We present fully traceable two-parameter Hückel equations (with parameters B and b 1) for the activity coefficient of sodium chloride and for the osmotic coefficient of water in aqueous NaCl solutions at temperatures from (0 to 80) °C. These equations apply within experimental error to all thermodynamic data available for these solutions at least up a molality of 0.2 mol·kg–1. In our previous study (J. Chem. Eng. Data 2016, 61, 286–306), these equations were successfully tested against the literature results of electrochemical, isopiestic, and cryoscopic measurements usually in the temperature range from (0 to 25) °C. There, a constant value was employed for B, whereas a linear model with respect to the temperature was utilized for b 1. The linear model was determined from the values of b 1 at 0 °C and at 25 °C obtained from freezing-point depression data and from isopiestic and cell-potential difference data, respectively. In the present study, these two b 1 values are utilized alongside the constant value of parameter B but a new quadratic model is presented for the temperature dependence of b 1. The third data point required for this model is obtained from the direct vapor pressure measurements of Gibbard et al. (J. Chem. Eng. Data 1974, 19, 281–288) at 75 °C. The results obtained with this quadratic equation for b 1 agree well with the test results of the linear model in the previous paper (see the citation above) up to 25 °C. The most important new test results above that temperature are reported here. Our quadratic model has additionally been tested with all the high-precision calorimetric data available in the literature for NaCl solutions. In this first part (Part 1) of the study, the test results from the thermodynamic quantities associated with partial molar enthalpy are reported. In the forthcoming second part (Part 2) of the study, the results of the quantities associated with the heat capacity of NaCl solutions will be considered. In the tests of these two parts, all calculations dealing with calorimetric data are performed in a new way. Both the calorimetric data and the vapor pressure data (from both direct and isopiestic measurements) can be predicted using the new Hückel equations within experimental error in dilute NaCl solutions from (0 to 80) °C. For comparison, also other Hückel models are considered and at best these apply up to the molality of the saturated NaCl solution at various temperatures. Following the success of the new models, new values for the activity coefficients, osmotic coefficients, relative apparent molar enthalpies, and relative partial molar enthalpies for NaCl solutions at rounded molalities are reported at the end of this Article. We have good reasons to believe that the new values contain the most reliable ones available for the given thermodynamic quantities.

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A novel calculation method is presented in this work to evaluate the ion interaction parameters for the Pitzer model from freezing points of aqueous solutions of pure electrolytes. The freezing-point depression data from aqueous solutions of sodium chloride and potassium chloride are used to epitomize this method. The parameter values obtained using this method predict more accurately the experimental data than the most often used values for the Pitzer method up to a molality of 1.5 mol·kg–1. The parameters obtained by the freezing-point technique are associated with the solutions which are below the temperature of 273.15 K. It is verified in the present study that the temperature dependences of these parameters between the two temperatures are, however, practically negligible at molalities less than 0.5 mol·kg–1. In addition to the freezing-point data existing in the literature, the validity of the parameters was also studied using the literature results from concentration cells without transference.

We present fully traceable two-parameter Huckel equations with parameters B and b 1 for the activity coefficient of sodium chloride and for the osmotic coefficient of water in aqueous NaCl solutions at temperatures from (353.15 to 383.15) K. In our most successful parametrization of these equations, parameter B is treated as a constant whereas b 1 is a quadratic function of the temperature. The new calculations extend the tables presented up to 353.15 K in our previous study (

In Part 1 of this two-part study (J. Chem. Eng. Data 2017, 62, 2617–2632), we presented fully traceable two-parameter Hückel equations (with parameters B and b 1) for the activity and osmotic coefficients in dilute aqueous NaCl solutions in the temperature range (0 to 80) °C, and these equations apply within experimental error to almost all thermodynamic data existing in the literature and used in the tests at least up to a molality of 0.2 mol·kg–1. These data include also molar enthalpies of the components in the solutions. In our model, parameter B is treated as a constant whereas parameter b 1 depends in a quadratic way on the temperature. No calorimetric data were used in the parameter estimation of the model. In this second part (Part 2) of the study, the results of the quantities associated with the heat capacity of NaCl solutions are considered. All heat capacity data available for NaCl solutions at least up to 0.2 mol·kg–1 can be predicted within experimental error using the same Hückel equations as those determined in Part 1 in dilute NaCl solutions from (0 to 80) °C. For comparison, also other parametrization (obtained in Part 1 for higher temperatures) was here considered and it applies better to less dilute solutions in the higher temperatures than the model recommended now primarily. Following the success of the new models, we supplement the thermodynamic tables of Part 1 with the relative apparent and partial molar hear capacities for NaCl solutions. We have good reason to believe that the new tables contain the most reliable values available for the heat capacity quantities in dilute NaCl solutions.