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In this paper we describe numerical simulations of a number of physico-chemical phenomena observed at disk microelectrodes polarized with a high frequency ac waveform. These phenomena include resistive heating of an electrolyte solution surrounding a microelectrode, electrothermal flow of the solution and electrochemical mass transport of dissolved redox species. The main purpose of doing these simulations was to provide the required theoretical background to the previously obtained experimental data. It is shown that the simulated linear flow rate of the solution is proportional to the voltage across the solution resistance in the power four and inversely proportional to the radius of the microelectrode. Thus this supports the idea that the convection at hot microelectrodes is driven primarily by the electrothermal flow. Simulations of the Faradaic current agree semi-quantitatively with the experimental observations. We have identified possible reasons for the observed discrepancy between the two sets of data.

A relationship between the tip response and the tip-to-substrate separation distance was investigated under alternating current scanning electrochemical microscopy (AC-SECM) conditions. For insulating substrates only negative AC feedback was observed. However, in the case of conductive substrates positive as well as negative AC feedback could be observed depending on experimental conditions. The theoretical model proposed in this work explains very well all observed trends and allows for qualitative predictions of the tip impedance in AC-SECM as a function of various experimental conditions (including: frequency, the supporting electrolyte concentration, the tip-to-sample separation distance and the tip diameter). In this theoretical model the double-layer capacitance of the tip and the sample is represented by a constant phase element (CPE), rather than an ideal capacitor. The paper also describes how raw experimental impedance data can be corrected for imperfections of the measuring system.

Convolutive voltammetry was employed in studies of the kinetics of sulfide ion adsorption at copper amalgam and mercury electrodes. Measurements were carried out for low electrode coverages to minimize the effect of the lateral interaction energy, and the results were extrapolated to zero coverage. In the case of a Cu amalgam electrode (7.4×10−4 mol dm−3 of Cu) kinetic and thermodynamic parameters of the adsorption/desorption process were determined in solutions of different pH (3.6 to 12.8). For pH

Oxidation of ferrocene (Fc) and reduction of p-dicyanobenzene (DCB) were examined by ac-voltammetry in benzene solutions containing tetra-n-hexylammonium perchlorate at a hemispherical (Hg) and a disk (Au) μm-electrode, respectively. The specific conductivity of benzene solutions indicates that the degree of dissociation of the supporting electrolyte is less than 1%. The low concentration of free ions combined with the low dielectric permittivity of the solvent cause large double layer effects evident from the values of the charge transfer coefficients (0.35 and 0.69 for Fc and DCB, respectively). In addition, the low concentration of ions causes problems in measurements of charge transfer kinetics if the concentration of the depolarizer exceeds 0.2 mM. The standard rate constants obtained in benzene solutions (0.22 and 0.15 cm s−1 for Fc and DCB, respectively) are not much different from values observed in common polar solvents. Apparently, in benzene solutions, very weak electrical coupling of the activated state with solvent molecules is compensated by very strong interactions with counter ions and large double layer effects. The experimental results indicate the necessity of taking into account ionic relaxation processes in a theoretical description of interfacial charge transfer dynamics.

A method for studying impedance spectroscopy of micrometer-sized electrodes in the frequency range from 5 kHz to 5 MHz has been developed and tested. Details of the electronic circuit and the data processing method are provided. Since the method involves high-speed data acquisition and fast Fourier transformation, the impedance data can be obtained within a 5-ms time window. This enables measurement of changes in the electrode impedance caused by rapid changes in the solution composition. In particular, the method is well suited to studying the performance of electrochemical microdetectors in flowing solutions. The performance of the method was tested in a brief study of the competitive adsorption of hydrogen and methionine on a Pt ultramicroelectrode. It has been shown that proper electrochemical conditioning can prevent any long-term changes in the properties of the electrode caused by its exposure to common organic or inorganic substances. Hence, it provides means for making an ultramicroelectrode with...

Problems associated with electrochemical ac impedance measurements at frequencies up to 50MHz are discussed in detail. Limitations in the impedance measurements arise from inductance and stray capacitance of leads connecting electrochemical cell to a measuring circuit. Theoretical considerations indicate that the best accuracy of measurements can be achieved when working with ultramicroelectrodes 2.5 × 10−7 to 8 × 10−6cm2. Construction of ultramicroelectrodes suitable for such measurements and a diagram of the electronic circuit is described. Preliminary results showing frequency dispersion of the double layer capacitance and solution resistance and the kinetics of adsorption of n-butanol are presented.

