Experimental and calculated complex formation curves as an optimization tool in speciation studies of polarographically labile metal-ligand systems

The Pb-II-N,N'-di-2-picolylethylenediamine (DPA-2) system was studied by differential pulse polarography (DPP) and glass electrode potentiometry (GEP) at fixed total-ligand to total-metal concentration ratio and varied pH. The polarographic experimental complex formation curve (ECFC) and calculated complex formation curve (CCFC) are used for modeling of a metal-ligand system and the refinement of stability constants. The ECFC is used instead of a shift in the peak potential plotted vs. pH and it appears to be a characteristic function for a particular metal-ligand system. The CCFC is a theoretical curve calculated for the assumed metal-ligand model from mass-balance equations. The final model of metal species formed is the one which is accepted by these two experimental techniques and for which stability constants of metal complexes obtained from DPP and GEP differ the least. Four lead complexes Pb(HL), PbL, PbL2(OH) and PbL(OH)(2) and their stability constants as log beta found from DPP 12.67 +/- 0.08, 9.68 +/- 0.07, 16.12 +/- 0.08 and 14.88 +/- 0.06, respectively, are reported. Results obtained from GEP and DPP differ only within experimental errors typical for these techniques.