Electrochemical characteristics of a cobalt pentacyanonitrosylferrate film on a modified glassy carbon electrode and its catalytic effect on the electrooxidation of hydrazine

Electrochemically prepared thin films of cobalt pentacyanonitrosylferrate (CoPCNF) were used as surface modifiers for glassy carbon electrodes. The electrochemical behavior of a CoPCNF-modified glassy carbon electrode was studied by cyclic voltammetry; the modified electrode shows one pair of peaks with a surface-confined characteristic in 0.5 M KNO₃ as supporting electrolyte. The effect of different alkali metal cations in the supporting electrolyte on the behavior of the modified electrode was studied and the transfer coefficient (α) and charge transfer rate constant (k_s) for the electron transfer between the electrode and modifier layer were calculated. The experimental results show that the peak potential and peak current vary with different alkali metal cations, but anions such as Cl^?, NO₃^?, CH₃COO^?, H₂PO₄^?/HPO₄^2? and SO₄^2? at 0.5 M concentration have no effect on the peak potential and peak current. An extensive study showed that the response of the modified electrode is not affected within a pH range of 2–8. The CoPCNF films on glassy carbon electrodes show excellent electrocatalytic activity toward the oxidation of hydrazine in 0.5 M KNO₃. The kinetics of the catalytic reaction were investigated by using cyclic voltammetry, rotating disk electrode (RDE) voltammetry and chronoamperometry. The average value of the rate constant for the catalytic reaction and the diffusion coefficient were evaluated by different approaches for hydrazine.     

Electrocatalytic oxidation of NADH at graphite electrodes modified with osmium phenanthrolinedione

We report here a detailed study concerning the electrochemical behavior of Os(4,4′-dimethyl, 2,2′-bipyridine)₂(1,10-phenanthroline 5,6-dione) complex, adsorbed on spectrographic graphite, and about its electrocatalytic activity for NADH oxidation. Cyclic voltammetric measurements, performed in aqueous phosphate buffer solutions, at different scan rates and pH values, allowed us: (i) to relate the redox response of the o-quinone ligand (phendione) to that of the Os(II) central ion; (ii) to confirm that, in aqueous solutions, the phendione based redox process globally involves two electrons and two protons; (iii) to estimate the rate constant for the heterogeneous electron transfer corresponding to the phendione redox couple (k_s≈20.1 s^?1). The second order rate constant for electrocatalytic oxidation of NADH (k_1,[NADH]=0=1.9×10³ M^?1 s^?1, at pH 6.1) as well as its pH dependence (from pH 5.5 to 8.1) were evaluated from RDE experiments, using both Koutecky–Levich and Lineweaver–Burk data interpretations.     

Electrochemical properties of a tetrabromo-p-benzoquinone modified carbon paste electrode. Application to the simultaneous determination of ascorbic acid,dopamine and uric acid

The electrochemical study of a tetrabromo-p-benzoquinone modified carbon paste electrode (TBQ-MCPE), as well as its efficiency for electrocatalytic oxidation of ascorbic acid, dopamine and uric acid, is described. Cyclic voltammetry was used to investigate the redox properties of this modified electrode at various solution pH values and at various scan rates. Three linear segments were found with slope values of ?58.4 mV/pH, ?28.1 mV/pH and 0.0 mV/pH in the pH range 2.0–7.1, pH 7.1–9.0 and pH 9.0–11.0, respectively. The apparent charge transfer rate constant, k_s, and transfer coefficient, α, for electron transfer between TBQ and CPE were calculated as 3.79 ± 0.10 s^?1 and 0.55, respectively. The electrode was also employed to study the electrocatalytic oxidation of AA, using cyclic voltammetry, chronoamperometry and differential pulse voltammetry as diagnostic techniques. It has been found that the oxidation of AA at the surface of TBQ-MCPE occurs at a potential of about 430 mV less positive than that of an unmodified CPE. The diffusion coefficient of AA was also estimated using chronoamperometry. The kinetic parameters such as the electron transfer coefficient, α, and heterogeneous rate constant, $k_{\mathrm{h}}^{\prime }$, for oxidation of AA at the TBQ-MCPE surface was determined using cyclic voltammetry. Differential pulse voltammetry (DPV) exhibits two linear dynamic ranges and a detection limit of 0.62 μM for AA. In DPV, the TBQ-MCPE could separate the oxidation peak potentials of AA, DA and UA present in the same solution, though at the unmodified CPE the peak potentials were indistinguishable. This modified electrode was quite effective not only to detect AA, DA and UA, but also in simultaneous determination of each component concentration in the mixture.     

