The Gibbs-Donnan model for ion-exchange resins is used to describe and predict the sorption equilibria of metal ions on chelating resins. These are characterized by their intrinsic complexation constants, which, according to the model, are related to the complexation constants in solution with Ligands having structure similar to that of the active groups in the resin. The intrinsic complexation constants are independent of the experimental conditions, for instance the composition of the solution phase. The model is applied to two different kind of chelating resins: a strong anion exchanger loaded with sulfonated azo dyes, and a weak cation exchanger with iminodiacetic groups (Chelex 100) covalently linked to the mainframe. In most cases the complexation inside the resin exactly corresponds to that in aqueous solution, while sometimes the formation of 1-to-2 complexes is promoted inside the resin. In any case the intrinsic complexation constants allow us to evaluate the exchange coefficients, holding at the particular conditions, by the Gibbs-Doman model. Thus the sorption and elution of a metal ion by a batch procedure can be predicted. A particular application is described, which is useful in the field of analytical chemistry. It is the determination of total and free metal-ion concentration in an aqueous solution of unknown composition, at very low concentration of metal. This also is made possible by the knowledge of the exchange coefficients at the particular conditions.