Transforms and impedance

The last of these is the impedance which has been considered throughout this chapter. We now consider forced convection. For low frequencies the diffusion layer thickness due to the a.c. perturbation is similar to that of the d.c. diffusion layer in these cases convection effects will be apparent in the impedance expressions. For the rotating disc electrode these frequencies are lower than 40 Hz33. For higher frequencies where the two diffusion layers are of quite different thicknesses, the advantage of hydrodynamic electrodes is that transport is well defined with time, as occurs with linear sweep voltammetry. 

Applications of double hydrodynamic electrodes are particularly interesting because the change of phase in the current measured at the downstream electrode permits discrimination between the electron flow and the flux of electroactive species produced at the upstream electrode34. 

Modulation of the convective flux, originally proposed by Bruckenstein35, leads to the electrohydrodynamic impedance. It has been used for determining kinetic parameters and diffusion parameters in Newtonian and Ostwaldian fluids, and in corrosion. 

Whatever the excitation, the transformation of the response from the frequency to the time domain (Fig. 11.21) is done with the inverse Fourier transform, normally as the FFT (fast Fourier transform) algorithm, just as for spectra of electromagnetic radiation. Remembering that the Fourier transform is a special case of the Laplace transform with 

Macdonald, Transient techniques in electrochemistry, Plenum, New York, 1977, Chapter 7. 

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