Development of Binary Liquid–Vapor Phase Diagram Laboratory Procedures to Replace the Traditional Tetrachloroethylene/Cyclohexanone System

Kelly J. Gordon, Laura Kenkel, Stephanie Prescia and Marcy Towns*

Department of Chemistry, Ball State University, Muncie, IN 47306;*Department of Chemistry, Purdue University, West Lafayette, IN 47907, mtowns@purdue.edu

Received September 22, 2006. Accepted December 15, 2006.

Abstract: Two chemical systems were investigated and implemented as alternatives to the traditional tetrachloroethylene/cyclohexanone system used in the binary liquid–vapor phase diagram physical chemistry laboratory. Both the 2-butanone/cyclohexane and the ethyl acetate/cyclohexane systems reduce the cost of waste disposal and eliminate the use of chlorinated solvents. Student data showed that the azeotropic composition and temperature could be easily identified from a binary liquid–vapor phase diagram. For both systems, student data produced azeotropic compositions and temperatures that were in tolerably good agreement with known values. For example, in the 2-butanone/cyclohexane system the students found the azeotropic composition and temperature to be 0.52 mole fraction cyclohexane and 70 °C compared to the literature values of 0.5624 mole fraction and 71.8 °C.

The construction of a binary liquid–vapor phase diagram is a traditional physical chemistry laboratory and although the physical chemistry laboratory has evolved with the field, many faculty choose to retain classic laboratories in their curriculum [1–4]. One of the problems with this time-honored laboratory [5] is that the chemical system of choice uses a chlorinated solvent, such as tetrachloroethane [1], which poses disposal issues and health related challenges. Thus, the replacement of this system with one that reduces the health risks and the cost of waste disposal while providing reasonable data for the students is desirable.

Experimental Overview

Binary mixtures of two volatile liquids demonstrate a range of boiling-point behavior from the ideal solution that follows Rauolt’s law and yields a continuous change in boiling point with composition, to a nonideal system that deviates from Rauolt’s law. In some cases the deviation can be large enough to produce a maximum or minimum in a temperature versus composition phase diagram. This results in an azeotrope, a mixture that boils with constant composition. Chemical systems that produce an azeotrope cannot be completely separated into two pure components by distillation. In a physical chemistry laboratory focusing on Rauolt’s law, students can explore nonideal systems, create temperature versus composition phase diagrams, and determine the azeotropic composition and temperature for the system under investigation.

We evaluated proposed binary liquid systems for viability by examining the refractive indices and boiling points of the pure components, the boiling point of the azeotrope, the miscibility of the solvent system across the entire concentration range, and the toxicity and safety of each component [5– 7]. To be a reasonable system the boiling point of each of the pure substances and the azeotrope had to be distinct—differences easily measurable using a variety of temperature sensing devices with precision between ±0.5 °C to ±0.1 °C. The refractive indices of the components had to be dissimilar enough that a calibration curve could be constructed. We eliminated components from consideration for health reasons, (benzene and toluene for example), and safety concerns (methanol and ethanol can produce highly flammable binary mixtures). Table 1 presents a list of possible systems and their accompanying physical properties (some of the systems have not been described previously). We note in this table the systems that proved impractiable based upon our selection criteria as well as those we pursued.

We settled on two solvent systems, cyclohexane/2-butanone and cyclohexane/ethyl acetate, that proved to be practicable based upon our selection criteria and produced reasonable student generated data. For both systems the students were able to produce reasonably accurate binary liquid–vapor phase diagrams that demonstrated a deviation from Rauolt’s law. One benefit for Ball State’s chemistry department has been a reduction in the amount of chlorinated waste. In previous years this laboratory generated nearly six liters of chlorinated waste, which added to the financial waste disposal burden carried by the department.

Experimental

Hazards. MSDS sheets should be consulted for all the solvents used in this experiment. Proper technique for distillations should also be used. Departmental disposal procedures should be followed.

Procedure. For each system, students use an Abbe refractometer to prepare a refractive index versus weight percent calibration curve. Although the calibration can be performed using mole fraction as the composition variable, the use of weight percent allows the students to directly measure the independent variable for the calibration curve.

