John H. Wise, James K. Shillington William J. Wan, and R. D. Whitakerl
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Washington and Lee University Lexington, Virginia
A Eutectic Experiment For general chemistry laboratory
The system, biphenyl-naphthalene, has convenient properties for a simple eutectic experiment. The chemicals are readily available and inexpensive, the melting points to be determined are well within the range of the standard llO0C thermometer, and the system is nearly ideaL2 The melting point of the eutecthe phase tic is 39.4"Ca t 44.0 mole Oj, diagram is readily ~ b t a i n a b l e . ~ . ~ Fach student obtains four melting points, on weighed samples, a t approximately 0. 30, 60, and 100 mole % naphthalene. One gram of biphenyl is weighed into a 4-in. test tube, and the melting point is determined; then 0.35 g of naphthalene is added to the tube, and the melting point of the mixture is determined. One gram of naphthalene is weighed into a separate 4-in. test tube; after obtaining the melting point, 0.77 g of biphenyl is added, and the melting point of this mixture is determined. All weighings are to 0.01 g. The melting points are obtained by melting in a hot water bath, then cooling in air while stirring with a thermometer, and recording the temperature when about 25% of the sample has solidified. Present address: University of South Florida, Tampa. l M ~ C. ~M., ~ROBEN, ~ , B. W., AND SWIFT,R. M., J. CBEM. Enuc.., 18.473 . (19411., "'International Crit -~ ~ ~ ~ - - - ical - ~Tablq" ~ Vol. IV, McGmw-Hill Book Go., New York, 1928, p. 180. ' CAERONIS,N. D., "Semimicm Experimental Organic Chemistry," John de Graff, Inc., New York, 1958, p. 6.
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% / Journal of Chemical Educofion
The experimental melting points are plotted versus mole Oj, naphthalene as two straight lines, with the intersection point taken for eutectic data. One hundred and thirty-seven students obtained from plots an average eutectic melting point of 42.0°C with a standard deviation of *6.6, and an average eutectic composition of 42.4 mole Oj, naphthalene with a standard deviation of e10.3. This data was later analyzed with an IBM 1620 program to give an average eutectic melting point of 41.S°C with a standard deviation of *6.3, and an average eutectic composition of 43.3 mole % naphthalene with a standard deviation of e8.4. Data from another group of 67 students, on the 1620 program, resulted in a n average eutectic melting point of 403°C and an average eutectic composition of 42.0 mole % naphthalene. The procedure presented here is quite simple. In less than one laboratory period, each student may obtain sufficient data to determine the eutectic point on an individual basis. For this purpose, the simplified procedure is justified although experimental errors may he increased. If the eutectic data are used to calculate molecular weights by the freezing point depression method, the results are very poor since the solutions are so concentrated. The authors wish to acknowledge the assistance of Mr. J. B. Fox, Supervisor, Statistical Records, in the operation of the 1620 programs employed with this experiment.
