Henry G. Lutrick,' Kenneth C. Williams, and Russell W. Maatmanz University of Mississippi University
A Gas Adsorption Apparatus and Experiment
Even though there is a need for training in the principles and techniques of gas adsorption, the undergraduate student usually learns very little about this subject. It is difficult to obtain, operate, and maintain adsorption apparatus. These problems often arise because the desired adsorption experiments can be carried out only in vacuum systems. The purpose of this paper is to show how one or two upper level undergraduate students can inexpensively build in one academic year a gas adsorption apparatus which is easily operated and maintained. The apparatus can be used in succeeding academic years for a wide variety of adsorption experiments. The basic principles of the apparatus described, a "sorptometer," have been described by Nelsen and Eggertsen (I) and Lee and Stross (8). The gas chromatography principle is used: the column of the gas chromatograph is replaced by the solid adsorbent and the carrier gas is mixed, a t any desired partial pressure, with the adsorbate. The total pressure is the atmospheric pressure and the difficulties inherent in vacuum systems are thus avoided. The present paper emphasizes the low cost and the educational value of a stndentbuilt sorptometer. A more refined model is available from Perkin-Elmer.
needle valves permit a pressure regulator setting high enough to render pressure fluctuations in the regulator of negligible effect in the rest of the system. Gases pass through glass capillaries I and/or J, and K. Thus, for given settings of the regulators and the needle valves there is a certain flow rate of helium through capillary K, and, corresponding to the three different
The Sorptometer
The gas to be adsorbed is mixed with a suitahle carrier gas, usually helium, with each gas a t a known flow rate. The mixture passes through a Gow-Mac thermal conductivity cell, and the base line on a 10-mv recorder is established. An adsorption peak appears when the gas mixture passes over the solid adsorbent; a reverse, desorption peak occurs when the solid is heated. Either peak area is a measure of the amount of gas adsorbed. The apparatus for the adsorption of nitrogen a t liquid nitrogen temperature is shown in Figure 1. The figure shows the approximate relative positions of the components mounted vertically on a wooden panel. Our apparatus was built for the purpose of determining nitrogen adsorption isotherms, but experiments with more easily condensable gases can be performed on the same instrument. For example, the adsorption of n-butane a t 0°C may be preferred when liquid nitrogen is unavailable. Gases from tanks A and B pass through pressure regulators C and D and needle valves E and F. The 1 Participant in the 1962 Reaearch Participation Program for College Teachem, sponsored by the National Science Foundation. 'To whom correspondence concerning this paper should be addressed, at Dordt College, Sioux Center, Iowa.
Figure 1.
Gascircuit forthe adrorpti~napparatus.
positions of three-way stopcock G there are (assuming capillaries I and J are of different size) three different nitrogen flow rates. Consequently three different nitrogen partial pressures are possible for a given setting of the regulators and needle valves. The liquid nitrogen trap, L, removes condensables. The mixing chamber, N, which insures gas mixing, is a 6-XJ/,-in. copper cylinder connected to the cold trap and the reference side of the Gow-Mac "Pretzel" thermal conductivity cell, 0. The connecting tube is S/ls-in. copper tubmg, M. The cell is almost completely submerged in a white oil bath, P ; it is unnecessary to control the bath temperature to keep the reference and test sides of the cell a t the same temperature. Volume 41, Number 2, February 1964
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The gas then passes through copper tubing Q to the part of the system involving the sample tube, calibration tubes, and several three-way stopcocks; with each of these stopcocks two of the three tubes are on one side of the stopcock. Stopcocks R and S can permit flow of the gas through either T, the sample cell, or U, a calibration tube of known volume. (The single outlets of stopcocks S and V point upward, and the single outlets of stopcocks R and W point downward.) The gas then passes through copper tubing X, behind the panel, to Y, the hall type flowmeter. Copper tubing Z connects the expansion chamber AA (similar to the mixing chamber, K) with the flowmeter and the sample gas side of the thermal conductivity cell. Copper tubing BB connects the cell with the soap bubble type flowmeter, CC, a t the vent of the system. The electrical circuit is shown in Figure 2. The actual markings on the Gow-Mac cell and the Varian G-10 Recorder, with the range selector, are shown. The control panel is next to the gas expansion chamber, and in a very approximate way Figure 2 can be superimpsed on Figure 1. For the sake of convenience connections were color-coded, and these are shown. Because good Edison batteries were available, they were used; the cheaper lead storage batteries can also be used.
loon POT.
20R POT
u VARUN 6-10 RECORDER
Figure 2.
Electrical circuit for t h e adsorption apparatus.
