We study the behaviour of the interstellar R_n_=n(Zn^+^)/n(Na^0^) ratio in ionization equilibrium conditions. The ratio is very sensitive to variations of the physical state of the gas and is only marginally affected by variations of interstellar depletion. We find logR_n_ {in} [=~-2,=~0] dex in cold molecular clouds, logR_n_ {in} about [0,+1] dex in cold atomic gas, and logR_n_ {in} about [+1,+3] dex in warm gas. We compare the theoretical R_n_ ratio with the column-density ratio R_N_=N(ZnII)/N(NaI) measured in a sample of 48 interstellar lines of sight. The comparison supports the validity of the assumption of ionization equilibrium and indicates that R_N_ is a good indicator of the physical state of the gas. The few deviations from ionization equilibrium that we find are all located in the general direction of β CMa, where studies of the local ISM revealed the presence of an extended (=~ 2x10^2^ pc) tunnel of hot gas. Finally, we report the results of a correlation analysis logR_N_-logN(NaI) and logR_N_-logN(H). The correlations that we derive imply the presence of a mechanism linking the physical state of interstellar clouds to their column density. A practical consequence of the correlation with N(NaI) is that a NaI measurement alone is sufficient to estimate the physical state of the gas in many situations of interest.