Abstract
Monte Carlo (MC) simulations were used to calculate single ion and mean ionic activity coefficients and water activity in concentrated electrolytes and at elevated temperatures. By using a concentration dependent dielectric constant, the applicability range of the MC method was extended to 3 mol·L−1 or beyond, depending on the salt. The calculated activity coefficients were fitted to experimental data by adjusting only one parameter, i.e., the cation radius. Fitted ionic radii obtained by such a procedure indicate the extent of cation–anion interaction in a salt solution. For example, the fitted radii of Li+ and Na+ in LiClO3 and NaClO3 indicate that Li+ is strongly hydrated and has a weak interaction with the ClO3− ion whereas Na+ forms ion pairs and loses its hydration. The single ion activity coefficients for protons and chloride ions in HCl were calculated by MC simulations and compared with experimental values obtained by ion selective electrodes. The calculated single ion activity coefficients for protons and chloride ions are much lower and higher, respectively, than the experimental values. However, the mean activity coefficients of HCl obtained by the MC simulations, ion selective electrodes and vapor pressure measurements are in good agreement. In the case of NaCl and KCl the calculated single ion activity coefficients of Na+, K+, and Cl− are much closer to the values obtained by ion selective electrodes. The results in HCl indicate that the hydrated proton is large and includes the chloride ion within the hydration shell, i.e., the apparent size of the chloride ion is negligible.
Highlights
In many industrial processes concentrated salt solutions are of absolute necessity
The results reported in this study are based on long simulations, i.e., 95 million Monte Carlo (MC) configurations were performed to generate the thermodynamic data
The fitting procedure adopted in this study, i.e., the best fitting of calculated activity or osmotic coefficients to the experimental data by adjusting the cation radius while keeping the anion radius fixed at crystallographic value, has its merits in experimental evidence
Summary
In many industrial processes concentrated salt solutions are of absolute necessity. One such process is the production of sodium chlorate that requires concentrated salt mixtures up to 6 mol·L−1 [1]. In order to understand the reactions occurring in these concentrated electrolytes, basic thermodynamic data such as activity and osmotic coefficients are needed. Such data can be acquired experimentally or through calculations. The best system of equations available at present is the Pitzer model by which activity and osmotic coefficients of single as well as mixed salts at different temperatures can be modelled up to ionic strengths as high as 6 mol·L−1 [3]. The Pitzer model is very useful it requires quite a number of fitting parameters, physical transparency of the fitted parameters can be lost
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