Lipoic acid has a carboxylic group (pKa 4.7) and forms stable complexes
with Cu
2+
, Fe
2+
, Mn
2+
, and Zn
2+
with the complex being almost entirely with the carboxylate group [26,27]. However, in the presence of
excess copper LA reduces Cu
2+
to form the Cu+
–DHLA complex. In
this complex, two Cu+
ions are coordinated to the two thiolic sulfur
atoms of every ligand and the product is adsorbed strongly obtaining
a sensitive method to determine LA [25].