The important features of ion exchange reactions are that they are stoichiometric, reversible and possible with any ionizable compound. The reaction that occurs in a specific length of time depends on the selectivity of the resin for the ions or molecules involved and the kinetics of that reaction.

The stoichiometric nature of the reaction allows resin requirements to be predicted and equipment to be sized. The reversible nature of the reaction, illustrated as follows:

allows for the repeated reuse of the resin since there is no substantial change in its structure.

The equilibrium constant, K, for Eq. (1), is defined for such monovalent exchange by the equation:

In general, if AT is a large number, the reverse reaction is much less efficient and requires a large excess of regenerant chemical, HC1 in this instance, for moderate regeneration levels.

With proper processing and regenerants, the ion exchange resins may be selectively and repeatedly converted from one ionic form to another. The definition of the proper processing requirements is based upon the selectivity and kinetic theories of ion exchange reactions.

2.1 Selectivity

When ion B, which is initially in the resin, is exchanged for ion A in solution, the selectivity is represented by:

where Z, is the charge and Vi is the partial volume of ion /. The selectivity which a resin has for various ions is affected by many factors. The factors include the valence and size of the exchange ion, the ionic form of the resin, the total ionic strength of the solution, the cross-linkage of the resin, the type of functional group and the nature of the non-exchanging ions.

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 solution 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 < Cs Mg < Ca < Sr < Ba F < CI < Br < I

Table 2. Ionic Size of Cations[21]


Crystallographic Radius (â)

Hydrated Radius (â)

Ionization Potential


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