Challenges in Modelling and Optimization of Stability Constants in the Study of Metal Complexes with Monoprotonated Ligands. Part II

The influence of 2‐hydroxy‐3‐(2‐hydroxy‐1,1‐dimethylethyl)amino]propane‐1‐sulfonic acid (AMPSO=HL) on systems containing copper(II) was studied by glass‐electrode potentiometry (GEP) and direct‐current polarography (DCP), at fixed total‐ligand‐to‐total‐metal‐concentration ratios and various pH values (25°, 0.1M KNO₃ medium). The predicted model (CuL]⁺, CuL(OH)], CuL₂], CuL₂(OH)]^?, CuL₂(OH)₂]^2?, and CuL₃]^?) and the overall stability constants for species found were obtained by combining results from both electrochemical techniques. The last five complexes are reported for the first time. For the species CuL]⁺, CuL₂], CuL₃]^?, and CuL₂(OH)₂]^2?, it was possible to determine stability constants with reasonable certainty and their values, as log β, were found to be 4.62±0.04, 9.5±0.1, 13.4±0.1, and 21.2±0.1, respectively. For the species CuL(OH)] and CuL₂(OH)]^?, stability constants 11.7±0.2 and 15.6±0.2, respectively, are presented as indicative values. It was demonstrated that AMPSO buffer may decrease the Cu²⁺ concentration by ten orders of magnitude by forming complexes with Cu²⁺. For the first time, the correction in DCP waves for the adsorption of the ligand and quasi‐reversibility of the metal allowed to determine stability‐constant values that are in good agreement with the values obtained by GEP. The importance of graphic analysis of data and significance of employing two analytical techniques was demonstrated; neither GEP nor DCP would be able to provide the correct M/L/OH^? model and reliable stability constants when used independently.