Protonconstants of Per (I) and Amin (II) ligands and the formation constants of their complexes with some transition metal ions (Zn(II) and Cu(II)) and some trivalent lanthanides (Ce(III), Pr(III), and Er(III)) ions have been determined potentiometrically in aqueous methanol medium. The effect of temperature was studied at 20, 30, and 40ºC. The thermodynamic parameters (∆Hº, ∆Gº, ∆Sº) were evaluated and discussed. The formation curves for the metal complexes were obtained and the formation

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... ants were determined. Experimental Reagents and Materials: Metal ion solutions (0.01M) were prepared from Analar metals nitrate in bidistilled water. Standardization of metal ions were performed by complexometric titration using standard solution of EDTA (1). The ligand solutions (0.01M) were prepared by dissolving the accurate weight of the solid in aqueous methanol medium. Solutions of 0.1M HCl and 0.1M of nitric acid and potassium nitrate were prepared by dissolving A.R. grade chemicals in bidistilled water. The acids were standerdized by using primary standard base. Potentiometric Measurements: pHmeasurements were carried out using Fisher scientific equipment pH meter model 825 mp.