Equation ( ) shows the dissociation of the salt used for the electrolyte conductivity, which acts as the main proton carrier (Eq. ( )) as PF6 -has a much higher proton affinity than ACN (Eq. ( )). In our system, the acidity is generated from the electrodes while the conjugated base remains fixed and isolated from the medium by the Nafion. The proton concentration is similar to the one of Bu4PF6, but 100 times higher than the concentration of FAM (FAM 0.5µM, Bu4PF6 10mM vs prot ons electrogenerated calculated from the Faradaic currents ~14.5mM). Thus, as the acid generated by the Faradaic currents reaches the electrolyte, the protonation of PF 6 -(Eq. 2) and the migration of protons to FAM (Eq. 4) (f aster than the protonation of the acetonitrile (Eq. 3)) assures the availability of all the protons to interact with the acid-base pairs of the FAM. Also, due to the planar conf iguration of the electro chemical cell and the electrodes, the typical diff usion length is limited by the height of the cell, which is in t he order of f ew μm. Thus, the f luorescence in our electrolyte was mainly determined by the equilibrium in Eq. ( ). Then we can interpret the reversibility of the f luorescence spectra f rom a complete quenching during the oxidation and its total recovery after the reduction bias as the reporting of FAM f luorescence of the proton activity. In the inset of f ig.5 (c), we observe a delay of the f luorescence respect to the f aradaic charge, which we attribute to the delay of protons to cross the Naf ion membrane. However, as the time interval between the oxidation and reduction is much longer (30s), the equivalence in f ig 5 pH* vs. FAM f luorescence is a good approximation over a f ull cycle. We can def ine the proton activity of electrochemically generated acid in the cell, the pH* using the Faradaic currents (-log(Q/F*V)). Assuming the delay of the protons to cross the Naf ion barriers, we used the correlated temporal dependence of Faradaic currents and f luorescence shown in the inset of f ig. 5 (c) to generate FAM f luorescence vs. pH* shown in black in f ig. 5(c). We used the relation of the pH* with FAM f luorescence to calibrate the dissociation of acids in a solvent with the same composition of our electrolyte. We titrated diff erent acids including strong and weak acids (HCl and TFA f or the strong and acetic and formic for the weak acids, respectively). We recorded the FAM f luorescence signal corresponding to the titration of the acids using a solvent with the same composition as our medium (insets f ig 5(d) and 5(e)). To obtain the pKa dependence, we considered the acid dissociation constant: