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From the rate data obtained with impedance and demodulation voltammetry at varied concentrations of chloride ion acting as the catalyst, it is concluded that the cadmium ion reduction proceeds according to a CECEC mechanism. The second and third “chemical” steps show up because the catalyst accelerates specifically the first chemical and the first electrochemical step and because it does not affect the other steps to a significant extent. It is argued that the addition of a catalyst may serve as a general tool to unraveling the mechanism of an electrode reaction in a way that has some resemblance to variation of the electrode potential. The rate data, after correction for the Frumkin effect and complex formation, are treated as a function of the concentration, the surface excess and the surface activity of the catalyst, leading to a preference for the latter quantity. The way in which the catalyst exerts its activity is found to be greatly different from the results obtained earlier in this laboratory with thiourea catalyzing the Cd(II) reduction and with iodide ions catalyzing the Zn(II) reduction.

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 particles and that intermediates are present only at the interface. From the analysis of the impedance dat it followed that at the interface the Cd(II) species is first subject to a heterogeneous chemical transfer and there-after is reduced according to two discrete one-electron transfers, both obeying the Butler-Volmer equation. The kinetic parameters of the steps are reported. A number of other conceivable mechanisms were considered, but fortunately they all either could be definitely ruled out or were found to be less acceptable.

The reduction of Zn(II) has been studied by means of the faradaic impedance method in wide potential and frequency ranges and at varied water activity. The results obtained could be quantitatively explained on the assumption of a mechanism including at least the following consecutive steps: 1. (1) fast partial dehydration of the electroactive species; 2. (2) fast transport from the solution into the double layer; 3. (3) slow transfer of the first electron; 4. (4) a fast second partial dehydration step; 5. (5) slow transfer of the second electron. The hydration number of the Zn2+ ions in the solution was obtained from the shift of E 1 2 r and Eox with water activity, and was found to agree fairly well with the results obtained from the kinetic study.

The views developed in an earlier publication (J.H. Sluyters, E. Bosco and M. Sluyters-Rehbach, J. Electroanal. Chem., 241 (1988) 79) on the early stage of a nucleation process by means of a computer simulation are worked out. It appears that the value of the induction time, too, can be used to obtain information on the kinetics of nucleation.

A theoretical study is presented concerning the application of a high-frequency alternating current, amplitude modulated by a low-frequency sine wave, to a galvanic cell. Based on the correlation with the faradaic rectification technique, expressions are given for the low-frequency demodulation voltages of the faradaic process and for the double-layer capacity. Both responses, which are in-phase with the modulation signal, are combined by considering them as voltage sources, having respectively the faradaic impedance and the double-layer impedance as internal impedances. The resulting demodulation voltage appears to possess an in-phase and a quadrature component, from which information can be obtained of the high-frequency as well as the low-frequency behaviour of the cell.

Summary The theory of faradaic rectification polarography is extended to systems of higher order stoichiometry. The couples Zn 2+ /Zn(Hg) in 0.5 M MgSO 4 and Cd 2+ /Cd(Hg) in 1 M KCl are studied with the aid of this technique in a wide potential range. The Zn 2+ /Zn reaction is interpreted as a multistep mechanism. The Cd 2+ /Cd reaction did not appear to be a simple linear reaction. Some higher order mechanisms are tested using the measurements.

After defining probabilities for growth, decline and being idle as a function of cluster size, the behaviour of a unit volume or a unit surface as for nucleation is studied by a simple simulation program. An interpretation of the induction time and the critical cluster size is obtained together with the size distribution and the distribution of the time to attain a certain size. It is also found that in the earliest stage of the process, its rate can pass through a minimum. Adsorption and underpotential deposition can also be described formally in this way.

A critical study is made of the mathematics needed to describe the current response towards a potential step perturbation of an electrode at which phase formation takes place by the process of nucleation and diffusion-controlled hemispherical growth. As a result, an expression for the current-time transient is derived which is more general than the expressions for the limiting cases used hitherto. Procedures are discussed for fitting experimental data to this new expression in order to obtain the number density of nucleation sites and the nucleation rate constant. The theory is extended to cases where overlap of growing nuclei has to be accounted for, resulting in a new expression as well. From a comparison with existing theories it is concluded that the present derivation, and its result, although also not rigorous, is to be preferred because it is unique in extrapolating to both short-time and long-time limiting behaviour exactly.

A theory is developed of how to obtain the reaction pathway(s) and the kinetic parameters of a metal ion reduction if the reduction proceeds in several heterogeneous steps and via several co-existing metal complexes in a so-called scheme of squares. The theory is applied successfully to the results obtained using the demodulation technique on the reduction of Cd(II) from aqueous NaClO 4 + NaF mixed base electrolyte at a dropping mercury electrode.

The adsorption of Cd(II) has been studied by measuring the interfacial admittance spectra at different dc potentials of the dropping mercury electrode (DME) in solutions of: (a) 1 M KBr+0.48 mM Cd(NO3)2,(b) 1 M KI+0.105 mM CdI2 and (c) 0.1 M NaI+0.9 M NaClO4 +0.116 mM CdI2 The experimental data were analysed using the procedures described in Part I of this series. The frequency dependence of the admittance corresponds to that of a reversible electrode reaction with reactant adsorption. The potential dependence of the resulting adsorption parameters can be mathematically described by a Langmuir isotherm for adsorption of Cd(II) with the adsorption energy being at least a quadratic function of potential. Detailed checks on the consistency of this model have been applied and were found to be satisfactory.

The general principles worked out in Part II of this series are applied to the theory of demodulation polarography. The cases of both single and double side band modulated ac perturbation are considered. The advantages of the latter are stressed. The theory is worked out for the four methods possible in principle. Galvanostatic perturbation control with detection of the demodulation voltage is believed to be the most suitable technique for fundamental work, whereas potentiostatic perturbations together with detection of the demodulation current appear to be more suitable for purposes within electrochemical analysis. The potentialities in the elucidation of electrode processes of the first method are shown, especially for its complementary properties towards the faradaic admittance technique.

The dropping mercury electrode and the hanging mercury drop electrode are advocated as objects in ellipsometry and reflectometry. Practical details are given and the method is demonstrated with some qualitative off-null measurements.

A number of theories, developed to predict the d.c. polarographic behaviour of adsorbed species, is discussed by considering them as special cases of the general theory of Reinmuth and Balasubramanian. Conclusions are drawn with respect to the validity of these treatments, especially concerning the frequently used diffusion layer approach. It is shown that this approach is fundamentally in error, but still suitable as an approximation, because of the non-stationary character of the dropping mercury electrode. Numerical calculations for Langmuirian adsorption are performed, showing that for not too extreme adsorption this approximation is sufficiently close for correct predictions of polarograms. However, in reverse, if deviation from normal behaviour is used to obtain information about adsorbed quantities, the more rigorous approaches will be required.

The analytical theory for adsorption-influenced limiting currents in normal pulse polarography is extended to conditions where the adsorption isotherm is non-linear. The case of Langmuirian adsorption at a stationary planar electrode is treated in detail, using the Reinmuth series for the dependence of surface coverage on time, and it is explained how the measured currents are affected by the isotherm parameters. The approach to Langmuirian adsorption at stationary spherical and expanding plane electrodes is briefly indicated. Finally it is pointed out how the problem can be solved for an arbitrary type of isotherm.

