Temperature and pressure dependence of the volume of ionization of acetic acid in water from 25 to 225°C and 1 to 3000 bars

Abstract

The standard partial molal volume change ΔV° for the ionization of acetic acid in water has been determined over the range 25-225°C and 1-3000 bars from measurements of the pressure dependence of the dissociation constant. The standard partial molal volumes and partial molal compressibilities of ionization at atmospheric pressure ΔV°1 and Δχ°1 were obtained from linear plots of [RT/(P– 1)] In (KP/K1) against (P– 1). Both ΔV° and Δχ° became more negative with increasing temperature at constant pressure, and less negative with increasing pressure at constant temperature. These changes can be correlated qualitatively with the temperature and pressure dependence of the dielectric constant of water. At constant volume, the volume of ionization and pressure becomes more negative with increasing temperature, but the isochoric temperature dependence is smaller than the isobaric one. The difference between the isobaric and isochoric standard partial molal expansibilities of ionization is considered in terms of the equation ΔE°p–ΔE°V=αΔχ°T/β.