Modelling of cadmium(II) species formed with N,N'-bis(2-pyridylmethyl)-1,2-diaminoethane and optimisation of stability constants by the use of polarographic complex formation curves for labile metal-ligand systems

The ligand N,N'-bis(2-pyridylmethyl)-1,2-diaminoethane (DPA-2), containing pyridyl and saturated nitrogens as donor groups, was studied with Cd-II by differential-pulse polarography (DPP) and glass electrode potentiometry (GEP) at fixed total ligand to total metal concentration ratios and various pH values, The polarographic experimental complex formation curve (ECFC) and calculated complex formation curve (CCFC) were used for modelling the metal-ligand system and the refinement of stability constants, The ECFC, in which experimental parameters of polarographic peaks are included (a shift in a peak potential and a variation ina peak height) appears to be a characteristic function fora 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, Five cadmium complexes, Cd(HL)(3+), CdL2+, Cd(HL2)(3+), CdL22+ and CdL(OH)(+), and their stability constants as log beta found from DPP to be 12.70 +/- 0.08, 10.18 +/- 0.04, 21.06 +/- 0.05, 13.74 +/- 0.03 and 13.40 +/- 0.06, respectively, are reported, Results obtained from GEP are also reported and they differ from the polarographic results only within experimental errors typical for these techniques.