The Eh-pH Diagram for N-H₂O System at 623 K Calculated by the Revised HKF Equation

The Eh-pH diagram (Pourbaix diagram) for N-H₂O system was constructed at 623 K based on the revised HKF (H C. Helgeson, D. H. Kirkham and G. C. Flowers) equation. The fundamental thermodynamic data such as Cp° and ΔG° were derived from the HKF equation directly for ionic species, and from the combination of HKF equation and the Electrostatic Non-Electrostatic Model (H. C. Helgeson) for neutral species. Comparing the Eh-pH diagram at 623 K with that at 298 K, all boundaries between gaseous species were found to slant to the lower E with increasing pH. The stable regions of neutral species, i.e. NH₄OH(aq) and HNO₃(aq), expanded toward the neutral pH at 623 K, possibly because of the formation of ion pairs.