Pressure-volume-temperature behavior of carbon tetrafluoride using a variable-volume cell of bellows design

Abstract

This paper presents data observed for the pressure-volume-temperature (P-V-T) behavior of carbon tetrafluoride (CF 4 ) below room temperature, and uses these data in conjunction with higher temperature data from the literature to evaluate an equation of state. The P-V-T measurements were made via a variable-volume technique without using a confining liquid. The variable-volume design makes it possible to cover a range of specific volumes with a single charge, resulting in an efficient operating procedure. A unique bellows design with a 14-fold volume variation was developed. The bellows contained the compound under study (CF 4 ) and was surrounded by a hydraulic liquid whose pressure could be varied causing the bellows to expand or contract. The volume of the bellows was determined by measuring the volume of the hydraulic liquid. Calibrations were made using a gas (CO 2 ) of known thermodynamic properties. Pressure measurements were made via a differential pressure transducer on three Heise gauges calibrated in place against a Ruska dead-weight tester. The bellows cell was maintained in a well-agitated bath whose temperature was measured by a platinum resistance thermometer. The data were taken by charging a known amount of the compound into the bellows and observing the pressure-volume relationship at a fixed bath temperature. At the end of a run, the charge was recovered, the amount recovered agreeing within 0.01% with the amount charged. The observed data were analyzed graphically on pressure-volume, pressure-temperature, and compressibility diagrams. Very good consistency was observed with the higher temperature data from literature. The data were correlated analytically with the Martin equation of state. This equation represents all the data with an average deviation of 0.25%.