Adsorption kinetics of small organic molecules was studied by ac voltammetry with mercury ultramicroelectrodes in the frequency range 0.22-6.25 MHz. It was found that the activation energy of the adsorption process can be expressed as a linear combination of the electrical component of the standard free energy of adsorption (a major contribution) and the energy of lateral interactions (a minor contribution). The results suggest that, during the adsorption process, work against an electrical field must be performed before excess energy (due to the change in the hydrophobic interactions) is released. The final state is reached by a rotational and translational motion of the surfactant molecule. At the zero charge potential the rate constant reaches the maximum value of (4.6 ± 0.3) x 10 9 s -1 . This value indicates that the viscosity of interfacial water at mercury near the zero charge potential is about 1.7 times higher than the bulk viscosity.

The computations described in the paper are based on the Frumkin adsorption isotherm. Variation of the electrosorption valency with potential is taken into account. The rate-determining step can be either diffusion or interfacial kinetics. The kinetics of charge transfer associated with the formation of a chemical bond between an adsorbate and an electrode is described in terms of the Butler-Volmer equation. The effect of lateral interactions between adsorbed molecules and the activated complex is also considered. The presented dependences of the peak width and peak potential on the electrode coverage and the sweep rate allow the determination of various kinetic and thermodynamic parameters of the adsorption process. Computations were carried out for both linear and spherical symmetry of diffusion.

A method for determination of the thermodynamic and kinetic parameters of electrosorption from fast sweep rate cyclic voltammograms was developed and tested using a simulation program. The method was then used for determination of the thioglycol adsorption kinetics on Hg in KOH and H 2 SO 4 aqueous solutions. Cyclic voltammetric experiments were carried out with a hemispherical Hg ultramicroelectrode (5 μm radius) at sweep rates ranging from 1000 to 100 000 V s −1 . The experimental data fit relatively well to a theoretical model based on the Frumkin adsorption isotherm, despite the fact that the current model assumes independence of the electrosorption valency and the interaction parameter of the potential and the electrode coverage, and that it neglects possible reorientation of adsorbed molecules on the surface with a change in electrode coverage and/or potential. It was found that the electrosorption valency for thioglycol in both solutions studied is similar and close to unity; the standard energies of adsorption are 2.93 and −46.6 kJ mol −1 in 1 M H 2 SO 4 and 1 M KOH respectively; the interaction parameters are −3670 J mol −1 (in 1 M H 2 SO 4 ) and 2450 J mol −1 (in 1 M KOH); the rates of adsorption and desorption steps extrapolated to the zero charge potential are k a 0 = 4.7 × 1O 5 s −1 , k d 0 = 9.8 × 10 6 s −1 (in 1 M H 2 SO 4 ) and k a 0 = 1.7 × 10 12 s −1 , k d 0 = 2.12 × 10 4 s −1 (in 1 M KOH). The interaction parameter for the activated complex in both studied solutions is 3000 J mol −1 larger than the corresponding interaction parameter of the product. The results are discussed in terms of participation of protons in the adsorption-desorption process and the effect of potential on the orientation of adsorbed molecules.

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.

Ammonium amalgam formation has been studied at mercury ultramicroelectrodes in various organic solvents and at standard size mercury electrodes in aqueous solution. Numerical simulations were carried out in order to establish the kinetics of the process. Ammonium amalgam is formed via a fast charge-transfer process, but it undergoes a decomposition according to the first-order rate law with a rate constant of about 0.6 s−1. In addition, some NH4+ ions are irreversibly discharged at the electrode surface, presumably to form H2 and NH3. This competing process is affected strongly by the nature of the solvent. Matching of experimental and simulated curves suggests that the diffusion coefficient of NH4 in mercury is about 1.8 × 10−5 cm2 s−1. The standard potential of NH4+(aq)/NH4(Hg) was determined to be −1.723 V/SHE. The absolute chemical potential of ammonium ions in mercury was calculated and compared with that of alkali metal ions.

The methodology for carrying out ac voitammetric measurements with ultramicroelectrodes is discussed. Electron-transfer rate constants for the oxidation of ferrocene (FeCp2) at a Pt ultramicroelectrode and cobaltocene (CoCp2) at Hg, Au and Pt ultramicroelectrodes were measured at frequencies from 2 to 50 kHz. It was shown that the method can provide accurate information about fast interfacial charge-transfer kinetics in organic solvents. The rate of CoCp2 oxidation on Hg or Au electrodes was found to increase with a decrease in the supporting electrolyte concentration, but in the case of the Pt electrode no effect of the supporting electrolyte concentration was observed for oxidation of either CoCp2 or FeCp2. The state of the Pt—solution interface was examined by studying the H+ adsorption—desorption process in aqueous H2SO4 and the double-layer capacitance in nonaqueous media.

