Electrostatic Theory of Ion Association

The mass action law is applicable to the equilibrium between the free and associated ions and the association constant K is 

Here a is the fraction of the ions that are free with activity coefficients and 1 - a is 

If the ion pair is charged (e.g MgC or NaSO ), may be estimated according to In Xip = (l+B af ) but if it is neutral (e.g., Csl or MgSO ), it has traditionally been ignored, that is, set equal to unity. This is incorrect, however, and of neutral ion pairs can be estimated according to two approaches on the assumption that the ion pairs are salted-out by the free ions or of them to be zwitterions as shown by Marcus [30]. 

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