Recently the phenomenon of reactant and/or product adsorption in electrochemical systems has received renewed at tention, especially with a view to the explanation of anomalous d.c. polarograms [ 1--4]. For this explanation a theory is needed to describe the mass transport under the circumstances dictated by the system. The primary aim of such a theory is to find a relation between the surface excesses of the adsorbing species and time. Reinmuth and Balasubramanian have shown that for reversible systems this relation can be given by the same expressions, which were already derived for adsorption of electro-inactive species, provided that only the dimensionless parameters are given their appropriate meaning [5]. In this connection the fairly rigorous solutions in the form of a series expansion, presented by Holub [6] for linear adsorption and by Reinmuth [ 7] for Langmuirian adsorption, are of great importance [4]. The principle is that one attempts to write the surface excess as a polynomial of a dimensionless parameter containing time. The coefficient of the first term appears as a simple function of the adsorption coefficient, the other coefficients are given by a recurrence formula. While applying Reinmuth's solution for Langmuirian adsorption at an expanding plane (as a model for a dropping electrode), we found that it is quite suitable at short times, but not at large times, because of non-convergency of the series. Thus, it was possible to calculate only small coverages, up to ca. 60 to 80% of the equilibrium coverage (somewhat depending on the adsorption coefficient). The same problem was evidently encountered by Holub for the linear isotherm. However, he gave also expressions for an expansion in negative powers of the dimensionless time parameter [6], which could be applied at long times. In the present communication we want to show the results of such an expansion for the Langmuir isotherm. In Reinmuth's t reatment [7] an electrode is considered growing into a solution at constant potential E. The solution contains a surface-active species, the adsorption of which is controlled by diffusion only. In Reinmuth and Balasubramanian's t reatment [5] the same conditions are assumed with this difference that at the applied potential an electrode reaction

It is reported that, after injection of a solution of pentavalent niobium in 5 M HC104 into a 1.3 M citric acid+ 1 M NaC104 solution, only a small portion of the niobium is electrochemically active initially. This portion increases with time until after 2000 min an equilibrium is established where most of the niobium is electroactive. This transfer is shown to proceed with (pseudo) first order kinetics and its rate constant is reported together with estimates of the two equilibrium constants. In a solution in which this transfer is completed, the kinetics of the Nb(V)/Nb(IV) redox couple are studied at the DME by the impedance method. The operational transfer coefficient is found to be strongly potential dependent. This leads to the conclusion that the electron transfer is both preceded and followed by potential independent processes. The following process could be absent if the trnsfer coefficient of the electron transfer is allowed to deviate from 0.5.

Impedance measurements were performed to study the reduction of cadmium(II) at a DME from 1 M KF solutions containing 5, 20 and 50 m M thiourea. The frequency dependence of the interfacial admittance was found to be in agreement with the expressions valid for an ac-reversible electrode reaction complicated by reactant adsorption. The potential dependence of the admittance parameters was analysed in terms of a Langmuir isotherm with a potential-dependent adsorption coefficient. The surface excess of cadmium(II) increased with the thiourea concentration. The nature of the adsorbing species, the relation between cadmium(II) and thiourea adsorption, and the possible role of co-adsorption in the catalysis of the electrode reaction by thiourea are discussed tentatively. Back-extrapolation to zero concentration of thiourea also leads to the conclusion that in its absence the reduction of Cd(II) proceeds via the adsorbed state.

It is shown both theoretically and experimentally that light scattering by a homodisperse system of spheres on a metallic substrate gives rise to interference phenomena. If the spheres are growing, e.g. electrolytically, the light scattered in a certain direction varies with time in an oscillatory way. Perfect agreement between theory and experiment is obtained for mercury droplets growing electrolytically on a platinum substrate. Experimental results obtained at various scattering angles are reported, both for parallel and normally polarized light, incident perpendicularly to the substrate. From the oscillating light intensity pattern the size of the spheres can be determined as a function of time directly, without the use of a theory on electrodeposition. Peculiarly, this type of interference also occurs with non-coherent light.

It is argued that the use of the Avrami theorem, S(t) = 1−e−Sx(t), is in principle not allowed when applied to overlapping diffusion zones or, more generally, in all cases where the phantom nuclei overtake the front of their parent nuclei. Via computer simulations the overtake effect is shown to exist and is found to be surprisingly small. The use of a modified Avrami equation, S(t) = FSx(t)·[1 −e−Sx(t)], is suggested for such cases and the function F[Sx(t)] pertaining to diffusion-controlled growth is reported.

