The selectivity of resins in the hydrogen ion or hydroxide ion form, however, depends on the strength of the acid or base formed between the functional group and the ion. The stronger the acid or base formed, the lower is the selectivity coefficient. It should be noted that these series are not followed in nonaqueous solutions, at high solute concentrations or at high temperature.

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,

1 + Ba°JJi where y± is the mean activity coefficient, A and B are constants, and n is the ionic strength of the solution. The mean activity coefficient in this instance represents the standard free energy of formation (-AF°) for the salt formed by the ion exchange resin and the exchanged ion. Figure 4[23] 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.

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