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The phase speed of an ultrasound wave depends on the elastic properties of the material. It is a significant source of information on the molecular structure of the liquid and on physical and chemical processes occurring in the liquid phase. Speed of ultrasound measurements carried out as functions of temperature and pressure enable also the determination of pρT relationships. Moreover, the measurements of the speed of ultrasound permit the investigation of very subtle effects such as, for example, stacking effect [1] or the effect of the position of OH groups in diol isomers on the physicochemical and thermodynamic properties of the pure compounds or their mixtures [2]. However, it requires measurements of adequate high accuracy and precision [3–6]. One of the convenient methods for the determination of the internal pressure is the acoustic one, i.e. the use of speed of ultrasound in conjunction with the density, isobaric heat capacity, and isobaric expansibility. The starting point is the combination of the low-frequency speed of ultrasound and density, which yields the isentropic compressibility (the Newton–Laplace equation). Then the latter may be combined with the isobaric expansibility and the isobaric heat capacity to yield the isothermal compressibility and finally the internal pressure (more details can be found elsewhere [7–11]). The so-called direct methods of determination of internal pressure are very inconvenient. Therefore, these methods are rather rarely used.

Speed of Ultrasound and Internal Pressure of Propanediol and Butanediol Isomers under Elevated Pressures