Chronopotentiometry at very small stationary disk electrodes

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=4nFc*_RDa, (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.     

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.     

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.     

Analysis and physical significancy of the kinetic parameters associated with albumin adsorption onto glassy carbon obtained by electrochemical impedance measurements

The kinetics of albumin adsorption onto a glassy carbon rotating disk electrode in a phosphate buffer, is reported from the time variations of the double-layer capacitance C_d, of the charge transfer resistance R_t and of the Tafel coefficient of the electrochemical reaction b; these three electrical quantities are determined by electrochemical impedance and faradaic current I measurements. The variations of C_d, 1/R_t, b  (R_tI)⁻¹ and I can be written under the form: α₀ + α₁ e^−t/τ1 + α₂ e^−t/τ2, where α₀, α₁, α₂, τ₁, τ₂ characterize a given electrical quantity. We demonstrate that this type of variation validates a two-step adsorption mechanism already proposed, starting from simply taking into account C_d(t). We analyze and discuss comprehensively various possibilities of interpreting the shifts between the time-constants associated with thevarious quantities as well as the correlations between these constants and those associated with physical adsorption phenomena.     

Application of a Power Time Current to the Study of a Catalytic Mechanism in Chronopotentiometry and Reciprocal Derivative Chronopotentiometry. Advantages of a Cyclic Stationary Response

This paper develops the theory for a pseudo‐first order catalytic mechanism in chronopotentiometry with a power time current, I(t)=I₀t^u (u≥?1/2), applied to a spherical electrode of any size, and the advantages of the use of small electrodes and ultramicroelectrodes are discussed. The advantages of using a cyclic power time current to reach a stationary response suitable for characterizing a catalytic mechanism easily and accurately are reported. The reciprocal derivative (dt/dE?E) curves, which present peaks quantitatively related to the kinetic parameters of the chemical reaction, have been obtained from the potential‐time responses. The influence of the homogeneous kinetic, the electrode radius and the power of time current in the achievement of a stationary response is analyzed. Methods for determining thermodynamic and kinetic parameters of the chemical reaction are proposed.     

Effect of the activation of a graphite-epoxy composite electrode and the buffer capacity of a solution on the reversibility of the hydroquinone electrode reaction

Peak potentials and the kinetics of hydroquinone oxidation at an electrode from a graphite-epoxy composite in activated and passivated states were studied in supporting electrolytes of different buffer capacities and pH varying from 0.1 to 8.8, using methods of direct-current and cyclic voltammetry. The electrode was activated before its polarization by mechanically cutting a 0.2–4-μm surface layer directly in a test solution. The electrode was passivated by storing in air for two or more days. The behavior of hydroquinone in its oxidation at the passivated and activated electrodes was compared using diagnostic criteria for the following functions: I _a?v ^1/2, logI _a?logv, I _a/v ^1/2?v ^1/2, and I _a/c, where v is the rate of the potential sweep and c is the volumetric concentration of hydroquinone. The potential difference of anodic and cathodic peaks in cyclic voltammograms indicated the reversibility of the electrode reaction in all supporting electrolytes.     

Raman spectroscopic study of dye adsorption on TiO2 electrodes of dye-sensitized solar cells

The state of dye adsorption on TiO₂ electrodes in dye-sensitized solar-cell (DSSC) systems is important for its power-conversion efficiency (PCE). We propose a non-destructive and quantitative method to evaluate the amount of adsorbed dye on TiO₂ electrodes by using micro-Raman spectroscopy. The Raman peak intensity ratio of adsorbed dye to TiO₂, I_d/I_t, is defined as a dye adsorption parameter. Based on a comparison between I_d/I_t and the amount of dye evaluated from UV–vis absorption, the quantitativity and reproducibility of our method are verified.We investigated the change of I_d/I_t spatial distribution of TiO₂ electrodes immersed in a dye solution for different time scales. The statistical analysis of I_d/I_t distribution suggests that dyes adsorbed on TiO₂ electrodes with chemical coordination increase at first, and after their saturation, dye aggregations are formed over the chemisorption layer. We also describe the effect of the I_d/I_t distribution on PCE. From a comparison of PCE and I_d/I_t distribution obtained from various immersion processes, it was considered that the PCE of DSSCs can be optimized by minimizing the I_d/I_t dispersion.     

