It was not easy for the oxygen to get electrons in neutral or basic media for the reason that E20 was less than E10, and the amounts of oxygen that was to be reduced at the cathode decreased. Accordingly, the amounts of oxidized UA and FA also decreased at the anode based on the electrons getting-losed conservation in the oxidation–reduction reactions. As a result, the oxidation peak currents of UA and FA decreased when the pH of the solution exceeded 6.00. Hence, pH 6.00 was selected for their determination at the PAIUCPE.