Edwin F. Meyer DePaul Universitv Chicago. 1L 60614 Joseph A. Mever Evanston owns ship High !School Evanston, IL 60204
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A Binary Solid-Liquid Phase Diagram Experiment Including Determination of Puritv, ~ n t h aofl ~Fusion ~ and True Melting Point
In a study of the thermodynamics of solution of various solutes in liquid mixtures of n-tetracosane (r?,) and di-ndodecyl thioether (Cj4S)a question arose regarding the extent of non-ideality of the solvent mixture. Because ol the low vapor pressures of these substances, and the fact that we were working in the temperature range only slightly above the meltinenoints of the mixtures. i t b a s decided that an examinationif the solid-liquid bin& phase diagram of Cna-CzaS mixtures mieht the information we needed. .. orovide . The experimental set.up used waq sosimple, and the results sopuod, that we think it o f f e n a n attractivealternative to the undergraduate experiments described in popular lahuratory texts. These seem limited to high temperature metal systems, or lower temperature systems involving objectionable or unD-dichlostable materials (e.e. navhthalene:di~henvlamine: . . .. rohenzene: o-cresolj. Advantages of the experiment described herein are as follows: (1) The systemswere never heated over 80'C. Cooling in undisturbed, room-temperatureair leads to very satisfactory temperature-time ~. curves. (2) The compaunds in question are stable for years when stored without any precaution heyond using a closed container. (3) They are eommereially available at better than 97% purity1 (Humphrey Chemical Co., New Haven, CT) (4) They are not hazardous chemicals. (5) Only small amounts are required (1-l'Iz g per cooling curve). (6) Quite respectahle-looking phase diagrams are obtained without stirring. due to the thinness of the sample under -. ~resumahlv . study. An extension of the usual exneriment. which to our knowledge has not been described in undergraduate laboratorv texts. is the estimation of . nuritv " of each "oure" comoonent of the series of mixtures. It involves no extra data collecting and is based on the same principle as the rest of the experimenh2
ated in O.1° interval~.~The test tuhe full of the solid mixture is heated carefullv ,with s heat lamo until all of the ssmole is melted. For the teat tubes we used, theshrinkaged the\amplem melting wnsrompenrated for hy insertion of the thermometer. The lattrr 1s remwed and inserted ~everaltimes to prwide ndequate mixing the wmponents, and a wire (common hell-wire is gwd) looped around the test tuhe under its lip is attached to the top of the thermometer to hold the sample in place. T h e unit is then placed in a clamp on a ring stand. The heat lamp is used to heat the sample to about 7 5 T , turned off and removed from the vicinity of the experiment. When the mercury hits 65°C. - a. stoo ~~~. ~ watch or timer is turned on. and readines ~.are taken every It1 or 20 sec until room rcmperature is appnrachrd Interpretation of the curves is dercrilwd adequately in many Iahoratory texts. We observed the euteetic mixture at a C d 3 molefraetion of 0.83, and t = 34.9%. (See Fig. 1.)
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Estimation of Purlty The estimation of purity will he illustrated with our data for "pure" C& The cooling curve is shown in Figure 2. The mole fraction of the impurity in the sample is related to the slope of a plot of equilibrium temperature versus the inverse of the fraction. F. of the samnle melted. The relationshin. mavbe derived as follows: The fundamental equation on which the experiment is based may be written dT -RT2 (1) d l n r AH, where in the present instance, x is the mole fraction of the
Experimental
Convenient mole fractions of the thioether are 0.0.0.2.0.4.0.6.0.8. . . . 0.9.ll.95,and 1.00 Samplesareweigheddirertly inL,:
