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.     

Current challenges related to the deployment of shape-controlled Pt alloy oxygen reduction reaction nanocatalysts into low Pt-loaded cathode layers of proton exchange membrane fuel cells

The reduction of the amount of platinum used in proton exchange membrane fuel cell cathodes at constant power density helps lower the cell stack cost of fuel cell electric vehicles. Recent screening studies using the thin film rotating disk electrode technique have identified an ever-growing number of Pt-based nanocatalysts with oxygen reduction reaction Pt-mass activities that allow for a substantial projected decrease in the geometric platinum loading at the cathode layer. However, the step from a rotating disk electrode test to a membrane electrode assembly test has proved a formidable task. The deployment of advanced, often shape-controlled dealloyed Pt alloy nanocatalysts in actual cathode layers of proton exchange membrane fuel cells has remained extremely challenging with respect to their actual catalytic activity under hydrogen/oxygen flow, their hydrogen/air performance at high current densities, and their morphological stability under prolonged fuel cell operations. In this review, we discuss some of these challenges, yet also propose possible solutions to understand the challenges and to eventually unfold the full potential of advanced Pt-based alloy oxygen reduction reaction catalysts in fuel cell electrode layers.     

Theory of a.c. voltammetry at a rotating disk electrode: Part III. Redox-electrode reactions coupled with first-order chemical reactions

A theory of a.c. voltammetry at a rotating disk electrode is developed for systems with coupled first-order chemical reactions. A general expression for a.c. waves, together with that for d.c. waves, is derived. It is shown that the effect of hydrodynamics on the phase angle is negligible under the condition of p≤0.35, where p is a parameter defined by the frequency of of a.c. wave, the rotation speed of the electrode and Schmidt number.     

The response surface of an amperometric flow cell detector based on a rotating disk electrode for h.p.l.c.and continuous flow analysis

The amperometric detector flow cell based on a rotating disk electrode can be used in conjunction with continuous flow analysis as well as with h.p.l.c. The response surface of the detector as a function of flow rate, electrode rotation speed and concentration of electroactive species (hexacyanoferrate(II)) is measured in combination with continuous flow analysis. When the electrode is stationary, the detector behaves as a wall-jet detector. Rotating the electrode results in a completely different hydrodynamic flow pattern in the flow cell. The response becomes independent of the flow rate and is linearly related to the electrode rotation speed. The influence of nozzle height in the flow cell on the detector response in combination with h.p.l.c. is described. With certain combinations of nozzle height and rotation speeds, a favourable flow pattern appears to be created in the cell and the sensitivity is increased considerably.     

The anodic oxidation of 2,4,5-triarylimidazoles in aprotic medium

The anodic oxidation of 2-(p-dimethylaminophenyl)-4,5-diphenylimidazole and 2,4,5-tris-p-methoxyphenylimidazole in benzonitrile solutions has been studied, using rotating disc electrode, rotating double ring electrode, and coulometry as electrochemical techniques. ESR and UV-vis. spectroscopy were used for product analysis. The triarylimidazoles exhibit two or more oxidation waves. The first one is interpreted as a non-conventional e.c.e. mechanism where the parent molecule is involved in the chemical step. Further anodic waves are due to the oxidation of protonated triarylimidazole, produced in the e.c.e. mechanism.     

Voltammetric,potentiometric and amperometric studies with a rotated aluminum wire electrode : A comparison of the baker and morrison cell with the raie.

In the amperometric determination of fluoride at the RAIE a half cell composed of 5% cadmium amalgam in equilibrum with a solution 1 M in cadmium sulfate and saturated with potassium chloride can be used as a reference electrode in a short-circuited cell instead of applying a potential of -0 75 V versus the saturated calomel electrode The standard addition method can be used in the presence of air, although removal of oxygen is recommended Using the Baker and Morrison electrode versus the above half cell and following their directions (10 ml solution, magnetic stirring) proportionality between current and fluoride concentration in a range between 1 · 10^-5 and 1 · 10^-4M was found in oxygen-free solutions Halides and perchlorates do not interfere. The standard addition technique can be used in the determination of fluoride in an unknown.     

