In the use of first order kinetic equation to fit the kinetic data of ion adsorption, the crux is how to determine the value of N_∞ (the adsorbed quantity at equilibrium). Both erroneous N_∞ value and unsuited N_∞ value to the mathematical equation would lead to a wrong conclusion. In this work, based on a theoretical analysis of the first order kinetic equation of chemical adsorption mechanism and physical diffusion mechanism, a way to determine the value of N_∞ was advanced and verified by the ion diffusion and ion adsorption experiments.