Activity Coefficients of Neutral Ion Pairs

1 Activity Coefficients of Neutral Ion Pairs Neutral ion pairs can be treated as if they were nonelectrolytes being salted-out by the free ions of the electrolyte. For the salting-out of nonelectrolytes (Section 7.3) according to McDevit and Long [31] (see Section 7.3.2), the treatment requires the isothermal compressibility of the solvent Kj, the intrinsic volume of the electrolyte V(. its standard partial molar volume y , and the latter quantity of the nonelectrolyte (ion pair) V . The value of can be estimated from values for the ions tabulated by Marcus et al. [32] and in Table 2.8 and are independent of the solvent, and values are known for many salts in many solvents (Table 4.6). The value of 17 must be estimated from the dimensions of the ions and may be assumed to be equal to for the symmetrical ion pair. Then its activity coefficient becomes according to Marcus [30]  

Contrary to most studies of ion pairing that ignore deviations of from unity, a recent study by Marcus [35] of the change of the static permittivity of aqueous electrolytes with their increasing concentration did calculate the activity coefficient of the ion pair, y., values iteratively, using Equation 7.26. The fraction of 

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