  We discuss problems associated with the notion of pH in heterogeneous
systems. For homogeneous systems, standardization protocols lead to a well
defined quantity, which although different from S\o rensen's original idea of
pH, is well reproducible and has become accepted as the measure of the
``hydrogen potential". On the other hand, for heterogeneous systems, pH defined
in terms of the chemical part of the electrochemical activity is
thermodynamically inconsistent and runs afoul of the Gibbs-Guggenheim principle
that forbids splitting of the electrochemical potential into separate chemical
and electrostatic parts -- since only the sum of two has any thermodynamic
meaning. The problem is particularly relevant for modern simulation methods
which involve charge regulation of proteins, polyelectrolytes, nanoparticles,
colloidal suspensions etc. In this paper we show that titration isotherms
calculated using semi-grand canonical simulations can be very different from
the ones obtained using canonical reactive Monte Carlo simulations.