The electrical circuit can be understood by the student easily if he keeps in mind the Wheatstone bridge circuit. The four filaments of the thermal conductivity cell are the four resistances of the bridge; they are not shown in Figure 2. The two leads to the galvanometer usually shown in Wheatstone bridge diagrams are here the two leads to the recorder ("Rec"). The battery leads of Wheatstone bridge diagrams are here B-, to one side of the bridge, and the two B+ leads, to the other side. The resistances in the battery circuit enable the operator to obtain the correct cur94
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rent in the arms of the bridge. Students are often surprised that the measurements of interest are made when the Wheatstone bridge is not balanced; this is contrary to the usual situation in elementary physics experiments. Operution of the Instrument
The desired flow rate of one of the gases alone is obtained by using either the soap bubble flowmeter or the previously calibrated ball flowmeter. Flow rates are adjusted either with stopcock G or the needle valves. The second gas is thcn added to the gas stream, and where only one flowmeter is used, the flow rate of the second gas must he measured with the soap buhble flowmeter. The mixture passes through either the sample tube or the calibration tube of known volume. The sample tube is a t a temperature well above the temperature permitting adsorption; with materials upon which nitrogen will adsorb physically but not chemically, such as silica gel, the sample may be at room temperature. When the system is flushed, the switch on the control panel is turned on and the resistances are adjusted so that the current does not exceed 270 ma. To prevent destruction of the cell filaments, gas should flow over the filaments whenever the current is on. The recorder is turned on and the base lme is established for the particular mixture of gas being used. If both adsorption and desorption are to be measured, the base line should he near the middle of the chart. The sample tube is then immersed in liquid nitrogen. The area of the peak is a measure of the amount of nitrogen which adsorbs. Since peak area depends upon flow rate and the flow rate changes in an irregular fashion as adsorption occurs, an error is introduced. For small flow rate changes a correction can be made by assuming peak area proportional t o flow rate. Even a t high nitrogen partial pressure the flow rate change can be kept small by using the by-pass stopcock DD to divide the gas stream between the sample tube T and tube X a t the time the sample is cooled; e~entually the stopcock DD is turned to eliminate the by-pass. Prior to desorption measurement, reestablishment of the base line indicates the sample is in equilibrium with the gas stream. To desorb, the liquid nitrogen flask E E is removed from the sample tube as the bypass stopcock DD is turned to vent the gas mixture from the tanks to the atmosphere; either stopcock R or S is turned to prevent desorbing gas from backing up to the vent. The gas which desorbs expands through stopcocks V and W and into the expansion chamber AA; its flow is limited by manipulation of either stopcock V or W. As long as the desorbed gas does not expand beyond the expansion chamber AA into the thermal conductivity cell, the flow rate need not be constant. When desorption is nearly complete, stopcock DD (and either R or S) are manipulated to return the carrier gas to the system a t its initial flow rate. The best calibration is carried out not only by keeping the flow rate and the gas mixture constant, but also by using a calibration tube of about the volume of nitrogen adsorbed in the sample tube. Calibration can be carried out without removing the sample.
Dry nitrogen enters the system a t FF and passes through the previously unused tube of stopcock W, through V, through calibration tube U, and through S and R ; it is vented through the previously unused tube of stopcock R. Both the sample tube and the calibration tube are connected to three-way stopcocks V and S. As the calibration tube is filled with pure nitrogen, the by-pass stopcock D D is turned so that the other gas stream continues to pass through both sides of the thermal conductivitv cell. The Dure nitroeen flow is stopped by turning stopcocks V and S in that order. The gas mixture is flushed through the sample cell; there is then pure nitrogen in the calibration tube up to stopcoeks V and S. The calibration peak is obtained by turning stopcocks V and S so that the pure nitrogen sample flows through the sample side of the thermal conductivity cell. The volume of the calibration tube is determined by filling it with mercury from a beaker up to stopcocks V and S; the beaker is weighed before and after pouring into the nitrogen vent above R
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Notes on Construction and Operation Some glass working is necessary. U-tubes for sample and calibration are needed; the bottom of the sample U-tube should be large enough to hold the sxmple. Some of the tubes attached to stopcocks must he bent. To make the capillaries Pyrex capillary tubing is further drawn out. The soap bubble flowmeter ia a 50-ml Pyrex buret with two side-arms; a discarded buret, with most of the barrel intact, can be used. The upper side-arm is connected to the bottom of the buret with rubber tubing containing Soap solution. The solution is squeezed up to the lower side-arm when needed.
or tygan. Samples usually require preheating. This can be done in the system venting the effluent gas through the extra tube above s t o p cock W; heating tape or a small furnace can be used. I t is often possible to heat the sample tube in a nemby furnace prior to a rsoid insertion into the svstem.
neeted to s mercury manomet&. Nitrogen is allowed to enter the ~ y s t e muntil the pressure is about 10 cm above atmospheric pressure; under these conditions nitrogen condenses. The inlet is closed, and the equilibrium manometer reading indicates the difference between atmospheric pressure and the vapor pressure of nitrogen at the temperature of the flask EE. I t is assumed the U-tube and the sample cell attain the same temperature.