Electrochemical sensing of tumor suppressor protein p53–deoxyribonucleic acid complex stability at an electrified interface

Electrochemical biosensors have the unique ability to convert biological events directly into electrical signals suitable for parallel analysis. Here we utilize specific properties of constant current chronopotentiometric stripping (CPS) in the analysis of protein and DNA–protein complex nanolayers. Rapid potential changes at high negative current intensities (I_str) in CPS are utilized in the analysis of DNA–protein interactions at thiol-modified mercury electrodes. P53 core domain (p53CD) sequence-specific binding to DNA results in a striking decrease in the electrocatalytic signal of free p53. This decrease is related to changes in the accessibility of the electroactive amino acid residues in the p53CD–DNA complex. By adjusting I_str and temperature, weaker non-specific binding can be eliminated or distinguished from the sequence-specific binding. The method also reflects differences in the stabilities of different sequence-specific complexes, including those containing spacers between half-sites of the DNA consensus sequence. The high resolving power of this method is based on the disintegration of the p53CD–DNA complex by the electric field effects at a negatively charged surface and fine adjustment of the millisecond time intervals for which the complex is exposed to these effects. Picomole amounts of p53 proteins and DNA were used for the analysis at full electrode coverage but we show that even 10–20-fold smaller amounts can be analyzed. Our method cannot however take advantage of very low detection limits of the protein CPS detection because low I^_str intensities are deleterious to the p53CD–DNA complex stability at the electrode surface. These data highlight the utility of developing biosensors offering novel approaches for studying real-time macromolecular protein dynamics.     

Variations in the Overall Charge and Open-Circuit Potential of a Palladium Electrode during the Adsorption of Iodine and Iodide Anions

Transients of the current and the open-circuit potential during the adsorption of I^– and I₂ on Pd/Pt (supporting electrolyte 0.5 M H₂SO₄) are measured and so are potentiodynamic and galvanostatic charging curves, in the presence and absence of adlayers. Dependences of the overall electrode charge on potential are constructed. After the adsorption of both I^– and I₂, values of the electrode surface charges at potentials of the double-layer region turn negative and considerably differ in their magnitude. The latter is connected with different character of adlayers, which is shown by experiments on the adsorption of silver atoms at underpotentials after a preliminary adsorption of I^– and I₂ on the electrode surface. Certain discrepancies between experimentally-found and theoretically-expected values of transients are attributed to a substantial increase in the irreversibility of sorption and desorption of hydrogen and oxygen in the presence of iodine atoms adsorbed on the surface.     

Studies of electrochemical nucleation by means of standard and modified pulse potentiostatic techniques

This report compares experimental data for the overpotential dependence of the stationary nucleation rate I_st and of the induction time t_o obtained in two different ways: (i) by varying the potential E_n of the working electrode at a constant concentration of metal ions (standard pulse potentiostatic technique), and (ii) by varying the concentration of metal ions at a constant electrode potential E_n (modified pulse potentiostatic technique). It is shown that the latter experimental method allows us to fix the surface state of the working electrode and to create well-defined conditions for nucleus formation. Information is obtained on the rate-determining step of the nucleation process.     

Voltammetric Characterization of Grafted Polymer Modified with ZnO Nanoparticles on Glassy Carbon Electrode

In this study, a grafted polymer (GP) with ZnO nanoparticles (GP/ZnO NPs) was attached on the surface of glassy carbon electrode (GCE), in order to produce a new modified electrode (GP/ZnO NPs-GCE). The gamma irradiation method was used to grafted polystyrene (polymer) with acrylonitrile (monomer), while slow evaporation process was used to prepare the new modified electrode. The cyclic voltammetry (CV) of K₄[Fe(CN)₆] was used to study the electrochemical properties GP/ZnO NPs-GCE. The peak separation (ΔE_pa-c) was 500 mV between the redox peaks of Fe(II)/Fe(III) in an aqueous solution of 1 M KCl and the current ratio of redox current peaks (I_pa/I_pc) was ≈ 1 for the modified electrode. This indicated that the modified electrode has s good reversibility and conductivity, wherefore; it was applied in the voltammetric filed. It was found that the modified electrode GP/ZnO NPs-GCE have a reasonable solubility and stability at various pH medium. Additionally, the sensitivity of the electrochemical analysis by cyclic voltammetric (CV) method is extensively subjected to the pH medium and the scan rate (SR). A couple of redox current peaks of K₄[Fe(CN)₆] in KCl solution was observed with a reversible process: Fe³⁺/Fe²⁺. Finally a good diffusion coefficient of electroactive species (D) for the new modified electrode was found in this study by chronoamperometry method using Cottrell equation.     

