Critical evaluation and development of thermodynamic models for ionization of H2O, HCl and NaCl at high temperatures and pressures

Ionization of water and speciation of aqueous electrolytes exerts important control on chemical composition, acid-base interaction, and fluid-mineral equilibria in hydrothermal systems. Several competing approaches are available to describe H₂O, HCl and NaCl speciation under hydrothermal conditions: (i) electrostatic models using the Born theory (Tanger and Helgeson 1988; Shock et al. 1992, Djamali and Cobble 2009), (ii) semi-empirical correlations with H₂O density (Marshall and Franck 1981, Mesmer et al. 1988, Holland and Powell 1998); (iii) hydration models (Pitzer 1982; Tanger and Pitzer 1989) and (iv) diverse theoretical models (e.g., Akinfiev and Diamond 2003, Lvov et al. 2018). Available models were compared to experimental data (n = 606) covering temperature from 0 to 800 °C and pressure of 1 to 8000 bar, with several measurements up to 1000 °C and 133 kbar. Various approaches consistently and accurately reproduce the experimental H₂O ionization up to 400 °C and above 300 bar. By contrast, at higher temperatures or lower pressures the models substantially diverge and reveal conceptual deficiencies. For HCl dissociation, the electrostatic, density and virial models are accurate to 400 °C and 1500 bar, at higher temperatures remain consistent but rapidly inaccurate and at higher pressures they diverge. This critical evaluation identifies density models as most promising for formulating new equation of state for ionic species. Here we combine individual thermodynamic contributions representing intrinsic species properties, mechanical and electrostatic solute-solvent interaction during hydration, and standard-state conversion term. We use the experimental datasets for H₂O, HCl and NaCl ionization to evaluate the performance and accuracy of optional functional forms for heat capacity, hydration compression and its coupling to the standard-state conversion term. Our preliminary results indicate that intrinsic enthalpy, entropy and volume coupled with heat capacity linearly dependent on temperature, compression and standard-state conversion terms, without involvement of electrostatic contribution or chemical hydration afford the most accurate description of thermodynamic properties of ionic species. This model provides basis for accurate prediction and modeling of mineral-fluid and melt-fluid equilibria in upper-crustal magmatic and hydrothermal systems.

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