The Ionization Constant of Water at Elevated Temperatures and Pressures: New Data from Direct Conductivity Measurements and Revised Formulations from T = 273 K to 674 K and p = 0.1 MPa to 31 MPa

Experimental values for the ionization constant of water, pKw,m, from T = 373 K to T = 674 K and from p = 5.75 MPa to p = 31.15 MPa, have been derived from direct measurements of the electrical conductivity of very pure water at the University of Guelph, the University of Delaware, and the Oak Ridge National Laboratory using high-precision high-temperature flow-through AC electrical conductance instruments based on the design by Wood and co-workers [J. Phys. Chem. 99, 11612 (1995)]. The results compare well with published high-temperature potentiometric and calorimetric studies up to 573 K and are consistent with the 1981 and 2006 IAPWS (International Association for the Properties of Water and Steam) pKw,m formulations to within better than 0.1 pK units up to 598 K and to better than 0.2 pK units at 623 K. Above 623 K, the 2006 and 1981 IAPWS formulations showed systematic deviations from the new results, which reached two and five orders of magnitude near the critical point, respectively. Based on these conductivity studies and critically evaluated literature data, revised parameters for the Marshall–Franck and Bandura–Lvov equations of state are reported, which reproduce the experimental data with standard uncertainties u(pK) = 0.018 and u(pK) = 0.016, respectively, over the experimental temperature range at water densities from 1.00 g cm−3 to 0.20 g cm−3, which corresponds to T = 373 K–674 K from psat to p = 31 MPa, and over the range T = 273 K–373 K at p = 100 kPa. These new experimental conductivity results are the most accurate values to be reported under near-critical conditions for densities between 0.50 g cm−3 and 0.20 g cm−3.