When ion exchange is represented according to thesite-binding model for adsorption, the ion exchangereactions the ions are envisioned to bind to individual sitesin the ion exchange matrix. The bound ions are therebyremoved from solution and exist as adsorbed species. Thismodel implicitly assumes that the solution inside the ionexchange matrix is identical to the bulk solution, so that theinterior ion exchange sites are exposed to and equilibratedwith the same solution as the exterior sites.An alternative model treats the counter-ions inside the ionexchange matrix not as specifically adsorbed to individual