The Experimental Determination of Activity Coefficients for Electrolytes

Having obtained the solvent vapor pressure, one may immediately relate it to the activity of the electrolyte using the Gibbs-Duhem relationship. Thus, 

At this point, it is convenient to introduce a quantity called the osmotic coefficient, (p. It is defined according to the equation 

One often finds values of p tabulated for electrolyte solutions as well as the mean activity coefficient y [Gl, G5]. Taking the total derivative of equation (3.7.3), one obtains 

Remembering that both y and p go to unity as goes to zero, one obtains 

This relationship is used to calculate mean ionic activity coefficients using values of the osmotic coefficient, cp. Alternatively, one may calculate the osmotic coefficient given the mean ionic activity coefficient. From equations (3.7.2) and (3.7.3) 

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