THE SORPTION OF BROMINE AND IODINE BY SILICA GEL ANGUS E. CAMERON'
AND
L. H. REYERSON
School of Chemistrv, University of Minnesota, A l i n n e a p o h , Minnesota Received A p r i l 26, 193.4
In the literature on the adsorption of gases and vapors by porous adsorbents there is lacking a reasonably complete study on the halogens. There is recorded a set of measurements by Magnus and Muller (3) on the adsorption of chlorine by silica gel in an all-glass system a t 0", 20" and 40°C. They obtained adsorption isotherms which were slightly concave toward the pressure axis. However their results did not fit the Freundlich equation. Some time ago one of us decided to undertake a rather complete study on the sorption of the halogens and the hydrogen halides on various surfaces. This and the following paper give the results for bromine and iodine sorption on silica gel and charcoal. APPARATUS AND MATERIALS
Measurements of the sorption were made by the static method. The vapor pressure of pure halogen vapor in a glass and quartz system was controlled, and the sorption determined by means of a McBain sorption balance (4). Figure 1 shows the essential portions of the apparatus. The quartz spring balance was suspended from a hook on a glass ball resting upon three projections in the sorption tube, A. The sorbent was contained in a small glass bucket (l),8, and was suspended from the spring by a quartz fiber. The pressure in the apparatus was determined by the 4.5 turn Bodenstein quartz spiral gauge, B, using this instrument as a null point indicator. The pressure in the jacket of the gauge was varied until the index needle coincided with the fixed reference point viewed in the field of a low-power microscope. The reference point, which was an integral part of the gauge, was attached with wax to the end of a glass tube sealed through the bottom of the jacket in order to prevent damaging vibration. The gauge was mounted in the jacket with a graded seal. Pressures were read from the large bore, closed-end, mercury manometer with a cathetometer. The manometer readings were corrected to 0°C. 1 The material here presented formed a part of a thesis submitted to the Graduate Faculty of the University of Minnesota by Angus E. Cameron in partial fulfillment of the requirements for the degree of Doctor of Philosophy, December, 1932.
169
170
ANQUG E. CAMERON AND L. H. REYERSON
Zero readings were taken with the apparatus evacuated or the halogen frozen out with liquid oxygen and the quartz gauge a t the temperature at which a run was t o be made.
9
TO
A
J
i
AIR VACUUM
FIQ.1. SORPTIONAPPARATUS
The entire apparatus, with the exception of the lower portion of the sorption tube, A, and the bottom of the quartz gauge, was enclosed in an air bath built of wood and lined with asbestos. A multibladed fan at
SORPTION OF BROMINE AND IODINE BY SILICA GEL
171
the top of the bath blew air down through a grid of resistance wire. A long, thin-walled glass regulator bulb filled with mercury and connected through a capillary to a control “U” was inserted through the wall of the oven, and operated a thermionic relay device t o vary an external resistance in series with the heating grid. The halogen was introduced into the bottom of tube, A, as will be described, and its temperature was controlled by means of a thermostated bat,h in a half-gallon Dewar jar. The space between the lid of this small bath and the bottom of the air bath was insulated by a glass tube of large diameter, which was wound with asbestos paper. A resistance wire wound on the inside of this tube permitted heating of this portion of the tube to prevent condensation of halogen when working a t high vapor pressures. The small bath was stirred by a multibladed paddle driven by an induction stirring motor. A special thermoregulator bulb, with large external surface and filled with toluene or mercury, operated a thermionic relay device to control the bath temperature. Above room temperature a small, direct-immersion heating element with a series resistance was used. Water or paraf5.n oil were used as bath liquid, depending upon the temperature to be obtained. Below room temperature the temperature was controlled by an automatically operated cryostat (2). The sorption tube was cut at the point indicated by the arrow, and a quart>zfiber of sufficient length to extend below the floor of the oven was attached to the one already hanging from the spring. This fiber was shielded from air currents inside the oven by a glass tube of somewhat greater diameter attached to the sorption tube with a ring of cork. A mica pan, weighing about 0.03 g., was hooked to this fiber, and calibrated fractional gram weights were placed upon the pan. The long fiber and pan were weighed on an analytical balance. Measurements of the spring length were made with a traveling microscope rigidly mounted on a vertical rod supported permanently on the side of the air thermostat. Observations were made through a narrow glass window. Other windows permitted observations of the interior of the thermostat. To reduce thermal disturbances on the inside the latter were double windows. Illumination for reading the spring points was furnished by a long show-case bulb behind an opal glass plate. The opal plate was removed for the studies with iodine vapor and the filament of the bulb lined up behind the spring points. The traveling microscope was especially designed for vertical mounting and was constructed in the instrument shop of the University of Minnesota. The screw was cut with a 1-mm. pitch and the micrometer head was marked with 100 divisions. The total useful range of the instrument was 100 mm.
