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.