Speed-of-Sound Measurements and Derived Thermodynamic Properties of Liquid Isobutane

Abstract

This article reports comprehensive and accurate measurements of the speed of sound in liquid isobutane. The measurements were carried out by a double-path-length pulse-echo technique and cover the temperature range between 200 and 420 K with pressures of up to 100 MPa. The expanded measurement uncertainties (at the 0.95 confidence level) are 2.1 mK for temperature, 0.007% for pressure, and 0.009% for the speed of sound with the exception of a few state points at low pressures and in the vicinity of the critical point, where it increases up to 0.035%. Furthermore, densities and specific isobaric and isochoric heat capacities were derived from the speed-of-sound data in the temperature range between 200 and 340 K and up to 100 MPa by the method of thermodynamic integration. Very accurate results for the derived properties were obtained by determining values of the isobaric heat capacity on the initial isobar for the integration by a well-known thermodynamic relation among the isobaric heat capacity, density...