Protonation thermodynamics of 1,10-phenanthroline in aqueous solution. Salt effects and weak complex formation

The protonation of 1,10-phenanthroline has been studied potentiometrically in different aqueous salt media (LiCl, NaCl, KCl, Me(4)NCl, Et(4)NI, MgCl2, CaCl2, SrCl2, and BaCl2) in the ionic strength range 0 less than or equal to I less than or equal to 1 M. This ligand forms two protonated species, [H(phen)](+) and [H(phen)(2)](+); the monoligand species shows protonation constants strongly dependent on the medium. Medium effects were explained by the formation of some weak species: [H(phen)Cl](0), [M(phen)](2+) (M = alkaline earth metal cations). Formation thermodynamic parameters are reported.