The column dynamics of simple ion-exchange processes involving acetic
acid and N-acetylmethionine were studied using a strong-base anion exc
hange resin to reveal the complex relation between pH, concentration,
equilibrium isotherms, and chromatographic separation in ion-exchange
chromatography of weak electrolytes. Changing either the influent pH o
r the concentration, while keeping the other variable constant, led to
considerable effluent concentration and pH-fluctuations, respectively
. This behavior is caused mainly by the uptake of undissociated acids.
To describe these phenomena quantitatively, a rigorous thermodynamics
-based equilibrium model for ion exchange, including Donnan potential
and reaction equilibria, was incorporated into a fixed-bed model. Expe
rimental results can be described well by this dynamic model, which is
superior to conventional models due to its explicit incorporation of
colons and partitioning of neutral species. It has great potential for
optimizing chromatographic separations of weak electrolyte solutions.