A faradaic impedance study has been made of the reduction of Cd(II) ions from a 1 M KF base electrolyte at the DME. It could be shown that only the Cd(II) present in the solution and the end product, viz. Cd(Hg), are the diffusing partmles and that intermediates are present only at the interface. From the analysis of the impedance data it followed that at the interface the Cd(II) species is first subject to a heterogeneous chemical transfer and thereafter is reduced according to two discrete one-electron transfers, both obeying the Butler--Volmer equation. The kinetm parameters of the steps are reported. A number of other conceivable mechanisms were considered, but fortunately they all either could be defimtely ruled out or were found to be less acceptable.

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

It is shown that in the whole range of the composition of this base electrolyte the Cd(II) reduction can be explained on the assumphon of a reaction mechanism that consists of a chemical step followed by two one-electron transfers, the transfer coefficients of which both appear to be 0.5. The rate constant of the chemical step is shown to increase rapidly with chloride ion concentration. The dependence of the electrochemical rate constants on the composition of the base electrolyte is also reported and discussed.

A faradaic impedance study has been made of the reduction of Cd(II) ions from a 1 M KF base electrolyte at the DME. It could be shown that only the Cd(II) present in the solution and the end product, viz. Cd(Hg), are the diffusing partmles and that intermediates are present only at the interface. From the analysis of the impedance data it followed that at the interface the Cd(II) species is first subject to a heterogeneous chemical transfer and thereafter is reduced according to two discrete one-electron transfers, both obeying the Butler--Volmer equation. The kinetm parameters of the steps are reported. A number of other conceivable mechanisms were considered, but fortunately they all either could be defimtely ruled out or were found to be less acceptable.

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