AN IMPROVED APPARATUS FOR DETERMINING VAPOR-LIQUID EQUILIBRIUM . JAMES W. ROGERS, JACK W. KNIGHT, and A. R. CHOPPIN Louisiana State University, Baton Rouge, Louisiana
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INTRODUCTION
Vapor-liquid equilibrium data may be determined to a high degree of precision using highly specialized types of apparatus, as that described by Othmer.' However, such apparatus is usually not available for use in elementary physical chemistry laboratories. Such experiments are usually performed with very simple app a r a t ~ s . ~ , We have found that students using this type of apparatus experience a great deal of difficulty in obtaining valid equilibrium data. We have constructed an apparatus for the determination of such vapor-liquid equilibrium curves by modifying a Ccttrell boiling-point apparatus as described below. The apparatus is easy to operate, comes to equilibrium very quickly, and yields excellent equilibrium data. In addition, i t can still be used as a Cottrell boiling-point apparatus for the determination of molecular weights from the elevation of the boiling point. ' ,
THERMOMETER, G R A D E , 0.1-C.
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DESCRIPTION AND OPERATION OF APPARATUS
The apparatus as shown in Figure 1 consists of an ordinary Cottrell a ~ u a r a t u to s which has been added a three-way, T-type stopcock. One arm of the stopcock serves as a reflux return, one arm opens to the bottom of the flask, and the third arm serves as a sampling tube. Standard taper joints are used throughout on the apparatus. For the determinations of vapor-liquid equilibrium data, a precision grade thermometer, graduated in 0.l0C., is used instead of a Beckrnanp thermometer. In order to determine vapor-liquid equilibrium curves it is necessary to establish equilibrium between the boiling liquid and the refluxing vapor. Samples of liquid and corresponding vapor must then be withdrawn and analyzed. In using this apparatus, a charge of approximately 100 ml. of the binary liquid is introduced into the boiling flask, with the three-way stopcock in position (1) as shown in Figure 2. .The liquid is heated with a microburner or electric hot plate a t such a rate that the Cottrell pump will eject a vigorous stream of liquid over the thermometer bulb, and the condensed vapor will return from the condenser a t a steady rate. Boiling is continued until the temperature becomes constant (from one to five minutes, depending upon the liquid pair se-
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lected). Samples are then withdrawn by the following procedure: The burner is extinguished and the stopcock is turned to position (Z), allowing the liquid standing in the vapor-return tube to flush out the sampling tube into a small beaker. This will require only two or three seconds, after which the stopcock is turned to position (3). Vapor will continue to condense and will partially fill the vapor-return tube above the stopcock. The outlet tube is then drained by touching the tip with a piece of filter paper, after which the vapor sample is collected in a small vial bv turnine the stoncock back to ' Othmer, D. F., Ind. Eng. Chem., 20, 743 (1928). position (2). ~pproximatelyhaE of the liquid in the JASPER, J. J., "Laboratory Methods of Physical Chemistry," boiling flask is drained into the beaker by turning the. Houghtan MiMin Company, New York, 1938, pp. 100-7. E.,AND W, G, FRANCE, MA Manual of stopcock to position (4). The stopcock is then turned Elementary Physical Chemistry," 2 ~ d d., D. van ivostrand to position (51, the tip is again drained, and the liquid . sample is collected by returning the stopcock to posiCompany, Inc., New York, 1934, pp. 126-31. ~L
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tion (4). The stopcock is returned to position (1) in preparation for the next samples. For the first.determination the apparatus is charged with one pure component. The composition of the liquid for the second determination may be altered by adding a small amount of the second component through the condenser. Enough of the liquid in the waste beaker is then added to bring the liquid in the apparatus back to its orifinal level. The composition of the liquid should be determined after each sample is taken. From this value and the known amount of liquid in the apparatus, the amount of the second component which must be added to shift the composition can be calculated. For the purpose of a laboratory experiment in elementary physical chemistry, it is desirable to use a binary liquid which can be analyzed by the determination of a simple physical property. In a number of laboratories binary liquids are used which may be analyzed by determining the refractive index versus composition for a series of liquids of known composition. Samples may then be analyzed by determining their refractive index and reading the composition from the reference curve.
