Resonant frequency measurements as an alternative to phase-selective A.C. polarography in tensammetry and pseudocapacitance determinations

Abstract

Several papers have appeared that describe the use of on-line digital computers in methods for the determination of the double-layer capacitance [l-5] . In this field, three lines of approach can be distinguished: (1) adaptation of phase-selective measurements with lock-in amplifiers or network analyzers to computer control as described by Bongenaar et al. [3] ; (2) the use of the computer to generate a non-sinusoidal cell excitation with a repeating sequence and to produce either Fourier or Hadamard transforms to obtain the cell impedance as first suggested by Creason and Smith [6] and improved by Seelig and de Levie [l] ; (3) the use of computer-generated potential steps as cell excitation and processing of the resulting cell current/ time profile as proposed by Bos [ 41 and Britz [ 51. A comparison of the three categories shows that the first requires expensive extra hardware such as programmable function generators and network analyzers or lock-in amplifiers. The second category relies on a stringent time relationship between the excitation signal and the sampling of the cell response and requires special computer hardware. Both the first and the second category require analog circuitry for the potentiostat, iR compensation and i-to-E conversion functions. The accuracy of the results will depend on the quality of this circuitry with regard to flat frequency response, linearity, stability and the magnitude of the phase shift introduced by the iR compensation circuitry. In the presence of electroactive compounds, the measurements in the first and second categories can provide information on the double-layer capacitance as well as on the kinetics of the electrode reactions.