On the impedance of galvanic cells—XVI The impedance of the dropping mercury electrode in aqueous 1 M KCl with K+ discharge

The discharge of K+ ions at the dropping mercury electrode (dme) in 1 M KCl solution at 25°C has been studied by means of analysis of the electrode impedance at different frequencies, using the complex plane method. It is shown that the electrode reaction is almost entirely diffusion-controlled, but a slight influence of activation polarization on the faradaic impedance can be detected at potentials between −1.95 and −2.0 V(sce). The heterogeneous rate constant ksh is estimated as ca 0.7 cm/s. Values of the double layer differential capacitance in the potential region where the electrode reaction occurs are calculated. Special attention is given to the inaccuracy of the results inherent in the method

The study of interfacial phenomena using impedance measurements in conjunction with complementary techniques

It is explained why and how in particular cases additional information from complementary methods should be used to validate and to improve the information obtained from impedance measurements. As an example we present the direct determination of charge density vs potential curves with a newly developed dropping mercury micro electrode, and its application to prove the specific adsorption of the alkali ions at the mercury / aqueous solution interface. As a second example the study of electrode reactions with a finite rate of charge transfer and complicated by adsorption of the reactant is discussed. Here the combination of impedance measurements with high frequency demodulation measurements is indispensable. This is illustrated by some results for the system (TI (I) / Tl (Hg) in 1 M HCI O4

On the meaning of the impedance concept in the case of an object that varies with time and in the case of a swept F frequency

Expressions are derived that describe the behaviour of a condenser the capacity of which varies linearly with time under potentiostatic and galvanostatic a.c. conditions. The “impedances are found to be different. Application to the dropping mercury electrode is indicated. Also the behaviour of a constant capacitor subject to a frequency swept a.c. potential or current is calculated. The admittance of the capacitor is found to have increased in the latter two cases

Some basic views on the influence of reactant adsorption on wave shapes in d.c., a.c. and linear sweep voltammetry

A comprehensive discussion is presented of the basic views and quantitative expressions which are needed to understand the influence of adsorption of electroactive species on the shapes of the waves in d.c. polarography, a.c. polarography and linear sweep voltammetry. It is shown that it is either the non-stationary nature of the techniques or the nonstationary nature of the electrode surface area (or both) which shows up the influence of adsorption

On the validity of the polarographic determination of stability constants of metal ions in the case of adsorbable electroactive species

The idea that anion-induced adsoprtion of the electroactive species might interfere with the determination of stability constants via shifts in the half-wave potential is critically examined. It is shown that this adsorption occurs only if the standard thermodynamic potential at the electrode-solution interface differs from that in the solution. The final conclusion is that the position of the reversible half-wave potential is not influenced by the adsorption of the electroactive species if the reactions between the complexes, and all adsorption phenomena, proceed infinitely fast

On the impedance of galvanic cells XXVIII. The frequency-dependence of the electrode admittance for systems with first-order homogeneous chemical reactions and reactant adsorption occurring simultaneously

Equations are derived for the interfacial admittance of an electrode at which the electrode reaction O+neR occurs assuming that the charge transfer is infinitely fast and that either O or R is involved in a first-order homogeneous reaction delivering the substance Y, or that both O and R are involved in a catalytic reaction. If, moreover, adsorption of O and R or Y at the electrode surface is assumed, rather, more complicated expressions are obtained. A number of simpler limiting cases are treated in more detail and it is concluded that in many practical cases the corresponding reduced equations describe the electrode admittance with sufficient precisio

On the impedance of galvanic cells XXVII. The temperature-dependence of the kinetic parameters of the hydrogen electrode reaction on mercury in concentrated HI

The impedance of a dropping mercury electrode in 57% HI (7.6 M) was measured at temperatures between −35° and +25°C. In a certain potential and temperature region, two reactions were found to be proceeding simultaneously: the reversible Hg/HgI4−2 reaction and the irreversible H+/H2(Hg) reaction. Analysis of the impedance data gave information about the impedance parameters of both electrode reactions, and about the double-layer capacity. The temperature-dependences of the transfer coefficient and the exchange current density of the H+/H2(Hg) reaction are reported. The transfer coefficient decreases with temperature. The apparent heat of activation is 21.6 kcal mol−1

On the impedance of galvanic cells XXII. Determination of the parameters of the double-layer impedance from the electrode impedance and the electrocapillary curve in the case of specific

The electrode impedance in the case of specific adsorption of the electroactive species is discussed, critically applying the charge separation model of Delahay. Comparison with experiments performed with the In3+/In(Hg) couple in 1 M KCNS, comprising impedance, chronopotentiometric and surface-tension measurements, is given. An improved theory of the relation between surface tension and double-layer parameters is presented, leading to a better agreement with experiment. Consistent values are obtained for the surface excess of In3+. It is shown that the linear relation between the value of thè double-layer capacity and Tl+ concentration that has been found earlier at low concentrations, also holds for concentrations up to 2 mM. The consequences of the application of a modified treatment given by Reinmuth, are discussed

A faradaic impedance study of the reduction of oxygen from aqueous alkaline solution at the dropping mercury electrode

The reduction of oxygen to hydrogen peroxide at a dropping mercury electrode in an aqueous solution of 1 M KNO3+0.04 M KOH (pH=12.35) has been studied by means of impedance measurements as a function of frequency and d.c. potential. The reaction appears to be nearly reversible in the dc sense, but quasi-reversible in the ac sense. The impedance data obey the Randles' equivalent circuit with the following apparent values for the kinetic parameters: standard heterogeneous rate constant ksha=0.035 cm s−1 and cathodic transfer coefficient αac=0.22. The results are interpreted in terms of a two-step charge transfer mechanism with the step O2+eO2− being rate-determining