Liquid-Vapor Equilibrium of a Binary System

Adapted by J. M. McCormick from an exercise used at the University of Kansas.

Last Update: October 15, 2008

Introduction

The understanding of the equilibrium between the liquid and vapor phases in a multi-component system is important industries ranging from brewing to petroleum refining. In this exercise you will be examining the chloroform-acetone system and comparing your results to literature values.  You are referred to references 1 through 3 for more information on the theory involved.^1-3

Procedure

The procedure is similar to that described in Garland et al.,¹ and in Halpern and McBane.²  However, instead of the apparatus described in these references, the one shown in Fig. 1 will be used.  The modified apparatus is designed so that there is very little liquid hold-up, and so the small amount of liquid that is condensed from the vapor has almost the same composition as the vapor. The apparatus described in the Garland requires that a substantial amount of liquid be vaporized and condensed.  This causes the liquid and vapor compositions to change continuously and so the condensate composition is a rather poor representation of the vapor composition. Moreover, in the distillation procedure given in Garland the temperature changes continuously, which introduces an unacceptably large uncertainty into the results.  The apparatus described in Halpern and McBane is better, but expensive.

Figure 1.  Distillation apparatus to be used in this exercise.

In the kit for this exercise you will find the items listed in Table 1.  Please check that everything is present and in good condition before beginning.

Table 1. Equipment needed for this exercise.

Calibrate the temperature probe at three points (0 °C, room temperature, and 100 °C are convenient). Helpful hint: the calibration of the temperature probe should be done while the apparatus is being assembled. Click here to review operation of the LabPro and the LoggerPro software that controls it.

Assemble the distillation apparatus as shown in Fig. 1 (check with the instructor for assistance). Wrap the glass portions of the reflux apparatus with glass wool or aluminum foil to ensure that a stable reflux temperature is obtained. Place a septum liner on the vapor condensate collection arm (A in Fig. 1) and a septum stopper in smaller arm of the round bottom flask (B in Fig. 1). Insert the temperature probe through the orange retaining nut and then through an o-ring.  Carefully place it in position (C in Fig. 1) such that the temperature probe is upright (a small clamp may be needed) and the o-ring is not kinked or bent and then gently tighten down the retaining nut.  In addition to the small clamp on the temperature probe, you should clamp the apparatus just above the neck of the round bottom flask and on the condenser.  The blue Keck clip should be used to hold the round bottom flask to the distillation head.  A heating mantle or oil bath may be placed below the round bottom on a lab jack.

The experiment can be performed during two periods with CHCl₃-rich solutions being investigated in the first week and acetone-rich solutions in the second.  As you are attempting to measure accurately and precisely the composition and boiling point of the azeotrope, it is advisable to overlap the regions studied and to use the third week to improve the quality of your data.  For example, keep adding CHCl₃ to the acetone when you are doing the acetone-rich system to make the solutions CHCl₃-rich and thus overlap the data with those you obtained in the first week.  It is easier to sequentially add one component to the flask to give a new mixture than it is to make up a fresh mixture each time.  However, it is critical that only the minimum amount (~0.05 ml) of the distillate and the solution in the flask be removed for analysis.  If the total volume is sufficiently large and amount withdrawn is small, you do not need to correct for the material removed, but this should be checked during the experiment.

Load the flask with 40 ml CHCl₃ (for the CHCl₃-rich solution), or 30 ml acetone (for the acetone-rich solution) and a few boiling chips. CAUTION! both CHCl₃ and acetone are highly flammable, and CHCl₃ has adverse health effects.

Carefully start heating the flask until reflux begins. Record the barometric pressure now, and several additional times during the course of the experiment. When liquid is seen refluxing in the lower portion of the condenser and the temperature reading is stable (i. e., remains unchanged for several minutes), record the temperature.  Confirm that the temperature corresponds to the boiling point of the liquid in the distillation apparatus. Using a clean, dry syringe equipped with the long needle, remove a sample (approximately 0.05 mL each) from the liquid that has collected below the condenser (representing the vapor's composition) and from the distillation flask (liquid composition).  It is advised that you first remove the condensate (through the top of the condenser), and then remove the sample from the distillation flask. Do not leave the needle poking through the septum afterwards. Carefully place the samples in marked vials and immediately measure the index of refraction of each sample using the Abbé refractometer. Evaporation of the sample in the refractometer can be a problem, so work quickly. Confirm that the condensate and the liquid in the distillation flask have the index of refraction and that both match the literature value for the solvent in the flask.  This procedure serves two purposes: 1) it allows us to find any systematic error (and determine any correction factor to apply), 2) the instrument will then be set near to the index of refraction to be measured, thus minimizing the time needed to make each subsequent measurement.

Add 5 ml of acetone or CHCl₃, as appropriate, to the flask through the septum stopper at B via syringe. After the system has stabilized, remove a sample of the distillate and a sample from the flask and measure the index of refraction of each.  Repeat the addition, equilibration and withdrawal of sample steps at least five or six times. 

The index of refraction as a function of the mass percent of chloroform in acetone-chloroform mixtures is given in Table 2 (click here to download this table as an Excel file).  If your index of refraction lies between two tabulated values, you may make a simple linear extrapolation from the two nearest tabulated points. An alternative method is to fit the index of refraction data to an empirical function in mass percent chloroform in Excel or LoggerPro, and then simply use this function to find the composition from a measured index of refraction.

Table 2. Refractive index of acetone-chloroform mixtures as a function of the mass percent chloroform in the mixture.

When work for the day has been completed, allow the flask and the reflux apparatus to cool to room temperature, disassemble the set up and return it to the storage box. Place all liquids in the appropriate waste solvent container. Remove and discard the septum. Examine the septum liner, and discard it if damaged.

Results and Discussion

Your results must include your experimentally determined phase diagram. Use the method described in Halpern and McBane to fit your data,² and present the results of this fit. The discussion should address the points raised in both literature procedures, as well as comparing your results to the accepted values⁴ and quantitatively assessing the overall quality of the measurements. 

References

1. Garland, C. W.; Nibler, J. W. and Shoemaker, D. P. Experiments in Physical Chemistry, 7^th Ed.; McGraw-Hill: New York, 2003, p. 208-215.

 

2. Halpern, A. M. and McBane, G. C. Experimental Physical Chemistry, 3^rd Ed.; W. H. Freeman: New York, 2006, p. 14.1-14.14.

 

3. Atkins, P. and de Paula, J. Physical Chemistry, 8^th Ed.; W. H. Freeman: New York, 2006, p. 179-184.

 

4. CRC Handbook of Chemistry and Physics, 64^th Ed.; Chemical Rubber Company: Boca Raton, FL, 1983, p. D13.