Interpretation of non–Nernstian slopes in graphic analysis of data collected in pH range close to deprotonation of a ligand: Part I. A glass electrode potentiometric and polarographic study of Cd–(TAPSO)x–(OH)y and Zn–(TAPSO)x–(OH)y systems

In this work, the complexation of cadmium and zinc ions by 3-[N-tris(hydroxymethyl)methylamine]-2-hydroxypropanesulfonic acid (TAPSO), a commercial biological buffer, was evaluated using three electrochemical techniques, at fixed total-ligand and total-metal concentration ratio and varied pH, at 25.0 ± 0.1 °C and ionic strength set to 0.1 M KNO3. For both metal–ligand systems, complexation was evidenced in the pH range close to deprotonation of the ligand and the final models were optimised after a meticulous graphical analysis. –(TAPSO)x–(OH)y system, two complexes, CdL and CdL2, were identified in the buffering region of the ligand. The proposed final model for this system is: CdL, CdL2 and CdL2(OH) with stability constants, as log β, of 2.2, 4.2 and 8.6, respectively. –(TAPSO)x–(OH)y system, the complex ZnL is the main species formed in the buffering pH range. The proposed final model is ZnL, ZnL(OH) and ZnL(OH)2 with overall refined stability constants (as log β) to be: 2.5, 7.2 and 13.2, respectively.