The intensity of the O IV intercombination lines around 1400 Å observed with
the Interface Region Imaging Spectrograph (IRIS) provide a useful tool to diagnose the electron number density (N e ) in the solar
transition region plasma. We measure the electron number density in a variety of solar features observed by IRIS, including an active
region (AR) loop, plage and brightening, and at the ribbons of the 22-June-2015 M 6.5 class flare. By using the emissivity ratios of
O iv and S iv lines, we find that our observations are consistent with the emitting plasma being near isothermal (logT [K] ≈ 5) and
iso-density (logN e [cm −3 ] ≈ 10.60) in the AR loop. Moreover, high electron number densities (≈ 10 13 cm −3 ) are obtained during the
impulsive phase of the flare by using the S iv line ratio. The S iv lines provide in fact a higher range of density sensitivity than the
O iv lines. Finally, we investigate the effects of high densities (above 10 11 cm −3 ) and non-Maxwellian electron distribution on the atomic
processes involved in the formation of the spectral lines used as density diagnostics. In particular, the ionization ratios are found to be
shifted towards lower temperatures compared to the low density case.