where φ D is the Donnan potential between the semigrand canonical system and its external reservoir. We note, however, that the standard implementations of cpH simulations do not provide us with the value of the Donnan potential since it cancels out in the pair insertion/deletion moves used to preserve the charge neutrality during the simulation . Recently, however, we have developed a new reactive grand canonical MC-Donnan (rGCMCD) method, which allows us to determine the Donnan potential directly within the simulation , allowing us to calculate both the titration isotherms of canonical and semi-grand canonical systems simultaneously -showing that for systems of low colloidal volume fraction and low ionic strength, the number of deprotonated groups can be 100% larger in an isolated system 40,41 compared to a system connected to a reservoir of exactly the same pH.