A high-precision network analyzer system for the measurement of the electrode impedance of both stationary and non-stationary electrode, with special attention to the dropping mercury electrode

Abstract

A largely automatic instrument for the measurement of the admittance of a galvanic cell is described. During a measurement the frequency of the alternating voltage and the dc voltage are scanned stepwise in a programmed sequence. A high precision has been obtained by applying a programmed correction procedure and an automatic correction for short-term drift.The performance of the instrument is demonstrated on a dummy cell, a cell with supporting electrolyte only and on the determination of the kinetic parameters of the fast Cd(II) ion reduction on the DME in 1 M KC1. (I) INTRODUCTIONIn the past the impedance method has proved to be a powerful method for the study of electrochemical cells. The method may be applied for several purposes, e.g. characterization of an electrochemical object to find possible mechanisms in electrochemical processes, or in electroanalytical chemistry. In the last two cases the electrochemical object is mostly the dropping mercury (DME), because of its advantages of reproducibility and relative chemical inertness.When accurate measurements were required, ac bridges [ 1--4] were long considered to be superior to direct-measuring, phase-sensitive devices [ 5--11 ]. However, the (manual) operation of an ac bridge is tedious and time-consuming, especially in the case of a non-stationary object. In particular, in studies of electrode processes, it is increasingly recognized that many data points are needed, in both a wide frequency and a wide dc potential range, in order to be able todistinguish the possible contributions of charge transfer, diffusional mass transport, coupled chemical reactions and adsorption phenomena. It is therefore understandable that in recent developments much effort has been devoted to design more or less automatically operated, directmeasuring devices. For example, Huebert [ 12], using several tuned amplifiers, was able to apply a number of frequencies simultaneously to the cell, but still

The mechanism of the Cd(II) reduction in 1 M fluoride, perchlorate and chloride solutions at the DME, a combined DC, AC admittance and demodulation study

“…The ac measurements were performed as usual with the network analyser system described earlier [11]. The values of the double layer capacitance C a in the supporting electrolyte were also measured with this instrument at 1 kHz and at potential intervals of 3 mV.…”

Reduction of Cd(II) Ions in the Presence of Tetraethylammonium Cations. Adsorption Effect on the Electrode Process

“…The Hg mass drop is indicated in Table 1. For these measurements, the network analyzer system described earlier [23,24] was used. The cell impedance was analyzed in the frequency range of 80-10,000 Hz at 5 mV intervals of the dc potential within the Faradaic region.…”

A study of the reduction of tris-oxalato Fe(III) from aqueous 1 M oxalate solution at a polycrystalline gold electrode

“…In view of our recent discussions about the posslbihty of multi-step mechanisms [18,19] and in view of the conclusion drawn for the reduction of tris-oxalato Fe(III) at the mercury electrode [12], it is also logical to consider the posslbdity of such a mechanism here. The steep change of ct with potential suggests that kf could possibly be adapted to a so-called CEC mechanism, i.e.…”

“…the electron transfer is both preceded and followed by a "heterogeneous chemical reaction step", the rate constant of which is independent of potential. If it is assumed that the rate constant of the electron-transfer step ~s related to the potential simply by k 1 = k~ ° exp(-al@ ), with a 1 close to 0.5, the overall rate constant will be given by [18,19] However, in view of the high negauve charge of the electroactlve spectes and the large potential range involved, it should be envisaged that the non-linear potential dependence of In kf is (partially) due to the variation in the double-layer structure. In order to estimate this effect, the Frumkln correction should be applied in a way analogous to eqn.…”