Summary Starting from Delahay's charge separation equations the consequences of adsorption of an electroactive species for its half-wave potential measured at a non-stationary electrode ( e.g. the dropping mercury electrode) are predicted. It is shown that the inconsistency between potentiometric and polarographic measurements recently reported by Bond and Hefter can be explained on this basis. Potential variations at open circuit to be expected at a dropping mercury electrode when both the Ox and the Red component are present and either or both of them are adsorbed are calculated with the aid of new derived theoretical expressions. The theory is confirmed by some epperiments concerning the Pb(II)/Pb(Hg) electrode in different media. It is concluded that the DME is not suitable for potentiometric work if reactant adsorption occurs.

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 KCl.

On the basis of the assumption that on a homogeneous surface area a probability 0 < P(E, c)

The enhancement of the dc polarographic reduction current of oxygen by adsorbed Pb(II) species first observed by Strnad in 1939 and later studied qualitatively by a number of other authors has been reconsidered both experimentally and theoretically. A number of possible reaction mechanisms is considered. The experimental results obtained in this work could be fitted well only to one of these mechanisms. It is concluded that the activation energy of the electrochemical reduction of H2O2 to OH− ions is lowered by an amount proportional to the surface excess of the Pb(II) species and that the participation of a Pb(II)−Pb(IV) oxidation-reduction cycle should be regarded as disproved.

On the basis of a recently published theoretical approach further proof is given for the CEE mechanism proposed earlier for the Cd(II) reduction in fluoride solution. It is shown that in perchlorate base electrolyte solution the same mechanism applies and that the rate constant of the chemical step is quite close to the value in fluoride solution. The theories available at present are found not to apply to the Cd(II) reduction in chloride solution because of the invalidity of the Randles equivalent circuit, contrary to experimental evidence published so far. The outstanding potentialities of the demodulation method for the study of charge-transfer mechanisms is clearly demonstrated by means of the systems mentioned.

The reduction of the Fe(III) oxalate complex at the dropping merury electrode in a solution of 1M K2C2O4+0.05 M H2C2O4 is studied, combining the results of dc, ac and demodulation polarography. It is shown that the double-layer capacity is influenced by the presence of the reactants due to their adsorption. A careful analysis is undertaken to explain the extreme value of the transfer coefficient. Contrary to earlier beliefs, the results indicated that the kinetics do not follow simple Butler-Volmer behaviour, but rather should be described in terms of an EC mechanism (electron transfer followed by a heterogeneous chemical step). The kinetic parameters are reported and compared with data in the literature.

An elaboration is made of the consequences of reactant or product adsorption for the shape and situation of d.c. polarographic waves at the dropping mercury electrode. The treatment is based on the diffusion-layer model. As an example the Langmuir isotherm is adopted, but the results will be representative in a more qualitative way also for other isotherms. Detailed features of the polarograms under various circumstances are given. It is concluded that the polarograms can be analyzed precisely if only one component is adsorbed. If both components are adsorbed, it may occur that the information concerning the adsorption cannot be obtained from the polarogram. The adsorption may even seem to be absent.

A comparative study has been made on the reduction of oxygen to hydrogen peroxide and of hydrogen peroxide to water in H2O and D2O. The first electron transfer does not show an isotope effect. A considerable isotope effect occurs for the subsequent protonation of the O2− ion. From the temperature dependence of the pertinent rate constants, activation parameters are evaluated. It is concluded that the reduction of hydrogen peroxide is slow due to the low value of the pre-exponential factor, the activation energy being surprisingly small for this irreversible reduction.

Explicit expressions are derived for NPP limiting current densities (it) influenced by reactant adsorption. Two cases are considered: (1) a lin

From impedance measurements at varied dc potential it is found that by the presence of thiourea the rate constant k t f is increased in such a way that the effect is mathematically expressed by k t f = k t,0 f (1 + B Γ TU , where B is a proportionality constant and Γ TU stands for the surface excess of thiourea. Because of the stepwise reaction mechanism an explanation in terms of the bridging or the surface reaction model fails. A possible alternative interpretation of the effect is given.

