T H E SORPTION OF CHLORIKE BY SILICA GEL)' L. H. REYERSON
ASD
A . W. WISHART
School of Cheinisiry. rniverszty of M t n n e s o t n , 3 f i n n e a p o l z s . M i n n e s o t a
Received J u n e 22, 1937
-1s wis observed by Reyerson and Cameron ( l ) , comparatively few studies had been made on the sorption of the vapors of the halogens by porous sorbents. Such a complete study on a family of elements YO closely related chemically might well throw new light on the problems of sorption by porous materials. -1 series of experiments was begun in this laboratory, and the results of the investigations on the sorption of bromine and iodine by silica gel and charcoal have been reported (1). The present work completes the study as far as silica gel is concerned. Upon completion of sorption studies on charcoal it will be possible to compare the behavior of the halogens toward these two porous sorbents. Magnus and Muller (2) carried out a series of measurements on the adsorption of chlorine by silica gel in an all-glass system a t 0, 20, and 40°C. However, no desorption measurements were made, and the method used differed considerably from t h a t described by us; this raised the question of the reversibility of the sorption. APPARATUS AND MATERIALS
The construction of the actual sorption system w s essentially the same as that previously described (1). It consisted of an all glass-quartz system having no stopcocks. For a complete description of this system reference may be made to the previous paper. Figure 1 shows the essential portions of the apparatus. The quartz spring balance (& was supplied I) by the Bell Telephone Laboratories (5). It was approximately 25 mm. in diameter and consisted of twelve turns. The Bodenstein quartz-spiral manometer (B), which was used as a null point instrument, and the travelling microscope were the same instruments used in the previous work. With the air pressure in the outer jacket of the Bodenstein manometer adjusted so that the pointers were matched, pressure measurements on the mercury manometer were read by means of a n accurate cathetometer. The manometer readings were corrected to 0°C. Zero readings n-ere taken with the apparatus evacuated and under actual run conditions. Presented a t the Fourteenth Colloid Symposium, held at Minneapolis, Minnesota. June 10-12, 1937. 943
944
E. H. RETERSON A S D
.4. W. U'ISHART
FIG.1. Sorption apparatus. A, sorption tube; B. Bodenstein manometer; C, lead t o high vacuum system; D. hreakoffskys; E, sample bulb; F, storage bulbs; G, cryostat.
FIG.2. Vertical section of cryostat. A , asbestos-Bakellte base; B, circulation windows; C, heating coil; D, platinum coil supports; E, Bakelite form; F, fan mount and bearings; G, brass support rods; H, brass ring; I, brass gears; J, Bakelite f a n ; K, outer Dewar; L, inner Dewar; M, platinum resistance coil; X, brass axle; 0, outlet t o vacuum; P, pulley wheel; Q, float contacts; R, leads t o platinum coil; S, leads t o heating coil;T, float.
SORPTION OF CHLORINE
945
The chlorine was introduced into the bottom of the adsorption tube (A) through the breakoffsky (E) and its temperature (or vapor pressure) controlled by means of a precision cryostat (4) shown in detail in figure 2. Constant low temperature was controlled in the following manner. A Dewar flask, K, containing liquid air surrounded a second Dewar flask, L, containing the constant-temperature bath liquid. The air pressure between the walls of L, which regulated the amount of cooling, was controlled by means of a vacuum system connected to 0 . Heat was supplied through the coil, C, mounted on the Bakelite form, E. The bath liquid
FIG.3. Thermoregulator circuit. C’, condenser (capacity, 2 microfarads); G , high sensitivity galvanometer; H,, cryostat heating coil; K, key; L, 6-volt lamp; I>,, 25-watt lamp (110 volts): L2, 75-watt lamp (110 volts); L,, La, 126, Le, lamps (110 volts); P, potentiometer; P.C’.,photoelectric cell, type P.J. 23; R , base of telephone relay, type E-W, S6; R1, RP,10,000-ohm variable resistance boxes; Rs)high resistance, graphite grid leak; R,, 200-ohm variable resistor; R6, 63-ohm slide-wire rheos t a t ; Rg, 400-ohm varirtble shunt; S,, solenoid; S,,mercury switch.
