During electrolysis, the hydronium ions [H3O+] and the hydroxide ions [OH-] are depleted at the cathode and the anode respectively. This depletion of the ions results in the evolution of hydrogen and oxygen at the anode and cathode leading to a change in pH in the electrolyte layer close to the electrodes . This pH gradient can be related to the concentration gradient to obtain valuable information about the mass transport and reaction kinetics. Though various techniques have been explored to investigate the pH gradients, the fluorescence microscopy have an edge over other techniques as it can produce visual images [2, 3]. Fluorescent microscopy makes use of the optical property of the fluorescent dyes to record changes in pH. Disodium fluorescein salt will be used in this study which produces fluorescence of varying intensity (sigmoidal curve) with the maximum intensity between pH 5 and 7. This optical analysis of the pH gradients will help quantify the mass transfer of electrolytes to the electrode surface which will complement the bubble growth measurement.
 A.T. Kuhn, C.Y. Chan, pH changes at near-electrode surfaces, Journal of Applied Electrochemistry, 13 (1983) 189-207, https://doi.org/10.1007/BF00612481.
 N.C. Rudd, S. Cannan, E. Bitziou, I. Ciani, A.L. Whitworth, P.R. Unwin, Fluorescence Confocal Laser Scanning Microscopy as a Probe of pH Gradients in Electrode Reactions and Surface Activity, Analytical Chemistry, 77 (2005) 6205-6217, https://10.1021/ac050800y.
 J. Pruchyathamkorn, M. Yang, H.M.A. Amin, C. Batchelor-McAuley, R.G. Compton, Imaging Electrode Heterogeneity Using Chemically Confined Fluorescence Electrochemical Microscopy, The Journal of Physical Chemistry Letters, 8 (2017) 6124-6127, https://10.1021/acs.jpclett.7b02925.