172
ANGUS E. CAMERON AND L. H. REYERSON
The spring was calibrated a t the various temperatures both before and after each run. The second calibrations were used in calculations of the sorption. The calibration was carried out in steps of 0.1 g. The sensitivity of the spring, expressed in milligrams per 0.01 mm. elongation, was calculated for each 0.1 g. interval a t the different temperatures (see table 1). This sensitivity and the calibration points were used to determine the load on the spring from the observed length. The weight of the bucket was determined by hanging it on the short fiber and measuring the spring length. About 0.2 g. of sorbent was added t o the bucket; after hanging it in place the sorption tube was sealed together. The system was evacuated through tube C. The vacuum system consisted of a n ordinary and a high speed Langmuir mercury vapor pump operating in series, with the customary McLeod gauge and liquid air trap. The portion of the sorption tube in which the sorbent hung was heated with a small resistance tube furnace operated a t 500-550°C. During TABLE 1 S p r i n g calibration LOAD
I
LENGTH
I
DIFFERINTIAL SENSITIVITY
grams
mm.
mg. per 0.01 nim.
0.33958 0.43944 0.53967
57.051 62.666 67,272
0,17783 0.178ao
evacuation the connecting tubing was flamed over as much of its length mm. pressure for 24 hours, as possible. The system was evacuated a t and tube C was sealed. The halogen in one of the ampoules, E, was frozen with liquid air, and the breakoffsky operated by a small direct current solenoid which had been placed around the tube before it was sealed in place. The halogen was allowed to remain in contact with the sorbent for a t least twelve hours. During this time the furnace was held a t a sufficiently high temperature t o prevent excessive condensation on the sorbent. The temperature was raised to 500°C. several times during the twelve hours to aid in the displacement of foreign gases from the sorbent surface. The ampoule E, containing most of the halogen, was then sealed off, and the system opened t o the vacuum line through one of the bank of breakoffskies, D, after the remaining halogen had been frozen out in a seal-off bulb, F, with liquid oxygen. Evacuation and activation were continued as before for 48 hours, at which time the open breakoffsky was sealed and the second sample of
SORPTION O F BROMINE AND IODINE BY SILICA GEL
d
173
halogen admitted, distilled into the bottom of tube A, and the empty bulb sealed off. The activation furnace was removed when all the halogen had been frozen down in the bottom of tube A, the air bath was brought up to temperature, and the vapor pressure thermostat was assembled and brought into operation. At the completion of a run the halogen was frozen into the remaining bulb, F, and sealed off before opening the system to the vacuum line. The temperature of the oven was read on a calibrated thermometer suspended beside the sorption tube. The temperature was constant t o &0.15"C., at any setting from 60°C. t o 220°C. When working with bromine the temperature of the liquid bath was read on a 100°C. thermonieter graduated in 0.1"C. steps and calibrated against thermometers with Bureau of Standards certification. A similarly calibrated 220°C. thermometer reading in 0.2"C. steps was used in the work with iodine. Below 0°C. the temperature of the bath was determined with a calibrated toluene thermometer which for later work was replaced with a ten-junction copperconstantan thermocouple. The temperature of the bath was held t,o f0.01"C. above room temperature and to within &O.O3"C. from room temperature t o 0°C. Below 0°C. the variation was found to be over a range not greater than 0.08"C. The same quartz spring was used throughout this work. It consisted of twenty-one turns and was 1.5 cm. in diameter. Its sensitivity was approximately 0.18 mg. per 0.01 mm. elongation. The calibration of the spring was found to be remarkably reproducible, spring lengths measured two months apart checking within a few thousandths of a millimeter under the same load and a t the same temperature. The bromine used was prepared from analytical reagent quality bromine. Further purification was carried out according t o the directions given by Scott (6), shaking the bromine twice with 1N potassium hydroxide and once with 0.5 N . It was then distilled once from dilute potassium hydroxide solution in an all-glass still with a short fractionating column, and again from the same still after cleaning and drying. The middle portion of the bromine was twice distilled in high vacuum through phosphorus pentoxide which had been resublimed in a current of oxygen. The pentoxide tube was then sealed out of the all-glass apparatus and the bromine thrice fractionated in high vacuum before being distilled into the capsules and sealed off. The iodine was of C.P. quality and was sublimed once in air from a n intimate mixture with C.P. potassium iodide and calcium oxide. The sublimed iodine was then ground with more potassium iodide and calcium oxide, and introduced into the bottom bulb of a series of four bulbs joined by strictured tubing. The column was pumped out and about two-thirds of the iodine driven into the second bulb by heating with boiling water.
