Lipoic acid has a carboxylic group (pKa 4.7) and forms stable complexes
with Cu2+, Fe2+, Mn2+, and Zn2+ with the complex being almost entirely with the carboxylate group [26,27]. However, in the presence ofexcess copper LA reduces Cu2+
to form the Cu+–DHLA complex. In this complex, two Cu+ions are coordinated to the two thiolic sulfuratoms of every ligand and the product is adsorbed strongly obtaining a sensitive method to determine LA [25].