Pitzer and Chen

As suggested by Pitzer, the dielectric constant used in calculating the Debye-Huckel constant is depended upon to reflect the effect of temperature on the activity coefficients. Either of the following methods may be used  

2) Use Chen s equation for calculating A, regressed to fit the A s calculated using Pitzer s dielectric constant  

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The methods for calculating activity coefficients have, up to this point, been presented for solutions at 25 C. This is due to the fact that most experimental data available, used for determining the various parameters of the acitivity coefficient models, was measured at 25°C. Recognizing that activity coefficients can be strongly affected by temperature, Bromley, Meissner, Pitzer and Chen have suggested methods of adapting their models and parameters to any temperature solution. 

In order to test Bromley. Meissner, Pitzer and Chen s methods for calculating activity coefficients, the models were coded on the HP-85 and HP-87 desktop computers. The calculated values for various electrolytes were plotted against smoothed experimental data published by the National Bureau of Standards and others. In the first plot for each electrolyte, the maximum molality to which they were plotted is the maximum molality of the published parameters. For some of the electrolytes, when there was experimental data available, the maximum molality was extended on a second plot. These plots illustrate the wide deviation from experimental data that may occur when using the published parameters for solutions with ionic strengths greater than the noted maximum molality. 

As mentioned earlier, most experimental data is measured at 25 C. This limits the applicability of the published parameters for the activity coefficient models discussed. The foDowing pages compare available experimental data and the activity coefficients calculated using Bromley, Meissner, Pitzer and Chen s models at temperatures other that 25"C. 

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