Potentiometric and Thermodynamic Studies of Some Schiff-base Derivatives of 4-Aminoantipyrine and Their Metal Complexes

4-(4-hydroxybenzalideneamine) antipyrine (L1) and 4-(2-hydroxybenzalideneamine) antipyrine (L2) have been synthesized and their structure have been confirmed by elemental analyses, mass spectrometry, IR and 1H NMR spectroscopy. The geometrical structures of these ligands are carried out by HF method with 3-21G basis set. The proton-ligand dissociation constant of the ligands (L1 and L2) and metal-ligand stability constants of their complexes with metal ions (Mn2+, Co2+, Ni2+ and Cu2+) have been determined potentiometrically in 0.1 mol.dm-3 KCl and 10 % (by volume) ethanol–water mixture. The stability constants of the formed complexes increases in the order Mn2+, Co2+, Ni2+ and Cu2+. The effect of temperature was studied at 298, 308 and 318 K and the corresponding thermodynamic parameters (?G, ?H and ?S) were derived and discussed. The dissociation process is non-spontaneous, endothermic and entropically unfavorable. The formation of the metal complexes has been found to be spontaneous, endothermic and entropically favorable.