A NEW VAPOR-PRESSURE APPARATUS S. 0. COLGATE and ROGER D. WHEALY West Texas State College, Canyon, Texas
T H E apparatus to be described was developed to simplify vapor-pressure studies in undergraduate laboratories. Data for pressuretemperature plots for liquids, solids, mixtures, and solutions may be obtained easily and rapidly. The apparatus (Figure 1) consists of a long manom-
eter with a side arm A supported inside a large Pyrex tube. Both manometer arms are graduated in millimeters and equipped with airtight clamps or stopcocks a t the top. The sample bulb R is a thin-walled bulb blown in 6-mm. tubing; in it is placed from two to three grams of the substance to be studied. The bulb and sample are connected tightly to the side arm by a short piece of Tygon plastic tubing. The arm omosite the bulb is evacuated with a vacuum pump-and closed tightly at C. With the sample bulb in place, the manometer arm supporting it is evacuated until the mercury levels are the same. This arm is then sealed a t D. At equilibrium the difference in heights of the mercury columns represents the vapor pressure of the sample a t the temperature indicated by the total immersion thermometer E. To vary the temperature, water from the temperature bath F is forced into the large outer jacket. An aspirator ronnected to the exposed end of the Y-tube G provides sufficient vacuum to fill the outer jacket. By control of the flow rate through the aspirator, the water level in the large tube ran easily be made to remain near the top. As soon as the mercury columns rome to rest, their heights and the temperature are recorded. The large tube is drained by shutting off the aspirator and opening clamp H. The bath temperature is increased 5-1O0C., and water is again drawn into the outer tube to obtain the vapor pressure a t this new temperature. By repetition of this process, curves similar to that shown in Figure 2 are quickly obtained. After all
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readings have been taken, bulb B is replaced with an empty bulb and the outer tube is filled with hot water. The vapors of the sample are then pumped out of the apparatus, which is now ready for the next sample. The working model built by the authors was constructed from 6-mm. Pyrex tubing and gave pressure
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readings up to 100 cm. I t has been successfully used by a group of undergraduate physical chemistry students. Their results were in excellent agreement with values taken from the literature. The experimental and accepted plots obtained for water (Figure 2) are typical.
