As noted previously, the mainsubstantive difference between the conventional modelingof ion exchange and other adsorption reactions is that, in ionexchange, the surface sites are assumed to be fully occupiedunder all circumstances; that is, qS ¼ 0. Additional practical differences between the two groups of reactions includethe types of adsorbents of interest, the centrality of charge,and the equal focus on the adsorbing and desorbing speciesin ion exchange.By solving the isotherm equations in conjunction with themass balance equations on the various adsorbates in an ionexchange system, we can determine the concentrations andadsorption densities of all the adsorbates at equilibrium.However, it is often desirable to characterize the relativeaffinities of a resin for different adsorbates in a more genericway, without reference to a particular set of conditions. Theparameter that is most often used for this purpose is knownas the separation factor for species B over species A,represented as aB/A and computed as follows: