  This work examines the geometric mean of concentrations and its behavior in
various situations, as well as the reversal permanent charge problem, the
charge sharing seen in x-ray diffraction. Observations are obtained from
analytical results established using geometric singular perturbation analysis
of classical Poisson-Nernst-Planck models.
  For ionic mixtures of multiple ion species Mofidi and Liu [{\em SIAM J. Appl.
Math. {\bf 80} (2020), 1908-1935}] centered two ion species with unequal
diffusion constants to acquire a system for determining the reversal potential
and reversal permanent charge. They studied the reversal potential problem and
its dependence on diffusion coefficients, membrane potential, membrane
concentrations, etc. Here we use the same approach to study the dual problem of
reversal permanent charges and its dependence on other conditions. We consider
two ion species with positive and negative charges, say Ca$^+$ and Cl$^-$, to
determine the specific conditions under which the permanent charge is unique.
Furthermore, we investigate the behavior of geometric mean of concentrations
for various values of transmembrane potential and permanent charge.
