Constructing a Complete Temperature-Composition Diagram for a Binary Mixture A New Experiment That Uses Quantitative Gas Chromatography Ru-Jen cheng,' Chih-Li Hsu, and Yu-Fang Tsai National Chung-Hang University, Taichung, Taiwan 40227, Republic of China Distillation is one of the most fundamental techniques of nurification. but in most cases one simple distillation is not enough. ~ractionaldistillation, which involves multiple simple distillations, is required ( I ) . One can explain the principles of fractional distillation of an ideal binary volatile mixture bv" a~nlvinz . . - .. Raoult's law and Dalton's law, which together yleld a "temperature-composition diagram" that is widely used to explain the processes involved in fractional distillation. Unfortunately, students frequently do not comprehend the diagram. To reinforce the physic& basis of thk diagram and its correlation to fractional distillation, asking students to generate this diagram seems useful (2). This experiment was developed with two goals: to help students appreciate fractional distillation; and to improve general chemistry experiments in our country. We basically followed the same idea and procedure published by Thorstenson (2).One major modification has been made to obtain experimentally the composition of the gas phase, instead of doing this by calculation. The success of this experiment depends on assuring liquid-vapor equilibrium. Our Apparatus We sought an appropriate apparatus for liquid-vapor equilibrium (31,but due to their cost and complexity, none of those available seemed suitable for freshman courses. For our purposes, we could sacrifice a certain degree of accuracy if a versatile, easily assembled, and simple setup were available. We now report such a new design that requires only the parts of a simple distillation assembly, plus a thermometer adaptor for approaching liquid-vapor equilibrium and for collecting the condensed vapor phase, as shown in Figure 1. Gas chromatography is a popular method for both qualitative and quantitative analysis (4), and the instruments are easily accessible in most laboratories. Thus, we have integrated GC into our design to determine the composition of the gas phase. From these two parts of the experiment, we hope that freshmen will learn both the concep-
tual basis of fractional distillation and a fundamental application of GC. Experimental Using the GC for Qualitative Analysis and the Construction of a Calibration Curve The experiment was performed on a silicone OV-101 column, installed in a Shimadzu GC-8A that was equipped with a thermal conductivity detector. Pure toluene and carbon tetrachloride are used individually for determination of retention times by GC. Then, a specific composition of the toluene-carbon tetrachloride system in mole percent (100:0, 90:10, ..., 10:90, 0:100) is assigned for quantitative analysis (4). When a complete set of data is available, the calibration curve of area percent versus mole percent can be plotted. The resultant calibration curve happens to be linear. To clarify the use of this calibration curve, an unknown sample can be given to determine its composition. Liquid-Vapor Equilibrium States Because the mixture of toluene and carbon tetrachloride can he approximated as an ideal solution, its theoretical boiling point can be calculated2by Raoult's law (5).Some boiling chips3 should be added to the previously prepared solution of known composition, and the apparatus can be assembled as shown in Firmre la. The solution is heated madually until the liquid temperto the theoretical boiling pbint ature is as close as and the vapor temperature. The liquid temperature and the vapor t k m p e r a k should be recorded as soon as the mixture is stable enough. Then the distillation head is turned slowly from the upward position to the downward position as shown in Figure lb. When three drops of distillate have been collected, the head is reversed quic!sly and the heating is stopped.
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Caution:The toluene-earbon tetrachloride mixture is toxic. Do not dismantle the apparatus until the solution has cwled to room temperature.
Some distillate is injected into the GC to obtain the area percentage. The composition of the condensed vapor can be determined using the previously obtained calibration curve. The experimental data can be compared with the theoretical composition calculated from Dalton's law. When all data are ready, we can construct a complete temperature-composition diagram of toluene and carbon tetrachloride. All data reported should include liquid composition, liquid temperature, vapor composition, and vapor Figure 1. (a)The apparatus for liquid-vaporequilibrium. (b) To collect three drops of distillate, move the distillation head from the upward position of Figure la to the downward position.
'Author lo whom correspondence sho~ldbe addressed. necessary vapor pressures can be found in ref 5, and the enernal pressLre sho~ldcorrespond to tne local one. 3Magnetic stirring does not work well. Volume 69 Number 7 July 1992
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male% of carbon tetrachloride
F~gure2. The complete temwrature-composltlon atagram of the toluene-carbon telrach or oe system from obr data. Tne contmous wives are lheoretca curves.
temperature. Graphing our data yields a satisfactory diagram, as shown in Figure 2.
Summary Due to the importance of GC in chemical analysis and due to the use of temperature-composition diagrams in illuminating the physical bases of fractional distillation, we combined these two separate but related freshman chemistry experiments in two 3-h laboratory sections.
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Here we obtain both qualitative and quantitative data by using GC to exploit variable adsorptions, instead of using techniques that exploit the refractive indexes, such as those commonly found in references. We have also used innovation in the apparatus we have chosen to achieve liquid-vauor eauilibrium: We have modified a sim~le-distillation issembly instead of using the complex unit found in most references. With such a versatile and easily constructed apparatus, the results are suffic~entlvsatisfactory for oedaeomc demonstration. From our experience, thk pedago&l value will be more than doubled by construction of a complete temperature-composition diagram a s suggested. First, students can appreciate that the compositions of the vapor phase and the solid phase a t equilibrium are differentthat the former is always richer in the low boiling point comoonent. Further.. thev. can double check the exoerimen~~~-~~ tal results by carrying out the calculations and proving the appl~cationof Raoult's law and Dalton's law. ~~~~
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Acknowledgment This work was supported by National Science Council of Republic of China, Grant No. NSC 77-0111-S005-04E. Literature Cited
5. Jordan, T E. Vopor~rpssu~ o f 0 r m i e Compamds; hte-enee:
19.
New Ymk, 1954; p