Theory of a.c. voltammetry at a rotating disk electrode: Part II. Quasi-reversible and irreversible redox-electrode reactions

Alternating current response at a rotating disk electrode to a small a.c. potential wave superimposed on the slowly changing d.c. potential is theoretically treated for an quasireversible and an irreversible redox-electrode process. Expressions for alternating current-d.c. potential curves are given for three types of a.c. signals, i.e., sinusoidal, rectangular and triangular waves. It is shown that the effect of the coupling of the diffusion layers due to hydrodynamics and a.c. polarization can be expressed in terms of a parameter which is defined by the ratio of the frequency of a.c. wave to that of the rotation of the electrode and Schmidt number.     

Hydrodynamic Modulation Voltammetry with a Dual Disk Chopped Flow‐Microjet Electrode (CF‐MJE)

A novel form of hydrodynamic modulation voltammetry (HMV) is described, based on the periodic variation of mass transport in a microjet electrode (MJE) system, in combination with phase‐sensitive detection techniques. In the configuration developed, a jet of solution is fired from a nozzle that is aligned directly over the surface of a dual disk Pt‐Pt ultramicroelectrode (UME). The potential at each electrode is controlled separately. A rotating blade, positioned between the nozzle and the UME probe, is used to periodically interrupt flow to the electrode surface, resulting in modulation of the overall mass transfer rate between two defined extremes. The use of a dual disk UME enables two transport‐limited current signals to be recorded simultaneously, one for the analyte of interest, and the other for a ‘reference species’ (oxygen for the studies described herein). The latter current response corresponds to the variation in mass transport rate in the chopped flow (CF) arrangement and is used as the signal for phase sensitive detection of the analyte current. Studies of potassium hexachloroiridate (III) [IrCl ] oxidation in aqueous solution are used to demonstrate the capabilities of the technique. HMV in the CF‐MJE arrangement allows quantitative concentration measurements, down to at least 5×10^?7 M.     

Determination of traces of metals by anodic stripping voltammetry at a rotating glassy carbon ring—disc electrode: Part 1. Method and instrumentation with evaluation of some parameters

The equipment and procedure are described for the determination without preconcentration of several heavy metals based on d.c. anodic stripping voltammetry at a rotating ring—disc glassy carbon electrode with in situ mercury plating. During stripping of the metals deposited on the disc, the current from the reduction of the ions collected at the ring is measured. Some parameters (scan rate, thickness of the mercury film, electrode rotation and deposition time) influencing the ring collection peak current are examined experimentally. The results are compared with the theoretical considerations given by de Vries and van Dalen for anodic stripping voltammetry on a stationary mercury film electrode and by Bakanov et al. for a rotating mercury film electrode.     

Application de la methode polarovoltrique : Interprétation du dosage de L'acide maléique par des bases organiques en milieu n,n-diméthylformamide

The determination of the voltammetric curves, I=f(E), of maleic acid and of a basic titrant in N,N-dimethylformamide using 3 electrodes allowed interpretation of the polarovoltric titration curves of the neutralization of this acid by sodium methylate and tetrabutylammonium hydroxide. The different variations of the potential between 2 polarized platinum electrodes observed in such a neutralization reaction were identified and a notation was developed so that the origin of the observed potential variations could be established. The signal E_c^H+→S, which is related to the initial acidity, is given by the cathode and corresponds to a jump from the potential of the H⁺ reduction curves to that of the solvent reduction curves. The signal E_A^S→OH- characteristic of the second acidity, is given by the anode and is related to a jump from the potential of the solvent oxidation curves to that of the titrant oxidation curves.The effect of the use of unsymmetrical polarized electrodes (a rotating filament electrode and a large-surface, stationary electrode) on the shape of the titration curve was also examined: a rotating cathode gave rise to a curve resembling an inverted U, whereas a rotating anode gave rise to a curve resembling a deformed M.     

Resistance effects in phase sensitive fundamental harmonic a.c. polarography

Equations relating the effect of cell resistance to the background current in phase sensitive fundamental harmonic a.c. polarography have been derived. Under normal experimental conditions the background current was found to be proportional to the specific resistance of the supporting electrolyte and the squares of the a.c. signal frequency and electrode double layer capacity.     

