A preliminary analytical model for prediction of the breakthrough behavior of Lewatite TP 214 chelating resin in adsorption of Ag+ ions was developed on the basis of the shrinking core model and constant pattern approximation (CPA). The model, instead of fractional residual capacity or "f" factor of classical CPA, uses a correlation between C-0(V-e - V-b) and Q - C0Vb. In order to find the adjustable parameters of the model, a semiempirical correlation between operational parameters and one of the adjustable parameters in the form of B (q(sat)v(.)/C-0) was devised. In addition, the available mass transfer correlations were also employed for estimation of the adjustable parameters. The results demonstrated the superiority of the first method from both simplicity and accuracy points of view. The model could successfully estimate the breakthrough and exhaustion points. Moreover, in contrast to the famous bed depth service time (BDST) model, it was capable of fitting the experimental breakthrough curves.