On the activity coefficients of charge-symmetrical ion pairs

The association constant KA of charge-symmetrical ion pairs in electrolyte solutions, obtained from experiments yielding the fraction present as free ions as a function of the electrolyte concentration α(c), is calculated from KA = (1 − α)fip / α2cf±2. The activity coefficients of the free ions f± are generally taken into account but those of the ion pairs fip are commonly set equal to unity, for lack of better knowledge. Two methods for obtaining the latter quantity, one based on salting-out (in) and the other based on an analogy with zwitterions, are suggested for the calculation of fip. The consequences are illustrated for cases taken from the literature of ion pairing of thallium (I) chloride in water and of sodium iodide and tetrabutylammonium bromide in acetonitrile that are characterized by low or moderate values of KA, and of magnesium sulfate in water that has a fairly high value of KA.