Depletion Layer Effects on the Response of the Electrochemical Quartz Crystal Microbalance

Electrochemical quartz crystal microbalance (EQCM) measurements in solutions of K₃Fe(CN)₆ or K₄Fe(CN)₆ reveal a significant frequency response due to changes in the density (ρ) and viscosity (η) of the depletion layer during cyclic voltammetric experiments. Frequency shifts exceeding 15 Hz are obtained with a 5-MHz AT-cut quartz crystal during the voltammetric oxidation of 100 mM Fe(CN)₆^4- (or reduction of Fe(CN)₆^3-) in 1.0 M Na₂SO₄ solutions. The observed frequency shifts are shown to result from small variations (<3%) in the depletion layer density and viscosity that accompany oxidation or reduction of the electroactive species. Even at smaller concentrations (20 mM), changes in the density and viscosity of the depletion layer are manifested as frequency changes of several hertz upon potential cycling. The measured data are in excellent agreement with a model that accounts for the experimentally measured values of (ρη) for the solutes and the exponential decay of the shear wave in the fluid, normal to the vibrating EQCM surface. Similar results are obtained with Ru(NH₃)₆³⁺, demonstrating that the observations made in Fe(CN)₆^3- or Fe(CN)₆^4- solutions are not due to the formation of adsorbed films. These results suggest that analysis of small frequency shifts during EQCM studies needs to account for changes in ρ and η within the depletion layer.