8
A CONVENIENT METHOD OF VAPOR PRESSURES
DETERMINING
J. M. LEONARD and J. D. BULTMAN Naval Research Laboratory, Washington, D. C. Dnmiv~the course of an investigation it became necessary to know the vapor pressures of a series of compounds, the data for which mere not to be found in the literature. Examination of the existing methods for measuring vapor pressures failed for various reasons to provide a suitable method. Accordingly an apparatus, shown in the figure, was designed that could be used with equal facility for solids and liquids. It is similar in construction to the static system of Smith and Menzies' and is composed of two sections. The outer section consists of a doublewalled glass tube as shown. In operation this double wall construction serves to minimize localized heating in the sample compartment. The inner section of the apparatus consists of an inverted U-tube (sample tube), the shorter, closed arm of which serves as the sample compartment. The U-tube and a thermometer are supported by a rubber stopper in the neck of the outer section. The entire apparatus is submerged to the side arm in an oil bath equipped with adequate stirring and heating facilities and the side arm is connected to a conventional vacuum distillation system, complete with dry ice trap, manometer, surge tank, and bleeder valve which is comected to the atmosphere. In a typical run the sample, solid or liquid, is introduced into the sample compartment of the U-tube and the tube is so adjusted that the thermometer bulb and sample are adjacent. The unit is then placed inside the outer section and the entire assembly is placed in the bath and the vacuum line is attached. During an experiment the-vacuum pump runs coutinuously. Pressure is controlled by manipulation of the bleeder valve. At the beginning this valve is closed and the entire system is evacuated to the capacity of the pump, thus removing most of the air from the sample tube. As heat is applied to the bath the increasing vapor pressure of the sample is kept nuder control by manipulation of the bleeder valve until the desired temperature is reached, at which time the heat to the bath is reduced and the sample compartment is bled several times by alternately opening and closing the bleeder valve. (Caution: bumping!) The evaporation of the sample c i s e s a lowering of the vapor pressure in the sample tube. Therefore, after the residual air has been blown out of the tube, the sys-
tern must be allowed to come to equilibrium before any measurements can be taken. To take a measurement the bleeder valve is manipulated until the meniscus of the mercury in the sample tube is level with that of the mercury pool. When the two menisci are a t the same level the pressure in the sample tube is equal to the pressure in the outer system. The manometer is isolated from the system and the temperature, along with the corresponding pressure, is recorded. The procedure is repeated at intervals as the temperature of the system decreases until a series of temperatures and corresponding pressures have been obtained. The measurements are taken with decreasing temperature in preference to increasing temperature because it is felt that more efficientsweeping out of the residual air from the sample tube is obtained by bleeding the tube when the sample vapor pressure is a t a maximum.
I
10 MU VENT 1 0 OUTER JACKET 4 MU GLASS CANE SUPPORT fOR SAMPLE TUBE
COMPARTKENT SAMPLE TUBE 8 H M OD TUBING
' SXITE,A,, AND A. W. C.MENZIEB,J. Am. Chem. Soe., 32,897,
1412 (1910).
Sample Tuba and Aslembly
623
181
624
JOURNAL OF CHEMICAL EDUCATION
Two corrections must be made. The first is to compensate for the approximate capillary depression of the Material mercury in the sample tube which is +1.2 mm. for the &mm. OD tube used here, and the second is the sub- Naphthalene traction of the vapor pressure of mercury a t tempera- d-Camphor tures where it is appreciable. pDichlarobenzene Constants for the equation log,,P = - ( A / T ) + B were determined from the vapor pressure data collected Pentachlorophenol . . on a series of known compounds. Comparisons of these data to accepted literature values for four of the com- C''~,~l$'$;~/f;4";lliterature pounds are given in the table.
ation -(AF) +B B
Constants of e
h g , , ~=
A
2438 + 151 7.%306 f 0.3723 2500" 7 .975OS 8.3840 f 0.1044 2628 f 36 26660 8.4779" 8.1695 =t0.1541 2336 f 59 2323' 8.0913" 3312 f 83 8.3515 =t0.4390 3495G 8.8153" data: STULL,D. R.3 I d E ~ P .
