  Optical methods for monitoring the electrochemical reaction at the interface
are advantageous because of their table-top setup and ease of integration into
reactors. Here we apply EDL-modulation microscopy to one of the main components
of amperometric measurement devices: a microelectrode. We present experimental
measurements of the EDL-modulation contrast from the tip of a tungsten
microelectrode at various electrochemical potentials inside a
ferrocene-dimethanol \ch{Fe(MeOH)2} solution. By the combination of the
dark-field scattering microscope and the lock-in detection technique, we
measure the phase and amplitude of local ion-concentration oscillations in
response to an AC potential as the electrode potential is scanned through the
redox-activity window of the dissolved species. We present the amplitude and
phase map of this response As such, this method can be used to study the
spatial and temporal variations of the ion-flux due to an electrochemical
reaction close to metallic and semiconducting objects of general geometry. We
discuss the advantages and possible extensions of using this microscopy method
for wide-field imaging of ionic currents.
