Polarographic Complex Formation Curves for Fully Inert Metal-Ligand System

The ligands N,N,N′,N′-tetra-2-picolylethylenediamine (TPEN) and 1,4,7-tris(2-hydroxyethyl)-triazacyclononane (THETAC) were studied with BiIII, CdII and PbII by differential pulse polarography (DPP) at fixed total ligand to total metal concentration ratios and varied pH. These metal ions formed fully inert ML complexes with the ligands. It has been shown that the polarographic experimental and calculated complex formation curves (ECFC and CCFC), used previously for modeling of the fully labile metal-ligand systems and the refinement of stability constants of labile metal complexes, can be employed without any modification to study fully inert metal-ligand systems. The ECFC, in which a variation in a height of a labile DPP peak is converted into millivolts, appears to be a characteristic function for a particular metal-ligand system where the inert ML complex is formed. It has been demonstrated that the excess of the ligand is not required at all to obtain the reliable stability constant of an inert complex ML that can be polarographically either active or inactive. Stability constants found here are compared with the literature data.