Voltammetry as Virtual Potentiometric Sensor in Modelling of a Metal/Ligand System and Refinement of Stability Constants. Part 5

Abstract

AbstractThe concept of virtual potential (employed here in modelling operations), a unique experimental setup designed and built in our laboratories, and new regression equations derived for nonlinear fitting of quasi‐reversible direct‐current polarograms were combined with the existing rigorous treatment and refinement of polarographic data to establish reliable metal/ligand models and accurate stability constants for the lead(II)/glycine/OH− and lead(II)/sarcosine/OH− systems (sarcosine = N‐methylglycine). In the case of glycine, the complexes [M(HL)], [ML], [ML2], and [ML3] were identified, and their stability constants (as log β) were established to be 10.51 ± 0.06, 4.58 ± 0.02, 7.19 ± 0.10, and 9.27 ± 0.02, respectively, the complex [ML3] being reported here for the first time (Table 2). The system with sarcosine involving [M(HL)], [ML], [ML2], [ML3], and [ML2(OH)2], with the stability constants (as log β) 11.01 ± 0.04, 4.18 ± 0.03, 7.23 ± 0.03, 9.1 ± 0.3, and 15.97 ± 0.07, respectively, is reported for the first time (Table 3). The log K1 value for PbII with sarcosine is a fraction of a log unit smaller when compared with the PbII complex with glycine, in agreement with the literature data for CuII, NiII, and ZnII showing the same trend for these two ligands. The proposed nonlinear curve‐fitting operations expand the applicability of polarography to study reliably and conveniently quasi‐reversible, on the polarographic time scale, metal/ligand systems (systems with involved heterogeneous kinetics).