In the Laboratory edited by
Cost-Effective Teacher
Harold H. Harris University of Missouri—St. Louis St. Louis, MO 63121
Equipment for a Low-Cost Study of the Naphthalene–Biphenyl Phase Diagram David Calvert University of St. Andrews, St. Andrews, Fife, Scotland Michael J. Smith* and Eduardo Falcão Department of Chemistry, University of Minho, Gualtar, Braga, Portugal
The use of cooling curves to characterize the phase diagram of a system forms the basis of an experiment that has been included in physical chemistry laboratory manuals (1) and described in undergraduate textbooks for many years (2). However, some of the disadvantages of the technique have been pointed out in a recent paper (3), and recourse to the more sophisticated techniques of instrumental thermal analysis and microscopic methods has also been proposed (4). In this paper a simple experimental assembly is described that allows rapid acquisition of data from a set of six samples of different compositions. The resulting experiment is straightforward, requires very little previous laboratory experience, and allows the eutectic temperature, the eutectic composition, and the locations of the liquidus lines to be identified with good precision and excellent reproducibility. Experimental Assembly Figure 1 shows a schematic representation of the assembly used in this experiment and identifies the various components of the equipment. Six test tubes (o.d. 24 mm) are uniformly distributed around a centrally located heat exchanger, supported by a transparent plastic disk1 which fits closely into the mouth of a 800-mL glass beaker. The heat exchanger is simply a metal cylinder (o.d. 15 mm, immersion depth 80 mm) through which a slow flow of cooling water is passed. The test tubes are held with silicone sealant in holes in the plastic disk and sample tubes (o.d. 17 mm) are supported within the test tubes by a small cube of polystyrene foam insulator and a Teflon sleeve. The dimensions of the test tubes and sample tubes are not critical and most suppliers of laboratory glassware list suitable items in their catalogues. An air gap of about 2 mm should be maintained between the sample tube and the inner surface of the test tube. Six mixtures of naphthalene and biphenyl, each with a total mass of 5 g and suitable naphthalene molar fractions, are located in the sample tubes, which are sealed using Teflon caps with neoprene o-rings. This type of seal between the cap and the inner test tube avoids loss of material during operation at temperatures up to 95 °C. The estimated workshop time necessary to prepare all the tube caps with seals, plastic disk, and heat exchanger for one experimental assembly is about 3–4 hours. Type K thermocouples are supported in 2-mm o.d. stainless steel tubes, which pass
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through the caps and locate the sensor tips close to the center of each sample. The six thermocouples are connected to a type K thermocouple switchbox or to an electronic thermometer with six inputs.2 Several other organic systems (4–6 ) are just as readily characterized as the naphthalene–biphenyl chosen for this experiment. However, most of these alternatives have higher liquidus temperatures, which means that less convenient and more expensive oil-baths or ovens would have to be used to obtain suitable results. Using the equipment described in Figure 1, a series of six samples can be analyzed simultaneously for a low initial setup cost. Procedure The experimental assembly is heated using the maximum power of a 650-W stirrer hot-plate and the stirring rate is adjusted to obtain a uniform temperature within the beaker and sample tubes. The warm-up period may be reduced to about 15 minutes by using hot water to fill the beaker at the beginning of the experiment. When the sample temperatures reach 75 °C the water flow through the heat sink is started and adjusted to about 150 mL/min and the heater is switched off. As a result of the thermal inertia of the assembly the temperature continues to rise to about 90 °C, then slowly decreases. The thermocouple selector is positioned to read the temperature of sample 1 and a stopwatch is started when the temperature reaches 80 °C. The first temperature is registered after a 10-second interval and the selector position is immediately changed to read the temperature of sample 2. After a further 10 seconds the temperature of thermocouple 2 is read, noted in the results table, and the process is continued with the other 4 samples so that a reading of the temperature of each sample is taken once a minute. This reading rate is sufficient to identify the changes in the gradient of the cooling curve that occur during component crystallization and eutectic solidification. The registration of temperatures is continued for a few minutes after the last sample has solidified to allow the temperature to decrease to several degrees below the system eutectic. Under these circumstances the eutectic temperature is more easily defined. Normally the data acquisition takes between 45 and 60 minutes.
Journal of Chemical Education • Vol. 76 No. 5 May 1999 • JChemEd.chem.wisc.edu
In the Laboratory
Experimental Results In view of the need to register data at a fairly high rate, it is suggested that students work in pairs. Typical cooling curves for selected sample compositions are shown in Figure 2a–d, and corresponding liquidus and eutectic temperatures are indicated on the phase diagram in Figure 2e. A good representation of the system phase diagram can be obtained from a study of only six samples with appropriately chosen compositions; however, the results obtained from three separate experimental assemblies with different sample compositions are shown in Figure 2e. Clearly the phase diagram may be more accurately defined by providing groups of student pairs with samples of different compositions and promoting an exchange of results after completion of the experiment. The data from the cooling curves was compared with results recorded using an instrumental technique, differential scanning calorimetry, and the locations of the eutectic temperature (about 37 °C) and composition (at a naphthalene mole fraction of 0.45) were found to be in good agreement. A useful extension to this experiment could be introduced by adding an appropriate computer interface (7 ) to allow automatic recording of sample temperatures after completing the initial setting-up procedure. While this modification would certainly simplify the data acquisition, and perhaps form the basis of an interesting science project, our principal objective was to develop an experiment that permits an expeditious definition of a relatively low-temperature phase diagram, by the cooling curve method, with a modest outlay of funds. Apart from providing cooling curves within a suitable temperature range, the naphthalene–biphenyl system is suitable for study because it is a system with behavior close to ideal. The data obtained from the experiment can also be used, as described by Williams (4 ), to calculate enthalpies of fusion for naphthalene (experimental 20.7 ± 1.5 kJ mol{1, lit. 18.726 kJ mol {1) and biphenyl (experimental 17.2 ± 1.7 kJ mol{1, lit. 19.569 kJ mol {1) giving a reasonable agreement with accepted values (8).
