Presence of the Donnan potential between the implicit reservoir and the simulation cell introduces significant difference between titration curves calculated for open and closed systems -the pH c of an isolated (closed) system is different from the pH R of the reservoir, for the same protonation state of the polyelectrolyte. When discussing pH of inhomogeneous systems, such as a polyelectrolyte solution separated from the reservoir of acid and salt by a semipermeable membrane, it is usual to define activity by separating the mean-electrostatic potential within the phase. Such definition is equivalent to placing the reference (saturated calomel) and hydrogen electrodes within the same phase when measuring the pH. The equivalence of electrochemical potential of hydronium ions across the membrane is then expressed as ln a S H + βq ϕ D = ln a R H , where a S H is the activity of hydronium ions inside the polyelectrolyte solution (system) and a R H is the activity inside the reservoir.