Hydrodynamic voltammetry at channel electrodes: Part III. Theory of kinetic currents

A theory of kinetic currents at a channel electrode is developed without any assumption on the magnitude of the rate constant of the preceding chemical reaction. An approximate equation with reasonable accuracy which relates the limiting current to the rate constant, flow velocity and the electrode geometries, is presented. The validity of the equation derived from the concept of the thin reaction layer is also discussed.     

Hydrodynamic voltammetry at channel electrodes: Part II. Theory of first-order kinetic collection efficiencies

The theory of first-order kinetic collection efficiencies at the double channel electrode is developed for the following two schemes: (I) A±n₁e→B (at the generator electrode), B→^kP (in solution), B±n₂e→Y (at the detector electrode), (II) A±n₁e→B, B→^kA, B±n₂e→Y. The exact expressions for the kinetic collection efficiencies are obtained as ascending and asymptotic series with respect to the kinetic parameter. Further, approximate formulae in exponential forms are given, which hold within an error of about 2% for conventional electrode geometry. Finally, the validity of the approximate procedure, which has been used previously to obtain the kinetic collection efficiencies for fast homogeneous reactions, is discussed in comparison with the present theory.     

Hydrodynamic voltammetry at channel electrodes: Part IV. Theory of chronopotentiometry for reversible electrode reactions

The theory of chronopotentiometric measurements at channel electrodes is developed for reversible electrode reactions, by rigorously solving the corresponding time-dependent boundary value problem, where the non-uniform accessibility of the channel electrode surface is taken into consideration. The theoretical equations of the transition time and the potential-time curve are derived as functions of (I_d/I), where I denotes the applied current intensity and I_d the limiting diffusion current obtained at steady-state. Finally, the time variation of the current density distribution at the electrode surface is given.     

Voltammetry at partially covered electrodes: Part II. Linear potential sweep and cyclic voltammetry

Effect of inhomogeneity of the electrode surface on the linear potential sweep and cyclic voltammograms is investigated theoretically and experimentally using model electrodes partially covered with photoresist layer. Good agreement between the theoretical and experimental results is obtained.     

Voltammetry at partially covered electrodes: Part I. Chronopotentiometry and chronoamperometry at model electrodes

Equations for chronopotentiometry and chronoamperometry at partially covered electrodes have been derived using a model of hexagonal array of cylidrical spaces terminated, at the electrode surface, by concentric active and inactive regions. The boundary value problem was shown to be analogous to that for a charge transfer preceded by a chemical reaction. Experiments with the reduction of ferricyanide on gold model electrodes partially covered with photoresist layer showed excellent agreement with the theory. Application of the equations to estimation of coverage and size of active sites distributed on a electrode surface is discussed.     

Voltammetry at microcylinder electrodes: Part III. Chronopotentiometry

Expressions for chronopotentiometry at stationary microcylinder electrodes are presented. The transition time, τ, is expressed as a function of a single parameter λ ( = nFc^*_RD / ia, where a is the radius of the electrode, D is the diffusion coefficient, i is the current density, n is the number of transferred electrons, F is the Faraday constant and c^*_R is the bulk concentration. When the values of λ are small, the transition-time constant, i√τ / c^*_R, depends linearly on λ, with the intercept predicted from the Sand equation. Conversely, when values of λ increase, the transition time also increases exponentially. Therefore transition necessarily occurs no matter how small a current flows. An approximate equation for the transition time is presented, from which one can evaluate the diffusion coefficient. Equations for the potential-time curve and the quarter-wave potential are also obtained. The equations were tested experimentally using carbon fiber electrodes (a = 4.1 μ m) and platinum wire electrodes (a = 10-100μ m). The transition times obtained experimentally were in good agreement with those predicted theoretically for various values of the applied current, for several different radii of the electrodes.     

Linear sweep voltammetry at the tubular graphite electrode: Part II. Totally irreversible processes

The theory of linear sweep voltammetry at the tubular graphite electrode has been developed for irreversible processes. The convective diffusion differential equations have been transformed into an integral equation which is solved numerically. The current-potential curves have been calculated theoretically and verified experimentally. The dependence of the current-potential curves on velocity has been studied. A procedure for the determination of kinetic parameters, i.e. standard rate constant and transfer coefficient, is presented.     

Limiting diffusion currents in hydrodynamic voltammetry: Part IV. Stationary disk and ring electrodes in a rotational flow produced by a rotating disk

A new electrolysis cell for the use in hydrodynamic voltammetry is devised, in which a stationary disk or ring electrode is immersed coaxially in a rotational flow produced by a uniformly rotating disk (rotor) and works as an indicator electrode. Theoretical equations of the limiting diffusion currents at such a disk or ring electrode are derived. The theoretical predictions for the dependence of the limiting diffusion current upon various experimental variables, i.e., the rotation speed of the rotor, the bulk concentration of depolarizer, the geometrical parameter of the electrode and the viscosity of the solution, are verified experimentally for the reduction of ferricyanide ions.     

