A potentiometric and differential pulse polarographic study of CdII with 1-hydroxyethylenediphosphonic acid

The complexation of the ligand 1-hydroxyethylenediphosphonic acid (HEDP) with Cd-II was studied by differential pulse polarography (DPP) and glass electrode potentiometry (GEP) at fixed total Ligand to total metal concentration ratios and varied pH values. Labile and non-labile metal complexes were analysed simultaneously by the use of a polarographic experimental complex formation curve (ECFC) and the calculated complex formation curve (CCFC). Complex formation curves were used for modelling of the metal-ligand system and the refinement of stability constants. The ECFC, in which experimental parameters of only labile DPP peak are included (a shift in the peak potential and a variation in the peak height) appears to be a characteristic function for a full metal-ligand model (labile and non-labile parts of a 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 confirmed by these two experimental techniques, and for which stability constants of metal complexes obtained from DPP and GEP differ the least. Six cadmium complexes M(H4L), M(H2L), M2L, ML (all labile), ML2 (non-labile), and ML(OH) (labile) and their stability constants as log beta found from DPP 25.04 +/- 0.06, 19.56 +/- 0.02, 12.67 +/- 0.03, 7.26 +/- 0.02, 10.39 +/- 0.06 and 10.93 +/- 0.04, respectively, are reported. Results obtained from GEP are also reported and they differ from polarographic results only within the experimental errors typical for these techniques. (C) 1999 Elsevier Science B.V. All rights reserved.