174
ANGUS E. CAMERON AND L. H. REYERSON
1
(CORRECTED) PRESSURE
TABLE 2 Sorption of bromine bu silica gel X/M
XIM
Isothermal a t 58.0"C. mm.
I
0.0 3.7 16.1 38.2 68.6 88.6 113.8 144.7 178.1 239.0 219.1 198.4 160.2 127.2 99.9 68.9 68.7 58.7 20.7 9.0 5.3 0.6
-
Isothermal a t 79.0°C.-Cont.
hours
mm.
millimolea per gram sorbent
hours
0.0000 0.0384 0.1238 0.2585 0.4337 0.5514 0.7094 0.9291 1.2134 1.8908 1.6580* 1.4130* 1.0650* 0.8188* 0,6347* 0,4433' 0.4423* 0.3629* 0.1705*
3.0 1.5 2.0 1.5 16.0 1.7 3.7 3.0 2.5 10.0 3.7 4.7 3.0 2.7 3.5 6.0 11.2 3.0 2.2 2.0 2.0 1.7
795.1 693.6 640.2 547.0 501.2 426.8 365.1 293.7 244.2 174.4 137.7 108.5 64.4 10.2 3.9 0.0
5.2766 4.4618* 3.6289* 2.4475* 2.0126* 1.5788* 1.2778* 0.9758* 0.7935* 0.5615* 0.4511* 0.3674* 0.2382* 0.0516* 0.0256 *
6.0 5.0 12.0 5.5 5.0 12.5 8.2 2.0 13.0 3.5 3.5 3.5 15.0 3.7 2.0 10.0
0.0890*
0.0635* 0.0230*
.
TIMEFORREADING
millimoles per gram sorbent
Isothermal a t 79.0"C. 0.0 2.4 11.5 69.8 112.5 162.4 218.6 267.7 326.9 395.4 458.0 520.5 566.6 617.7 671.6 718.7 756.0
I
0.0000 0.0105 0.0473 0.2342 0.3601 0.5034 0.6911 1.0144 1.0942 1.4103 1.7697 2.1890 2.5863 3.0396 3.6238 4.3252 4.6483
* Desorption readings.
2.0 1.0 1.2 12.7 1.o 1.2 1.7 11.0 2.5 4.0 2.5 1.7 13.0
'
4.0
4.0 7.0 6.0
11
o.oooo*
Isothermal a t 99.9"C. 1.7 12.9 72.6 118.3 151.3 214.2 265.9 324.4 392.6 455.6 336.1 488.4 564.9 629.6 694.7 744.4 793.0 673.8 567.0 473.4 364.9 289.2 235.1
0.0036 0,0267 0.1443 0.2210 0.2770 0.3801 0.4665 0,5649 0.6828 0.8028 0.5934* 0.8752 1.0350 1.1930 1.3616 1.4505 1.6324 1.2929* 1.0495' 0.8498' 0.6466* 0.5149* 0.4271*
2.5 0.7 1.2 4.2 1.5 2.0 5.0
3.0 1.0 1.0 3.0 2.0 3.2 18.0 4.5 3.5 11.0 26.5 3.0 2.0 6.0 10.5 2.0
175
SORPTION O F BROMINE AND IODINE BY SILICA GEL
TABLE 2-Concluded
Isothermal at 117.5"C.-Cont.
