We consider 2,2PEAQ at the pH range of 0 to 14. In this range, direct protonation of 2,2PEAQ does not occur (path PT58 is closed). In the presence of the electrode, the first ET appears at a potential of -0.464 V SHE . Depending on the pH of the solution, ET or PT may occur in the following step. If pH ≤ 5, the reduction reaction is followed by a PT reaction for an applied potential between -0.464 and -0.762 V SHE . In contrast, a PET reaction (2,2PEAQ to 2,2PEAQ-H) is even more likely to occur under strongly acidic conditions, as indicated by the E 0 value of -0.159 V SHE . According to the Nernst equation, protons along the ET-PET-PT path. The maximum applied potential required for the PET reduction reaction is -0.52 V SHE occurring at pH of 8 (i.e., the reduction potential at pH = 0, -0.048 V, lowers by -0.059/pH). To oxidize 2,2PEAQ-H 2 to 2,2PEAQ, the same 2PET reaction is anticipated to occur in the reversible pathway.