ON EXCHANGE EQUILIBRIA

When the percentage of DVB is low, 2 to 4%, the network is open and the extent of swelling with water is high. Resins with 8% DVB are very popular and are structurally stronger but still imbibe water. In its hydrated form, this resin acts like a gel which, by the levels of sulfonate or tetra-alkylammonium ions present, simulate a strong electrolyte of from 3 to 5M 

The exchange reaction can be described in terms of the following equations. 

Equilibrium expressions eorresponding to these reactions may be written as follows  

Values of K gx for various cation exchange reactions can be shown to be related to the ratio of activity coefficients in the resin phase by means of the Donnan theory developed by describing membrane equilibria. The Donnan theory, based in thermodynamics, states that the activity product of a salt on either side of a membrane to which it is permeable, is identical. We can consider the resin phase as being separated from the aqueous phase by the equivalent of a membrane which permitted the free passage of all ions except those which are chemically bound to the polymer. Applying this to a system consisting of a resin in the hydrogen form in equilibrium 

One can interpret this as signifying that ion exchange equilibria depend on the differences in the properties of the ions themselves rather than on any specific interaction with the resin. Ionic activity coefficients of ions of the same charge are very similar to one another, but their activity coefficients in the resin phase are not. Even though it is not possible to determine the coefficients of the cations in the resin phase directly, the resemblance of the resin phase to a highly concentrated electrolyte solution permits the comparison of experimentally determined values with the 

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P.W. Arnold (1-3) was an early pioneer of the work at Rothamsted. The present research programme was begun by 0. Talibudeen (4-6), with whom I began collaboration in 1974. This paper summarises our results and my more recent work on exchange equilibria (4-16). 

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