The values of the selectivity coefficients for various cations in strong cation exchangers are arranged as follows (decreasing order):įe 3+, Al 3+, Pb 2+, Sr 2+, Ca 2+, Co 2+, Ni 2+, Cu 2+, Zn 2+, Mg 2+, Mn 2+, Ag +, Cs +, Cd +, K +, NH 4 +, Na +, H +, Li +, Hg 2+ The values of the selectivity coefficients for various anions in amino base anion exchangers are arranged as follows (decreasing order): The ions with higher selectivity coefficient will replace the ions with lower selectivity coefficient in the resin molecules. Selectivity coefficient also determines a preference of an ion to another ion. The higher the selectivity coefficient the higher fraction of the ion B is replaced with the ion A.
Selectivity coefficient indicates the degree of the ions (A and B) exchanging. Where, ,, - concentrations of A, R nB, RA and B. The equilibrium constant of the reaction ( selectivity coefficient): Where: R - ionized resin molecule with an attached functional group A + / A - exchangeable ion B n+ / B n- cation/anion dissolved in the liquid n+ /n- electrical charge of the ion B.
The general chemical reaction of ion exchange: Cation exchange