Effect of pH on the catalytic electrooxidation of NADH using different two-electron mediators immobilised on zirconium phosphate

A detailed study is reported on the electrocatalytic oxidation of β-nicotinamide adenine dinucleotide at three different carbon paste electrodes modified with redox mediators commonly used in bioelectrochemistry, viz. Meldola blue, methylene green and riboflavin, adsorbed on zirconium phosphate. Cyclic voltammetric investigations of these chemically modified electrodes performed in aqueous tris buffer solutions at different pHs allowed us to conclude that the immobilised redox species presented very stable redox properties, with midpoint potentials (E_m, ?125, ?40, and ?140 mV versus SCE for Meldola blue, methylene green and riboflavin, respectively) virtually independent of the pH of the contacting solution. The second-order rate constant for electrocatalytic oxidation of NADH as well as its pH dependence (from 6 to 8) were evaluated from rotating disk electrode (RDE) experiments, using the Koutecky–Levich approach.     

Electrocatalytic oxidation of some amino acids on a cobalt hydroxide nanoparticles modified glassy carbon electrode

In this study we investigated the electrocatalytic oxidation of cysteine, cystine, N-acetyl cysteine, and methionine on cobalt hydroxide nanoparticles modified glassy carbon electrode in alkaline solution. Different electrochemical techniques such as cyclic voltammetry, chronoamperometry and steady-state polarization were used to track the oxidation process and its kinetics. From voltammetric studies we concluded that in the presence of amino acids the anodic peak current of Co(IV) species increased, followed by a decrease in the corresponding cathodic current peak. This indicates that amino acids were oxidized on the redox mediator which was immobilized on the electrode surface via an electrocatalytic mechanism. Using Laviron’s equation, the values of α_s and k_s for the immobilized redox species were determined as α_s,a = 0.63, α_s,c = 0.38 and k_s = 0.28 s⁻¹, respectively. The catalytic rate constants, the electron transfer coefficients and the diffusion coefficients involved in the electrocatalytic oxidation of amino acids were determined.     

Simultaneous determination of epinephrine and uric acid at a gold electrode modified by a 2-(2,3-dihydroxy phenyl)-1, 3-dithiane self-assembled monolayer

In the present paper, the use of a gold electrode modified by 2-(2,3-dihydroxy phenyl)-1,3-dithiane self-assembled monolayer (DPDSAM) for the determination of epinephrine (EP) and uric acid (UA) was described. Initially, cyclic voltammetry was used to investigate the redox properties of this modified electrode at various scan rates. The apparent charge transfer rate constant, k_s, and transfer coefficient, α, were calculated. Next, the mediated oxidation of EP at the modified electrode was described. At the optimum pH of 8.0, the oxidation of EP occurs at a potential about 155 mV less positive than that of an unmodified gold electrode. The values of electron transfer coefficients (α = 0.356), catalytic rate constant (k = 1.624 × 10⁴ M⁻¹ s⁻¹) and diffusion coefficient (D = 1.04 × 10⁻⁶ cm² s⁻¹) were calculated for EP, using electrochemical approaches. Based on differential pulse voltammetry, the oxidation of EP exhibited a dynamic range between 0.7 and 500.0 μM and a detection limit (3σ) of 0.51 μM. Furthermore, simultaneous determination of EP and UA at the modified electrode was described. Finally, this method was used for the determination of EP in EP ampoule.     

Fabrication of functionalized carbon nanotube modified glassy carbon electrode and its application for selective oxidation and voltammetric determination of cysteamine

The functionalized carbon nanotube electrode was fabricated by electrodeposition of 1,2-naphthoquinone-4-sulfonic acid sodium (Nq) on single-wall carbon nanotube (SWNT) modified glassy carbon electrode (GCE). This electrode was characterized by scanning electron microscopy (SEM) and the results showed that Nq can rapidly and effectively be deposited on the surface of SWNT film with high stability. The electrochemical properties of functionalized SWNT/GCE with Nq (SWNT–Nq/GCE) were studied using cyclic voltammetry, double step potential chronoamperometry and differential pulse voltammetry methods. The results indicated that SWNT could improve the electrochemical behavior of Nq and greatly enhances its redox peak currents. The SWNT–Nq/GCE exhibited a pair of well-defined redox peaks. The experimental results also demonstrated that the Nq deposited species on SWNT could catalyze cysteamine oxidation and SWNT–Nq exhibited a high performance with lowering the overpotential by more than 710 mV. The effect of pH value, number of scans and Nq concentration were investigated on the electrochemical behavior of cysteamine. The selectivity of the reaction has been assessed with no interference from tyrosine, lysine, methionine, tryptophan, alanine and glutathione. The presented method has highly selectivity for voltammetric detection of cysteamine in the dynamic range from 5.0 × 10⁻⁶ M to 2.7 × 10⁻⁴ M and with a detection of limit (3σ) 3.0 × 10⁻⁶ M.     