Equations for the electrode admittance in, the case of adsorption of electroactive species have been derived, considering both a.c. and d.c. aspects of adsorption. It was shown that due to the d.c. aspects the function also contains a drop-time dependent term f( t m ): = c O D O 1/2 + c R D R 1/2 +f( t m )⩾s Under normal polarographic conditions the value of is constant, so c O and c R are drop-time dependent and consequently also the value of the Warburg coefficient σ, as well as the adsorption parameters C LF and C HF .

The reduction of Cd(II) to Cd(Hg) has been studied by means of demodulation voltammetry in a wide potential range and at various water activities using NaClO 4 as the base electrolyte. From this study it is concluded that the reduction proceeds according to: (i) fast loss of 12.5 water molecules in a preceding equilibrium; (ii) a slow “chemical” step, which is not a desolvation; (iii) slow transfer of the first electron; (iv) fast loss of four water molecules; (v) slow transfer of the second electron; and (vi) fast loss of the remaining nine water molecules. These results and the values of the rate constants obtained are compared with results obtained earlier on the reduction of Zn(II).

It is shown that application of the impedance method allows a more detailed picture of the mechanism of nitromethane reduction to be obtained as compared with dc voltammetry. This is especially true at pH values where all electron transfers are fast. The necessary theory is developed and applied to the reduction of nitromethane from 1 M aqueous KCl at pH 9.0. It is concluded that the reduction proceeds via an ECCE mechanism. The kinetic parameters of the steps in the mechanism are reported.

A formalism is developed to describe the current—potential characteristic (“rate equation”) for electrode reactions that proceed via mechanisms consisting of a sequence of elementary reactions steps. Single electron transfer steps, as well as heterogenous monomoleculair chemical reactions are accounted for. this formalism is used to investigate catalytic and inhibitive effects resulting from variation of the electrolyte composition. A number of examples are discussed, showing that sometimes the effects considered act specifically on one or more of the individual steps. The results are found to be consistent with the theory and its underlying assumptions, and even to give additional information about the nature of the mechanism.

Summary The influence of a spurious frequency dispersion of the impedance of a dropping mercury electrode on the “apparent” kinetic parameters of an electrode reaction has been studied. Two extreme possibilities are chosen: “Shielding” impedance parallel to or in series with the electrode impedance. For both cases a correction method is given viz. (a) subtracting the “shielding” admittance, measured in the absence of a faradaic process, from the cell admittance to give the corrected electrode admittance and (b) taking for the ohmic resistance the measured resistance at each frequency (again in the absence of a faradaic process) instead of its high-frequency limit. Even if the wrong correction is applied (resistance correction in the case of an admittance error or the reverse) the frequency variation method for the impedance analysis gives good results (except at very low values for the transfer resistance and the Warburg coefficient). For the concentration variation method only the proper correction gives good results.

Summary Expressions are derived for the potential dependence of the Warburg parameters which determine the faradaic part of the electrode admittance in the case of electrode reactions with a coupled first order preceding, following or catalytic reaction under Nernstian conditions. The theory is based on a relatively simple diffusion-layer model of the d.c. processes occurring at the dropping mercury electrode, which appears to be a good approximation to the more complicated expanding plane model. It has been shown that, after the frequency analysis of the electrode admittance, examination of the potential dependence of the parameters obtained provides valuable diagnostic information about the reaction mechanism. Directions are given for the course of such an analysis. A short extension is made to systems under non-Nernstian conditions.

Summary The step-wise reduction of the cupric ion at the DME in aqueous fluoride solutions containing ammonia has been studied using phase sensitive a.c. polarography. The reaction path is described as two separate charge transfer steps, interrelated through a common variable, the concentration of the intermediate Cu+ ion. Values of the rate constant and the transfer coefficient pertaining to the different steps were determined and subsequently used in the calculation of the theoretical admittances in the potential region where the a.c. polarographic waves overlap. These agree closely with experiment.

