Speed-of-sound measurements in liquid n-heptane and 2,2,4-trimethylpentane (isooctane)

This paper reports comprehensive and accurate measurements of the speed of sound in liquid n -heptane and 2,2,4-trimethylpentane (isooctane). The measurements were carried out by a double-path-length pulse-echo technique and cover the temperature range between 200 K and 420 K with pressures up to 100 MPa. The expanded uncertainties (coverage factor k = 2 ) amount to 2.1 mK in temperature, 0.005% in pressure, 0.02% in speed of sound in n -heptane, and 0.015% in speed of sound in isooctane, with the exception of a few state points at low pressures, where it increases up to 0.03% for n -heptane and up to 0.035% for isooctane. Our data are more accurate than previously published data for both fluids. The measurements for isooctane extend the range in which the speed of sound had been measured before from 293 K down to 200 K and from 373 K up to 420 K. We also provide accurate correlations for the speed of sound as a function of temperature and pressure in the range of our measurements. Our data can contribute to developing new, more accurate equations of state for both fluids. • Speed-of-sound measurements in liquid n -heptane and isooctane were carried out. • The measurements cover temperatures from 200 K to 420 K with pressures up to 100 MPa. • The uncertainty in speed of sound is 0.02% for n-heptane and 0.015% for isooctane. • Correlations, which represent the data within their uncertainties, are provided.