Complex electrochemistry of flavodoxin at carbon-based electrodes: results from a combination of direct electron transfer,flavin-mediated electron transfer and comproportionation

Staircase cyclic voltammetry (SCV) and differential pulse voltammetry on fully oxidized flavodoxin from Desulfovibrio vulgaris Hildenborough at the bare glassy carbon electrode give one redox couple at a potential of ?218 mV (standard hydrogen electrode (SHE)) at pH = 7.0 with an SCV peak current proportional to the scan rate. This response is caused by flavin mononucleotide (FMN), dissociated from the protein and adsorbed onto the electrode. The midpoint potential and the pK of 6.5 are equal to the values measured with free FMN in solution. When the cationic promoter neomycin is added, one additional and diffusion controlled response is observed. The midpoint potential is ?413 mV (SHE) at pH 7.0 with a redox-linked pK of 4.8 for the reduced form. The temperature dependence is ?1.86 mV K^?1, yielding ΔS° = ?179 J mol^?1 K^?1 and ΔH° = ?12.4 kJ mol^?1. Although the starting material was 100% quinone, no response was observed around the midpoint potential of the quinone to semiquinone reduction of ?113 mV (SHE) at pH 7.0, determined in an EPR-monitored titration with dithionite. Digital simulation shows that the peak currents of the second reduction couple approach a maximum value after a few cycles if comproportionation of fully reduced and fully oxidized flavodoxin occurs in solution and a small amount of semiquinone is either present initially or is generated by mediation of electrode-bound FMN. In the latter case the heterogeneous electron transfer rate between adsorbed FMN and flavodoxin is 6.3 × 10^?6 m s^?1. The implications of this anomalous behaviour for electrochemistry on flavin enzymes like glucose oxidase are discussed.     

Electrocatalytic characteristics of ascorbic acid oxidation at nickel plated aluminum electrodes modified with nickel pentacyanonitrosylferrate films

Nickel pentacyanonitrosylferrate (NiPCNF) films have been deposited on the surface of an aluminum electrode by a simple electroless dipping method. The cyclic voltammogram of the resulting surface modified Al electrode prepared under optimum conditions, shows a well-behaved redox couple due to the [Ni^IIFe^III/II(CN)₅NO]^0/?1 system. The NiPCNF films, formed on the Al electrode show excellent electrocatalytic activity towards the oxidation of ascorbic acid in phosphate buffer solution of pH 7.2. A linear calibration graph is obtained over the ascorbic acid concentration range 2–50 mM using linear sweep voltammetry. The kinetics of the catalytic reaction were investigated using cyclic voltammetry, rotating disk electrode (RDE) voltammetry and chronoamperometry. The results were explained using the theory of electrocatalytic reactions at chemically modified electrodes. The rate constants for the catalytic reaction evaluated by three different approaches, are in good agreement and were found to be around 10^?3 cm s^?1. Further examination of the modified electrodes shows that the modifying layers (NiPCNF) on the aluminum substrate have reproducible behavior and a high level of stability.     

Electrochemical behavior of iodide at a nickel pentacyanonitrosylferrate film modified aluminum electrode prepared by an electroless procedure

The surface of an aluminum disk electrode was modified by a thin film of nickel pentacyanonitrosylferrate and used for electrocatalytic oxidation of iodide. The cyclic voltammogram of the modified Al electrode showed surface redox behavior due to the [Ni^IIFe^III/II(CN)₅NO]^0/1? redox couple. The modifying layer shows excellent catalytic activity toward the oxidation of iodide. Different supporting electrolytes containing different alkali metal cations affected the apparent formal potential of the redox films and thus, changed the thermodynamic tendency and kinetics of the modifying film toward the catalytic oxidation of iodide. This was explained by including the concept of a surface coverage normalized-catalytic current. The kinetics of the catalytic reaction were investigated by cyclic voltammetry and rotating disk electrode voltammetry in a suitable supporting electrolyte. The results were explained using the theory of electrocatalytic reactions at chemically modified electrodes. The heterogeneous rate constant for the catalytic reaction, k, diffusion coefficient of iodide in solution, D, and transfer coefficient, α, were found to be 5.8 × 10² M^?1 s^?1, 1.3 × 10^?5 cm² s^?1 and 0.66, respectively. In addition the effect of electrode surface coverage on the dynamic range of a calibration curve was investigated. Under optimum conditions a linear calibration graph was obtained over an iodide concentration range of 2–100 mM.     

