We revised the classical Stem equation for ion exchange at charged membranes using the statistical mechanics concept of chemical potential and ion activity in nonhomogeneous systems. Our proposed equation does not require an iterative procedure to obtain the self-regulated surface charge at the membranes. We predict significant differences with the classical results. When compared with experimental results, our approach not only gives a better fit but also predicts dissociation constants that are physically more reasonable than those from the classical approach,