Dissociation constants of some Schiff bases and stability constants of their copper,cobalt, nickel and zinc complexes

The stoichiometry and stability constant of metal complexes with 4-(3-methoxy-salicylideneamino)-5-hydroxynaphthalene-2,7-disulfonic acid monosodium salt (H₂L) and 4-(3-methoxysalicylideneamino)-5-hydroxy-6-(2,5-dichlorophenylazo)-2,7-naphthalene disulfonic acid monosodium salt (H₂L¹) were studied by potentiometric titration. The stability constants of H₂L and H₂L¹ Schiff bases have been investigated by potentiometric titration and u.v.–vis spectroscopy in aqueous media. The dissociation constants of the ligand and the stability constants of the metal complexes were calculated pH-metrically at 25 °C and 0.1 m KCl ionic strength. The dissociation constants for H₂L were obtained as 3.007, 7.620 and 9.564 and for H₂L¹, 4.000, 6.525, 9.473 and 10.423, respectively. The complexes were found to have the formulae [M(L)₂] for M = Co(II), Ni(II), Zn(II) and Cu(II). The stability of the complexes follows the sequence: Zn(II) < Co(II) < Cu(II) < Ni(II). The high stability of H₂L¹ towards Cu(II) and Ni(II) over the other ions is remarkable, in particular over Cu(II), and may be of technological interest. Concentration distribution diagram of various species formed in solution was evaluated for ligands and complexes. The formation of the hydrogen bonds may cause this increased stability of ligands. The pH-metric data were used to find the stoichiometry, deprotonation and stability constants via the SUPERQUAD computer program.