Electrochemical studies of the oxidation mechanism of polyamide on glassy carbon electrode

Polyamides containing N-methyl pyrroles and N-methyl imidazoles are a type of small molecule that can bind and recognize the bases of DNA with high affinity and specificity. Five polyamides were studied at glassy carbon electrode in acetate buffer by cyclic and differential pulse voltammetry to clarify their redox pathways. The polyamide electrochemical responses are compared by peak currents and peak potentials. The slopes of the three anodic E_p vs. pH plots of a typical polyamide are linear and show 0.059, 0.057, 0.056 V per pH in acid media, respectively, which correspond to a mechanism involving the equal number of electrons and protons. A possible mechanism for the redox pathway of various polyamides is proposed: the oxidation product of imidazole ring is acylamide and the results of in situ UV–Vis spectroscopy at Pt web electrode support the proposed mechanism. electrospray ionization mass spectroscopy (ESI-MS) indicates that one or two oxygen atoms are added into polyamide molecule after electrochemical oxidation.     

Preparation,characterization and electrocatalytic properties of polynuclear mixed-valent ruthenium oxide/hexacyanoruthenate film modified electrodes

Polynuclear mixed-valent ruthenium oxide/ruthenocyanide (ruthenium oxide/hexacyanoruthenate or mvRuO/RuCN) films were prepared using consecutive cyclic voltammetry directly from the mixing of Ru³⁺ and Ru(CN)₆^4? ions from solutions of two divalent cations (Ba²⁺ and Ca²⁺), and seven monovalent cations (H⁺, Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, and Ga⁺). The films exhibited three redox couples with Ba(NO₃)₂ or BaCl₂ aqueous solutions, and the formal potentials of the redox couples showed a cation and pH effect. An electrochemical quartz crystal microbalance (EQCM), cyclic voltammetry, UV–visible spectroscopy, and the stopped-flow method (SFM) were used to study the growth mechanism of the mvRuO/RuCN films. The results indicated that the redox process was confined to the immobilized ruthenium oxide/ruthenocyanide. The EQCM results showed a Ba²⁺ ion exchange reaction for the two most negative redox couples. The electrocatalytic reduction properties of SO₅^2?, and S₂O₈^2? by the ruthenium oxide/ruthenocyanide films were determined. The electrocatalytic oxidation of NADH and dopamine were also determined, and revealed two different types of properties. The electrocatalytic oxidations of SO₃^2?, S₂O₃^2?, and N₂H₄ were also investigated. The electrocatalytic reactions of the ruthenium oxide/ruthenocyanide films were investigated using the rotating ring-disk electrode method.     

Electrochemical behavior of quercetin: Experimental and theoretical studies

Electrochemical oxidation of quercetin, as important biological molecule, has been studied in 0.1 M phosphate buffer solution, using cyclic voltammetry, chronoamperometry, rotating disk electrode voltammetry as well as quantum mechanical calculations. The heterogeneous charge transfer rate constant, k′, transfer coefficient, α, and exchange current density, j₀, for oxidation of quercetin at the glassy carbon electrode are determined as 4.84 × 10^?2 cm s^?1, 0.65 ± 0.01 and (1.17 ± 0.39) × 10^?7 A cm^?2, respectively. The formal potential, E^0′, of quercetin is pH dependent with a slope of ?60.1 mV per unit of pH which is close to the anticipated Nernstian value of ?59 mV for a two electrons and two protons process. The standard formal potential, E⁰, of quercetin was found to be equal with 558 mV versus saturated calomel electrode (SCE). The mechanism of oxidation was deduced from voltammetric data in various pHs and also in different concentrations of quercetin. The diffusion coefficient of quercetin was calculated as 3.18 × 10^?6 cm² s^?1 for the experimental condition, using chronoamperometric results. The results of density functional theory (DFT) calculations for the oxidation of quercetin in aqueous solution, are also presented. The theoretical standard electrode potential of quercetin is obtained to be 568 mV versus SCE, which is in good agreement with the experimental value. The discrepancy between theoretical and experimental values is only 10 mV. The agreement verifies the accuracy of experimental method and the validity of mathematical model.     

