Base exchange. The reversible chemical reaction in which certain ions in a crystal lattice on particle surfaces or edges can become hydrated in the presence of aqueous solutions with the result that they can be replaced, equivalent for equivalent, by ions (usually cations) exhibiting similar charge from the solution without a loss in the crystal structure.
According to a widely held view, the cations do not occupy precise sites: some are located in a plane parallel to and within a few angstroms of the clay surface (Helmholtz layer); others are contained in a zone of diminishing concentration (called the diffuse layer) extending outward from the Helmholtz layer.
Materials, particularly clays, which exhibit this property support surface conductance with their exchangeable cations.