Polynuclear complexes of the edta analogues Original Research Article

Complex formation by O,O′-bis-(β-aminoethyl)-ethyleneglycol-N,N,N′,N′-tetraacetate (egta) with Ni2+ and Zn2+ and of 1,5-diaminopentane-N,N,N′,N′-tetraacetate (peta) with Ni2+ was investigated at 20°C and I = 1.00 (KNO3) using alkalimetric titrations of equimolar solutions of the components in the range 0.5–32 mM. While Zn2+ forms only mononuclear species, Ni2+ gives mono-, NiL2−, and polynuclear NiqHLq(q−1)− (q =1, 2, 3, 4, 5, 6) complexes. Complex formation by Hg2+ with edta and its homologues was investigated by emf measurements (glass and Hg electrode) under the same conditions. The total concentrations of mercury(II) was varied from 0.01 to 50 mM, using a metal-ligand ratios of 1 : 2. Only species with the same q value, of the type HgqLq2q−, were present in these solutions, in the pH range 5–8, irrespective of the total concentrations of the components. The value of q is one for edta and two for the other ligands. The stability constants increase as the number of methylene groups between N atoms increases from 3 to 5. The concentration stability constants of all the species formed are reported.