For a simplified view of ion exchange equilibrium for binary ion exchange assume the equilibrium constant can be determined by the law of mass action. Let A represent the hydrogen ion and B the sodium ion. For monovalent ion exchange

A^{+} + RB + X^{−} = AR + B^{+} + X^{−}

illustrated in Eq. (18-36), the mass action equilibrium expression simplifies to

**(18-40a)**

where we have assumed the local concentrations of co-ion X^{−} in the interior of the resin are both identical (and very low) in the numerator and denominator and hence cancel. (These concentrations are very low in the resin because of a phenomenon known as Donnan exclusion—the ...

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