Voltammetric determination of iodide by use of an interdigitated microelectrode array

An interdigitated microelectrode array (IDA) sensor has been applied to the determination of iodide in mineral water. It is based on reversible charge transfer in the redox system I2/2I- at a platinum microelectrode. The analytical signal from the IDA system was obtained by use of a bipotentiostat in dual mode. One segment of the IDA (generator) was polarized to the limiting current for oxidation of iodide to iodine in 0.1 mol L(-1) HClO4. The second segment (collector) was fixed at a potential value corresponding to the limiting current of iodine reduction. The geometrical arrangement of the IDA enables this transfer with high efficiency. Because the diffusion layer of both segments overlaps the iodide produced on the collector, the iodide diffuses back to the generator where it is reoxidized. Therefore, redox cycling will enhance the voltammetric signal of the IDA. The signal obtained with a vertically separated IDA was 20 times higher than that in the single mode. Because multiplication of the signal reduces the detection and determination limits, direct voltammetric determination of iodide in mineral water is possible.     

Diffusion coefficients at rotated microelectrodes

The limiting current for several electrode reactions at a rotated microelectrode is observed to vary with the two-thirds power of the diffusion coefficient at infinite dilution in accordance with the theories of TACHI, EISBNBERG, LIN, and others. However, the uncertainties involved in estimating diffusion coefficients at given ionic strength require, that for accurate work, the limiting currents for different electrode reactions be compared on the basis of electrode sensivity, k_ion= i₁/C, and without regard to the role of diffusion in the transport process. The method of LAITINEN AND KOLTHOFF using an electrode reaction with only linear diffusion is recommended for measuring diffusion coefficients at given ionic strength under conditions similar to those extant in voltammetry.     

The detection of formaldehyde in textiles using interdigitated microelectrode array diffusion layer titration with electrogenerated hypobromite

An interdigitated microelectrode array (IDA) was applied to the determination of formaldehyde released from textiles produced in industry. The proposed method is based on formaldehyde reaction with hypobromite which is formed in weakly basic media by control current electrooxidation of bromide on the generator segment of the IDA array. The unreacted hypobromite diffuses through the gap between individually polarisable IDA segments and it is amperometrically detected on the collector segment of the IDA. The efficiency of this nonconvective transfer process in the absence of formaldehyde was substantially higher (78%) in comparison with that when using the rotating ring disc electrode. The influence of the added formaldehyde on the transfer process can be utilised to develop a simple and sensitive analytical procedure for formaldehyde detection with a detection limit of 4×10⁻⁶ mol dm⁻³.     

Prussian Blue-coated interdigitated array electrodes for possible analytical application

Thin films of iron(III) hexacyanoferrate(II) (Prussian Blue) were electrochemically deposited on interdigitated array (IDA) electrodes, yielding systems which can be considered as chemiresistors in sensing alkali metal ion concentrations in an adjacent electrolyte. This is due to the fact that the conductivity of the film being measured by a steady-state current on application of a voltage to the two-fingered electrodes of the IDA depends on both the redox stare of the film and the cation concentration in the electrolyte. From the dependence of the steady-state current on the electrode (bias) potential at variable cation concentrations for different alkali metal ions and for mixtures of alkali metal ions, the possibilities of analytical application were elucidated. In addition, by using the methods of staircase coulometry and scanning conductivity, the electron diffusion coefficient D_e was determined as a function of the redox state of Prussian Blue. It is concluded that Prussian Blue-coated IDA electrodes are, in principle, suitable as chemiresistors for the determination of alkali metal ion concentrations with increasing selectivity in the series Li < Na < K < Rb < Cs.     

Checking of applicability of conditions for reaching the limiting-current mode to flow-by porous electrodes

For the flow-by porous electrodes, which differ in thickness, specific surface area, solution flow rate, and a ratio between the phase conductivities, the conditions providing the limiting-current mode over the entire electrode surface at nearly 100% current efficiency are determined using the method of successive refinement of total current and profile of its distribution along the solution flow. The used values of electrode thickness L are compared with available estimates for the limiting thickness of porous electrode L _lim derived for the ideal limiting-current mode and calculated using real values of the width of the limiting-current plateau of overall polarization curve, solution conductivity, and the diffusion limiting current in the zone of solution input into the electrode. It is found that these values are close to each other in all cases. The largest error of estimation of L _lim does not exceed 10% indicating that it can be used for preliminary estimation of the conditions for reaching the limiting-current mode for the flow porous electrodes of this type.     

