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

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, and speed of sound from very accurate density data at low pressures of (Glos, S.; Kleinrahm, R; Wagner, W. J. Chem. Thermodyn. 2004, 36, 1037-1059) and our speed-of-sound data. Moreover, these initial values were manually adjusted to enforce physically correct behavior of the calculated derivatives of the equation of state. Comparisons of the experimental speeds of sound and derived properties with the Helmholtz energy formulation of Bucker and Wagner for isobutane (Bucker, D.; Wagner, W. J. Phys. Chem. Ref. Data 2006, 3.5, 929-1019) and data from the literature demonstrate the high accuracy of the results and reveal potential to improve the formulation.