Competitive “binding” of Ca²⁺ and Na⁺. Average numbers of Ca²⁺ (increasing curves) and Na⁺ (decreasing curves) absorbed into the central region of the pore. (A) CaCl₂ added to 0.03 M NaCl (closed symbols) or to 0.03 M NaCl + 0.09 M MCl (M⁺ is a “large” cation of radius 2.5 Å). (B) Varied amounts of CaCl₂ added to baths containing different fixed concentrations of NaCl.