A differential pulse voltammetric method for simultaneous determination of ascorbic acid,dopamine, and uric acid using poly (3-(5-chloro-2-hydroxyphenylazo)-4,5-dihydroxynaphthalene-2,7-disulfonic acid) film modified glassy carbon electrode

A stable modified glassy carbon electrode based on the poly 3-(5-chloro-2-hydroxyphenylazo)-4,5-dihydroxynaphthalene-2,7-disulfonic acid (CDDA) film was prepared by electrochemical polymerization technique to investigate its electrochemical behavior by cyclic voltammetry. The properties of the electrodeposited films, during preparation under different conditions, and their stability were examined. The homogeneous rate constant, k_s, for the electron transfer between CDDA and glassy carbon electrode was calculated as 5.25(±0.20) × 10² cm s⁻¹. The modified electrode showed electrocatalytic activity toward ascorbic acid (AA), dopamine (DA), and uric acid (UA) oxidation in a buffer solution (pH 4.0) with a diminution of their overpotential of about 0.12, 0.35, and 0.50 V for AA, DA, and UA, respectively. An increase could also be observed in their peak currents. The modified glassy carbon electrode was applied to the electrocatalytic oxidation of DA, AA, and UA, which resolved the overlapping of the anodic peaks of DA, AA, and UA into three well-defined voltammetric peaks in differential pulse voltammetry (DPV). This modified electrode was quite effective not only for detecting DA, AA, and UA, but also for simultaneous determination of these species in a mixture. The separation of the oxidation peak potentials for ascorbic acid–dopamine and dopamine–uric acid were about 0.16 V and 0.17 V, respectively. The final DPV peaks potential of AA, DA and UA were 0.28, 0.44, and 0.61 V, respectively. The calibration curves for DA, AA, and UA were linear for a wide range of concentrations of each species including 5.0–240 μmol L⁻¹ AA, 5.0–280 μmol L⁻¹ DA, and 0.1–18.0 μmol L⁻¹ UA. Detection limits of 1.43 μmol L⁻¹ AA, 0.29 μmol L⁻¹ DA and 0.016 μmol L⁻¹ UA were observed at pH 4. Interference studies showed that the modified electrode exhibits excellent selectivity toward AA, DA, and UA.     

Oxidation kinetics of NADH by heteropolyanions

A series of selected Dawson-type mixed heteropolyanions readily oxidize NADH in buffered aqueous pH 7 medium. The process was monitored by UV-visible spectroscopy, which helps to establish the 2:1 stoichiometry for the oxometalate/NADH couple. The starting system for electrochemistry consists of the one-electron reduction product of the heteropolyanions in the presence of various amounts of NADH. Cyclic voltammetry confirms unequivocally that the oxidized forms of the selected heteropolyanions are capable of catalyzing efficiently the oxidation of NADH. The kinetics were studied quantitatively by double step chronocoulometry. The logarithm of the second order rate constant was a linear function of the E^o of the first redox systems of the heteropolyanions with a slope of 16.4 V^?1. This result indicates that the oxidation of NADH proceeds by a multistep mechanism involving an initial rate-limiting one-electron transfer. An estimate of the E^o value for the one-electron NADH/NADH^·+ redox couple has been obtained.     

Preliminary electrochemical study of phenothiazines and phenoxazines immobilized on zirconium phosphate

The electrochemical properties of one phenothazine (Methylene Blue) and one phenoxazine (Nile Blue) dye immobilized on zirconium phosphate were investigated by incorporation into a carbon paste electrode. The dyes presented a very stable redox property. The midpoint potentials (E_m) were 150 and 50 mV vs. SCE for Methylene Blue and Nile Blue, respectively. The composition and pH of the supporting electrolyte did not affect the E_m, in contrast to those observed for the dyes in solution phase or when adsorbed on carbon. These electrodes showed a good activity in a test made for electrocatalytic oxidation of NADH.     