was circulated by means of the Bakelite fan, J, driven by the motor-pulley system P, Tu’, I. Windows, B, were cut into the frame to insure proper circulation of the liquid. The forni, E, was mounted on the base, A, and around this was mounted the Dewar flask, L, by means of a brass ring, H, and three brass rods, G. The current applied to the heating coil through the leads, S, was controlled by means of a thermoregulator, the circuit of which is shown in detail in figure 3. A constant supply of heat, nearly sufficient to balance the cooling, was supplied through the lamp bank LB, Ld, Ls, and slide-wire
946
L . H. REYERSON AND A. 1%'. WISHART
rheostat, RE,,from a 110-volt A . C . source. The bath was held to constant temperature in the following manner. h coil of platinum wire, R,, was used as a thermosensitive element and its leads were connected to a potentiometer, P, set to the electromotive force of the resistance thermometer circuit corresponding to the desired temperature. A high sensitivity galvanometer, G, was connected to the potentiometer through the shunt, Rg. An image of the lamp, L, was formed by the mirror of the galvanometer upon a narrow slit in a screen which covered the photoelectric cell, P. C. Thus when the temperature of the cryostat dropped, the galvanometer mirror was deflected and the beam from the lamp fell upon the sensitive plate of the cell. The photoelectric current generated was amplified by means of a U.X.171A radio tube, t o the grid of which the photoelectric tube was connected. In the plate circuit of the amplifier tube was placed a telephone relay, type E-W, SG, R, and a mercury switch, S,, which closed a circuit to the lamp bulb, LE,,thus supplying sufficient additional current to the heating coil, H,, to bring the temperature back to the desired value. At this point the light beam passed off the slit onto the screen shading the photoelectric cell and the circuit to the balancing heater then opened, which allowed cooling to begin. The bath liquid used in these studies was ethyl bromide, which proved to be highly satisfactory. The liquid air supply in the outer Dewar flask was maintained at constant level by use of a device shown in detail in figure 4. The float, F, operated through the contacts, K, and the relay, R, a solenoid, S, which in turn actuated an air valve, Y, This valve opened and closed a low-pressure air supply to the storage Dewar flask, D,, thus forcing liquid air over into the Dewar flask, D, which contained the cryostat. With this apparatus it was possible to control the temperature of the chlorine within less than 0.01"C. of the desired value. This variation produced no measurable change on the Bodenstein manometer. The procedure followed in calibrating the spiral and in preparing the adsorption system and adsorbent for the sorption studies was nearly identical with that described by Reyerson and Cameron (1). For t h a t reason, a very brief discussion will be given a t this time. The spring balance used had a seiisitivity of approximately 0.065 mg. per 0.01 mm. The calibration was made using a mica pan suspended from the spiral, which weighed approximately 0.04 g. Spring length measurements made under the same load were duplicable, after an interval uf t n o months, to a few thousandths of a millimeter. All measurements were made by means of the travelling microscope previously mentioned. The sorption system was evacuated for approximately one hundred hours a t a pressure of 10-5 nim. of mercury. During this period the temperature of the sorbent was raised at intervals to several hundred degrees Centigrade. A preliminary flushing out of the system with halogen n as carried out before admitting the sample to be used in the studies.
947
SORPTION OF CHLORINE
F I G .4 . ('onstant level liquid air device. A-I, low-pressure air supply; B, iron base; C, iron core; 11, outer Dewar of cryostat (1 gallon); DI, storage Dewar (d gallon); F, float; J, brass jacket; K, float contacts; 01, air outlet; Oz-Tz, tube for air rondurtion to storage Dewar; P, brass plunger; R , base of telephone relay, type E-W , 86; S, solenoid; Sp, spring; T I ,tube for liquid air conduction; V, air valve.
