Proton-ligand dissociation constants of 4-(4-acetophenyl)hydrazono-3-methyl-2-pyrazolin-5-one (AHMP) and metal-ligand stability constants of its complexes with some transition metal ions were calculated potentiometrically in 0.1 M KCl and 50% (v/v) ethanol-water mixture. The order of stability constants was found to be Th4+ > UO22+ > Ce3+ > La3+ > Mn2+ < Co2+ < Ni2+ < Cu2+ > Zn2+. The dissociation constant, pK1H, of AHMP and the stability constants, log K, of their complexes were determined at different temperatures (298, 308, and 318 K). The corresponding thermodynamic parameters (ΔG, ΔH, and ΔS) were derived and discussed. The dissociation process is nonspontaneous, endothermic, and entropically unfavourable. The formation of the metal complexes has been found to be spontaneous, exothermic, or endothermic (depending the metal), and entropically favourable. The stoichiometries of these complexes were determined conductometrically and indicated the formation of 1:1 and 1:2 (metal:ligand) complexes. Key words: 4-(4-acetophenyl)hydrazono-3-methyl-2-pyrazolin-5-one, dissociation and stability constants, thermodynamic parameters, stoichiometries.
