A Study of Depletion Layer Effects on Equivalent Circuit Parameters Using an Electrochemical Quartz Crystal Impedance System

An electrochemical quartz crystal impedance system (EQCIS) was used to investigate depletion layer effects on equivalent circuit parameters of piezoelectric quartz crystal resonance in electrochemical processes of CoSO4 aqueous solutions containing 0.4 mol/L ethylenediamine + 0.5 mol/L Na2SO4, 0.2 mol/L 1,10-phenanthroline + 0.5 mol/L Na2SO4 and K3Fe(CN)6, or K4Fe(CN)6 aqueous solutions containing 1.0 mol/L KCl in potential cycling experiments, respectively. The impedance data were analyzed according to Martin's model. For the former two systems, motional resistance R 1, series resonant frequency f s, and motional inductance L 1 changed reversibly with potential, and the observed values of ΔR 1, ΔL 1, and Δf s roughly satisfied equations reflecting liquid loading effects, suggesting that changes in these parameters might be mainly governed by local variations in solution density and viscosity near the electrode surface. For the latter two systems, reversible changes in R 1 with potential coincided well with the simulated results from variations in solution density and viscosity near the electrode surface; however, besides reversible changes of ΔL 1 and Δf s with potential, rising drifts of f s and decreasing drifts of L 1 were found at oxidation potentials. The drifts of f s and L 1 observed are believed to result from mass changes of the gold electrode, and cyanide and chloride corrosion of the gold electrode at oxidation potentials may play an important role for the drifting phenomena. It is concluded that quantitative analyses of ΔR 1, ΔL 1, and Δf s obtained from the EQCIS provide the possibility for differentiating the net depletion layer effect with the change in electrode mass, and the ΔR 1 response may be well used for evaluating local changes in liquid loading inside the depletion layer compared with responses of Δf s and ΔL 1.