The results of this experiment are subject to errors associated with the temperature sensing. Apart from the inherent error of the thermocouples and electronic thermometer, the inertia imposed by the sample mass, and the relatively low thermal conductivity of the sample mixtures, supercooling also causes slightly low values to be obtained for the temperature coordinate. The difficulties involved in estimating the liquidus temperature are discussed in some detail by Skau et al. (6 ). Experimental manipulations are straightforward. The total time necessary for initial temperature equilibration and data acquisition is less than two hours. The students do not handle toxic materials or spend time on tedious weighing procedures or complicated assembly of the laboratory equipment. The time invested in the initial preparation of the equipment is rewarded with safe, reliable performance. Safety Considerations CAUTION: The compounds used in this experiment are classified as toxic or irritant (9) and may be harmful by inhalation, skin absorption or if swallowed. Relatively small amounts of the compounds are used, and as the instructor or technical staff prepare the experimental assembly no danger to the students is foreseen. Clearly the initial preparation of the samples should be carried out with care using suitable protection in a fume hood. Assembly maintenance involves only periodic substitution of the contents of the sample tubes. The suggested service interval is two years; however, sample lifetime depends on the quality of the sample-tube seal. As the samples are sealed into glass tubes and no significant vapor loss arises during the experiment, this system seems very suitable for student experiments. Notes 1. Polymethylmethacrylate (PMMA) is a suitable material for manufacturing this disk. This polymer is a transparent, rigid, machineable thermoplastic that allows visual access to all regions of the assembly and is readily available from engineering materials suppliers as Perspex, Plexiglas, Oroglas, or Diakon. 2. RS Components Limited, PO Box 99, Corby, Northants, NN17 9RS, UK or SIKA, Dr. Siebert & Kuhn, Struthweg 4-9, P.O. Box 1113, D-3504 Kaufungen 1, Germany.
Literature Cited
Figure 1. Schematic diagram of experimental assembly. 1: heat exchanger; 2: type K thermocouple; 3: Teflon cap; 4: neoprene o-ring seal; 5: thermocouple in steel tube; 6: sample stirrer bar; 7: stirrer hot-plate; 8: water-bath stirrer bar; 9: polystyrene support; 10: sample; 11: air gap.
1. Shoemaker, D. P.; Garland, C. W.; Nibler, J. W. Experiments in Physical Chemistry, 5th ed.; McGraw-Hill: New York, 1989; pp 238–246. 2. Ferguson, F. D.; Jones, T. K. The Phase Rule; Butterworth: London, 1966; pp 46–54. Levine, I. N. Physical Chemistry; 4th ed.; McGraw-Hill: New York, 1995; pp 335–342. Atkins, P. W. Physical Chemistry, 5th ed.; Oxford University Press: Oxford, 1995; pp 345–347. Alberty, R. A.; Silbey, R. J. Physical Chemistry; Wiley: New York, 1991; pp 219–220. 3. Williams, K. R.; Eyler, J. R.; Colgate, S. O. J. Chem. Educ. 1987, 64, 499. 4. Williams, K. R.; Collins, S. E. J. Chem. Educ. 1994, 71, 617. 5. Mettler Applications Note number 812; Mettler Toledo AG, CH-8606 Greifensee, Switzerland; Mettler Toledo, Inc., P.O. Box 71, Hightstown, NJ 08520-0071, USA. 6. Skau, E. L.; Arthur, J. C. In Physical Methods of Chemistry, Part V; Weissberger, A.; Rossiter B. W., Eds.; Wiley-Interscience: New York, 1971; pp 105–197. 7. Bailey, R. A.; Desai, S. B.; Hepfinger, N. F.; Hollinger, H. B.; Locke, P. S.; Miller, K. J.; Deacutis, J. J.; Van Steele, D. R. J. Chem. Educ. 1997, 74, 732.
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In the Laboratory 8. CRC Handbook of Chemistry and Physics, 76th ed.; Lide, D. R., Ed.; CRC: Boca Raton, FL, 1995. 9. The Sigma Aldrich Library of Chemical Safety Data, 2nd ed.; Lange, R.; Ed.; Sigma-Aldrich Corporation, PO Box 355, Milwaukee, WI 53201, 1988.
(b)
(c)
(d)
Temperature / °C
(a)
Time / min
Temperature / °C
(e)
a d
c
b
Figure 2. Typical cooling curves for samples with naphthalene mole fractions of (a) 1.0; (b) 0.75; (c) 0.55; and (d) 0.25. (e) Experimental phase diagram.
Mole Fraction Naphthalene
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Journal of Chemical Education • Vol. 76 No. 5 May 1999 • JChemEd.chem.wisc.edu
