Model I: Summary and Abstract

Summary

The inorganic element of Model I is an equilibrium thermodynamic model of the system H⁺ - SO4²⁻ - NO3⁻ - Cl⁻ - Br⁻ - H2O which is valid from 328 K to about 200 K and includes the following species:

• Liquid phase:  water and the ions H⁺, SO4²⁻, NO3⁻, Cl⁻, and Br⁻.
• Gases:  water vapour and HNO3, HCl, HBr, and H2SO4.
• Solids:  H2O (ice), H2SO4 · H2O, H2SO4 · 2H2O, H2SO4 · 3H2O, H2SO4 · 4H2O, H2SO4 · 6.5H2O, HNO3 · H2O, HNO3 · 2H2O, HNO3 · 3H2O, and HCl · 3H2O.

See the model description page for an explanation of how organic compounds, and an additional liquid phase, are included in the chemical system.

Publications

The development of the model is described in the following paper:

Carslaw K. S., Clegg S. L. and Brimblecombe P. (1995) A thermodynamic model of the system HCl - HNO3 - H2SO4 - H2O, including solubilities of HBr, from <200 K to 328 K. J. Phys. Chem. 99, 11557-11574.

Abstract: a multicomponent mole-fraction-based thermodynamic model, together with Henry's law constants and the vapour pressure of pure water, is used to represent aqueous phase activities, vapour pressures (of H2O, HNO3, HCl, HBr) and saturation with respect to solid phases (ice, H2SO4 · nH2O, and HNO3 · nH2O) in the system HCl - HBr - HNO3 - H2SO4 - H2O.
The model is valid from 330 K to <200 K, and up to approximately 40 mol per kg total solute molality for solutions containing mainly H2SO4 and HNO3. Model parameters for pure aqueous H2SO4 were adopted from a previous study, and values for HNO3 - H2O, HCl - H2O and HBr - H2O obtained by fitting to activity and osmotic coefficients, electromotive force (emf) measurements, vapour pressures, freezing points and thermal (enthalpy and heat capacity) data.
The model was tested using measured partial pressures and solubilities of HCl in aqueous H2SO4 from 330 K to 200 K, HBr solubilities in aqueous H2SO4 from about 240 K to 205 K, and HNO3 partial pressures and freezing points for HNO3 - H2SO4 - H2O mixtures from 273.15 K to <200 K. Ternary (mixture) parameters were required only for HNO3 - H2SO4 - H2O. Solubilities of HNO3, HCl and HBr in liquid stratospheric aerosols are calculated.

Revisions

Since the above paper was published the inorganic thermodynamic model has been revised and extended as follows:

• The solid HNO3 · 2H2O has been added, and the prediction of HBr solubilities in aqueous H2SO4 improved, as described by Massucci et al. (J. Phys. Chem. A 103, 4209-4226, 1999).
• Vapour-liquid partitioning of H2SO4 has been added, see Wexler and Clegg (J. Geophys. Res. 107, No. D14, art. no. 4207, 2002).
• The prediction of HBr solubilities in aqueous H2SO4 has been further revised to improve predictions for H2SO4 concentrations less than about 60 wt%, see Wexler and Clegg (J. Geophys. Res. 107, No. D14, art. no. 4207, 2002).