By varying the total and the relative concentrations of a strong acid (HClO4) and a pH-sensitive fluorescent dye (8-hydroxypyrene-1,3,6-trisulfonate), this work demonstrates that both the hydrogen evolution reaction or the oxygen reduction reaction can be selectively and optically studied at an electrochemical interface. The local pH shift driven by the redox reaction can be visualized through fluorescence imaging of the interface. The use of finite strong acid concentrations further serves to constrain the pH change to a thin layer adjacent to the surface. This chemical confinement of the fluorophore improves the system's resolution and enables micrometer scale heterogeneity on the electrode surface to be readily visualized.