Normality of the resin phase:
The normality of the resin phase (C r) will depend upon the swollen volume of the resin and thus, is a function of cross linkage. Same, the apparent selectivity coefficient has this same form for exchanges between divalent and trivalent ions. Therefore, in the case of exchange between monovalent and trivalent ion, the expression takes the form (K)A+ + +B+ ( Cr/C)2
That means that the selectivity of the resin for the trivalent ions is inversely related to the square of total solution concentration.
It can be concluded that this polyvalent ion effect makes efficient water softening possible. The divalent cations (Ca++ and Mg2+) are easily picked up from hard water (a very dilute solution) and yet are easily displaced by Na+ ions of a relatively concentrated (10 - 15%) salt solution. For the same reason, rehardening of water i.e., replacement of Ca++ / Mg++ ions by Na+ is difficult by ion exchange. That polyvalent ion effect holds fine for both cation and anion exchange.