Acoustic determination of phase boundaries and critical points of gases: Carbon dioxide, carbon dioxide-ethane mixture, and ethane

The acoustic resonance technique developed in this laboratory is a remarkable tool for the determination of the thermophysical properties of materials. The measured frequencies of radially symmetric modes of vibration are simply related to the sonic speed in fluids enclosed in the spherical cavity. To minimize experimental perturbations a relative measurement against argon gas used as a standard reference was employed. The sonic speed in turn is utilized as a sensitive probe for detecting phase boundaries and critical points of substances. Location of the boundary is revealed by a discontinuity in the variation of sonic speed with temperature for a sample confined isochorically in the resonator. The corresponding temperature at which this discontinuity occurs was taken as a phase boundary temperature. This indicator is very pronounced, especially when the system is near the critical condition for which strong local density fluctuations disperse sound and cause the sonic speed to dip sharply toward zero. This research reports on such studies for two pure gases CO{dollar}sb2{dollar}, and C{dollar}sb2{dollar}H{dollar}sb6{dollar} and one mixture of these two. For each system, data on resonance frequency, amplitude and pressure as a function of temperature were collected under computer control for a series of different loading densities around the critical value. Portions of the phase diagrams revealed by the measurements were recorded in tabular and graphical formats.