Linear sweep voltammetry—Bandpass limitations andmaximum usable sweep rate in the study of faradaic processes

Abstract

Summary The bandpass limitations of the potentiostat and current measurer may affect the shape and characteristics of the LSV polarization curves. This problem is studied in the case of a nernstian faradaic reaction controlled by linear and semi-infinite diffusion. Three independent parameters are in general required to characterize the effects of ohmic drop through the uncompensated resistance, double layer charging and bandpass limitations. Under critical conditions the number of parameters is reduced to two. The solution is given in the form of an integral equation which was numerically computed for various sets of parameter values mainly in the case of critical conditions. The main effects of bandpass limitations on the faradaic peak are a shift in the same direction as that due to ohmic drop and an enhancement of the peak height opposite to the effect of ohmic drop. These effects increase rapidly with the sweep rate. Working curves are given that allow the maximum sweep rate corresponding to a specified permissible error to be computed. With potentiostat and current measurer amplifiers having gain-bandwidth product pulsations of the order of 107 rad s−1, the effect of bandpass limitations in current practice of LSV falls into the range of experimental errors up to about 1000 V s−1. Such amplifiers having moreover high input impedance specifications and 6 db/octave operating gain-frequency characteristics are required anyway in order to ensure the transfer function to be second order with a good accuracy. The theoretical analysis was experimentally verified with respect to peak portion and peak height by introducing purposely an additional resistor in the potentiostat loop. Second order conditions appear to be the most convenient to reach the maximum analogical compensation keeping the possibility of measuring accurately the residual uncompensated resistance and of correcting the residual ohmic drop mathematically. The best operating conditions are shown to involve a slightly overoscillatory double layer response.