Determination of acetic acid dissociation constants to 400°C and 32 MPa by potentiometric pH measurements

pH of 0.01 mol.kg(-1) CH3COOH aqueous solutions at subcritical and supercritical conditions was measured at temperatures from (21.8 to 400.4) degreesC and pressures from (23.0 to 32.3) MPa by a flow-through electrochemical cell, and it changed from 3.38 at 21.8 degreesC and 23.0 MPa to 6.90 at 400.4 degreesC and 28.1 MPa. The molal equilibrium constant for the dissociation reaction of CH3COOH was determined on the basis of the measured pH, and the value varied from 4.29 x 10(-10) at 400.4 degreesC and 28.1 MPa to 1.79 x 10(-5) at 21.8 degreesC and 23.0 MPa. At subcritical conditions, the values showed good agreement with the literature values determined from potentiometry with a static cell, from calorimetry, and from electrical conductivity. An empirical density model for the description of the temperature and pressure dependence of the equilibrium constants was used to correlate the data.