Electron densities derived from [S ii] λλ6717/6730 (black filled circles), [O ii] λλ3726/3729 (blue triangles), and [Ar iv] λλ4711/4740 (red stars) compared to the one derived from [Cl iii] λλ5517/5537. There is a clear correlation between all quantities. The trend n_e([Ar iv])  >  n_e([Cl iii])  >  n_e([S ii])  >  n_e([O ii]) is clear. Solid line represents the equality. Pointed, solid, and dashed lines are the fits to the n_e([Ar iv]), n_e([S ii]), and n_e([O ii]) vs. n_e([Cl iii]) data points, respectively.