Karel Holub

Microelectrodes. definitions, characterization, and applications: (Technical report)

Theory, preparation,and applications of microelectrodes and microelectrode arrays are critically reviewed, and future trends in the field are outlined. An operational definition of a microelectrode is also recommended.

The influence of Reactant Adsorption on Limiting Currents in Normal Pulse Polarography

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

Coupling of charging and faradaic processes: Electrode admittance for reversible processes

Summary Coupling between double-layer charing and a faradaic process is examined in a very general way for an electrode at which a charge transfer reaction takes place. it is shown that coupling follow directly from the general forms of the equations describing the effect of an external perturbation on the charging and faradaic processes. It is not necessary to introduce the concept of charge separation or recombination at the interface. The electrode admittance is derived and discussed for reversible processes (infinite exchange current). Six double-layer parameters are required in the analysis.

Ac impedance analysis of tetraethylammonium ion transfer at liquid/liquid microinterfaces

The kinetics of ion transfer reactions at the microhole-supported water/nitrobenzene interface have been studied by an ac impedance technique. A thin layer approximation of diffusional impedance has been derived for the geometry of the hole. In contrast to the large-area interface, the charge transfer resistance at the liquid microinterface is comparable in magnitude with the electrolyte resistance, which makes its determination more reliable. The apparent rate constant of tetraethylammonium ion transfer, kapp= 0.22 cm s–1, as determined at the liquid microinterface with an area ranging from 100 to 1000 µm2, has been found to be about twice as high as that previously reported.

The double layer at the interface between two immiscible electrolyte solutions—IV. Solvent effect

Differential capacitance of the electrical double layer at the water/nitrobenzene and water/1,2-dichloroethane interfaces was measured by an ac impedance technique. Potential difference across the diffuse layer in water, nitrobenzene and 1,2-dichloroethane and the corresponding capacitances, were calculated by using the modified Poisson-Boltzmann theory (version MPB4), which accounts for both the finite ion size and image forces. The two diffuse layers, one in water and the other in organic solvent, were assumed to be independent, ie they were associated only through an equal and opposite surface charge density. As compared with the Gouy-Chapman theory, the MPB theory provided a more reasonable description of the interface over the whole range of electrolyte concentrations and surface charge densities even in the system comprising the solvent of a low dielectric permittivity, such as 1,2-dichloroethane. By using the MPB theory, the interfacial potential difference and the inverse capacitance for both the systems studied were shown to split into two contributions, of which one is essentially independent of the electrolyte concentration and can be attributed to the presence of an ion-free inner (or compact) layer. The effect of the surface charge is to reduce the thickness of this layer, eg through the liquid phase interpenetration.

Accurate double-layer capacity and impedance measurements with the dropping mercury or amalgam electrode

Summary Spurious frequency dispersion of the impedance of a dropping mercury or amalgam electrode was studied. Poor cell and electrode geometry was unimportant in 1 M aqueous electrolyte solution in the 500 Hz–20 kHz range, but caused pronounced dispersion in 0.1 M solution. Apart from the well-established necessity of silicone-coating of the capillary, it seems essential that the drop diameter, at balance of the bridge, is at least 4–5 times larger than the tip diameter. Preparation and use of the dropping electrode are described in detail, and experiments supporting the proposed technique are discussed. Application was made to the Zn2+/Zn(Hg) electrode in 0.1 M NaClO4+0.002 M HClO4 in the 50 Hz–20 kHz range. Determination of the electrolyte resistance and double-layer capacity is discussed in detail. Results are accounted for by the classical theory of the faradaic impedance.

Relaxation of electrode processes with simultaneous consideration of double-layer charging and faradaic current. Part I

Summary Non-steady-state electrode processes are analyzed with simultaneous consideration of the double-layer charging and the faradaic process in the treatment of the diffusion problem. A treatment is given from a unified point of view for any galvanostatic or potentiostatic perturbation. The general formulas thus derived are applied to galvanostatic and potentiostatic step-perturbations and to one type of coulostatic perturbation. Differences from the results previously derived by the classical approach were found in the region where the double-layer charging influences the overall process.

Voltammetry of chemically heterogeneous metal complex systems. Part II: Simulation of the kinetic effects induced on polarographic waves

The present work formulates a general expression for the effect of the kinetics of association/dissociation of chemically heterogeneous metal complexes on polarographic curves and discusses the best numerical strategy to simulate them. The concepts are based on standard dc and pulse polarography. The treatment holds for (i) Freundlich-type of metal–ligand binding; (ii) different diffusion coefficients of complexed and uncomplexed species; (iii) an overall excess of ligand; and (iv) an association rate constant invariant for a given type of metal ion.

Study of first order chemical reaction following charge transfer using the double potential-step chronocoulometric method: II. Experimental study of reactions inactivating the product of the charge transfer reaction — Derivatives of tetrahydropterins as a model system

Abstract The theory of double-step chronocoulometry has been verified for the e.c. electrode reaction type, a reversible electron transfer being followed by a chemical reaction in solution leading to inactivation of the primary product. In contrast to methods proposed previously, the inactivation reaction rate constants were calculated using the charge values obtained by integration up to current decay after the second potential step. The anodic oxidation of tetrahydropterin derivatives on a Pt electrode was employed as a model system. It has been found that the rate of inactivation of the anodic product decreases with increasing pH of the solution and is different for various derivatives studied. A special case of zero inactivation rate was verified using the Fe 3+ /Fe 2+ redox system in an acidic medium.