Somp'e Bulb
-
Fio. 5 . Chlorine purification system
948
L. H . REYERSON AND A. W . WISHART
The temperature of the oven containing the sorption system was read on a thermometer suspended beside the sorption tube. This thermometer had been previously calibrated against one bearing a Bureau of Standards calibration certificate. The oven temperature was controlled by means of a long thin-walled glass regulator bulb filled with mercury and terminating in a control capillary. This operated a thermionic relay system varying an external resistance in series with the heater grid. Temperatures were constant to fO.l"C. The chlorine used was prepared from high grade tank chlorine. X diagram of the purification system is given in figure 5. A considerable quantity of chlorine was released from the tank in order to flush out the system before a sample was collected. I t was passed through a heater, a t about 400"C., to break down the oxides of chlorine, then through a preTABLE 1 Spring calibratzon TEMPERATERE
1
LOAD
"C.
grams
35 9
0 0558 0 1558 0 2558
51 0
1 ~
81 5
I I
0 0558 0 1558 0 2558
0 0558 0 1558 0 2558
I
I ~
I I
I
1
LEXQTE
RENSITKVITP
niin.
nig. per 0.01 mm.
50 497 66 092 81500
0 064124 0 064909
50 425 65 984 81 364 51 128 66 473 81 800
1
:
0 064271 0 065019
0 065167 0 065244
cooler at - 25"C.,and frozen down in a liquid air trap under vacuum. The system was then sealed off between the precooler and the trap and pumped for several hours at a pressure of 10-5 mm. of mercury. h vacuum distillation was then carried out a t this pressure and the middle portion of the chlorine collected. The first and last portions were sealed off and disposed of. This was repeated several times. The final sample was collected in the sample bulb equipped with a breakoffsky and sealed off under high vacuum. The purification system was protected from the mercury vapor pumps by a liquid air trap. The silica gel used was of the commercial glassy type. It was a sample of the same gel used in the bromine and iodine research, and a complete discussion of its preparation may be found by a reference to that work (1). RESULTS
Sorption isotherms were carried oiit with the adsorbent maintained at temperatures of 35.9, 51.0, 86.5, and 81.5"C. Equilibrium was established
949
SORPTIOK OF CHLORIXE
TABLE 2 Sorption of chlorine b y silica gel
Isothermal a t 35.9”C.: adsorption nim
of Hg
0 4 2 9 6 18 3 41 1 114 0 171 1 238 1 329 1 456 7 586 8 727 3
1
I
t i i t h n o l e s p e r gram of sorbent
i
i
I i
I
0 0 0 0 0 0 0 0 0 0 I 1
0 0399 0576 1064 2226 3468 4728 5710 7316 91S7 1218 2921
1 I
i
hours
2 4 2 2 2 2 3 2 3 3 2 3
0 0 5 6 2 5 2 1 2 5 8 0
2 2 2 2 1 1 2 2
3 1 0 0 7 8 0 0
2 4 2 2 2 3 2 2 2
1 3 1 4 7 4 1 0 3
I s o t h ~ r m a la t 35.0”C.: desorption 646 6 511 0 401 1 263 0 138 7 68 1 34 4 0
I
1 2053 1 0288 0 8514 0 6554 0 4284 0 2891 0 1880 0 0
Isothermal at 66.5Y’. : adsorption
0 0 0 0 0 0 0 0 0
0 15 5 45 0 99 9 198 8 337 0 484 7 575 3 721 2
0 0267 0890 1540 2695 3995 5144 5954 7013
Isothermal a t 6 6 . 5 T . : desorption 625 9 411 6 273 0 155 7 69 1 45 0 0 THE J O U R N A L OF €“1SIO.\L
, 0 1255
2 7 2 2 22
0 0890
0 0 CHEMISTRY, VOL.
41,
NO.
