The inorganic element of Model III is an an equilibrium thermodynamic model of the system H+ - NH4+ - Na+ - SO42− - NO3− - Cl− - H2O. The model is valid for 298.15 K only, and includes the following species:
(Note that NaH3(SO4)2 · H2O and NH4NO3 · 2NaNO3 are apparently metastable phases that have been observed by some workers, but are not included in the model on this website.)
See the model description page for an explanation of how organic compounds, and an additional liquid phase, are included in the chemical system.
The development of the model is described in the following paper:
S. L. Clegg, P. Brimblecombe, and A. S. Wexler (1998) A thermodynamic model of the system H+ - NH4+ - Na+ - SO42− - NO3− - Cl− - H2O at 298.15 K, J. Phys. Chem. A 102, 2155-2171.
Abstract: A multicomponent mole-fraction-based thermodynamic model of the H - NH4 - Na - SO4 - NO3 - Cl - H2O system is used to represent aqueous phase activities, equilibrium partial pressures (of H2O, HNO3, HCl, NH3 and H2SO4), and saturation with respect to nineteen solid phases. The model is valid for concentrations from infinite dilution to saturation (with respect to the solid phases), and to about 40 mol per kg for acid sulphate systems which can remain liquid to concentrations approaching the pure acid. Parameters for H2SO4 -H2O interactions were adopted from a previous study, and values for other binary (water-electrolyte) and ternary (water and three ions) interactions were determined from extensive literature data for salt solubilities, electromotive forces, osmotic coefficients and vapour pressures. The model is compared with solubility measurements for the quaternary systems H - Na - SO4 - Cl - H2O and NH4 - Na - SO4 - Cl - H2O.
Since the paper was published the thermodynamic model has been extended as follows:
The above extension is also present in Models I and II.
We have added aqueous NH3 to the model for all calculations except the "Simple" type. This allows systems that are alkaline to be treated – those in which the total ammonia present (NH4+ + NH3) is only partially neutralised by H+. However, the model is not intended to be applied to systems containing high concentrations of aqueous NH3 relative to other dissolved solutes.
The Gibbs energies of formation of the NH3(g) and NH3(aq) species are based on the Henry's law constant and acid dissociation constants used by Clegg and Brimblecombe (1989) (J. Phys. Chem. 93, 7237-7248). The dissociation of water in the aqueous phase is also modelled for some compositions. The activity coefficient of the OH− produced by the dissociation is at present estimated using only the Debye-Huckel limiting law expression in E-AIM. Gas liquid partitioning is only affected by this reaction at pH very close to neutral. The Gibbs energy of formation of the OH− ion was calculated using standard values of the equilibrium constant, and ΔH° and ΔCp° for the reaction.
In E-AIM the molality-based activity coefficient of dissolved NH3 is assumed to be unity at all concentrations and temperatures. The known "salting-out" influence of ions such as SO42− (sulphate) and Na+ (sodium) on aqueous NH3 has not yet been included in the model. This means that, for non-acidic systems in which the concentration of NH3 is significant relative to that of NH4+, the equilibrium vapour pressures of ammonia above concentrated solutions will be too low – by as much as 60% for an ammonium sulphate solution at 80% relative humidity and room temperature, for example. However, the effect was only seen at H+ concentrations below about 10-5 mol kg-1. For more acid systems, in which the dissociation of NH4+ is negligible, the lack of salting out parameters does not affect the accuracy of equilibrium pNH3 predictions or calculated ammonia partitioning.
See Clegg and Brimblecombe (1989) for a discussion of the available data, calculations of ammonia partial pressures and the acid dissociation of the NH4+ ion, and an activity coefficient treatment using the Pitzer molality-based model.