Isothermal a t 99.Q0C.-Conl. mm.
millimoles per gram sorbent
173.9 139.6 86.6 68.0 27.1 0.0
0.3262* 0.2698* 0.1670* 0.1443* 0.1081 *
1.7 1.5 1.7 3.0 2.0 4.5
o.oooo*
Isothermal a t 117.5"C. 0.0 67.7 110.2 139.4 174.2 215.7 266.0 323.5 392.0 470.6 563.1 669.3 790.4 740.0 614.2 517.1 429.8 356.5 292.1 241.2 194.4' 155.6 124.0 74.9 68.6 21.1
0.0000 0,0809
0.1262 0 1585 0.1889 0.2295 0.2846 0.3423 0.4076 0.4852 0.5780 0.6888 0.8315 0.7756* 0.6393* 0.5330* 0.4511* 0.3753* 0.3118* 0.2611* 0,2103* 0.1806* 0.1476* 0.1059* 0.0838* 0.0326* I
'
mm,
hours
11.0 9.5 2.7 2.0 1.5 1.o 3.7 1.5 1.7 2.0 2.0 4.0 1.5 9.7 2.5 2.5 2.2 4.0 8.2 11.0 3.2 1.5 2.2 2.5 10.7 1.7
millimoles per gram sorbent
3.9 1.2 0.0
'
0.0109* 0.0054* 0. oooo*
2.0 2.0 2.0
Isothermal a t 137.7"C.
I1
0.0 68.1 109.F 139.5 173.3 215.7 257.3 323.0 393.8 472.5 563.4 668.6 789.8 751.6 613.8 507.3 $32.2 356.0 294.5 239.3 193.5 152.8 122.9 67.7 24.3 9.3 1.7 0.0
0.0000 0.0570 0,0857 0.1039 0.1297 0.1577 0.1907 0.2249 0.2663 0.3186 0.3753 0.4429 0.5214 0.4959* 0.4102* 0.3412* 0.2928* 0.2456* 0.2071* 0.1744* 0.1446* 0.1199* 0.0988* 0.0559* 0.0280* 0.0171* 0.0080* 0.-*
3.0 11.0 1.5 1.7 5.5 16.2 3.0 3.0 2.7 3.0 3.5 3.0 10.2 4.0 2.7 2.5 14.5 3.7 5.5 22.0 2.0 1.5 3.0 11.0 2.0 3.0 2.0 2.5
The bottom bulb was sealed off and the system further evacuated with a mercury pump protected by liquid air trap. Two-thirds of the iodine was then sublimed into the third bulb and the remainder sealed off. The sample tubes, each with a breakoffsky attached, were previously sealed to the top bulb and the iodine driven'into them following a third fractional sublimation. Silica gel used was of the commercial glassy type. It was thoroughly
176
ANGUS E. CAMERON AND L. H. REYERSON
TABLE 3 Sorption of iodine by silica gel PRESSURE (CORRECTED)
I
1
X/M
T I M B FOR R E A D I N G
Isotherm a t 98.2%. mm.
0.031 5.6 10.6 18.0 23.3 13.6
millimoles per gram sorbent
0.0143 0.0231 0.0423 0.0787 0.0294*
hours
1.7
0..
2.0
10.7 3.0 4.0
1.7
Isotherm a t 137.6"C.
2.21. 47.4
. 80.0 120.0 99.6 15.3 0.03t
0.01462 0.03483 0.06375 0.2011 0.1129* 0.0217* 0.0009*
0.00
0.-
0.03t 15.0 48.2 100.0 144.4 199.6 169.7 0.03t
0.-
o.oot 2.6 48.0 101.4 235.1 319.1 428.4 368.4 169.6
0.0041 0.0120 0.0269 0.0492 0.1280 0.0743* 0.-*
0.-
0.0009 0.0071 0.0156 0.0436 0.0905 0.3411 0.2443* 0.0278*
3.0 2.0 10.0 9.5 6.2 11.5 5.0 3.5
3.0 1.2 1.5 5.5 9.5 3.5 1.2 2.7
14.0 1.5 1.2 2.0 1.5 2.5 9.7 8.5 2.2
SORPTION O F BROMINE AND IODINE BY SILICA GEL
TABLE 3-Concluded PRESBURE (CORRECTED)
I
x/.u
I
177
T I M E FOR R E A D I N Q
Isotherm at 198.5"C. mm.
