Volume density of H₂ in various cold dark clouds determined through LVG calculations using different tracers. Green points correspond to densities derived from ¹³C isotopologues of HC₃N (see values in Table 1), while blue and magenta points correspond to densities obtained from the anion C₆H⁻ and the radical C₆H, respectively (see values in Table 3). H₂ densities derived through C₆H⁻ are close to those derived by ¹³C isotopologues of HC₃N, while H₂ densities derived through C₆H are systematically higher by factors of 10-50.