Optimization of electrolysis in the cylindrical electrochemical cell rotating in the magnetic field

The relationships between the rotational rate of electrochemical cell, its ohmic resistance, a ratio between the radii of inner and outer cylindrical electrodes, a sum of cathodic and anodic potentials, and the mechanical energy, which is consumed for passing a prescribed current and overcoming friction forces, are analyzed. It is shown that the relationships can be used to optimize the operation of rotating electrochemical cell.     

Étude d'un modèle périodique de diffusion non linéaire en régime stationnaire pour une réaction réversible

For a partially inhibited solid electrode a periodic model of non-linear diffusion is defined, permitting the calculation of the attenuation factor, ?, of the diffusion current density as a function of the size and the closeness of active or inactive sites and of the diffusion layer thickness. Values of ? are compared with those obtained from the Landsberg's model [3]. In the case of a disk electrode rotating at N r.p.s., the influence of the characteristic parameters of the inhibiting coverage on the relationship between the diffusion current and N^1/2 is analysed in the experimentally accessible laminar flow domain.     

Dynamic tracking of pathogenic receptor expression of live cells using pyrenyl glycoanthraquinone-decorated graphene electrodes

Expression of specific transmembrane receptors by cells frequently represents an important signature of diseases, but this dynamic event can hardly be monitored directly with live cells due to the limitation of current biochemical techniques. Here we develop a pyrenyl glycoanthraquinone construct that can be firmly immobilized on a graphene-spotted screen printed electrode via strong π-interactions. The inherent current signal produced by the surface-confined glycoquinone can be used to detect selective sugar–protein recognitions with simple electrochemical techniques and portable facilities. Importantly, we demonstrate that the level of pathogenic receptors expressed by different types of live cells can be tracked with the electrode system in a label-free manner, providing a useful tool for the on-demand disease diagnosis as well as basic biochemical studies.     

Influence de la distribution de potentiel électrochimique sur l'interprétation de la courbe courant-tension anodique d'une électrode tournante d'alliage Fe-31 % Ni en milieu sulfurique

Influence of the electrochemical potential distribution on the interpretation of the anodic current-voltage curve of a rotating electrode of a Fe-31 % Ni alloy in normal sulfuric acid solution. The anodic steady-state polarization curve of a Fe-31 % Ni rotating-ring electrode in normal sulfuric acid was studied, and the dissolution profiles obtained at different points of the curve were surveyed. We showed that the use of a rotating-ring electrode allowed to obtain the entire J(E) relationship between the current density J and the electrochemical potential E, whereas this relationship cannot be obtained from the polarization curve of the classical rotating-disk electrode because of the non-uniform potential distribution over the disk surface. With the ring electrode, additional information on the transitions between three different states of the metal (homogeneous dissolution, intergranular dissolution and passivity) could be obtained.     

Spatially resolved temperature measurements in a furnace atomization plasma excitation spectrometry source

Spatially resolved atomic emission intensities from helium, and molecular emission intensities from OH and N⁺₂ have been measured in a furnace atomization plasma excitation spectrometry (FAPES) source. He I emission at 388.86 nm was used to monitor the spatial structure of the plasma in the source while increasing the radio frequency (r.f.) power applied to its center electrode. At higher r.f. power the He I emission intensity increased significantly while its spatial structure remained relatively unchanged. The He I emission was found to be most intense adjacent to the center electrode. Some less intense emission was observed adjacent to the graphite cuvette wall and some very weak emission was seen throughout the volume of the source. These observations suggest that the FAPES source operates as an r.f. glow discharge.Emission intensities from the OH (0-0) rotational A ²Σ⁺ → X ²Π_i and N⁺₂ (0-0) rotational B ²Σ⁺_o → ²Σ⁼_g bands were used to monitor the effects of increasing the r.f. power applied to the center electrode of the source. From these measurements, rotational temperatures for these molecules were calculated. The intensity measurements showed that there is a significant thermal gradient in the source with OH rotational temperatures ranging between 680 and 1050 K and N⁺₂ rotational temperatures ranging between 580 and 1920 K with 60 W r.f. power applied to the center electrode. At higher r.f. powers there is an increase in rotational temperatures and an increase in the dissociation of molecular species in the FAPES source.Lead excitation temperatures were calculated using the line ratio method by measuring the emission of the Pb I 280.119 and 283.306 nm lines at different r.f. powers. The temperature was found to increase monotonically with r.f. power over the range of 35 to 75 W.     