millimoles per gram aorbent
hours
0.03t 46.2 168.7 318.2 423.6 560.0 233.3 119.5 0.03t
0.0.0043 0.0160 0.0333 0,0530 0.1205 0.0236* 0.0119* 0.-*
1.0 1.2 10.0 2.5 1.7 3.5 1.5 14.0 2.0
,
washed with nitric acid and water and electrodialyzed for two days. The gel was dried and air-activated a t 600°C. It was crushed in an agate mortar and sieved, the portion passing 60 mesh and retained by 100 mesh being used in this work. A sample of this gel was ignited to constant weight in a platinum crucible and showed a loss in weight of 3.32 per cent. RESULTS
Measurements on the sorption of bromine by silica gel were carried out with the adsorbent maintained a t 58", 79", 99.9", 117.5", and 137.7"C., respectively. The vapor pressure of the bromine was regulated by the constant temperature cryostat, G. Equilibrium was established rapidly in the case of silica gel at all but higher pressures. However sufficient time was always taken to insure several constant readings before changing the pressure of the bromine. Data for the measurements are given in table 2. It will be noted that sorption studies were always followed by desorption determinations down to zero pressure. The spring calibrations for the measurements a t 58°C. are given in table 1. Similar calibrations were made a t the other temperatures, but they are not included in the results. A calculation of the correction for the buoyancy effect of the halogen vapor was made assuming the gas laws and it was found to be less than one-half of 1 per cent of the bromine sorbed at atmospheric pressure. Accordingly no buoyancy corrections were made. Table 3 gives similar results for iodine. The sorption-desorption isotherms were measured a t 98.2", 137.6", 158.3", 178.4", and 198.5"C., respectively. Readings were possible u p to about 600 mm. pressure. Above this pressure the density of the iodine vapor rendered it impossible to see the spring points even against the bare filament of the lamp. An experimental determination of the buoyancy correction for iodine showed it to be within the limits of experimental error, so no correction was made for this factor.
178
ANGUS E. CAMERON AND L. H. REPERSON DISCUSSION
The five isothermals recorded in table 2 are shown graphically in figure 1 of the following paper. Millimoles of bromine sorbed per gram of silica gel are plotted as ordinates against vapor pressure of bromine as abscissae. Similar results are presented in figure 2, also of the following paper, for the sorption isotherms of iodine on silica gel. It is evident from the results that adsorption and desorption are fully reversible. There is no evidence for the hysteresis so often found in sorptions on silica gel. The curves exhibit more or less convexity toward the pressure axis, which indicates loose binding forces and physical adsorption. Except for slight discontinuities the isotherms for bromine a t the higher temperature are almost straight lines. The discontinuities seem t o be greater than the probable experimental error and appear t o be real. The isotherms for iodine appear to rise more abruptly, but this is in the main due t o the change in the ordinate scale. Under comparable conditions many times as many bromine molecules are sorbed per gram of silica gel as iodine molecules. At its boiling point 1.5 millimoles of bromine are adsorbed per gram of gel under a pressure of 200 mm. Under the same pressure at its boiling point only 0.03 millimole of iodine is sorbed per gram of the gel. The International Critical Tables give the critical temperatures for bromine and iodine as 302°C. and 553"C., respectively. It is evident from the data that the amount of sorption here found is not related to the difference between the critical temperature and the temperature a t which the sorption was measured, as'is the case in so many sorption studies on silica gel. The data here presented do not fit the exponential equation of tlie classical adsorption isotherm. Neither do the data fit the Langmuir equation for monomolecular adsorption. The expression which best fits these experimental results is that given by McGavack and Patrick (5). A fuller discussion of the characteristics of the sorption of bromine and iodine by silica gel will be given a t a later date. A rather complete study on the sorption of chlorine by porous sorbents is in progress in this laboratory. SUMMARY
1. An apparatus combining the sorption balance of McBain and Bakr and a quartz spiral manometer is described. 2. Using this apparatus the sorption of bromine by silica gel was measured a t 58", 79", 99", 117.5", and 137.7"C. 3. Using similar technique the sorption of iodine was measured a t 98.2", 137.6", 158.3", 178.4", and 198.5"C.
SORPTION OF BROMINE AND IODINE BY SILICA GEL
REFERENCES (1) CAMERON: J. Am. Chem. SOC.63, 2646 (1931). (2) CAMERON: Rev. Sci. Instruments 4, 610 (1933). (3) MAGNUSAND MULLER:Z. physik. Chem. 148A, 241 (1930). (4) MCBAINAND BAICR:J. Am. Chem. SOC.48, 690 (1926). (5) MCGAVACK AND PATRICK: J. Am. Chem. SOC.42, 946-78 (1920). J. Chem. SOC.103, 847 (1912). (6) SCOTT,ALEXANDER:
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