Resin Selectivity:

The strong cation exchanger like Dowex 50 is comparable in acid strength with hydrochloric acid and will form stable "salt like" bonds with any cation. Similarly, a strong anion exchanger like Dowex 1 is comparable to sodium hydroxide and will form stable bonds with any anions. The only ions that cannot be held strongly through one or the other of these resins are complex ions or organic ions which because of their size or configuration are hindered from entering the interior of resin particle.

The above statement does not mean that all bonds between the strong resin and the different ions are of equivalent strength. An ion exchange resins will have preference for the particular types of ions they will like to hold if given the choice. It is this preference which is defined as the selectivity of the resin. In the resin systems, the typical physical chemistry equilibrium constant is not strictly applicable. It is substituted by a selectivity coefficient. To a resin holding B ion placed in a solution of ion A and allowed to come to equilibrium, the selectivity coefficient (K)^B_A for monovalent exchange is given as follows.

(K)^B_A   = (Conc. of A in the resin) × (Conc. of B in solution)/ (Conc. of B in the resin) × ( Conc. of A in solution)

It can also be written as

A + B_r  ↔  A_r + B

( K )^B_A =   [A]_r [B] [A][B] _r

Here, r in the subscript represents the resin phase. This definition ignores the activity coefficient of the ions in the two phases. There are no fully satisfactory techniques for determining the activity coefficient of ions in the resin phase and are thus omitted. The activity coefficient of ions in solutions can be obtained from the literature and can be applied in the above expressions for accurate results while working with other than dilute solutions. In that case of concentrated solutions while the activity coefficient is significantly altered, the selectivity coefficients values should be applied with caution.

It should be kept in mind that selectivity is dependent upon many factors. It varies with temperature and pressure. The effect of pressure has not been investigated due to the nature of the ion exchange technique.