Challenges in modelling and optimisation of stability constants in the study of metal complexes with monoprotonated ligands

The influence of 3-[N,N-bis(2-hydroxyethyl)amino]-2-hydroxypropanesulfonic acid (DIPSO) on solutions containing copper(II) was studied by two analytical techniques, glass electrode potentiometry (GEP) and direct current polarography (DCP). The readings were taken at fixed total DIPSO to total copper(II) concentration ratios and various pH values, at 25.0(plus-minus)0.1 °C and ionic strength 0.1 M KNO3. Combined interpretation of data from DCP and GEP indicated the formation of six main species, CuL+, CuL(OH), CuL(OH)2-, CuL2, CuL2(OH)- and CuL2(OH)22- for which stability constants (as log (beta)) were found to be 4.2(plus-minus)0.2, 11.8(plus-minus)0.2, 17.6(plus-minus)0.1, 8.1(plus-minus)0.1, 14.5(plus-minus)0.2 and 20.6(plus-minus)0.2, respectively. These results warn users of DIPSO that this buffer should not be used for studies involving this metal ion, unless the extent of complexation is accounted for. In addition, the magnitude of stability constants of the species present in the buffer pH range suggests that DIPSO can be used simultaneously as pH and metal ion buffer.