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VAPOR PRESSURE APPARATUS STEPHEN W. TOBEY University of Wisconsin, Madison
T H E apparatus described below for the measurement of vapor pressures of liquids is elementary both in its construction and operation yet is capable of giving quite precise results. The principle of operation is an obvious one, but one which has found little use in practice. A liquid is vaporized at the lower end of a long column and recondensed a t the upper end. Measurements of the temperature of the vapor in equilibrium with recondensing liquid are made with a thermometer suspended in the column. The inherent advantage of this apparatus over the Ramsay-Young dynamic type1 is that there is no loss of material during the course of the experiment. I t is superior to the static isoteniscope type of Smith and Menzies2in that no preliminary degassing of the sample is required, and no thermostat of any kind is required. Figure 1 shows the construction and critical dimensions of the column in use in this laboratory. It consists of three parts: a reservoir in which liquid is vaporized, a column in which the vapor is in equilibrium with condensing liquid, and a cold finger condenser to prevent vapor from escaping into the remainder of the apparatus. The reservoir is normally half filled with liquid. It is heated by means of a heating mantle controlled with a Variac. The flash guard a t the mouth of the reservoir prevents liquid from bumping up into the column with consequent superheating of the thermometer bulb. With this design i t is unnecessary to use any boiling chip. The thermometer is a 0.0llO.O°C. complete immersion precision type graduated to O.l°C. The bulb is wrapped in a muslin sleeve to assure thermal equilibrium with the refluxing vapor. The cold finger condenser is easily removable, making it a simple matter to change the liquid in the reservoir. If the available water supply is not cold enough to provide adequate cooling of the cold finger, a copper tubing coil immersed in an ice bath can be introduced in the cooling water inlet circuit. The column is connected to the remainder of the system, consisting of the usual mercury manometer, ballast tank, pump, and air inlet system via the side-arm ball joint connection. When organic liquids are used, the condenser joint should be lubricated with an organic insoluble type g r e a ~ e . ~
OPERATING PROCEDURE
The system is evacuated to about 10 mm. Hg internal pressure, or until the liquid in the reservoir begins to boil of its own accord with the heating mantle off. The RLMSAY, W., AND S. YOUNG, J. Chem. Soc. (London), 47, 42 (1885). SWTH,A., AND A. W. C. MENZIES,J. Am. Chem. Soe., 32, 1412 (1910). Nonaq grease, available from the Fisher Scientific Co., Pittsburgh, Pa., is satisfactory.
WL FLASK
Fiaure 1.
Column Dotail
Variac is then turned on and adjusted until there is a continuous dropwise return of liquid from the column through the flash guard, and a slower dropwise flow of liquid off the thermometer bulb sleeve. Continuous condensation of vapor will then be observed on the tip of the cold finger. The column will equilibrate within half a minute and can be maintained without further adjustment a t any desired equilibrium temperature and pressure for an indefinite period. Stepwise increases in the Variac setting will maintain the desired reflux rate as the column temperature is raised. Data are taken a t approximately 5°C. intervals up to the boiling point of the liquid a t atmospheric pressure. A series of data points is then taken on a descending run. Thus, between 25 and 40 readmgs can be easily obtained. From these, the usual plot of log P versus 1/T . is made and the heat of vaporization calculated. As described, the apparatus is capable of providing data over a range of pressures from 10 to 760 rnm. Hg with an accuracy of *1 mm. using a conventional meterstick manometer scale. Among the liquids that have been used in the apparatus are chloroform, carbon tetrachloride, benzene, toluene, water, methanol, and acetone. These liquids were chosen because of their appropriate vapor pressures over the range covered by JOURNAL OF CHEMICAL EDUCATION
the thermometer. I n all cases the experimentally determined vapor pressure curves agree with reliable literature data within the limits of experimental uncertainty.
VOLUME 35, NO. 7, JULY, 1958
ACKNOWLEDGMENT
The author wishes to express his particular appreciation to Professor Paul Bender for his helpful suggestions concerning the construction of the apparatus.
