Ton Exchange 39 The dependence of selectivity on the ionic strength of the solution has been related through the mean activity coefficient to be inversely proportional to the Debye-Huckel parameter, a [22 1+Ba° where, is the mean activity coefficient, A and B are constants, and u is the ionic strength of the solution. The mean activity coefficient in this instand represents the standard free energy of formation(-AF)for the salt formed by the ion exchange resin and the exchanged ion. Figure 4(231 shows this dependence as the ionic concentration of the solution is changed. As the concentration increases, the differences in the selectivity of the resin for ions ofdifferent valence decreases and, beyond certain concentrations, the affinity is seen to be greater for the lower valence ion 1.5 s1.2 MOLARITY OF SOLUTIONIon Exchange 391 The dependence of selectivity on the ionic strength of the solution has been related through the mean activity coefficient to be inversely proportional to the Debye-Huckel parameter, where 'y* is the mean activity coefficient, A and B are constants, and ,u is the ionic strength of the solution. The mean activity coefficient in this instance represents the standard free energy of formation (-W) for the salt formed by the ion exchange resin and the exchanged ion. Figure 4[231 shows this dependence as the ionic concentration of the solution is changed. As the concentration increases, the differences in the selectivity of the resin for ions of different valence decreases and, beyond certain concentrations, the affinity is seen to be greater for the lower valence ion. f I I I I 0 0,4 008 la2 106 2,o MOLARITY OF SOLUTION Figure 4. Dependence of the activity coefficient on the ionic concentration of aqueous