Validation of Eq. (6) using our Cloudy models. Panel α: comparison of [Ο i]/Ηα calculated with this relation assuming the mean f_ion (Eq. (16)) to the Cloudy output. Panel b: same comparison, but applied to the individual local zones in the Cloudy models. Here f_ion corresponds to the local values of n(H⁺)/n(H). Panel c: same test, but applied to results using Eq. (A.23) that are accounting for variations of f_ion and the spatial extent of the [O i] and Hα emitting zones.