THERMODYNAMIC STUDIES ON THE COMPLEXATION OF ORTHO-PHENYLENEDIAMINE-N,N,N',N'-TETRAACETATE WITH DIVALENT CADMIUM, MERCURY, AND LEAD IONS IN AQUEOUS-SOLUTIONS

Potentiometric studies have been carried out on the complexation of o-phenylenediamine-N,N,N',N'-tetra-acetate (phdta, L4-) with the cadmium and lead ions (M2+) in aqueous solutions at 25+/-0.1-degrees-C and I=1.00 M (NaClO4). The ligand buffer method was successfully applied to the determination of the formation constant (M2++L4=ML2-: K(ML)) by use of Cd and Pb ion-selective electrodes. This method gave log K(ML)=13.37 for CdL and 13.89 for PbL. Calorimetry and potentiometry gave the values of enthalpy and entropy of complexation (-DELTAH-degrees/kJ mol-1=18.9, DELTAS-degrees/J K-1 mol-1=193 (CdL); 34.8, 149 (PbL); and 50.6, 165 (HgL), respectively). Thermodynamic parameters of the protonation to ML, were also examined for the PhDTA complexes of these and some divalent transition metal ions. The metal complexes are classified into two categories according to the thermodynamic parameters: A) Mn2+, Cd2+, and Pb2- (-DELTAH(MHL)-degrees=-1.1-1.1 kJ mol-1; DELTAS(MHL)-degrees=43-49 J K-1 mol-1). B) Co2+, Ni2+, Cu2+, and Zn2+(-DELTAH(MHL)-degrees=8.0 -11.3 kJ mol-1;DELTAS(MHL)-degrees=18-37 J K-1 mol-1). The smaller value of -DELTAH(MHL)-degrees for A has been interpreted in terms of simple protonation to a carboxylate oxygen in the complex ML, with any acetate group not being dissociated. The entire process of protonation for B, however,may be composed of two steps: i) Detachment of one carboxylate group from the metal ion, followed by the hydration resulting in M(L') (H2O)2-, where L' represents a ligand anion with a free carboxylate group. ii) Protonation to an oxygen atom of the detached carboxylate group.