Reduction kinetics of glycinate complexes of palladium(II) on a dropping-mercury electrode in acid perchlorate solutions

Polarograms for the reduction of glycinate complexes of palladium(II) (5 × 10^?5 M) are obtained in equilibrium solutions of pH 0.8–3.0 with different protonated-glycine concentrations c _Hgly (supporting electrolyte, 0.5 M NaClO₄). It is established that the irreversible wave of reduction of complexes Pd(gly)₂ corresponds to the diffusion limiting current I _d ⁽²⁾ . A similar wave at pH 1.5 and c _Hgly = 0.005 M, as well as at pH 1.0 and c _Hgly = 0.05–0.5 M is preceded by the diffusion limiting current I _d ⁽¹⁾ . Values of the I _d ⁽²⁾ /I _d ⁽¹⁾ ratio are close to the ratio between equilibrium concentrations of Pd(gly)₂] and [Pdgly⁺], calculated using the step stability constant for Pd(gly)₂. This fact testifies to the reduction of complexes Pdgly⁺ in the vicinity of I _d ⁽¹⁾ and complexes Pd(gly)₂, in the vicinity of I _d ⁽²⁾ . At pH 0.8–1.2 and [H₂gly⁺] = 1 × 10^?4 to 5 × 10^?3 there is observed the diffusion-kinetic limiting current of the first wave I ₁ ⁽¹⁾ , which increases with increasing [H⁺] and decreasing [H₂gly⁺]. The nature of the slow preceding chemical stage that occurs during the reduction of complexes Pdgly⁺ is discussed.     

Application of pattern recognition techniques to qualitative electrochemical analysis: Part I. Feature extraction of non-linear characteristics of the electrode process by the two-dimensional fast Hadamard transform

The feature extraction on non-linear characteristics of the electrode process is described. A model consisting of a linear element and a static non-linear element (ZNL=zero-memory non-linear system) is used to acquire the representation of the electrode process, from which the features of the electrode process should be extracted. That is, the electrode process is assumed to be composed of two processes, a solution process which has a dynamic characteristic due to the accumulation of materials in solution, and a surface process which has a non-linear characteristic like a kind of Butler's relation, etc. Previous work showed that a linear diffusion model with and without pseudo-first order chemical reactions made it possible to obtain the representation of the former process as a cross-power-spectral pattern (linear transfer function) between the current (I) and the surface concentration (c). For the latter process, it should be pointed out that its non-linear characteristics are nearly static due to the absence of material accumulation along the normal direction to the surface at the electrode surface. On the basis of this static non-linear model, this paper shows that the surface process can be represented by a pattern of a static curved surface, E-I-c (E= electrode potential) obtained by use of the computational measurements of the surface concentration. A two-dimensional fast Hadamard transform achieved high information compression in the feature extraction of the observed E-I-c pattern.     

Voltammetric study on interaction of cysteine with type I-collagen in the aqueous medium

Interaction of cysteine with type I-collagen from bovine achilles tendon in the aqueous solutions has been examined using square wave voltammetry (SWV) and cyclic voltammetry (CV) techniques. In the absence of cysteine, type I-collagen gives a reversible peak at ?0.114 V in Britton-Robinson (B-R) buffer (pH 4.0). The electrochemical parameters (I _p/f, E _p/f, E _p/pH, I _p/pH, I _p/v, I _p/v^1/2) of type I-collagen have been also studied. In addition, it has been determined that there is a linear relationship between current and concentration of type I-collagen. On the other hand, cysteine exhibits a reversible peak at ?0.383 V due to the reduction of mercurous cysteine thiolate. By using a hanging mercury drop electrode in aqueous solutions, SWV and CV voltammograms obtained for type I-collagen in the presence of cysteine indicate that there is an interaction between type I-collagen and cysteine. In the presence of cysteine, peak current of type I-collagen decreases and a new peak is observed at ?0.121 V for cysteine which is bonded to type I-collagen.     

Polarography of cadmium(II) in presence of substituted alkanolamines

A study has been made of the effect of an irreversibly adsorbed iodide layer on the anodic oxidation of formic acid at a platinized platinum electrode. It is shown that, in the presence of the preadsorbed iodide layer, the oxidation process obeys the following rate expression, i=nF k^→c^α g(θ_I) exp(α_anFφ_r/RT where α≈0.75 and α_an≈0.5. This is explained in terms of the following rate-determining step, (HCOOH)_ads→C_*OOH+H⁺+e and involves the adsorption of formic acid on the iodide covered surface. A strong catalytic effect of the iodide layer is observed; the function g (θ_I) passes through two maxima at θ_I values of 0.15 and 0.53. It is suggested that these effects arise from a coverage-dependent variation of bond strength between the adsorbed iodide and platinum.     

