Ionization fraction versus the best column density ratio, C₂H/HCN (left panel), and the best line intensity ratio, C₂H (1–0) / HCN (1–0) (right panel) for tracing the ionization fraction in translucent medium conditions. The model grid is shown as a scatter plot, with the central crowded regions replaced by PDF isocontours containing 25, 50, and 75% of the points. Superimposed are the RF model (red line), the analytical fit (solid black line, presented in Sect. 5), the analytical fit of the 1σ uncertainty (dashed black lines, presented in Sect. 5), and the bounds of the validity range of the analytical fit (vertical lines, presented in Sect. 5). The quality estimates of the two models are indicated on the figure.