  Atomic ions are mostly neutralized by small grains (or PAH molecules) in
current theories of heating and cooling in cool diffuse clouds; in the main
they do not recombine with free electrons. This alters the ionization balance
by depressing n(H+) and n(He+) while carbon generally remains nearly fully
once-ionized: charge exchange with atomic oxygen and formation of H2 and OH
also depress n(H+) in partly molecular gas. Seemingly restrictive empirical
limits on the cosmic ray ionization rate of hydrogen ($\zeta_H$) are relaxed
and faster rates are favored in a wide range of circumstances, when grain
neutralization is recognized. Maintenance of the proton density at levels
needed to reproduce observations of HD requires $\zeta_H$ at least 2x10^{-16}
s^{-1}, but such models naturally explain the presence of both HD and H3^+ in
relatively tenuous H I clouds. In dense gas, a higher ionization rate can
account for high observed fractions of atomic hydrogen, and recognition of the
effects of grain neutralization can resolve a major paradox in the formation of
sulfur-bearing compounds.