7
950
1,. H. REYERSON A N D A . W. WIYHART
T.4BLE 2-Cqncluded
___..___.
-.________
I
PRESSURE CORRECTED
X/.V
TIME FOR READIKG
Isothermal a t 51.0"('.: adsorption Hg 0 5 5 24.5 55 5 115 6 184 3 299 5 444 2 684 0
nim. of
I
i~iill?.rnolesper gram of sorbent
1
hoiira
0 0 0 0266 n 0746 0 1359 0 2318 0 3234 0 4611 n 6086 0 8360
I
2 1 3 3 2 2 2 3 2 1 1 7 2 2 2 0 27
~
i __
-
1
~-
Isothermal a t 51 O Y ' . : desorptinn 730 1 551 5 348 3 258 2 131 7 79 4 36 0 14 n 5 5 0
I
I ~
1 I
0 8707 0 7099 n a49 0 398n 0 2470 n 1626 0 0884 0 0338 0 0240 0 0
2.4 1 8 1 7 1 7 2 1 1 9 18 2 0 2 1 2 1
Isothermal a t 81.5"C.: adsorption 0 4.8 33 2 125 8 218.2 353.3 491.9 638,9
0 0 0 0 0
0 0098 0570 1373 2113 n 3138 0 3940 0 4859
2 3 2 1 2 2 3 2
3 2 2 7 4 1 1 8
Isothermal a t 81.5"C.: desorption 720.1 560.3 425.9 353.3 286.8 165.2 75.2 24.5 0
0.5301 0.4350 0.3593 n . 3138 0.2648 0.1810 0.1034 0,0446 0.0
2 3 2 1 2.2 1 8 1.9 1.7 22 1 7 22
FIG.6. Sorption of chlorine by silica gel
3
F I G .7 Sorption o f vhlorinr, hloniine, a n d iodine by silica gel
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1,. H . REYERSON AND A . W . WISHART
rapidly in most cases; no measurable change in the amount adsorbed occurred after fifteen minutes. However, to insure equilibrium being established, approximately two hours werr allowed for each determination. Adsorption measurements were followed by desorption measurements at the same temperature. Spring calibrations for the various temperatures are given in table 1. The weight of the calibration pan and fibers was 0.0558 g. The calibration was then made in steps of 100 mg. The sensitivity is expressed in milligrams per 0.01 mm. elongation of the spiral. The data for the sorption measurements are given in table 2. X o buoyancy correction was made, since calculations showed it to be negligible. DISCUSSION
Figure 6 shows graphically the four isothermals recorded in table 2. Millimoles of chlorine sorbed per gram of silica gel are plotted as ordinates against vapor pressure of chlorine as abscissae. The rcbults indicated that adsorption and desorption were fully reversible and that there was no evidence of hysteresis. The isotherms show a somewhat greater increase in amount sorbed per unit pressure change at the lower vapor pressures. A. the pressure increases, the curves tend to flatten out somewhat. The isotherms for chlorine are concave toward the pressure axis, while those for bromine and iodine were convex. However, the amount of convexity diminished with increasing temperature in the case of bromine and iodine. Hence for comparable temperatures, considerably above the boiling point.., the curves are more nearly similar. The amount of halogen sorbed under nearly comparable conditions decreases with increasing atomic weight, as shown in figure 7 . The irotherm for chlorine was approximately 70°C. above its boiling point, for brominp approximately 60"C., and for iodine approximately 1.5"C. No more coniparable data w'crc available in the case of iodine. The boiling points of chlorine, bromine, and iodine a i given hy thc Intemational Critzcal Tables are --34.6, 58.78, and 184.30°C., respectively. &it35.9"C., 0.85 millimole of chlorine is sorbed per gram of gel under a pressure of 400 m m . Under the same pressure, 0.41 millimole of bromine i- 4 o r b d at 117.5"C. and 0.05 niillimole of iodine at 198 ZOC., per gram of gel. The data for chlorincl fit fairly well the Freundlich empirical isotherm. A plot of log S in against log p givcs a reawnably straight line. The expression given by AIcGavack and Patrick (3) likewise fit:, the tqxriniental results. Both of theie exprch4onh are nio:,t satisfactory a t higher vapor pressures. The data d o not fit thv Langmuii* equation for monomolecP ular adsorption. A plot of -- against P gives a curve which is concave X /m toward the pressure axis.
SORPTIOX OF CHLORINE
953
SUMMARY
-4dsorption and desorption of chlorine by silica gel were measured a t 35.9, 51.0, 66.5, and 81.5"C. The adsorption occurred rapidly and was fully reversible in all cases measured. REFERENCES (1) CAMERON ANDREYERSOS: J. Phys. Chem. 39,169 (1935). (2) MAGNUS AND MCLLER:Z. physik. Chem. A148,241 (1930). (3) MCGAVACK AND PATRICK: J. rlm. Chem. SOC.42,946 (1920). (4) SCOTTAND BRICRWEDDE: Bur. Standards J. Research 6,401 (1931). (5) WEINHART, H.K.:Rev. Sei. Instruments 4,350 (1933).
