Analytical Chemistry

exchanging ion and the hydrogen ion within the resin structure. Thus, the greater the affinity for a
particular ion, relative to hydrogen, the greater the value of K. Selectivity coefficients are functions of
the proportions of the exchanging ions, the total concentration of the solution and the degree of cross-
linking. In dilute solutions (<0.1 M) values of K increase with increasing formal valency, i.e. M4+ > M3+

M2+ > M+. There is a further variation within each charge group as shown by the examples given in
Table 4.17.

Table 4.17 Selectivity coefficients for some common cations and anions

Cations 4% DVB 8% DVB

Li+ 0.76 0.79

H+ 1.00 1.00

Na+ 1.20 1.56

1.44 2.01

K+ 1.72 2.28

Ag+ 3.58 6.70

Mg2+ 0.99 1.15

Zn2+ 1.05 1.21

Cu2+ 1.10 1.35

Ni2+ 1.16 1.37

Ca2+ 1.39 1.80

Sr2+ 1.57 2.27

Pb2+ 2.20 3.46

Ba2+ 2.50 4.02

Anions

6 – 10% Cross-linking

OH– 0.09

F– 0.09

CH 3 COO– 0.17

Cl– 1.0

CN– 1.6

Br– 2.8

3.8

I– 8.7

Values are quoted relative to the hydrogen ion for cations and to the chloride ion for anions. As shown,
selectivity also increases with increasing degree of cross-linking. At concentrations greater than 0.1 M