Mass of the ionised gas in the selected regions assuming different prescriptions for electron density: lower limit () and upper limit () derived from the reference H II regions using Eq. (6) and from R_[S II] = [S II]λ6717Å/[S II]λ6731Å flux ratio, respectively, and n_e, SII derived from R_[S II] of the observed spectra of the region itself. Different morphological types introduced in Sect. 4.1 are shown by different colours. Diagonal lines show 1-to-1 relation. Spearman correlation coefficients ρ are given on the plots.