INTERFEROMETRIC MEASUREMENT OF DEPLETION LAYER STRUCTURE AND VOLTAMMETRIC DATA IN CONCENTRATED ORGANIC REDOX SOLUTIONS

Phase-measurement interferometric microscopy (PMIM) of the depletion l ayer surrounding 15-mu m-radius Pt disk electrodes is described. PMIM images record spatial variations in the optical height (Delta h) resul ting from refractive index gradients generated by an electrochemical r eaction. Images obtained during the voltammetric reduction of 0.1-1.5 M nitrobenzene in benzonitrile solutions containing 0.2 M tetra-n-buty lammonium hexafluorophosphate are compared to predictions based on the Lorentz-Lorenz formula and Saito's analysis of transport to a disk-sh aped electrode. Good agreement between theory and experiment is obtain ed at distances within +/-10 radii of the electrode; at larger distanc es, density-driven fluid convection reduces the refractive index (and concentration) gradients to negligible values, Optical voltammograms, i.e., plots of Delta h vs electrode potential (E), are shown to faithf ully mimic the true i-E response, allowing a purely optical measuremen t of the electrochemical response of a microdisk electrode.