that can be covered, and the fact that the overvoltage-time curve can be obtained in one experiment. In the last respect, the pulse methods compare especially favourably with the complex impedance method where variation of frequency, the equivalent of variation of time, requires numerous time-consuming measurements. However, an analysis of experimental data indicates a disadvantage of the pulse methods compared with the impedance method because of the cumbersome expressions for the overvoltage-time relations. The expressions contain exp erfc functions of sometimes complex quantities. Only for short and large times are simpler approximations possible; these were used by Berzins and Delahay 1 as the basis for an extrapolation procedure to determine the interfacial parameters. This procedure leaves, therefore, a considerable time region unused. The situation has been improved by Kooijman and Sluyters 5 who proposed a numerical analysis procedure using nomograms. In these nomograms the properties of the overvoltage-time curve are precalculated in terms of dimensionless quantities on the basis of the classical theory, i.e. accounting for the presence of a purely capacitive double layer and an electrode reaction kinetically controlled by charge transfer and diffusional mass transfer. This method is advantageous for the detection of deviation from classical behaviour but a theory accounting for complications would probably be rather involved. Kooijman developed another method 6, pointing out that at one electrolysis time, the overvoltage r/and its derivative with respect to time t, provide two relations between the interfacial parameters. This resembles in a way the complex impedance method, where at one frequency the two components of the impedance are available. However, the determination of dq/dt may be rather inaccurate. A quite different approach has been made by Levart and Poirier d'Ang6 d'Orsay 7'8 who elaborated on an idea introduced by Wijnen 9, to make use of the Laplace transformation of experimental pulse data. It is proposed to determine the Laplace transforms of voltage and current for various values of the Laplace parameter s, by means of a graphical procedure. The quotient of the two transforms can be expressed as a function of s with much simpler algebra than the overvoltage itself as

Summary A set-up is described for the study of rectification by a galvanic cell with external application of a d.c. potential. With this set-up the rectification of the double layer and a simple electrode reaction have been studied. The values of the double-layer capacitance obtained at the DME in 1 M KCl and the kinetic parameters of the Fe(III)/Fe(II) electrode reaction in 1 M oxalate buffer are in good agreement with earlier reported results.

Interfacial tension values have been measured in order to calculate the electrical charge density as a function of potential. The results are in accordance with those obtained from double-layer capacitance data reported earlier2,3, also at highly positive potentials.

Several electrochemical methods have been developed in recent years for the study of the kinetic parameters of electrode reactions. These methods have been used for obtaining an abundance of experimental data for the standard heterogeneous rate constant, k sh , of electrode reactions, mostly limited to reactions at a mercury electrode. As early as 1933, Frumkin recognized the essential correlation between the double layer structure at the electrode interface and electrode kinetics. As enough data for the kinetic parameters are yet available, interest is growing in this correlation, because it may result in a better understanding of heterogeneous reaction rates. According to Frumkin the potential difference existing in the double layer, influences the kinetic parameters for two reasons: (a) It influences the effective potential difference which favours or hinders the reaction, (b) The effective concentration of reacting species is different from the “bulk” concentration. The Frumkin effect with its limitations is discussed. Adsorption of electroinactive species, e.g. organic adsorbates, may have two effects: variation of the potential difference in the double layer and hence of the Frumkin correction and secondly a blocking effect. Some examples, illustrating these effects, are presented. If the anions of the supporting electrolyte were specifically adsorbed at the interface, it appeared that the Frumkin correction did not suffice. For some reactions, e.g. the zinc system in mixed halide solutions, we found a linear relation between log k sh and the amount of specifically adsorbed anions q − 1 , from which it is inferred that the energy of activation for the electrode reaction decreases linearly with q − 1 . Quite frequently, the electroactive species themselves are adsorbed at the interface. In that case the adsorption complicates matters considerably, e.g. the mass transport equations change, so that the electrochemical methods, as normally used, are inapplicable. Recently Delahay has presented expressions based on the concept of the coupling between faradaic and double layer charging processes. These expressions can be used successfully for systems with adsorption of the electroactive species. Such systems have been investigated with sine wave methods and some results are presented. Almost all systems with adsorption of the reacting species were found to be mass transfer controlled, so that no influence of the double layer structure on the kinetic parameters could be investigated for these cases.

