In the Laboratory
Vapor Pressure Measurements in a Closed System
W
Mark Iannone Department of Chemistry, Millersville University, Millersville, PA 17551;
[email protected] The use of an isoteniscope to determine vapor pressures and ∆Hvap is a common experiment in physical chemistry labs (1, 2). This article describes an alternative method that uses a simple apparatus to measure vapor pressure versus temperature in a closed system, in which the total pressure is the vapor pressure of the liquid sample. The apparatus is inexpensive and can be constructed for under $200. However, the procedure requires a vacuum line equipped with a liquid nitrogen-cooled trap. Apparatus The apparatus (Figures 1 and 2) is assembled from Swagelok and Cajon fittings.1 (An all-glass apparatus would also work.) An inexpensive solid-state pressure sensor2 is used that puts out a 0–40 mV signal proportional to absolute pressure in the range 0–1 atm. The silicon piezoresistive sensor contains a thin silicon diaphragm with a built-in reference vacuum on one side and system pressure on the other side. A strain gauge produces a signal according to applied pressure. The diaphragm and gauge are protected by a silicone gel that is resistant to most liquids and gases (3). The output of the pressure sensor is amplified by a differential amplifier3 to give a signal linearly related to pressure. We use a voltmeter to obtain readings that are later converted to pressure; computer acquisition of temperature and pressure readings could easily be added.
Experimental Overview The apparatus is charged with the liquid of interest, which is degassed by freeze-pump-thaw cycles. By this technique, air is removed so that the total pressure in the sample chamber is the vapor pressure of the liquid. The apparatus is then detached from the vacuum system and suspended in a water bath; pressure and temperature readings are taken as the water temperature is increased. Finally, the liquid is again frozen and the pressure measured to verify that no leakage has occurred. Voltage readings are converted to pressures by a linear calibration. The results are analyzed using the Clausius-Clapeyron equation with constant ∆Hvap, or the modified equation ∆H vap,298K + ∆Cp (T − 298 K) dp dT = p RT 2
(1)
in which ∆Cp is Cp(gas) − Cp(liquid) and R is the gas constant. Procedure
Calibration A two-point calibration is made by noting the sensor reading at ambient pressure and under vacuum. The latter
pressure sensor Cajon 1
4
⬙ copper tubing
1
4
⬙ union
valve to vacuum
Swagelok T
Cajon reducing union
sample container 1 ⬙ o.d. glass 2
Figure 1. Schematic diagram of vapor pressure apparatus.
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Figure 2. Vapor pressure apparatus, shown with connector for vacuum line (right). For vapor pressure measurement, the apparatus is submerged in a constant temperature bath up to about 5 cm below the pressure sensor (top).
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In the Laboratory
can be done during the last freeze-pump-thaw cycle. We assume that the sensor’s response is linear.
Degassing About 2 mL of sample is introduced into the apparatus, which is then sealed. The vacuum line is evacuated with the valve of the apparatus closed. The sample is immersed in liquid nitrogen. Liquid N2 must not reach the metal vacuum fitting as the seal may be affected. After the sample has equilibrated with the liquid N2, the valve is opened to evacuate the space above the sample. Then the valve is closed and the sample melted by warming to near room temperature. The freeze-pump-thaw cycle is repeated twice more. The third time through, a pressure reading is taken during the pumping step as the second point in the linear calibration. Measurements With the valve closed, the apparatus is detached from the vacuum line and immersed in a constant-temperature water bath as much as possible without wetting the sensor. Some liquid may condense in the cooler parts of the apparatus, but this does not affect pressure readings. Pressure and bath temperature are recorded periodically as the temperature of the bath is slowly increased. Leak check At the conclusion of the experiment, the sample is refrozen in liquid nitrogen, with the valve still closed, and the pressure measured. The reading should not have changed by more than 1 Torr from the initial reading.
Results The pressure and temperature data for acetone are plotted in Figure 3. A linear fit to the data gave ∆Hvap = 30.9 ± 0.6 kJ mol᎑1 (95% confidence limits). The average accepted value of ∆Hvap over this temperature range is 30.4 kJ mol᎑1 (4). Discussion We still use the isoteniscope experiment as well. The present procedure leaves the vacuum line free after sample preparation, it does not require a large ballast in the vacuum line, and no sample is volatilized into the vacuum line during the procedure. Use of this apparatus perhaps gives students a more direct picture of vapor pressure than the isoteniscope method. The experience with freeze-pump-thaw cycles is useful in later photochemistry labs. Results have generally been quite accurate with various liquids, such as cyclohexane, toluene, and ethanol. If liquid nitrogen is not available, the sample could be degassed by briefly opening the valve to vacuum several times with the sample at room or dry ice temperature, but the zero of the pressure sensor would have to be measured in a separate step, with no sample present. Acknowledgments The author wishes to thank Samuel Courbis for initial testing of this method and Regina Cody and Robert Wismer for helpful discussions.
Hazards W
The ordinary hazards of handling small quantities of volatile organic solvents, liquid nitrogen, and vacuum apparatus are present. Students should be cautioned not to heat the sample above its normal boiling point.
Supplemental Material
A parts list and a schematic diagram of the sensor and amplifier are available in this issue of JCE Online. Notes
5.8
1. Available from Swagelok, http://www.swagelok.com (accessed Oct 2005) and local distributors. 2. Motorola MPX2100AP. Tolerances are: linearity ± 1% and offset ± 1 mV. Available from Allied Electronics, 1-800-433-5700 or http://www.alliedelec.com (accessed Oct 2005). 3. Analog Devices AD620, also available from Allied Electronics.
5.7
Literature Cited
6.0
ln( p / Torr)
5.9
5.6
5.5 3.18
3.20
3.22
3.24
1 T
(10
3.26
ⴚ3
K
3.28
3.30
3.32
ⴚ1
)
Figure 3. Student’s vapor pressure data for acetone from 303 K to 313 K along with a linear fit to the Clausius–Clapeyron equation. The slope of the line is ᎑3722 ± 70 K (95% confidence limits).
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1. Wismer, R. K. Laboratory Manual for Physical Chemistry; Millersville University: Millersville, PA, 1997. 2. Shoemaker, D. P.; Garland, C. W.; Steinfeld, J. I.; Nibler, J. W. Experiments in Physical Chemistry, 4th ed.; McGraw-Hill: New York, 1981; pp 197–205. 3. Motorola Semiconductor Technical Data document MPX2200/D. 4. National Institute of Standards and Technology Chemistry Webbook. http://webbook.nist.gov/chemistry (accessed Oct 2005).
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