  We perform 3D hydrodynamical simulations to study recombination and
ionization during the common envelope (CE) phase of binary evolution, and
develop techniques to track the ionic transitions in time and space. We
simulate the interaction of a $2\,M_\odot$ red giant branch primary and a
$1\,M_\odot$ companion modeled as a particle. We compare a run employing a
tabulated equation of state (EOS) that accounts for ionization and
recombination, with a run employing an ideal gas EOS. During the first half of
the simulations, $\sim15$ per cent more mass is unbound in the tabulated EOS
run due to the release of recombination energy, but by simulation end the
difference has become negligible. We explain this as being a consequence of (i)
the tabulated EOS run experiences a shallower inspiral and hence smaller
orbital energy release at late times because recombination energy release
expands the envelope and reduces drag, and (ii) collision and mixing between
expanding envelope gas, ejecta and circumstellar ambient gas assists in
unbinding the envelope, but does so less efficiently in the tabulated EOS run
where some of the energy transferred to bound envelope gas is used for
ionization. The rate of mass unbinding is approximately constant in the last
half of the simulations and the orbital separation steadily decreases at late
times. A simple linear extrapolation predicts a CE phase duration of
$\sim2\,\mathrm{yr}$, after which the envelope would be unbound.
