The pH inside the system is then defined as pH S = -log 10 (a S H ) and inside the reservoir as pH R = -log 10 (a R H ). In a recent paper we have discussed the problems that may arise due to the violation of the Gibbs-Guggenheim principle, when mean-electrostatic potential is excluded from the definition of activity. Nevertheless, since this is the usual convention, we will adopt it in the present paper. The pH inside the system is then different from that of the reservoir . Furthermore, if after the equilibrium is established, the system is disconnected from the reservoir (is closed) its pH S will not be affected. Therefore, if we would perform a canonical reactive simulation on such a closed system, we will obtain exactly the same pH, i.e. pH c = pH S . The relation between pH inside the system and pH in the reservoir is then: