In this paper we have explored the validity of the usual RR-cpH algorithm. Constant pH simulations are now a standard tool for investigating charge regulation in coarse-grained models of proteins, polyelectrolytes, and colloidal systems. It is commonly believed that RR-cpH algorithm can be applied to isolated (canonical) systems. This, however is not the case. The pH specified in the RR-cpH algorithm refers to that of the reservoir, which because of the presence of the Donnan potential can be very different from that of the system.