Factors influencing the interfacial width of copper gradients on gold produced by spatiotemporal control of the in-plane electrochemical potential distribution: electrode geometry and plating solution composition

Injecting an in-plane current through a thin Au film results in the formation of a linear electrochemical potential gradient at the surface of the electrode. Coupling the surface potentials to a redox species in solution then effects spatially dependent redox chemistry. This strategy has been used to map the electrochemical deposition of Cu⁰ onto the surface of Au. Because the local electrochemical potential, V(x), varies spatially in the plane of the electrode, a transition region is established on the surface in an area corresponding to the anodic peak (strip-out) potential. The width of this transition region is of both technological and theoretical interest, and the effects of electrode geometry and plating solution modifiers on the interfacial width of electrochemically deposited thin Cu films have been investigated. Careful calibration of the surface electrochemical potential gradient shows that the Au–Cu transition occurs at a spatial location corresponding to V(x)=125±14 mV in agreement with the observation of an anodic peak potential centered at ca. 125 mV vs. Ag|AgCl from cyclic voltammetry. Interfacial widths are dramatically affected by different solution modifiers, with l-(+)-tartaric acid and NaCl producing much sharper transitions and different morphologies in comparison to Cu gradients prepared in the presence of citric acid or malonic acid. Data reported here strongly support the notion that nucleation/growth, not electrical effects, play the dominant role in determining interfacial width and final morphology of the deposited film.     

Ligand-catalysed metal ion reduction. Voltammetric determination of rate and formation constants for the nickel complex with 6-mercaptopurine-9-d-riboside

The catalytic reduction of nickel ions on the hanging mercury drop electrode in the presence of traces of 6-mercaptopurine-9-d-riboside (MPR) was investigated by linear scan voltammetry in the physiological pH range. A nickel complex is reduced at potentials more positive than the reduction of the hydrated ion. The reduction of the complex produces a voltammetric pattern which is typical for the electrode processes controlled by a parallel chemical reaction. The theoretical treatment for the regeneration of the reactant by a redox reaction was adapted to the investigated electrode process. The kinetic equation thus derived was used for the determination of both the rate (k₁=3.11×10⁶ mol^?1 dm³ s^?1) and formation (log β₁=3.40) constants for the reducible complex by means of a multiparametric curve fitting approach. Under the same conditions traces of MPR catalyze the oxidation of Ni amalgam producing both an increase of the anodic peak current and the shift of this peak towards more negative potentials.     

Electrochemical polymerization of aniline and toluidines on a thermally prepared Pt electrode1

The polymerization of aniline (AN) and toluidines (o-toluidine (OTO)/m-toluidine (MTO)) was performed electrochemically on a Pt-coated titanium electrode in 1.0 M HCl using cyclic voltammetry. The results showed that the rate of polymerization was strongly dependent upon the type of monomer employed. The polymerization rate of these monomers on a Pt-coated electrode is in the order: AN>OTO>MTO. A kinetic expression for the autoacceleration process in the electrochemical polymerization of these aniline derivatives is expressed as v=k[p][M]+k′[M] in which k′, k, [M] and [p] are the initiation (nucleation process) rate constant, the rate constant when the polymer is deposited on the electrode, monomer concentration, and the total amount of polymer, respectively. The ratios of k′/k are 6.64×10^?6, 3.23×10^?6 and 3.15×10^?6, respectively, for AN, OTO and MTO polymerization. The polymerization rate of these aniline derivatives is reduced with agitation of the monomeric solution. The experimental results of E_1/2 for the first redox process showed that the AN–OTO or AN–MTO comonomer system generated true copolymers rather than the mixture of the corresponding homopolymers. The polymerization rates of these aniline derivatives on three different electrodes are in the order: IrO₂>RuO₂>Pt-coated electrode.     

Consequences of a potential-dependent transfer coefficient in ac voltammetry and in coupled electron–proton transfer for attached redox couples

The Marcus density-of-states model for simple electron transfer predicts that the transfer coefficient is dependent on overpotential. The nature of the potential dependence is a function of the reorganization energies associated with oxidation and reduction processes. A fifth-order polynomial expression accurately yields the potential dependence of the transfer coefficient and the resulting curved Tafel plots. With this polynomial expression, the effects of the potential-dependent transfer coefficient are examined for two cases, ac voltammetry of an attached redox molecule with simple electron transfer and the kinetic behavior of the 1-electron/1-proton redox system. Simulations of ac voltammograms indicate that the effects are minor and that ac voltammetry is poorly suited for determination of the reorganization energy of the redox molecule. In the coupled electron–proton redox case, the effects are marked. As expected, the apparent standard rate constant decreases dramatically at pH values between the pK_a values of the two oxidation states. More surprisingly, the simulated Tafel plots exhibit asymmetry between the anodic and cathodic branches depending on the pH. The path of electron transfer from the oxidized to the reduced species (electron–proton or proton–electron) at a fixed pH depends on the electrode potential.