390 Fermentation and Biochemical Engineering Handbook of functional group and the nature of the non-exchanging iong n, the type he total ionic strength of the solution, the cross- linkage of th The ionic hydration theory has been used to explain the effect of some ofthese factors on selectivity. 20 According to this theory, the ions in aqueous lution are hydrated and the degree of hydration for cations increases with increasing charge and decreasing crystallographic radius, as shown in Table 2. 21] It is the high dielectric constant of water molecules that is responsible for the hydration of ions in aqueous solutions. The hydration potential of an ion depends on the intensity of the change on its surface. The degree of hydration of an ion increases as its valence increases and decreases as its hydrated radius increases. Therefore, it is expected that the selectivity of a resin for an ion is inversely proportional to the ratio of the valence/ionic radius for ions of a given radius. In dilute solution, the following selectivity series are followed Li< Na<K<Rb<o Mg Ca sr Ba F<Cl< Br <I Table 2. Ionic Size of Cations[21] Crystallographic Hydrated Ionization Radius(A) Radius(A) Potential 0.68 10.00 1.30 0.98 K 33 5.30 0.75 NH4 1.43 5.37 1.65 5.05 0.61 0.89 2.60 1.34 960 1.60 Ba 149 8.80 140 The selectivity of resins in the hydrogen ion or hydroxide ion form, however, depends on the strength of the acid or base formed between the functional group and the ion. The stronger the acid or base formed the lower is the selectivity coefficient. It should be noted that these series are not followed in nonaqueous solutions, at high solute concentrations or at high temperature390 Fermentation and Biochemical Engineering Handbook the total ionic strength of the solution, the cross-linkage of the resin, the type of functional group and the nature of the nonexchanging ions. The ionic hydration theory has been used to explain the effect of some ofthese factors on selectivity.[20] According tothis theory, the ions in aqueous solution are hydrated and the degree of hydration for cations increases with increasing charge and decreasing crystallographic radius, as shown in Table 2.r2l] It is the high dielectric constant of water molecules that is responsible for the hydration of ions in aqueous solutions. The hydration potential of an ion depends on the intensity of the change on its surface. The degree of hydration of an ion increases as its valence increases and decreases as its hydrated radius increases. Therefore, it is expected that the selectivity of a resin for an ion is inversely proportional tothe ratio ofthe valencehonic radius for ions of a given radius. In dilute solution, the following selectivity series are followed: Li <Na < K < Rb < Cs Mg < Ca < Sr < Ba F < C1< Br < I Table 2. Ionic Size of Cations[21] Crystallographic Hydrated Ionization Ion Radius (A) Radius (A) Potential Li Na K NH4 Rb cs Mg Ca Sr Ba 0.68 0.98 1.33 1.43 1.49 1.65 0.89 1.17 1.34 1.49 10.00 7.90 5.30 5.37 5.09 5.05 10.80 9.60 9.60 8.80 1.30 1 .oo 0.75 0.67 0.61 2.60 1.90 1.60 1.40 - The selectivity of resins in the hydrogen ion or hydroxide ion form, however, depends on the strength of the acid or base formed between the functional group and the ion. The stronger the acid or base formed, the lower is the selectivity coefficient. It should be noted that these series are not followed in nonaqueous solutions, at high solute concentrations or at high temperature