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The apparatus may be used to determine the molecular weights by the elevation of the boiling point. This is accomplished by placing the stopcock in position (1)and operating as a conventional Cottre!l apparatus. The condensing vapor will then be returned to the boiling flask through the stopcock instead of through the vapor outlet tubes a t the top of the apparatus. EXAMPLES OF DATA ~- ----~
Vapor-liquid equilibrium curves were determined for two binary liquids. The System BenzeneMethyl Alcohol. A series of mixtures of benzene-methyl alcohol were prepared as follows: 1, 2, 3, . . . 9 ml. of methyl alcohol were measured into nine small vials from graduated pipets. Then 9,8, 7,. . . 1 ml, of benzene were pipetted into the vials using graduated pipets. From the known densities of methyl alcohol and benzene the weight per cent benzene in each sample was calculated from the volume per cent. The refractive index of these mixtures was determined a t 25°C. with an Abbe refractometer. The temperature was controlled to fO.l°C. by circulating thermostated water through the prism housing. The reference curve for a refractive index is presented in Figure 3. Equilibrium samples were obtained by following the procedure outlmed above, and compositions of liquid and vapor were determined from the reference curve. Vapor-liquid equilibrium curves are shown in Figure 4. The System Benzene-Acetone. A reference curve was constructed in the same fashion as described for benzene-methyl alcohol. The vapor-liquid equilibrium . curves are shown in Figure 5. Molecular Weight Data. To. iliustrate molecular weight determinations, the molar elevation of the boiling point, K,, was evaluated for two different solvents using two solutes in each case. A 100-ml. sample was pipetted into a flask and weighed to determine the weight of the solvent. This weight w.as then assumed to be the same for 100-ml. samples which were pipetted into the apparatus. Solutes were weighed on an analytical balance and introduced through the large standard taper joint. Temperature differences were determined with a Beckmann thermometer. Data for the K, are presented in Table 1. No attempt was made to correct for hold-up of solvent in.the condenser. Inasmuch as the boiling rate. total weight of solvent, and,temperature of the cooling water are kept constant for all determinations involving the same solvent, the hold-up is consistent for each series of runs. Therefore, if the constant, K,;is determined using a known solute in the same fashion that the molecular weight is determined for an unknown solute, no error will result in the molecular weight. If absolute values of K, are desired, it is necessary to correct for the hold-up. This may be done by determining the boiling-point elevation in the usual manner; the flame is then extinguished and the stopcock quickly turned to position (5). When the temperature of the solution has dropped sufficiently to prevent loss of sol-
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vent by evaporation, a sample of the solution is withdrawn by turning the stopcock to position 4. The weights of solute and solvent axe then determined by appropriate analytical methods. Data obtained in this fashion should give true values for the constant, K,.
,, BENZENE-METHYL
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ALCOHOL
TABLE 1 Evaluation of Mold Elevation of the Boiling Point z
Weight of solvent Solvent: Acetone 78.1 Solute: Ben~oicacid, M . W. 78.1 = 122.12 78.1
Weight of solute 2.082 4.382 4.293
0.390. 1.79 0.815 1.78 0.795 1.77
Solvent: Acetone Solute: Salicylic acid, M . W. = 138.12
78.1 78.1 78.1 78.1
2.148 2.112 2.164 2.160
0.360 0.360 0.367 0.368
Solvent: Carbon tetrachloride Solute: Bend, M. W. = 210.n Soluat: Carboqtetrachloride Solufe: Benzoin, M. W. = 212.24
157.4 157.4
2.115 4.049
0.310 4.86 0.610 4.99
157.4 157.4
2.175 2.025
0.322 4.95 0.299 4.93
1.5000
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VAPOR LIQUID
z 7 0
5
1.80 1.83 1.83 1.83
56 0
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WEIGHT 1.4800
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40 50 6 0 70 80 PER CENT B E N Z E N E
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REFERENCE CURVE FOR BENZENE -METHYL ALCOHOL
Acknowledgment. The authors are indebted to E. H. Sargent and Company for fabricating this apparatus.
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WEIGHT PER CENT BENZENE
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