Janet Osteryoung

Diffusion-controlled current at the stationary finite disk electrode: Theory

Abstract : Rigorous expressions for diffusion-controlled currents at a stationary finite disk electrode are derived through use of the Winer-Hofp technique. The chronoamperometric curve obtained varies smoothly from a curve represented by the Cottrell equation to a steady state value similar to that obtained for a spherical electrode as time elapses. The solution can be expressed also as the Cottrell term multiplied by a power series in the parameter sq. rt (Dt/a), where a is the electrode radius. The present work is discussed in terms of the coefficient of the first term in this formulation. (Author)

Linear sweep voltammetry at very small stationary disk electrodes

An expression for linear sweep voltammograms at very small stationary disk electrodes is presented. From numerical calculations theoretical voltammograms are obtained for various values of the dimensionless parameter, p = (nFa2v/RTD)12, where a is the radius of the electrode, v the potential sweep rate and D the diffusion coefficient. The maximum current and the half-maximum potential are evaluated from the voltammograms as functions of p and are expressed by approximate equations with high precision. In order to examine the validity of the equations, an experimental study was made at platinum small disk electrodes (a = 0.025 to 0.10 mm). The experimental voltammograms were in good agreement with the ones theoretically predicted for various values of the sweep rates and for several different radii of the electrodes.

Diffusion controlled current at a stationary finite disk electrode: Experiment

Abstract Microdisk electrodes were constructed from glassy carbon electrodes coated with photoresist by using conventional photolithography. Chronoamperometric measurements were made at these electrodes in the ferrocyanide/ferricyanide system. Dependence of currents on time and on radii of the electrodes was in good agreement with that predicted by the theory presented previously.

Square wave voltammetry at small disk electrodes: Theory and experiment

Abstract An equation is derived for the current response of a reversible electron transfer reaction for square wave voltammetry at an embedded disk electrode. Peak shape and position are invariant to the dimensionless parameter D τ/ r 2 where D is the diffusion coefficient, τ the square wave period, and r the radius of the disk, whereas peak current density increases without limit with increasing D τ/ r 2 . A simple empirical equation predicts the peak current for any value of D τ/ r 2 for square wave amplitude 50/ n mV and step height 10/ n mV. Experimental results for oxidation of ferrocyanide at small platinum electrodes agree well with the theory and demonstrate the practical utility of the experiment.

Reversible square-wave voltammograms independence of electrode geometry

Abstract The square-wave potential-time sequence treated here is composed of a square wave with a large amplitude superimposed on a staircase with a small step height. Current measurements are made at the end of the pulse in each square-wave half cycle and the difference of these two currents is plotted against the staircase potential. An equation describing the resulting voltammetric curve is derived for a reversible electrochemical system assuming arbitrary electrode geometry, and is expressed as a simple product of a geometrical term and a potential term. Therefore the normalized voltammetric response, which is a bell-shaped curve with a peak at a formal potential, does not depend on shape or size of electrodes. This finding is verified with numerical calculation for various electrode geometries. Approximate equations for the peak current and the peak width at half height are presented.

Differential normal pulse voltammetry-theory

Abstract The theory is presented of a modified form of differential pulse voltammetry, in which two potential steps with similar short duration are superimposed on the rest potential. Expressions for the current response to the double potential step are derived for quasireversible and totally irreversible charge-transfer processes of simple electrode reactions at stationary electrodes. Some approximate equations are given for i-E curves, peak currents, peak half-widths, peak potentials and potentials at zero current.

General equivalence of linear scan and staircase voltammetry

Linear scan voltammetry is specified by one parameter, the scan rate, ν, whereas staircase voltammetry requires three parameters, the step height, ΔE, step width, τ, and fraction of the step width at which current is sampled, α. Here it is shown that the response in linear scan is the same (within acceptable limits) as that for staircase, for ν = ΔE/τ, for a wide variety of mechanisms, when certain simple conditiosn obtain. For all mechanisms described by an Abel integral equation (e.g. reversible system with semi-infinite linear diffusion) this correspondence holds for α = 0.25. For systems described by non-Abellian equations (e.g., quasi-reversible processes), the correspondence obtains for α = 0.50. In general the quality of agreement deteriorates with increasing step height and is adequate for nΔE ≤ 8 mV.

Cathodic reduction of nicotinamide adenine dinucleotide and other adenine-containing compounds in acidic media

Abstract Both nicotinamide adenine dinucleotide (NAD+) and acid-hydrated NADH, as well as adenine, adenosine, adenosine mono-, di-, and tri-phosphate and adenosine diphosphoribose, undergo four-electron reductions of the protonated adenine ring in acidic media. The values of αna (transfer coefficient times the number of electrons involved in the rate-determining step), n (total number of electron transferred), and p (number of protons involved in the rate-determining step) agree well with values previously reported for adenine. Cathodic stripping voltammetry of an adsorbed film can be applied to these compounds. Rapid scan rates are required to eliminate the slow desorption step at −1.1 V vs. SCE for some of these compounds. Hydration of the nicotinamide ring of NADH appears to inhibit this desorption step, but does not appear to be related directly to the electroactivity of the hydration product.

Chronopotentiometry at very small stationary disk electrodes

Abstract Expressions for chronopotentiometry at very small stationary disk electrodes are presented. The transition time, τ, is expressed as a function of I ss / I , where I is the applied current and I ss is the steady-state diffusion current for the disk electrode given by I ss / I =4 nFc * R Da , (where n is the number of electrons transferred, a the radius of the electrode, D the diffusion coefficient and c * R the bulk concentration). When values of I ss / I are small, the transition time constant, , depends linearly on I ss / I and gives the intercept of the Sand equation. When I ss / I approaches unity, the transition time increases rapidly and diverges to infinity at I ss / I =1. If I ss / I exceeds unity, the transition does not occur and the electrode potential approaches a steady-state value corresponding to I . 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 platinum small disk electrodes ( a =0.025–0.10 mm). The transition times obtained experimentally were in good agreement with these predicted theoretically for various values of the applied current, for several different radii of the electrodes.

Mercury-coated carbon fiber microelectrodes : Preparation and some properties

Abstract Carbon fibre microelectrodes were made by sealing the fibres into glass and by using heat-shrinkable tubing. The electrodes can be coated with mercury by deposition at −0.9 V vs. SCE from 0.1 M thiocyanate containing 0.05 mM mercury(II) at pH 2.5. Coulometric measurements and square-wave voltammetry were used to establish the properties of the deposit. Conditions for the deposition and stripping of cadmium are outlined.