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.

Summary The shape of faradaic rectification polarograms is calculated on the assumption of a single step charge transfer, allowing deviations from the absolute rate theory. As a result, the conditions necessary for obtaining significant information about the kinetic parameters from measured rectification signals, are reported. Also, the conditions for which simplifications are allowed, are given.

Summary The consequences of the occurrence of a polarographic maximum on the faradaic impedance are studied in the case of the Ba2+ ion reduction from 1 M LiCl. It is shown that a substantial deviation from normal behaviour of the charge transfer resistance exists in the potential region of the maximum. The decrease of the thickness of the diffusion layer obtained from the charge transfer resistance is in reasonable agreement with the value obtained from the d.c. polarogram. The standard rate constant is 2.5×10−3 cm s−1 and the cathodic transfer coefficient β equals unity. The standard potential is −1.917 V vs. SCE. It is shown that in a.c. polarography large deviations from linearity will occur.

Summary A kinetic study of the Hg22+/Hg reaction is made by high-frequency faradaic rectification measurements at various frequencies and for a number of Hg22+ concentrations. The results are compared and combined with those of two former studies, viz. low-frequency impedance measurements and galvanostic pulse measurements. In a critical discussion it is shown that the combined results are consistent neither with the most simple model of diffusion+charge-transfer control, nor with the more complicated, but still simplified model of adsorption+diffusion+charge transfer, neglecting some of the adsorption parameters. The theory for the most rigorous model of adsorption+diffusion+charge transfer, including all adsorption parameters, is developed and applied to the results of the three methods mentioned. It appears that this model fits unambiguously to the experimental data.

Summary The impedance of the dropping mercury electrode in aqueous solutions in 1 M HCl, 7.5 M HCl and 5.2 M HClO4, saturated with hydrogen at one atmosphere was measured at temperatures between −39° and +72° both in and outside the potential region where the electrode reaction, e+H+ H2, proceeds. Analysis of the impedance data gave information about both the kinetics of the electrode reaction and the double-layer capacity. Exchange current densities and transfer coefficients are reported. The apparent heat of activation is 21.1 kcal mole−1 for 1 M HCl, 19.5 kcal mole−1 for 5.2 M HClO4, and 11.1 kcal mole−1 for 7.5 M HCl. In the latter solution the transfer coefficient is found to be a linear function of temperature. In 1 M HCl and 5.2 M HClO4 the transfer coefficient is virtually independent of temperature. In 1 M HCl the hump in the capacity curve is situated anodically to the potential of zero charge but in 7.5 M HCl and 5.2 M HClO4 this is reversed. In both 1 M HCl and 7.5 M HCl the capacity-potential curves show one potential where the double-layer capacitance is independent of temperature. In the case of 5.2 M HClO4, this occurs at two potentials, situated at either side of the potential of zero charge.

In normal sinusoidal a.c. polarography 1 the sensitivity is in the first place determined by the magnitude of the base line current or capacity current, caused by the existence of the electrical double layer at the electrode-solution interface. In order to suppress the capacity current ic, which is proportional to the frequency, low frequencies are employed. This has, however, the disadvantage that also the faradaic current i F is lowered, since iv is proportional to the square root of frequency. A better way of eliminating the capacity current is effected by an a.c. polarograph patented by Jessop 2, which measures only the in-phase component of the sine wave current. This signal contains theoretically no contribution of the capacity current if the a.c. voltage loss over the ohmic resistance Ra is properly compensated 3. Another, probably more expensive, technique is square-wave a.c. polarography 4, in which the capacity current is diminished to a large extent by measuring the current at the end of each half period of the applied small amplitude square-wave voltage. In this paper a new a.c. polarographic technique is proposed, with which theoretically the capacity current is eliminated, so that the base line in the a.c. polarogram is reduced to zero. The principle is the;application of a triangular a.c. voltage across the electrode interface, followed by full-wave rectification of the a.c. current. The effect of this on the a.c. polarogram to be obtained will be described in the next section.