Hydrodynamic voltammetry at channel electrodes: Part VII. Current transients at double channel electrodes

Expressions for the transient current at the downstream electrode in response to galvanostatic electrolysis at the upstream electrode in the channel flow cell were derived by applying double Laplace transformation when the electrode reaction at the upstream electrode is kinetically controlled. The ratio of the transient current to the steady state current or the transient collection efficiency was calculated as a function of electrode geometry and θ

, where U_m is the mean flow velocity in the channel cell, D the diffusion coefficient of the electroactive species, b the half height of the channel, x₁ the length of the upstream electrode and t the time elapsed since the beginning of the galvanostatic electrolysis at the upstream electrode. Curves for the transient collection efficiency can be applied to evaluating the amount of adsorption at the upstream electrode when metal at the electrode is anodically dissolved in solution. Digital simulation was carried out. Transient curves, obtained analytically, were in good agreement with those evaluated from the digital simulation. In order to allow one to draw transient curves readily, we derived a simple approximate equation.     

Rotating-disc electrode studies of the anodic oxidation of iodide in concentrated iodine + iodide solutions

The anodic oxidation of iodide on platinum in concentrated iodine + iodide solutions has been investigated using a rotating disc electrode. The conventional limiting diffusion current, which is produced by the diffusion of iodide ions towards the electrode, was not observed due to the formation of an iodine film on the electrode. On the other hand, the steady-state anodic current after a current/time transient is the genuine limiting diffusion current in the anodic oxidation due to diffusion of iodine species from the electrode surface towards the bulk solution. Thus, the dissolution-diffusion control mechanism of the iodine film is confirmed. This is interesting as a typical example of an anodic process in a redox system governed by diffusion of the anodic product species from the electrode surface towards the bulk solution. When an iodine film is formed on the electrode, the maximum driving force of the iodine species is Δm_I2,max, which is defined as the extent of unsaturation of the iodine, and the limiting current of the anodic oxidation of iodide is always directly proportional to Δm_I2,max, regardless of the forms of iodine species in the solution, which may be I₂, I⁻₃, i₅⁻, etc. δm_I2,max is clearly determined by the solution composition and temperature, and it is different in definition and value from the usual degree of unsaturation of iodine.     

二茂铁及其与DNA复合物的电化学行为

在Tris-NaCl(pH=7.2)缓冲溶液中, 应用循环伏安法、微分脉冲伏安法、旋转圆盘电极实验、电化学阻抗谱等技术研究了二茂铁在旋转碳纳米管(CNT)修饰电极上的电化学行为及其与小牛胸腺DNA的相互作用. 结果表明, 二茂铁及其与双链DNA的电活性产物在静止的CNT修饰电极上均呈现一对基本可逆的氧化还原峰；在旋转电极上呈现出明显的极限扩散电流, 电化学阻抗谱呈现一个压扁的半圆. 二茂铁与DNA的作用在扩散控制过程中表现为峰电流和极限扩散电流随DNA浓度增大而减小；电化学控制过程则表现为电化学反应电阻随DNA浓度增大而增大, 条件电位下的速度常数也有一定程度的减小.     

The applicability of phase-sensitive alternating current measurements in flow-through detection

The applicability of phase-sensitive alternating current (a.c.) measurements in flowthrough detection is discussed. The differences between flow and non-flow conditions are examined for copper(II) and thiourea at a horizontal dropping mercury electrode in an “open system”. The possibility of using this detection system in flow-injection procedures is indicated. A glassy-carbon electrode cell is used to test the feasibility of a.c. measurements with hydroquinone as the example.