SUBLIMATION CURVE, VAPORIZATION CURVE, AND TRIPLE POINT DETERMINATION' An Undergraduate Experiment NORMAN 0. SMITH and CONSTANCE M. COSTICH Fordham University, New York, N. Y. -
THE
determination of the vapor-pressure curve of a thermometer and the standard portable type of manompure liquid by the Ramsay and Young method (meas- eter ( M 4 cm.) are adequate, although a manometer urement of the boiling point under various pressures) covering a wider range increases the range of measureis a familiar experiment in the undergraduate curricu- ments. When assembled, the apparatus containing the solid lum in physical chemistry. That the method is also applicable, with some modification, to the sublimation is evacuated to a few millimeters and the pump conneccurre of an appreciably volatile solid seems to have tion turned off. The solid is heated gently by brushing been largely overlooked, perhaps because it appears with a free flame. The temperature r i d e a c h e s doubtful that equilibrium b e t ~ e e usolid and vapor a coustant value a t the temperature for which the subwill be reached readily. Strangely enough, equilib- limation pressure equals the pressure given by the rium is often readily attainable, as Ramsay and manometer. Happily, the solid condenses in an adherYoung2 themselves showed for several solids, and the ent coating on the thermometer bulb so that the prowriters have found that their technique can be modified and simplified to give an experiment suitable for the standard laboratory course in physical chemistry. It permits the measurement of the sublimation pressure of a solid, followed without interruption by that of the vapor pressure of its liquid. The resulting data lead immediately to location of the triple point temperature and pressure and thence to the unary phase diagram. The time required for assembling the apparatus and making the measurements is not more than two hours. Solids with appreciable sublimation pressures at easily accessible temperatures are not common, but at least half a dozen have been found feasible in the experiment t o be described. The one recommended is salicylic acid: its triple point is conveniently located, it equilibrates quickly, it is nonhazardous and inexpenmanometer sive, and it allows easy cleaning afterward. The apparatus is shown in Figure 1. The powdered solid A is placed in a container made by removing the bulb and side arm from a 500-ml. Claisen flask. Sufficient solid is used so that, when molten, it will fill only the portion of the vessel below the crotch, and yet, when solid, wi11:lcover the bulb of the thermometer B completely. The air condenser C leads to a tank or carboy D (for a buffer against pressure changes) and thence to a manometer, three-way stopcock E, airleak, and pump. (A Hyvac is necessary for work with salicylic acid but a materaspirator can be used with cedure adopted by previous workers,' of coating the F~~salicylic acid a (t260° bulb by dipping it repeatedly into the molten material, certain other is quite unnecessary. The temperature and pressure ' Presented before the Meeting-in-Miniature oi the New York are recorded and a small amountof air allowed to leak Section of the American Chemical Society, February, 1954. ' RAMSAY,W., AND S. YOUNG,Trans. R o y . Soc. London, in to raise the pressure by 2 or 3 mm. The solid is ~ 1 7 5 , 3 7(1884); J. Chem. SOC., 47, 42 (1886); 49,453 (1886); "0" heated further until a new constant temperature is reached and again temperature and pressure are see nlw LINDER.E. G., J. Phya. Chem.. 35, 531 (1931).
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recorded. This procedure is continued until, when 2 gives the results of typical runs with iodine and anthe pressure reaches about 20 mm. and the tempera- thraquinone and includes literature values quoted by ture about 160°, the solid melts and exposes the bulb StulL3 The data on a p-t diagram, with the addition completely. The technique is unaltered by melting, of a fusion curve (vertical straight line), yield the phase however, for pressures are increased (by gradually iu- diagram immediately. Although salicylic acid is water-soluble, ethanol will creasing amounts) and the corresponding constant temperatures (boiling points) observed as before. be found better for cleaning the apparatus, and a t least The normal boiling point of salicylic acid, 256", is 20 minutes should be allowed the student for this easily accessible, although the temperature range can- purpose. not be extended beyond 230" with a manometer limited to pressures under 44 cm. I n any case, the acid gradually decomposes with rise in temperature with a corresponding increased error in measurement. The resulting data are now plotted (log p versus l/T), giving two nearly straight lines, the intersection of which locates the pressure and temperature of the triple point, and the slopes of which lead to calculation of heats of sublimation, vaporization, and fusion. Figure
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Lines: valuer of Stull (ref. 31. Circles: prcscnt technique.
Apparatus Modified for Rapidly Subliming Solid.
Substances t o which the above technique has been found applicable are salicylic acid, hexachlorobenzene, iodine, hexachloroethane, camphor, anthracene, and anthraquinone. U. S. P. grade chemicals are satisfactory. For one reason or another, all but the first two listed are less appropriate for student use. Iodine gives excellent results but is, of course, obnoxious. Camphor requires too careful heat regulation. Both are removed from the apparatus with difficulty but have an advantage in that a water aspirator can be substituted for the Hyvac. Hexachloroethane works well but gives only the sublimation curve. Anthracene and anthraquinone volatilize so rapidly, even though their vapor pressure is lower than that of all the other substances mentioned, that a portion of an 18-mm. test tube, tapered a t its open end, must be used to jacket the thermometer bulb and contain the solid, as in Figure 3. A convenient summary of literature values for sublimation and vapor pressure will be found in Stull's c~mpilation,~ although we question the data given there for hexachloroben~ene.~ It should be pointed out, in conclusion, that the above technique is not applicable to all volatile solids: it is feasible only when their sublimation pressures are sufficiently large and when they equilibrate readily. STULL, D.R.,Ind. Eng. Chem., 39,517 (1947). 4 See, however, SEARS, G.W., AND E.R.HOPKE, J. Am. Chem. Soc., 71, 1632 (1949).
