Predicted a) OH⁺/H₂O⁺ and b) OH⁺/H₃O⁺ ratios as a function of χ_e × 10³/f_H2. Symbols show results for all generated chemical models, while the solid lines show the analytic curves by Gerin et al. (2010) and Neufeld et al. (2010). The inferred OH⁺/H₂O⁺ ratios in the nuclear regions of NGC 4418 and Arp 220, and in the C_extended of Arp 220, are indicated with the shaded dark and light regions, respectively. Gas temperatures of 150 K (black) and 550 K (red) are assumed.