Cu²⁺, Pb²⁺, and Cd²⁺ were individually and simultaneously determined using a novel and effective electroanalytical approach that has been devised and improved. Cyclic voltammetry was used to examine the electrochemical properties of the selected metals, and their individual and combined concentrations were determined by square wave voltammetry (SWV) using a modified pencil lead (PL) working electrode functionalized with a freshly synthesized Schiff base, 4-((2-hydroxy-5-((4-nitrophenyl)diazenyl)benzylidene)amino)benzoic acid (HDBA). In a buffer solution of 0.1 M tris–HCl, heavy metal concentrations were determined. To improve the experimental circumstances for determination, scan rate, pH, and their interactions with current were studied. At some concentration levels, the calibration graphs for the chosen metals were linear. The concentration of each metal was altered while the others remained unchanged for both the individual and simultaneous determination of these metals, and the devised approach was proven to be accurate, selective, and rapid.