Evaluation of a Peak Potential of an Uncomplexed Metal Ion in Speciation Study of Labile Metal-Ligand Systems by Polarography. Stability Constant Determination of CdII and PbII with Picolinic Acid at a Fixed LT:MT Ratio and Varied pH

The ligand picolinic acid was studied with CdII and PbII by differential pulse polarography (DPP) at fixed total ligand (LT) to total metal (MT) concentration ratios, varied pH at 25 °C and an ionic strength of μ = 0.1 and 0.5 mol dm−3. The corrected peak potential of an uncomplexed metal ion Ep(MFree) is defined for the first time and used, instead of experimentally available value, to evaluate the shift in the peak potential. The polarographic experimental and calculated complex formation curves (ECFC and CCFC) were used for modeling of the metal–ligand systems and the refinement of stability constants. It has been shown that with the use of the corrected value of Ep(MFree) together with the ECFC and CCFC it is possible to reproduce values of stability constants obtained by glass electrode potentiometry. Three cadmium complexes CdL, CdL2 and CdL3 and their stability constants as log β′ found from DPP at μ = 0.1 mol dm−3 4.34 ± 0.04, 8.01 ± 0.04, 10.79 ± 0.03, and at μ = 0.5 mol dm−3 4.29 ± 0.03, 7.89 ± 0.03, 10.49 ± 0.02, respectively, are reported. Four lead complexes PbL, PbL2, PbL3 and PbL2(OH) and their stability constants as log β′ found from DPP at μ = 0.5 mol dm−3 4.49 ± 0.02, 7.58 ± 0.03, 9.59 ± 0.02 and 11.46 ± 0.03, respectively, are reported. All stability constants at μ = 0.5 mol dm−3 and two lead complexes PbL3 and PbL2(OH) are reported for the first time, whereas the formation of the complex CdL3 has been confirmed by DPP. Results obtained in this work are compared with the literature data.