K = Selectivity compared to Li H = Hydration (g H20/eq resin) DVB = divinylbenzene

The degree of cross-linking can affect the equilibrium level obtained, particularly as the molecular weight of the organic ion becomes large. With highly cross-linked resins and large organic ions, the concentration of the organic ions in the outer layers of the resin particles is much higher than in the center of the particle.

The selectivity of the resin for a given ion is also influenced by the dissociation constants of the functional group covalently attached to the resin (the fixed ion) and of the counter-ions in solutions. Since the charge per unit volume within the resin particle is high, a significant percentage of the functional groups may not be ionized. This is particularly true if the functional group is a weak acid or base. For cation exchange, the degree of dissociation for the functional group increases as the pH is increased; however, the degree of dissociation for the ions in solution decreases with increasing pH. Therefore, if a cation resin had weak acid functionality, it would exhibit little affinity at any pH for a weak base solute. Similarly, an anion resin with weak base functionality exhibits little affinity at any pH for a weak acid solute.

The influence of pH on the dissociation constants for resin with a given functionality can be obtained by titration in the presence of an electrolyte. Typical titration curves are shown in Fig. 5 for cation resins and in Fig. 6 for anion resins.[26] For sulfonic acid functional groups, the hydrogen ion is a very weak replacing ion and is similar to the lithium ion in its replacing power. However, for resin with carboxylic acid functionality, the hydrogen ion exhibits the highest exchanging power. Table 4[271[28] summarizes the effect different anion exchange resin functionalities have on the equilibrium exchange constants for a wide series of organic and inorganic anions.

The selectivity can also be influenced by the non-exchanging ions (co-ions) in solution even though these ions are not directly involved in the exchange reaction. An example of this influence would be the exchange of calcium ascorbate with an anion resin in the citrate form. Although calcium does not take part in the exchange reaction, sequestering of citrate will provide an additional driving force for the exchange. This effect, of course, would have been diminished had a portion of the ascorbate been added as the sodium ascorbate rather than the calcium ascorbate.

For nonpolar organic solutes, association into aggregates, perhaps even micelles, may depress solution activity. These associations may be influenced by the co-ions present.

Meq NaOH per gram Resin Figure 5. Titration curves of typical cation exchange resins.[26]

Meq HCl per Gram Resin

Figure 6. Titration curves of typical anion exchange resins.[26]

Table 4. Selectivity Coefficients for Strongly Basic Anion Resin[27][281

Type I Anion

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