To preserve the charge neutrality, usual grand canonical simulation methods involve insertions of neutral pairs of ions into the system -making the Donnan potential cancel in the acceptance probability of such pair moves. This prevents us from obtaining the Donnan potential using such approaches. Therefore, a corrected constant pH algorithm -in which the charge neutrality is properly taken into account through the pairwise insertions -will only allow us to calculate the protonation state of polyelectrolyte as a function of pH R inside the reservoir and not the pH S inside the system (simulation cell). On the other hand, most experiments are conducted on closed systems, which according to Eq. ( ) will have different pH from that of the reservoir. Recently we have developed a new reactive grand canonical Monte Carlo Donnan (rGCMCD) simulation method that allows us to calculate both the Donnan potential and the protonation state of polyelectrolyte solution in contact with a reservoir of acid and salt. Eqn. ( ) then permits us to simultaneously calculate the titration isotherms for an open system, as a function of pH in the reservoir; and for a closed system, as a function of pH inside the simulation cell.