Thermodynamic Properties of Ion-Pairs

From the thermodynamic equilibrium constant, the estimate of the standard molar free energy of ion-pair formation is sanctioned by the well known relationship  

The heat of dissociation of an ion-pair may be experimentally estimated calorimetri-cally from the heat of dilution. This method presents some difficulties because at very high dilution it needs extrapolation procedures, while at higher concentrations, it is hard to distinguish electrostatic and pairing effects. A more straightforward method is study of the van t Hoff plot that represents the expression below that follows from Equation 2.25  

Ion pairing is also predicted to produce a positive volume change since the sum of the standard partial molar volumes of the free ions is lower than the standard partial molar volumes of the ion-pair. The difference between these two qnantities can be obtained by the pressure derivative of (corrected for isothermal compressibility if 

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