Revised Parameters for the IAPWS Formulation for the Ionization Constant of Water Over a Wide Range of Temperatures and Densities, Including Near-Critical Conditions

The literature database for the ionization constant of water, pKw, has been critically reevaluated to include new accurate flow conductivity data recently reported at near-critical and supercritical conditions. Recently published equations to express the limiting conductivity of fully ionized water were used to correct the conductivity data and yield more accurate pKw values at water densities below 0.6 g cm−3. The ability of the functional forms adopted by the 1980 and 2006 International Association for the Properties of Water and Steam releases to fit the near-critical and supercritical data was tested. Revised parameters for the 2006 “simple” function were derived to improve the accuracy of the model under these conditions. The data fitting procedure made use of estimated standard uncertainties as well as a weighting parameter for each dataset to minimize potential bias due to the very large amount of flow conductivity data now available. Calculations based on the revised formulation were found to be consistent with independent high-temperature data measured using calorimetry and density methods. The revised equation is accurate to within the estimated standard uncertainty limits over the range 0–1000 °C, p = 0–1000 MPa.