Impedance of the copper electrode in isopotential solutions containing complexes of Cu(II) and glycolic acid

In the frequency interval from 0.01 Hz to 50 kHz, the impedance of copper electrodes is investigated in a series of isopotential solutions, which contain various amounts of ligands and of complexes of Cu(II) and glycolic acid, but a constant concentration of Cu²⁺ ions. An equivalent electrode scheme is analyzed, in which two successive one-electron steps and the diffusive flows of both the charged particles and the intermediate are taken into account, and the presence of a homogeneous chemical reaction is also considered. It is established that the exchange current density for the first electron in a series of solutions, which contain 5.3 mM Cu²⁺, is essentially independent of the concentration of free ligand and is equal to 0.1 mA/cm². This indicates that the electrically active particles are aqua complexes of Cu²⁺. It is proposed that the chemical step, which is suggested by the impedance data, is the dissociation of glycolic acid, which releases the active form of the ligand.