Gapon Selectivity Coefficient

Based on Reaction 4.38, the Gapon exchange selectivity coefficient (AfG) can be expressed as... 

With an arbitrary definition of KNaX as equal to unity, thus establishing a reference half reaction, the equilibrium constant for any other half reaction can be determined from measured selectivity coefficients. The Gapon equation can be readily implemented in this manner. Implementation of the Vanselow equation, however, requires modification of the general equilibrium models to account for the more complex dependence of mole fractions on the molar concentrations. An example ion-exchange calculation using the half reaction approach to represent the Gapon equation is presented in Appendix 2. 

Note that it is written as the exchange of a single positive charge, irrespective of the valence of the ions the Gapon selectivity coefficient is written as... 

Evangelou, V. R, and R. E. Phillips. 1988. Comparison between the Gapon and Vanselow exchange selectivity coefficients. Soil Science Society of America Journal 52, no. 2 379-382. 

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