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

pH of 10−3 mol kg−1 H2SO4 aqueous solutions at subcritical and supercritical conditions was measured at temperatures from 375.0 to 400.6 °C and pressures from 25.8 to 32.5 MPa by a flow-through electrochemical cell and it changed from 3.90 at 375.2 °C and 32.5 MPa to 6.37 at 399.8 °C and 28.1 MPa. The molal equilibrium constant for the dissociation reaction of H2SO4 was determined based on the measured pH and the value varied from 1.67×10−7 at 399.8 °C and 28.1 MPa to 4.53×10−4 at 375.2 °C and 32.5 MPa. The values were between the literature values determined from UV-Vis spectroscopy and 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.