Electrodeposition of alkali metals on Hg/Au electrodes from non-aqueous solutions was studied using cyclic voltammetry and chronoamperometry. At temperatures of 20 to − 70°C the formation of bulk intermetallic compounds preceded by nucleation phenomena was observed. Stoichiometry of the solid phases formed under these conditions was determined. At very low temperatures, when the mobility of mercury atoms was restricted, the underpotential deposition of alkali metal ions on solid mercury electrodes was noted. The Gibbs energies of electrosorption are discussed in terms of interactions between alkali metals and mercury.

Electrode processes involving the formation of a heterogeneous amalgam of iron during the electroreduction of Fe(II) ions on mercury and the oxidation of the iron amalgam were investigated by cyclic voltammetry, chronoamperometry and cyclic chronopotentiometry. The hanging mercury drop electrode and an iron plate placed on a mercury pool electrode were used in the experiments as working electrodes. It was established that particles of the electrodeposited iron are wetted by mercury and penetrate into the bulk of mercury electrodes forming a suspension. However, during electroreduction of Fe(II) ions from concentrated CaCl2 and Ca(ClO4)2 solutions a small amount of metallic iron remains on the surface of mercury. Oxidation of the heterogeneous amalgam of iron occurs at potentials about 100–150 mV less positive than the onset potential of mercury oxidation in the solutions studied. At these potentials oxidation of mercury may become significant enough to uncover some parts of the iron microcrystals. These iron particles become partially passivated and their surface energy at the Fe/solution interface decreases; consequently, the spreading coefficient of mercury on iron decreases, microcrystals of iron are expelled from mercury and finally oxidized.

Fundamental aspects of alternating current Scanning Electrochemical Microscopy (AC-SECM) have been studied. In particular, the flow of AC currents in the cell and simplified equivalent circuit diagrams are discussed. In the case of conductive substrates, the local (1st and 2nd order) impedance characteristic of the substrate is presented as a function of DC potential. 3-D imaging of mixed substrates (containing both conductive and non-conductive areas) was performed in the supporting electrolyte alone, without an added redox mediator.

The mechanism, of electrodeposition of CdS on Hg, and Pt electrodes has been studied in dimethylsulphoxide solutions by performing cyclic voltammetry at stationary electrodes and pulse polarography at dropping Hg electrodes in solutions containing both Cd(II) ions and elemental sulphur. Evidence is presented to demonstrate that sulphur is chemisorbed at these electrodes and that formation of at least a monolayer of metal sulphide is probable. As a result, formation of the initial layer of CdS involves reaction of Cd(II) ions with chemisorbed sulphur or with a pre-existing layer of metal sulphide. Data for the formation of CdS are compared with those obtained for the formation of PbS and ZnS.

Polarographic half-wave potentials for two reduction steps of 49 cyclopentadienyliron complexes of substituted arenes or heterocycles in dimethylformamide were determined. A fast cyclic voltammetry (10–40 V/s) was used to study the electron transfer kinetics of some of these complexes. After correction for the double layer effects, the rate constants of all the complexes studied show that the transfer of electrons in the second reduction step occurs significantly faster than in the first one. This was interpreted as a result of greater delocalization of electrons in the 20-electron complexes of iron compared to the 19-electron complexes.

Electrolysis of arene and heterocycle cyclopentadienyliron hexafluorophosphates led to the liberation of arenes or heterocycles in 69–90% yield. The procedure described gives yields comparable with or superior to the yields obtained from pyrolytic sublimation used previously.

Electrochemical reduction of the cyclopentadienyliron hexafluorophosphates of benzophenone, o-chlorophenyldibenzoylmethane and diethyl ethylphenylmalonate led to the liberation of arenes without reduction of keto or ester groups in over 85% yields.

Polarographic half-wave potentials for two reduction steps of 49 cyclopentadienyliron complexes of substituted arenes or heterocycles in dimethylformamide were determined. A fast cyclic voltammetry (10–40 V/s) was used to study the electron transfer kinetics of some of these complexes. After correction for the double layer effects, the rate constants of all the complexes studied show that the transfer of electrons in the second reduction step occurs significantly faster than in the first one. This was interpreted as a result of greater delocalization of electrons in the 20-electron complexes of iron compared to the 19-electron complexes.