MM. OF MERCURY
700
SORPTION ISOTHERMS O B T A I N E D B Y U S E OF THE L.4NGYUIR
800
EQUATION
wliicli by rearrangement becomes
which is the equation of a straight line. The values of P / X / M were calculated from the data and plotted against ,P. The results are shown in figure 7, and the agreement with the Langmuir equation is excellent. Only when plotted on a much enlarged scale is any deviation from the straight-line relationship found, and this occurs at the very lowest pressure.
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L. H. REYERSON AND A. E . CAMERON
It seems evident that the sorption of bromine and iodine by activated charcoal is of the monomolecular type called “persorption” by McBain. In this type of sorption all of the sorbed molecules are bound directly by the atoms of the surface of the sorbent. If the surface of the charcoal is completely covered by bromine molecules, then a simple calculation shows that a gram of this activated charcoal has a surface area of about 350 square meters. This is of the same order of magnitude as found by other investigators. SUMMARY
1. Sorptions of bromine and iodine on silica gel and activated charcoal have been measured in an all-glass-quartz system at widely different tempera tures. 2. Sorptions by silica gel indicate a loose physical binding or capillary condensation. 3. Sorptions by charcoal indicate activated sorption that is monomolecular in depth of layer. REFERENCES (1) BODENSTEIN AND KATAYAMA: Z. Elektrochem. 22, 331 (1916). (2) BOSSHARD: Helv. Chim. Acta 12, 105 (1929). (3) CAMERON: Rev. Sci. Instruments 4, 611 (1933). (4) MAGNUS AND MULLER:Z. physik. Chem. 148A, 241 (1930). (5) MCBAINAND BAKR:J. Am. Chem. SOC.48, 690 (1926). AND ZEUMER: Kolloid-Z. 37, 270 (1925). (6) RUFF,RIMROTT,