Oxygen reduction on rotating porous cylinder of modified reticulated vitreous carbon

A rotating cylinder porous electrode (RCPE) of reticulated vitreous carbon (RVC) matrix was used for oxygen reduction reaction (ORR) in H₂SO₄ solutions. Cyclic voltammetry and hydrodynamic voltammetric techniques were used for electrochemical characterization of the ORR. Cyclic voltammograms in stationary solutions showed better performance of the anodically oxidized RVC (for periods of 1 and 5 min) for the ORR than the untreated RVC in which the first scan (ORR) after the surface treatment was of no utility, and the second scan was presented here. The hydrodynamic voltammograms obtained at the treated RCPE gave well-defined limiting current plateau with positively shifted onset potential as compared with the untreated (plain) RVC electrode. The analysis of the limiting current data on RCPE and the determination of a limiting current enhancement factor α enabled us to quantify the enhancement extent exerted by the anodic oxidation treatment. An enhancement factor of up to ∼3 was obtained at the RCPE electrode anodically oxidized for 5 min. It was found that the α slightly decreased with the rotation speed depending on the extent of anodic oxidation of RVC. This was attributed to the different mode of mass transfer (diffusion) to the interior of the micropores with different microstructure resulting from different extent of anodic oxidation. X-ray photoelectron spectroscopic and scanning electron microscopic measurements helped us to characterize the anodically oxidized RVC surface.     

Fabrication and electrochemical characterization of interdigitated nanoelectrode arrays

Interdigitated nanoelectrode arrays with controlled electrode bandwidth and gap geometries ranging from 30 nm to 1 μm were fabricated on glass substrates by a planar process involving high resolution electron beam lithography and lift-off, and their characteristic electrochemical responses to an aqueous ferrocene derivative solution were examined using fundamental electrochemical techniques. Despite the comparatively large electrode area of electrode arrays containing 10 bands to a single band electrode, quasi-steady-state currents with high current density were obtained at a slow potential sweep rate in cyclic voltammograms of ferrocene derivative since the lateral dimension of the nanoelectrode arrays was considerably less than the scale of the diffusion layer of redox species. Additionally, it was demonstrated that the electrode thickness influenced limiting currents of voltammograms in the case of nanoelectrode arrays. In generation-collection mode experiments, furthermore, a collection efficiency as high as ∼99% was attained by 100 nm wide electrode arrays with a gap dimension of 30 nm.     

Identifying Diffusion Limiting Current to Unravel the Intrinsic Kinetics of Electrode Reactions Affected by Mass Transfer at Rotating Disk Electrode

Rotating disk electrode systems are widely used to study the kinetics of electrocatalytic reactions that may suffer from insufficient mass transfer of the reactants. Kinetic current density at certain overpotential calculated by the Koutecky-Levich equation is commonly used as the metrics to evaluate the activity of electrocatalysts. However, it is frequently found that the diffusion limiting current density is not correctly identified in the literatures. Instead of kinetic current density, the measured current density normalized by diffusion limiting current density (j/j_L) has also been frequently under circumstance where its validity is not justified. By taking oxygen reduction reaction/hydrogen oxidation reaction/hydrogen evolution reaction as examples, we demonstrate that identifying the actual diffusion limiting current density for the same reaction under otherwise identical conditions from the experimental data is essential to accurately deduce kinetic current density. Our analysis reveals that j/j_L is a rough activity metric which can only be used to qualitatively compare the activity trend under conditions that the mass transfer conditions and the roughness factor of the electrode are exactly the same. In addition, if one wants to use j/j_L to compare the intrinsic activity, the concentration overpotential should be eliminated.     

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.     

Convective Instability of the Limiting Current of the Triiodide Reduction in a Vertical Channel

The instability of the limiting current of the cathodic reaction is studied in the I^– ₃–I^–/Pt system with excess of potassium iodide at the electrode situated at the bottom of a vertical cylindrical channel. Dependence of parameters of the oscillation process on the concentration of electroactive and supporting ions and solution viscosity is analyzed. The deceleration of the transport processes in solutions containing glycerol is shown to be due to a considerable decrease in the triiodide ion diffusion coefficient.     

