Thermodynamic properties of aqueous sulfuric acid were modeled with the Pitzer equation. Both the second dissociation constant K 2 for sulfuric acid and the Pitzer parameters were fitted simultaneously. The most accurately considered experimental data including electrochemical cell, osmotic, enthalpy, and vapor pressure data at temperature range (0 to 170) °C were used in the assessment. After variations of the used experimental data and the temperature dependency of the Pitzer parameters, the thermodynamic values for the bisulfate dissociation in aqueous sulfuric acid at 25°C were re-evaluated. The obtained values for the dissociation reaction are (11.00 ± 0.27) and (25.11 ± 0.80) kJ·mol–1 for Gibbs energy and enthalpy change, respectively, yielding to 0.0119 for K 2. The obtained thermodynamic values for the cell reactions are in good agreement with CODATA and NBS literature values as well as with other Pitzer based thermodynamic models for aqueous sulfuric acid. The best model selected by using the obtained new thermodynamic data was further tested and compared to the recent Pitzer models with excellent agreement as well as with good extrapolating capabilities with respect to temperature and acid concentration. The total number of fitted terms in Pitzer parameters is eight.

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Calcium sulfate is one of the most common inorganic salts with a high scaling potential. The solubility of calcium sulfate was modeled with the Pitzer equation at a temperature range from 273.15 to 473.15 K from published solubility data, which was critically evaluated. Only two Pitzer parameters, β (1) and β (2) , with simple temperature dependency are required to model the solubility with excellent extrapolating capabilities up to 548.15 K. The stable temperature range for gypsum is 273.15−315.95 K, whereas above 315.95 K the stable phase is anhydrite. Hemihydrate is in the metastable phase in the whole temperature range, and the obtained metastable invariant temperature from gypsum to hemihydrate is 374.55 K. The obtained enthalpy and entropy changes at 298.15 K for the solubility reactions are in good agreement with literature values yielding solubility products of 2.40 × 10 −05 , 3.22 × 10 −05 , and 8.75 × 10 −05 for gypsum, anhydrite, and hemihydrate, respectively. The obtained Pitzer model for the CaSO 4 −H 2 O system is capable of predicting the independent activity and osmotic coefficient data with experimental accuracy. The mean absolute average error of activity coefficient data at 298.15 K is less than 2.2%. Our model predicts the osmotic coefficient on the ice curve within 1.5% maximum error.

A simple model for thermodynamic properties of water from subzero temperatures up to 373 K was derived at ambient pressure. The heat capacity of supercooled water was assessed as lambda transition. The obtained properties for supercooled water such as heat capacity, vapor pressure, density and thermal expansion are in excellent agreement with literature data. Activity of water on ice curve, independent of used electrolyte and Debye−Hückel constant applied in modeling, is also calculated. Thus, the ice curve activity of supercooled water can be used as a universal basis for thermodynamic modeling of aqueous solutions, precipitating hydrated and anhydrous solids. A simple model for heat capacity, density and thermal expansion of ice are also derived from 170 K up to melting point.