Automated Impedance Measurements in Electrochemical Cells
Abstract In this paper a system is described for impedance measurements in electrochemical cells, integrated by a fast potentiostat, signal generator, digital transient recorder and a computer interface. The treatment of the signal is made by obtaining the correlation between the response cell signal with the in-phase and in-quadrature signals, as is made in the frequency response analysis systems, but the quadrature component being numerically generated. This allows one to obtain the d.c. first and second harmonic cell responses in a single experiment, as well as the a.c. phase selective response by programming the reference signal phase angle. The results obtained with a dummy cell showed the validity of the procedure since the deviation of the measured results from the theoretical data is low. The C/E curve of the interface 1 M KCI/Hg at 25°C is concordant with that previously reported in the literature. Moreover, the system has been tested with the reduction of Cd(II) ion.
Fourier transform square-wave voltammetry
Abstract In this technique a potential excitation waveform is obtained by a superposition of a small amplitude square-wave modulation on a staircase function. The square-wave cycle is repeated several times (usually 16) at each potential step. The electrode response is sampled at even time intervals 2 n (where n = 2, 3, 4…) times per square-wave cycle, and the fast Fourier transform of the electrode response at each potential step is performed. This allows the electrode admittance to be calculated for various odd harmonic frequencies and plotted as a function of the staircase potential. In addition, the average current at each potential step as well as the even harmonic response generated by non-linearity of the studied system can be obtained from collected data. The admittance data are almost identical with those obtained from traditional ac voltammetric experiments, but the equipment is less expensive and the experiments are run much faster and provide more information. The technique was tested by studying kinetics of Cd 2+ reduction from NH 4 + | NH 3 (aq) buffer on the static mercury electrode. Experimental results were compared with those obtained by digital simulation. Diagrams of electronic circuits used in the experiments are also presented.
Étude et réalisation d'un dispositif d'enregistrement numérique de la réponse transitoire d'une interface électrode-électrolyte
La reponse transitoire a une perturbation electrique d'une interface metal-electrolyte doit etre enregistree pendant un temps assez long (de 10 -5 a 10 2 s) car plusieurs processus, ayant des echelles de temps differentes, sont impliques dans une telle experience. Les methodes conventionnelles utilisant un oscilloscope sont, a la fois, longues et peu precises. Les auteurs decrivent un dispositif electronique qui permet d'echantillonner la reponse et de la stocker sous forme numerique, a des instants choisis d'avance. Le nombre d'echantillons etant limite (1 000 dans l'exemple present), il est necessaire d'optimiser la sequence des instants d'echantillonnage. Il est aussi necessaire d'utiliser un circuit echantillonneur-bloqueur pour eviter des erreurs systematiques provenant de la conversion analogique-numerique.