Data quantifies water’s ionization behavior in near- and supercritical conditions

Water exhibits unusual properties that can change drastically in near-critical conditions, for example, the self-ionization of pure water. A better understanding of the self-ionization behavior over a wide range of conditions is crucial but difficult to accomplish. Arcis et al. report a direct measurement of water’s ionization constant under conditions approaching its critical point at 374 degrees Celsius and 22 MPa.

“The major contribution of this work is to fill an important knowledge gap at near-critical and supercritical conditions for water’s ionization constant, where accurate data have been extremely scarce,” said author Hugues Arcis. “The conditions are extremely aggressive, resulting in exceptional experimental challenges.”

In a study of water’s near-critical ionization behavior, the authors found large discrepancies near the critical point compared to models recommended by the International Association for the Properties of Water and Steam, which mostly utilize data from the subcritical density region. They used their results to revise and update the models’ parameters and improved the near-critical and supercritical low-density predictions.

To obtain these results, they studied the impedance spectra of very pure water under flow at high temperatures and pressures. Based on pioneering work by R.H. Wood, the experimental design precisely controls flow rate, pressure and uniform temperature, allowing for very accurate sub-critical and supercritical conductivity measurements.

“This work adds a significant new understanding to a key aspect of our physical world,” Arcis said. “The new results presented here provide more accurate data that will help scientists and engineers improve their ability to understand and quantitatively model aqueous systems at extremes of temperature and pressure.”

Source: “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,” by Hugues Arcis, Jane P. Ferguson, Jenny S. Cox, and Peter R. Tremaine, Journal of Physical and Chemical Reference Data (2020). The article can be accessed at https://doi.org/10.1063/1.5127662 .