Author: J. M. McCormick
Last Update: October 16, 2008
Introduction
Solid-liquid phase diagrams show the phase relationships in mixtures of two or more components and are very important in understanding the behavior of mixtures in metallurgy, material science and geology. In this exercise, you will measure cooling curves of either the napthalene-biphenyl system (group 1) or the napthalene-durene (1, 2, 4, 5-tetramethylbenzene) system (group 2) using a Vernier temperature probe. From these data generate the phase diagram for the system.1 For a general introduction to solid-liquid phase diagrams and the theory behind them, please see Gallus,1 Garland,2 Atkins and de Paula,3 and Boardman and Youngblood.4
Procedure
CAUTION! Pay careful attention to the health and safety cautions accompanying the substances used in this exercise. Take care when handling these compounds to minimize your, and your labmates, exposure to these substances.
You will be recording the temperature as a function of time using a Vernier temperature probe interfaced to a computer through the LabPro interface and the LoggerPro software package. Click here to review the use of the temperature probe and program. Each day you will need to perform a a three-point (0 °C, room temperature, and 100 °C are convenient) calibration of the the temperature probe to assure accurate and precise readings.
Precisely weigh out the appropriate amount of each substance to achieve the desired mole fraction (maximum mass should be less than about 3 g). Place the substances in the sample holder and heat to melt (a beaker of boiling water is sufficient). Mix well. Place the temperature probe in the sample and start recording the temperature. Choose the LoggerPro acquisition parameters so that you take enough data to clearly see the break and arrests in the cooling curve. The key to high quality results in this exercise is slow cooling to prevent supercooling2,3 and to allow for observation of the breaks and arrests. The rate may be about 1 °C/min, but it may be more, or less, depending on the system. Both Garland and Gallus suggest using an ice bath to speed the cooling process; and you can decide whether this is a wise choice. The sample must also be stirred as it cools, which is best accomplished by manually stirring (a long piece of wire coiled at one end to fit around the temperature probe works well).
Prepare enough samples to cover the range of mole fractions required to prepare the phase diagram. If possible, obtain several points at low and high mole fractions of one component of the mixture (to determine ΔfusH for each component). Note that you do not need to prepare fresh samples each time. One can just add more of one component to generate a new mole fraction and then just follow the procedure given above on the new sample. Take care when doing this, however, because if any sample is lost during a previous run the new mole fraction (and thus your phase diagram from there one) will be erroneous.
Dispose of the waste in the appropriate bottle. It may be necessary to use a solvent (such as acetone) to thoroughly clean the sample container, and this must also be disposed of properly.
Results and Discussion
From your data, reproduce the solid-liquid phase diagram for your system, as shown in Gallus.1 Using the equations given in the literature,1,2,4 determine ΔfusH for the two pure substances at 95% confidence (note that this requires a sufficient number of points at each extreme of the phase diagram), and compare these to the literature results. Determine the eutectic's melting point and its composition, along with your best estimate of their uncertainties. Is the uncertainty in the mass or the temperature responsible for the uncertainties in your calculated quantities, or is some other factor responsible? Be sure to include your experimentally-derived phase diagram and example cooling curves in your results.
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