Vol. 7, Issue 7, Part A (2025)

This study explores the electrochemical reduction behavior and complexation priority of Cu²⁺, Cd²⁺, and Ag⁺ ions in the presence of L-cysteine across a pH range of 1.0 to 6.0 using cyclic voltammetry. L-cysteine, a biologically significant thiol-containing amino acid, forms coordination complexes with transition metal ions via its -SH, -NH₂, and -COOH functional groups. The voltammetric analysis reveals pH-dependent changes in peak potential and peak current, indicative of metal-ligand interactions. At low pH (1-2), Cu²⁺ exhibits the strongest complexation with cysteine, as evidenced by significant suppression of its cathodic peak current. From pH 3 to 6, Cd²⁺ shows the most pronounced peak current suppression, reflecting the formation of stable Cd-cysteine complexes. Ag⁺, in contrast, demonstrates relatively weak complexation throughout the pH range. The observed complexation trend follows the order Cd²⁺ > Cu²⁺ > Ag⁺, consistent with thermodynamic stability data and Pearson’s HSAB theory. The findings emphasize the critical role of pH in modulating ligand nucleophilicity and complex stability, with implications for metal ion speciation, bioavailability, and electrochemical sensing applications.