Dependence of S/O on electron density. Panels a and c show the theoretical behaviour of the emission-line ratios, [S III]33 μm/[S III]18 μm and [Ne V]24 μm/[Ne V]14 μm, respectively, as a function of density for different values of the electron temperature, as computed by PYNEB (Luridiana et al. 2015). Panel b shows the behaviour of log(S/O) as a function of [S III]33 μm/[S III]18 μm for SFGs. Panel d shows the behaviour of log(S/O) as a function of [Ne V]24 μm/[Ne V]14 μm for AGNs. The colour bar shows the values of 12+log(S/H) for both samples. Critical densities were computed assuming T_e = 10 000 K and are those associated with the emission line with the lowest value.