A procedure for studying the sorption of ethanol vapor on a monomolecular Langmuir-Blodgett arachic acid film

The paper presents the results of studies of the sorption of ethanol from the gas phase by a monomolecular arachic acid layer deposited by the Langmuir-Blodgett method onto the surface of a nickel substrate. Studies were performed using a working model based on a field-effect transistor. Sorption was accompanied by changes in the potential of the nickel substrate. The dependence of transistor current I _D on time t was related to the conditions of Langmuir-Blodgett film deposition. The value and characteristic time of signal variations as the atmosphere changed depended on the pressure of monolayer deposition and, therefore, monolayer phase state on the surface of water when it was transferred to the solid substrate. The experimental I _D (t) dependence was compared with the time dependence of arachic monolayer surface coverage with sorbent molecules calculated by the Langmuir model. The conclusion was drawn that the model was capable of describing the sorption of ethanol vapor by the Langmuir-Blodgett arachic acid monolayer at low partial pressures p ≤ 0.05p ₀, where p ₀ is the saturated vapor pressure.     

A non-linear approach to configuration interaction: The low-rank CI method (LR CI)

An alternative to the linear CI expansion is presented, which is based on the spectral resolution of the double excitation CI coefficients arranged as a symmetric matrix. The resulting energy expression is a quartic function of the new variables, where the double excitation operator is given as T̂₂ = ∑_I^Mω_I(d̂_I)², where d̂d_I is a single excitation operator: d̂_I =∑_Ia,d̂^I_iaÊ_ai. Since M is a small number, the number of variables is reduced considerably compared to normal SDCI, with only little loss in accuracy. A program for non-linear CI calculations has been written and is presented with results for H₂O and N₂. The method can easily be extended to include orbital optimization and cluster terms in the wavefunction, still yielding a fully variational approximation to the Schrödinger equation.     

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.     

Enhanced oxygen sensing properties of Pt(II) complex and dye entrapped core-shell silica nanoparticles embedded in sol-gel matrix

This paper presents a highly sensitive oxygen sensor that comprises an optical fiber coated at one end with platinum(II) meso-tetrakis(pentafluorophenyl)porphyrin (PtTFPP) and PtTFPP entrapped core-shell silica nanoparticles embedded in an n-octyltriethoxysilane (Octyl-triEOS)/tetraethylorthosilane (TEOS) composite xerogel. The sensitivity of the optical oxygen sensor is quantified in terms of the ratio I₀/I₁₀₀, where I₀ and I₁₀₀ represent the detected fluorescence intensities in pure nitrogen and pure oxygen environments, respectively. The experimental results show that the oxygen sensor has a sensitivity (I₀/I₁₀₀) of 166. The response time was 1.3 s when switching from pure nitrogen to pure oxygen, and 18.6 s when switching in the reverse direction. The experimental results show that compared to oxygen sensors based on PtTFPP, PtOEP, or Ru(dpp)₃²⁺ dyes, the proposed optical fiber oxygen sensor has the highest sensitivity. In addition to the increased surface area per unit mass of the sensing surface, the dye entrapped in the core of silica nanoparticles also increases the sensitivity because a substantial number of aerial oxygen molecules penetrate the porous silica shell. The dye entrapped core-shell nanoparticles is more prone to oxygen quenching.     

Steady State Reciprocal Derivative Chronopotentiometry with Programmed Currents at Microelectrodes

Equations corresponding to the application of Reciprocal Derivative Chronopotentiometry with programmed currents to spherical and disk microelectrodes under steady state conditions have been deduced. Traditional stationary dt/dE vs. E curves for a current time function of the form I(t)=I₀t^u, u>0, and the new dI/dE vs. E curves, which do not depend on the particular characteristics of the applied programmed current I(t) are shown for any degree of reversibility of the electrode process. The evolution from transient to steady state for both derivative techniques has been analyzed. Criteria for characterizing reversible and totally irreversible stationary derivative curves, together with easy methods for determining thermodynamic and kinetic parameters of the charge transfer process from the measure of peak parameters are proposed. The validity of the theoretical predictions has been experimentally tested with hexachloroiridate(IV) reversible oxidation and with iron(III) irreversible reduction in aqueous media.