Elektrochemische Methode zur quantitativen und kontinuierlichen Bestimmung von Ozon in Gasgemischen

The method is based on the measurement of the diffusion controlled limiting current obtained with the cathodic reduction of ozone according to O₃ + 2H⁺ + 2e^? → H₂O + O₂ in sulphuric acid. The electrolyte is vigorously mixed with the gas containing ozone and circulated through the electrolytic cell by the ascending gas flow operating as a gas-lift pump. The solution saturated with ozone moves along a cylindric electrode consisting of a smooth platinum foil in laminar flow. A constant potential is maintained at the electrode by means of a potentiostat in such a way, that the electrochemical reaction is proceeding under limiting current conditions. A lead anode of high capacitiy is used as a counter- and reference electrode. The current recorded continuously is directly proportional to the concentration of ozone in gas.     

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.     

Theory of linear sweep voltammetry with finite diffusion space: Part II. Totally irreversible and quasi-reversible cases

The theory of linear sweep voltammetry with finite diffusion space is developed, taking into account the electrode kinetics. As a result of numerical integration of the integral equation, which expresses the current-potential relationship, voltammograms are calculated for a series of values of the three parameters, the anodic transfer coefficient α_a, the kinetic parameter, =k^o′(RT/nFDV)^1/2 and the finite diffusion parameter, w=nFd²v/RTD, where k^o′ is the conditional rate constant of the electrode reaction, d the thickness of the finite diffusion space, v the potential sweep rate and D the diffusion coefficient of the electroactive species. Features of variations of the peak current and the peak potential with these three parameters are described. The diagnostics for classifications into the reversible, the quasi-reversible and the totally irreversible waves and into the semi-infinite diffusion case, the finite diffusion case and the case of surface waves are also presented. For four limiting cases, in which values ofand w are large or small, approximate but reasonably accurate equations are given for the peak current and the peak potential. The results obtained can be applied to analyzing voltammograms for redox polymer films or thin-layer cells.     

Electrochemical deposition of indium: nucleation mode and diffusional limitation

the electrochemical deposition of indium metal from InCl₃ solutions was investigated. Cyclovoltammetric experiments showed that the initial hydrogen evolution reaction, observed together with the metal deposition on Pt surface, is blocked when the surface is covered by In. At large cathodic potentials, the current is diffusion-limited. The scan rate dependence of cyclovoltammograms allowed the determination of the diffusion coefficient of In³⁺ ions, 8.18 × 10^–6 cm²/s, using the Delahay equation. The activation energy of diffusion, determined from the temperature dependence of cyclovoltammograms, is about 0.3 eV (23 kJ/mol). Chrono-amperometric experiments are consistent with the cyclovoltammetry; the In³⁺ diffusion coefficient determined using the Cottrell law is in good agreement with the value determined by the Randles-?ev?ik equation. Moreover the use of the nucleation models developed by Scharifker and Hills showed a progressive nucleation mode. Electron microscopy observations and X-ray diffraction patterns confirmed the formation of crystalline indium deposits.     

Computational electrochemistry: finite element simulation of a disk electrode with ultrasonic acoustic streaming

Computer simulations using the finite element method (FEM) are used to predict the correlation between the transport limiting current (I(lim)) and parameters such as diffusion coefficient, source to electrode separation, source power, and medium viscosity for a sonicated disk electrode in "face on" mode. The fluid dynamics and diffusion layer are modeled directly using FEM and predict that the electrode is uniformly accessible, I(lim) is proportional to the diffusion coefficient to the 2/3 power and I(lim) is proportional to the square root of the source power. Curves are also calculated relating I(lim) to the source to electrode separation and liquid viscosity.     

Amperometric detection of dopamine based on tyrosinase-SWNTs-Ppy composite electrode

A novel 3-dimensional single wall carbon nanotubes (SWNTs)-polypyrrole (Ppy) composite was prepared as an electrode by chemically polymerizing polypyrrole onto SWNTs using a LiClO₄ oxidant. This composite electrode was characterized by scanning electron microscopy (SEM) and cyclic voltammetry with 1 mM [Fe(CN)₆]⁻³/[Fe(CN)₆]⁻⁴. The SWNTs were thickly coated with chemically polymerized polypyrrole and the composite had many surface pores and crevices which could enhance mass transfer. The SWNTs-Ppy composite electrode showed a large specific surface area (30 m²/g) and a good reproducible current response, at about 100 times the peak current of a glassy carbon electrode (GCE). The diffusion coefficient was calculated to be 4.81 × 10⁻⁶ cm²/s. As a biosensor application, tyrosinase was immobilized on the functionalized SWNTs and tyrosinase-SWNTs-Ppy composite was prepared in the same manner. This tyrosinase-SWNT-Ppy composite electrode was used for amperometric detection of dopamine in the presence of ascorbic acid and showed high sensitivity (467 mA/M cm²) and lower detection limit (5 μM) compared to previous reports.     