though everything except a thermal conductivity cell and a helium source is usually available in a wellequipped laboratory, such a laboratory is not average. This project is, however, feasible in very many laboratories at low cost if a recorder is available. Besides the recorder, priced a t about $450, approximate costs are as follows: Gow-Mac "Pretzel" thermal conductivity cell, $55; dc source, $25; other components of electrical circuit, $15; helium and nitrogen tanks, $46; two pressure regulators, $90; two widemouth thermos bottles (mounted in old ether cans whose tops are cut off), $4; seven stopcocks, $32; flowmeter, $15; cold trap, $3; other glassware, $11; plywood panel and small table, $10. The total is $306 plus the cost of the recorder. The total is much less if certain common items are on hand. If the electrical components (except the cell) and the nitrogen source are available for occasional use, if stopcocks G and H are omitted, if the U-type flowmeter is constructed, if a discarded buret is used to make the soap bubble flowmeter, and if the means for mounting are on hand, the price is $171. The price is $101 if stopcocks and easily adaptable regulators are also available. Using the Sorptometer
The adsorption isotherms of light hydrocarbons on high surface area, porous oxides, easily purged by preheating, are probably the simplest t o obtain. Silica gel, porous alumina, and silica-alumina catalyst are typical adsorbents. The nitrogen adsorption isotherm near the nitrogen boiling point is usually used in the BET method of determining surface area (5) ; the n-butane isotherm at O°C, easier to obtain, is also used. The pore size distribution can also be determined from the nitrogen adsorption isotherm (4). Understanding the method of calculation in the determination of the pore sizes is difficult, and should not be attempted a t first. Both the surface area and pore size work teach the student something of the thermodynamics of surfaces. The apparatus is also useful in research problems; we determined both surface area and pore
Determination of the Surfoce Area of Code 70 Silica Gel w
Possible Simpliflcotions If gas mixing is t o be controlled entirely by the needle valves, stopcocks G and H and capillaries H, 3, and K can he replaced witha Y or T-tube joining the gas streams. Copper tubing is convenient hut not necessary. Mixing and eupansion chambers can be constructed from glass tubing and one hole stoppers if the gases to be used do not affert the stoppers. A U-tube flowmeter can be constructed a t virtually no cost. The U-tube is partially filled mith a heavy, non-volatile liquid, such as dibutyl phthalate. The gas flows through a capillary which joins the ends of the vertically mounted U-tube. The difference in the two liquid levels is a measure of the flow rate. Such a flowmeter should precede a cold trap.
Cost
Pb Po R,
Rat Rrr
V P PIP.
W e i ~ hof t gel after outg&sing cg) Outgasmng temperature (OC) Room temperature ( ' C ) Barometric pressure (mm) Vapor pressure of nitrogen Total flow rate (ml/min) Helium flow rate (ml/min) Nitrogen flow rate (ml/rnin, (RL- R d ) Volume of nitrogen adsorbed (ml) Partial pressure ol nitrogen, (mm, (Rx/Rt)Pa) Rela,tive oressure of nitroeen
100.84 81.08
93.75 81.08
18.76
12.67
11.64
10.55
140.14 101.79 212.78 0.1824 0.1325 0 2769
,f nitroaen equal to a monolaver from the
The availability of certain key components often determines whether or not a sorptometer is built. AlVolume 41, Number 2, February 1964
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size distributions, using complete nitrogen adsorption isotherms for the latter, for ion solvate and ion surface studies (6-7). Constructing and/or using the sorptometer helps in learning the principles of gas chromatography. The instrument described is not a gas chromatograph; yet a simple modification, whereby a "slug" of test gas is introduced instead of nitrogen (avoiding the cold trap), enables the student to perform a gas chromatography experiment. A Typical Experiment
The data and calculations for the determination of the BET surface area of Davison Code 70 Silica Gel are presented (according to the format of Nelsen and Eggertsen ( 1 ) ) in the table. The original calculation method is given in reference 3; Emmett discussed the method, modifications of it, and other methods (8).
% / Journal of Chemical Education
The \ d u r ulmined, :I;,? m' g, ngreea well wit11 t l w value of S.72 m2 g reported I)? the manufacturer. Literature Cited
(1) NELSEN,F. M., AND EGGERTSEN, F. T., Anal. Chem., 30, 1387 (1958). F. H., Division of Analytical Chem(2) LEE,C. F., AND STROSS, istry, 135thACS Meeting, Boston, Mass., April, 1959. S., EMMETT, P. H., AND TELLER,E., J. Am. (3) BRUNAUER, Chem. Soc.,60,309 (1938). R. W., AND INKLEY, F. A,, Advances in Caialwis, (4) CRANBTON, 9, 143, (1957). R. W., MCCLANAHAN, J. L., AND MAATMAN, R. W., (5) DALTON, J. Colloid Sci., 17, 207 (1962). J., AND MAATMAN, It. W., J. Colloid Sei., 18, 132 (6) STANTON, 119fi3). \----,-
(7) MAATMAN, R. W., NETTERVIL~, J.,HUBBERT, H.; AND IRBY, B., J . Miss.Acad. Sei.,8,201(1962). (8) EMMETT, P. H., "Catalysis," Volume I, Rheinhold Publishing Corp., New York, 1954, p p 31-74,
