Total solution ionic strength:

Mono-monovalent exchanges are usually little affected by the change in the total ionic strength. However, it becomes important if the exchanges are taking place in different valence say mono-divalent exchange.

Let us first consider mono-monovalent exchange

NaCl + RSO₃H ↔  HCl + RSO₃Na

(K^Na+_H+) = [Na⁺]_r[H⁺]/[Na⁺][H⁺]_r

By substituting the values as follows,

X^r_Na+ =Equivalent fraction of Na⁺ in resin = (Na⁺)^r/C^r

X_Na+ = Equivalent fraction of Na⁺ in solution =(Na⁺)/C^r

Where C^r =total capacity or normality of resin (equiv./L) and

C= total normality of solution

Eq. takes the following form

X^r_Na+/1-X^r_Na+ =K^Na+_H+ X_Na+/1-X_Na+

This equation gives the equivalent fraction of Na⁺ in the resin as a function of the solution with which the resin is in equilibrium. It may be noticed that the terms C_r and C do not figure in the Eq.

Now consider the exchange of monovalent ion with divalent ion.

CaCl₂ + 2 RSO₃Na ↔ 2 NaCl + (RSO₃)₂Ca

K^Ca++_Na+ =[Ca⁺⁺]_r[Na⁺]²/[Ca⁺⁺][Na⁺]²_r

When is expressed in terms of the equivalent fraction of Ca⁺⁺   in the resin as a function of the solution, it becomes

X^r_Ca++/(1-X^r_Ca++)² = K^Ca++_Na+ C^r/C X_Ca++/(1-X_Ca++)²

In Eq, the "apparent" selectivity coefficient is the term K^Ca++_Na+(C^r/C)

C^r is the total capacity of the resin per unit volume and, therefore, is fixed for particular resin; the selectivity of the divalent ion in this exchange is inversely related to the total concentration of the solution. It can be concluded that the more dilute the solution, a more selective the resin becomes for the divalent ions.