where the pK a represents the major species coexisting in the medium, and H 3 O + results f rom the protons generated electrochemically by the electrodes. At the equilibrium between t wo species, the pK a value is equivalent to the pH. The value of the pK a was calculated integrating the area of the oxidation peak f rom the beginning of the oxidation to the moment of the tautomerization. The pH for the minimum of f luorescence observed at 120s, calculated f rom the Faradaic currents was 4.2, which is in line with the pKa values reported in literature .