The experiment proceeds via two simple distillations. One begins with pure cyclohexane, then 2-butanone (or ethyl acetate) is added in specific volumes to approach the azeotropic composition. For each

Table 1. Proposed Azeotropic Systems for the Binary Phase Diagram Laboratory

Components		Refractive Index^a		Boiling point of pure liquids^a		Azeotropic composition and Temperature^a
A	B	A	B	A (T/°C)	B (T/°C)	Mole fraction A	T (°C)	Comments
Cyclohexane	2-butanone	1.425	1.377	80.7	79.6	0.5624	71.8	System meets all criteria.
Cyclohexane	Ethyl acetate	1.4266	1.3722	80.7	77.1	0.4714	72.8	System meets all criteria.
Hexane	Isopropanol	1.3751	1.3776	68.8	82.3	0.712	61	Refractive indices are too similar
Hexane	acetone	1.3751	1.3588	68.8	56.2	0.319	49.8	Refractive indices are too similar
Hexane	t-butanol	1.3751	1.3878	68.8	82.4	.7531	63.7	Refractive indices are too similar
Isopropanol	Ethyl acetate	1.3776	1.3722	82.3	77.1	.3046	74.8	Refractive indices are too similar
Cyclohexane	Isopropanol	1.425	1.3776	80.7	82.3	.5918	68.6	Miscibility issues; unable to produce calibration curve.

^aReferences 6–8.

Figure 1. Student generated data for the cyclohexane 2-butanone system. The reported azeotropic composition was 56% by weight cyclohexanone (0.52 mole fraction) and 70 °C.

mixture, the solution is heated until it is boiling. The temperature is recorded and a small amount of the vapor distillate and the remaining liquid in the pot is collected. The other distillation begins with pure 2-butanone or ethyl acetate and proceeds in a similar fashion by addition of cyclohexane to the azeotropic composition. The refractive index of each sample, liquid and distillate (vapor), is obtained and recorded. Using the calibration curve, the students can then determine the composition (weight percent) of the sample.

The temperature and composition of each liquid and vapor sample are plotted on the same graph using composition as the independent variable. The students then draw a smooth curve by hand through the liquid data points. Another smooth curve is drawn through the vapor points. The curves define areas that can be labeled as liquid, vapor, or liquid-vapor phase regions. Finally, students can identify the experimentally determined azeotropic composition and temperature and compare it to known values [6, 7].

Results and Discussion

The students can complete the data collection and analysis in two, three-hour laboratory periods and it is easily demonstrated that either experimental system does not follow Rauolt’s law. Both produce a low-boiling point azeotrope.

A student generated binary liquid–vapor phase diagram is shown for the cyclohexane/2-butanone system in Figure 1. In this case, the reported azeotropic composition and temperature, 0.52 mole fraction cyclohexane and 70 °C, were below the literature values cited in Table 1 [7]; however, the experimental data clearly shows a clustering of points about the azeotrope.

The experimental results were found to be agreeable for a number of practical reasons. First, the student-generated data based upon the instructor’s previous experience with this laboratory was acceptably close to the known azeotropic composition and temperature. Second, historically the traditional high-boiling-point tetrachloroethylene/ cyclohexanone system rarely produced data points close to the azeotrope. The procedure described herein yielded data points clustered about the azeotrope making it straightforward for students to identify the azeotropic composition and temperature. It was clear to the students that the system deviated from Rauolt’s law and a low-boiling-point azeotrope was produced. Third, the waste disposal costs were reduced due to the absence of chlorinated solvents. And fourth, the issue of dealing with chlorinated solvents as a health risk was eliminated.

Acknowledgments. We would like to acknowledge the physical chemistry students who tested this lab and the Ball State University chemistry department undergraduate research program and Ball State University Honors College for their support.

Supporting Materials. Student handouts and information for instructors are available (http://dx.doi.org/ 10.1333/s00897072025a).

References and Notes

1. Shoemaker, D. P.; Garland, C. W.; Nibler, J. W. Experiments in Physical Chemistry, 7th ed.; McGraw Hill: New York, 2002; Experiment 14.

2. Moore, Robert J.; Schwenz, Richard W. J. Chem. Educ. 1992, 69, 1000–1002.

3. Zielinski, T. J.; Schwenz, R. W. Chem. Educator 2004, 9, 108–131; DOI 10.1333/s00897040771a.

4. Kugle, R. W. J. Chem. Educ. 1998, 75, 1125–1129.

5. Smith, C. W., Cooke, J. B., Glinski, R. J. J. Chem. Educ. 1999, 76, 227–229.

6. Horsley, L. H. Azeotropic Data III; American Chemical Society: Washington, DC, 1973.

7. Ponton, J. WWW Chemical Engineer’s Toolkit. Homogeneous azeotrope databank. http://eweb.chemeng.ed.ac.uk/chemeng/azeotrope_bank.html (accessed May 2007).

8. NIST Chem Webbook. http://webbook.nist.gov/chemistry/ (accessed May 2007).