Electroreduction of bromate anion in acidic solutions at the inactive rotating disc electrode under steady-state conditions: Numerical modeling of the process with bromate anions being in excess compared to protons

The process of electroreduction of bromate anion BrO₃ in acidic solutions at catalytically inactive electrodes when bromate anions are in excess compared to protons has been studied by numerical integration of transport equations. Under these conditions, the maximum possible current is limited by the limiting diffusion current of protons (rather than bromate), since both ions are consumed in the comproportionation reaction. It has been demonstrated that the curve of maximum current versus diffusion layer thickness has an anomalous segment where the current increases with an increase of the latter.     

Gas diffusion hindrances on Ni cermet anode in contact with Zr<Subscript>0.84</Subscript>Y<Subscript>0.16</Subscript>O<Subscript>1.92</Subscript> solid electrolyte

The electrochemical behavior of Ni cermet electrode with CeO_{2 ? x} additive in contact with YSZ electrode was studied by means of impedance spectroscopy in H₂, H₂O, CO₂, CO, He, and Ar gas media of various composition within the temperature range of 700 to 950°C. Near the equilibrium potential, the electrochemical impedance spectra of the studied electrodes indicate to three stages of electrode reaction. The polarization conductivity of the low-frequency stage of electrode reaction (σ_lf) is characterized with the following regularities: (a) temperature dependence of σ_lf has a positive slope in Arrhenius coordinates; (b) σ_lf increases upon replacement of gas mixture with lower mutual diffusion coefficient by mixture with higher mutual diffusion coefficient, while polarization conductivity values of other stages remain practically invariable; (c) concentration relationships of 1/σ_lfrecorded for constant activity of oxygen in the gas phase are linear in the 1/σ_lf vs. 1/P _{CO 2} (P _CO) coordinates; (d) no low-frequency stage of the electrode reaction is observed upon electrochemical inflow (outflow) of the gas reagents (reaction products) to (from) the test electrodes (current passing through closely pressed specimens and central specimen impedance measurement); and (e) no change in the gas flow rate affects σ_lf value. The observed regularities were explained by assuming the gas diffusion nature of the low-frequency stage of the electrode reaction. The gas diffusion layer thickness was estimated.     

Kinetic analysis of the hydrogen oxidation reaction on Pt-black/Nafion electrode

The hydrogen oxidation reaction on Pt-black/Nafion electrode was investigated using a rotating disk electrode and cyclic voltammetry technique. The voltammetric results demonstrated that the electrode can be prepared with good reproducibility and that Pt-black particles without direct contact with Nafion were still electrochemically active in taking part in the H-adsorption/desorption process. For hydrogen oxidation, the limiting current density was reduced by the presence of Nafion coating. The H₂ diffusion resistance in Nafion film was avoided when the film thickness was less than 0.2 μm for a Pt-black loading of 20 μg. Moreover, the uncertainties in the kinetic results were discussed.     

Electrochemical Oxidation of Methionine Mediated by a Fullerene‐C60 Modified Gold Electrode

The usefulness of a C₆₀‐fullerene modified gold (Au) electrode in mediating the oxidation of methionine in the presence of potassium ions electrolyte has been demonstrated. During cyclic voltammetry, an oxidation peak of methionine appearing at +1.0 V vs. Ag/AgCl was observed. The oxidation current of methionine is enhanced by about 2 times using a C₆₀ modified gold electrode. The current enhancement is significantly dependent on pH, temperature and C₆₀ dosage. Calibration plot reveals linearity of up to 0.1 mM with a current sensitivity of close to 50 mA L mol^?1 and detection limit of 8.2×10^?6 M. The variation of scan rate study shows that the system undergoes diffusion‐controlled process. Diffusion coefficient and rate constant of methionine were determined using hydrodynamic method (rotating disk electrode) with values of 1.11×10^?5 cm² s^?1 and 0.0026 cm s^?1 respectively for unmodified electrode while the values of diffusion coefficient and rate constant of methionine using C₆₀ modified Au electrode are 5.7×10^?6 cm² s^?1 and 0.0021 cm s^?1 respectively.