Transport and kinetics at microheterogeneous electrodes: Part III. Application to the photoelectrochemical decomposition of water

The transport and kinetics in a system involving two sets of microheterogeneous electrodes, which are designed to use solar radiation to split water into H₂ and O₂, are described. A reaction scheme involving fourteen different processes is considered. The conditions for an efficient system are derived. It is shown that the main problems are electrode selectivity and the loss of H₂ and O₂ by back reactions.     

Polymer films on electrodes: Part III. Digital simulation model for cyclic voltammetry of electroactive polymer film and electrochemistry of poly(vinylferrocene) on platinum

The electrochemistry of electrodeposited poly(vinylferrocene) (PVF) and poly(vinylferrocene acrylonitrile) (PVFAN) films on platinum electrodes was studied in acetonitrile solutions using perchlorate or p-toluenesulfonate as the counter ion by cyclic voltammetry. A model is proposed for the cyclic voltammetric behavior of polymeric films on electrode surfaces. The model incorporates non-equivalent redox sites with interconversion between such sites, electron-transfer kinetics at substrate/film interface and diffusion within the film. Parameters are obtained which yield a good fit to the experimental results.     

Supramolecular phenomena in organic redox films at electrodes: Part I. The methylene blue/leucomethylene blue redox couple at the platinum electrode

It is possible to generate conductive multilayer structures by reduction of methylene blue cations at the interface: clean surface of a platinum electrode/1 M aqueous electrolyte solution.The characteristics of the multilayer ordered phase generated in fluoride and nitrate solutions are presented and the mechanism of a fast charge transfer between electrode/organic film and organic film/aqueous solution interfaces is discussed. The conductivity of the film is interpreted by postulating formation of a mixed valence structure with the generation of a cation radical intermediate which is favoured in the solid state at characteristic potentials.     

Chronoamperometric current at finite disk electrodes

The chronoamperometric current at a stationary finite disk electrode is studied using both analytical and digital simulation techniques. The exact long-time expansion of the current is obtained and its short-time behavior is considered. Digital simulation of the current using an explicit hopscotch algorithm is presented. In contrast to the usual explicit difference method, the ‘hopscotch’ algorithm is unconditionally stable, and thus, it is particularly suited for studying electrochemical problems at intermediate and long times. A simple analytic expressions for the current, which is accurate to 0.6% for all times, is presented.     

Anodic stripping voltammetry in a flow-through cell with a fixed mercury film glassy carbon disc electrode. Part I

Anodic stripping voltammetry at a glassy carbon disc electrode covered by a thin mercury film was adapted for use in a flow-through cell. The resulting system is characterized by extreme simplicity of set-up and operation, high sensitivity and excellent precision and stability. Its performance was tested via the determination of hydrated or labile complex ions of heavy metal ions in sea water, using short (2–10 min) deposition periods. The dependence of the stripping peak charge on metal ion concentration, length of deposition period, solution flow rate and other variables was examined and the reliability of the results obtained were evaluated under conditions resembling continuous monitoring.     

Stripping voltammetry with collection at a rotating ring-disk electrode

A new electroanalytical technique is described, called "stripping voltammetry with collection." The technique involves the use of a rotating ring-disk electrode and is an improvement over traditional voltammetric stripping at a single electrode in that it is characterized by a lower limit of detection and that the period of deposition before stripping can be shorter. The use of the technique is illustrated by the determination of 10(-10)M Ag(+) in 0.1 M H(2)SO(4) by use of a ring-disk electrode having a disk electrode constructed of glassy carbon and a ring electrode constructed of platinum.     

The acceleration of the hydrogen evolution reaction from acid solutions by sulfur compounds: Part II. Indium amalgam electrodes

Thiourea and its N-substituted derivatives accelerate the hydrogen evolution reaction at mercury, indium and indium amalgam electrodes. The differences in the effects observed with mercury and with indium amalgam electrodes can be accounted for in terms of differences in the adsorption of the organic compounds at the metal-solution interfaces.     

Ellipsometry of DNA adsorbed at mercury electrodes: A preliminary study

The adsorption of DNA at a mercury electrode can be detected ellipsometrically. Native DNA is probably more weakly absorbed at positive charges than at negative charges. Denatured DNA is more strongly adsorbed in the reduced form than in the protonated form and is desorbed at large negative charges. Adsorption of both forms is characterized by small negative values of δΔ/δ. It is suggested that these results should be interpreted in terms of an anisotropic film.     

Polymer films on electrodes: Part II. Film structure and mechanism of electron transfer with electrodeposited poly(vinylferrocene)

The structure and properties of electrodeposited poly(vinylferrocene) (PVF) films on platinum electrodes (PVF/Pt) were examined by electron microscopy, X-ray photoelectron spectroscopy, various electrochemical techniques and measurements of the film resistance. The data were consistent with a mechanism in which the polymer films are permeable to dis-solved reactants. A theoretical treatment of this situation for chronoamperometry is presented. The oxidation and reduction of a variety of dissolved reactants with redox potentials far removed from that of the PVF/PVF⁺ system at PVF/Pt occurred by diffusion of the electroactive species through the polymer film and subsequent reaction at the platinum surface.