In the Laboratory
Vapor Pressure Plus: An Experiment for Studying Phase Equilibria in Water, with Observation of Supercooling, Spontaneous Freezing, and the Triple Point Joel Tellinghuisen Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235
[email protected] Vapor pressure is an important thermochemical property of substances, and its measurement features prominently in the physical chemistry and general chemistry laboratory curricula (1-3). A recent article in this Journal notes 28 contributions on the measurement of vapor pressure (VP) and reviews the methods typically used in the teaching laboratory (4). Here I describe an undergraduate physical chemistry laboratory experiment that includes all the essentials of existing VP experiments plus much more. In this new experiment, the triple point (TP) of water is observed in the course of precise measurements of the pressure- and temperature-dependence of solid-vapor and liquidvapor equilibria. Cooling is achieved by vacuum pumping the sample, permitting observation of supercooling (sometimes to below -15 °C) followed by spontaneous freezing. The pressuretemperature data are collected on warm-up, using a calibrated thermistor and a capacitance manometer pressure gauge. Analysis of the data typically yields ΔsubH and ΔvapH within ∼1 kJ/ mol (2%) of accepted values, permitting fairly reliable determination of the small difference, ΔfusH = ΔsubH - ΔvapH (5). We have on occasion observed TP halts lasting several minutes, during which temperature held constant within ∼0.03 K and pressure within 0.05 torr (TP P = 4.58 torr; 1 torr = 133.3 Pa). The experiment can be described in terms of the typical pure-substance phase diagram used to discuss phase transitions in general chemistry and physical chemistry courses (Figure 1) (6). Starting at -20 °C on the solid-vapor equilibrium line, the system moves up in temperature and pressure with time, reaching the triple point in ∼15 min. At the TP, three phases are present at equilibrium, making this an invariant point. Thus temperature and pressure cannot change until one phase is eliminated;here the solid phase, which melts completely in ∼10 min. Then the system continues up the liquid-vapor line, slowing as temperature approaches the ambient room temperature. Except at the TP, temperature changes slowly with time on warm-up, so the collected data are called “pseudo-equilibrium”. However, vaporsolid and vapor-liquid equilibria obtain rapidly for most pure substances and the good agreement with literature vapor and sublimation pressure values confirms that such data are adequately treated as equilibrium data. The apparatus described here is the evolutionary product of design variations, mostly concerned with accommodating the thermistor in the sample cell in a way that yields reliable
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temperature measurements of samples of near-uniform temperature. With the present cell design, students can obtain all their data within 1.5 h, leaving sufficient time for data analysis and submission of final results within a single lab period. This sample cell requires the skill of a glassblower for fabrication; but an alternative design does not (discussed below). In our laboratory setup, the glass vacuum manifolds of two setups are evacuated with a single mechanical vacuum pump (Figure 2). Pressures are measured with capacitance manometer pressure gauges and temperatures with thermistors, the data being recorded every few seconds on a microcomputer. A single prototype of our system costs ∼$3000 to construct, largely from the pressure gauge and the transducer interface. However, the equipment is robust and has served ∼700 students over a 12-year period, with little additional expense. Much cheaper pressure transducers and analog-to-digital converters are available (3, 4), with which one should be able to prepare a setup for less than half our cost. Experiment The Laboratory Course Students in this laboratory course work in teams of three students and do six experiments during the semester. The lab is operated on the station method, with students using an online program to book experiments. This approach reduces labor in setup and takedown of apparatus, since most stations are in place for the full semester. Importantly, it also saves equipment costs, permitting us to use just two vacuum setups. The Cell The heart of the system is the borosilicate glass cell, diagrammed in Figure 3. A variation of this design includes another O-ring seal in the barrel of the large-diameter tube, permitting easy disassembly for drying if the sample bumps during pumpdown. Such bumping terminates a run because it distributes water over the cell walls, where its temperature is variable and no longer measured by the thermistor. However, when our current procedures are followed, bumping is a rare occurrence, so we prefer the simpler one-piece design shown in Figure 3. The O-ring seals are very effective, holding vacuum to better than 1 torr for several days.
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r 2010 American Chemical Society and Division of Chemical Education, Inc. pubs.acs.org/jchemeduc Vol. 87 No. 6 June 2010 10.1021/ed100138j Published on Web 04/01/2010
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Journal of Chemical Education
619
In the Laboratory
Figure 1. Phase diagram of water in the region of solid-vapor, solidliquid-vapor, and liquid-vapor equilibria studied in the experiment.
Figure 3. Diagram of borosilicate sample cell. The thermistor (glass cladded, 2 mm o.d.) is fed up through the 5 mm tube and glued into place in the tip of the sample receptacle using thermally conductive epoxy (Omega). With the cell removed from the vacuum manifold, sample (1 mL) is pipetted into the receptacle through the O-ring joint at top. The cell is then reattached to the manifold at the O-ring joint.
data for the thermistors (calibration precision ∼0.05 K). Details can be found in the supporting information. Procedures
Figure 2. The vacuum manifold showing one experimental setup: valve A is opened to permit evacuation of the system, but must be closed during all equilibrium measurements, including leak rate checks; valve B must be open during all equilibrium measurements (valves A an B are opened and closed stepwise in the initial pump-down procedure); and valve C will normally remain closed throughout the experiments; it is used to bleed air into the system and sometimes to connect other equipment, including absolute pressure gauges.
The cells and complete system can be seen in a PowerPoint document available on the Web (7) and in the supporting information to this article, both of which include a description of a alternative cell that can be constructed without the expert skills of a glassblower. Other Equipment Pressures are measured with an MKS Baratron capacitance manometer (current model equivalent 622A, cost ∼$1000). Temperatures are read with thermistors small enough to fit snugly into the receptacle tip in the cell. The pressure and temperature data are recorded on a microcomputer, using a LabWorks II interface (SCI Technologies,
