Challenges in modelling and optimisation of stability constants in the study of Cu-(TAPS)x-(OH)y system by polarography

This work describes the application of polarography, a technique scarcely used for modelling and optimisation of stability constants, in the study of copper complexes with (2-hydroxy-1,1-bis(hydroxymethyl)ethyl)amino]-1-propanesulfonic acid (TAPS). Direct current polarography (DCP), using low total copper ion and large total ligand to total copper concentration, enabled the full characterization of Cu-(TAPS)_x-(OH)_y system, whose complexation occurs in the pH range of copper hydrolysis and Cu(OH)₂ precipitation. Cu-(TAPS)_x-(OH)_y system was studied by DCP and glass electrode potentiometry (GEP) in aqueous solution at fixed total ligand to total metal concentrations ratios and varied pH values (25.0 °C; I = 0.1 M, KNO₃). The predicted model, as well as the overall stability constants values, are (as log β): CuL⁺ = 4.2, CuL₂ = 7.8, CuL₂(OH)⁻ = 13.9 and CuL₂(OH)₂²⁻ = 18.94. GEP only allowed confirming the stability constants for CuL⁺ and CuL₂ and was used to determine the pKa of TAPS, 8.342.Finally, a briefly comparative analysis between TAPS and other structural related buffers was done. Evaluation based on log β_CuL versus pKa revealed that TES, TRIS, TAPS and AMPSO coordinated via amino and hydroxymethylgroups forming a five-membered chelate ring. For BIS-TRIS and TAPSO, and possibly DIPSO, one or more five-membered chelate rings involving additional hydroxyl groups are also likely formed.