Modeling of a Cd-II-N-(2-hydroxyethyl)ethylenediamine system and optimization of stability constants by the use of complex formation curves. A polarographic study at a fixed L:M ratio and varied pH

The ligand N-(2-hydroxyethyl)ethylenediamine (HEEN) was studied with Cd-II by differential pulse polarography (DPP) at fixed total ligand (Lr) to total metal (M-T) concentration ratios and varied pH at 25 degrees C and an ionic strength mu = 0.5 mol dm(-3). The polarographic experimental complex formation curve (ECFC) and calculated complex formation curve (CCFC) were used for modeling of 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 in a peak height) appears to be a characteristic function for a particular metal-ligand system studied at a particular L-T:M-T ratio. The CCFC is a theoretical curve calculated for the assumed metal-ligand model from mass-balance equations. The Anal model of metal species formed is the one for which the CCFC fits best the ECFC. Four cadmium complexes CdL2+, CdL22+, CdL32+, and CdL2(OH)(+) and their stability constants as log beta found from DPP 5.08 +/- 0.03, 9.44 +/- 0.04, 11.25 +/- 0.05, 12.06 +/- 0.03, respectively, are reported. The complex CdL32+ was found for the first time. Results obtained in this work are compared with literature data.