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.     

A new linear ion trap with simple electrodes

Linear ion traps exhibit greater space charge capacity compared to conventional 3D Paul traps. Commercial ion traps are made of electrodes of hyperbolic shape. The present work investigates the possibilities of designing linear ion traps from simple electrodes, which are relatively cheap and handy for manufacturing and, at the same time, are comparable to commercial ion traps in resolving power of mass analysis. Using computer simulations and optimization of resonance ejection scan, it is shown linear ion traps can be made of electrodes of triangular cross-section with an ejection slit width of 16% of the trap inradius; the resolving power of the mass spectrum is more than 18000.     

Voltammetric behaviour of immunoglobulin G at stationary mercury electrodes

A.c. voltammetric measurements have shown that bovine and human immunoglobulin G are adsorbed at a mercury electrode over a broad potential range and at open circuit. A current signal at ca. -0.55 V was identified as being due to an interfacial process connected with reversible protein re-orientations in the adsorption layer and, possibly, with fast faradaic reactions of both adsorbed redox states of the immunoglobulin.     

Reduction of nitrate ion using hydrogen permeating Pd foil electrodes

The successive electrochemical reduction system using hydrogen-permeable Pd foil electrodes with Pd black catalyst was applied to reduction of nitrate ion. It was revealed that the second metals co-deposited to Pd black catalyst showed an excellent selectivity for decomposition of NO ₃ ⁻ to N₂, and was stable for at least 200 days. This system employing a compact and a simple electrolysis cell is expected to be an alternative or a supplement to conventional biological treatments for nitrate removal.     

Layer-by-layer thin film-coated electrodes for electrocatalytic determination of ascorbic acid

Multilayer thin films composed of poly(allylamine hydrochloride) (PAH) and carboxymethyl cellulose (CMC) have been prepared on the surface of a gold (Au) disk electrode by a layer-by-layer deposition of PAH and CMC and ferricyanide ions ([Fe(CN)₆]³⁻) were confined in the film. [Fe(CN)₆]³⁻ ions can be successfully confined in the films from weakly acidic or neutral [Fe(CN)₆]³⁻ solutions, while, in basic solution, [Fe(CN)₆]³⁻ ion was not confined. The [Fe(CN)₆]³⁻ ion-confined Au electrode showed clear redox peaks in the cyclic voltammogram around 0.35 V versus Ag/AgCl. The amounts of [Fe(CN)₆]³⁻ ions confined in the films depended on the thickness of the films or the number of layers in the LbL films. The [Fe(CN)₆]³⁻ ion-confined Au electrode was used for electrocatalytic determination of ascorbic acid in the concentration range of 1-50 mM.     

聚苯胺薄膜电极上示差脉冲伏安法可逆波理论研究

本文提出了聚苯胺(PANI)薄膜电极上示差脉冲伏安法可逆波理论,经验证理论和实验结果相符合.     

Triangular current-sweep chronopotentiometry at rotating disk and stationary,planar electrodes

A convenient series solution is presented for the evaluation of reactant and product concentrations during triangular current-sweep chronopotentiometry at a rotating disk electrode and at a stationary, planar electrode in the absence of free convection. The advantages of controlled-current processes, relative to controlled-potential processes, for otaining kinetic, thermodynamic, and transport information are elucidated. Theoretical predictions are compared with results obtained for the deposition of cadmium from a dilute, aqueous, cadmium sulfate + potassium sulfate electrolyte. A multidimensional optimization routine, the Levenberg-Marquardt algorithm, is used to evaluate physicochemical parameters.     

Kinetics of film-coated electrodes: Effect of a finite mass transfer rate of substrate across the film—solution interface at steady state

Previous models for describing the mediation kinetics of film-coated electrodes quantitatively are extended to account for a finite mass transfer rate of the substrate across the film—solution interface. Experimental data from polymer-coated rotating disk electrode experiments, analyzed by the extended model, provide evidence for finite interfacial mass transfer rates. Substrate size and charge contribute to this interfacial rate. Interfacial mass transfer resistance is more pronounced for polymerized films such as poly(vinyl ferrocene) and poly[Ru(vbpy)₃]²⁺ than for highly swollen polymers which bind electroactive species, e.g., poly(lysine).     

Reduction of electroactive polymers via polyelectron transfer processes at solid electrodes

The reduction of an electroactive polymer at an electrode involves the transfer of a large number of electrons to a single molecule. A quantitative study of this process indicates whether the system behaves ideally. The electron transfer process in polyvinylbenzophenone and in polyvinylbenzophenone-co-styrene in N,N-dimethylformamide solution was investigated by voltammetry at a rotating disk electrode. It is concluded that benzophenone (BP) groups attached to the polymer backbone are noninteracting electroactive centers and that every BP group is reducible at the electrode. The relationship between limiting current and molecular weight was examined quantitatively for a series of polymers of low dispersity with molecular weights ranging up to 300,000, and with a controlled, varied content of electroactive groups. The diffusion coefficient was determined as a function of molecular weight from the electrochemical data, and was found to be in good agreement with the theoretical model based on transport properties. The dependence of the diffusion coefficient on concentration of polymer and on temperature were investigated and activation energies determined.     

Kinetics of multiple electron transfer reactions at porous electrodes under stationary and flow conditions

The kinetics of consecutive two-electron transfer reactions at porous flooded electrodes are investigated under both stationary and flow conditions, where mass transfer is due respectively to diffusion and forced convection. The current-polarization relations were calculated for both modes of mass transfer as a function of the specific surface area of the electrode, the rates of the respective steps of the electron transfer reaction and the appropriate mass transfer coefficients. The computed solutions degenerate to the known limiting cases of single electron transfer control under conditions of very high or very low polarizations. Thus, at high anodic polarization, the electrochemical reaction is controlled by a single electron transfer step, the other step being too fast. Under conditions of 0.1<i/i_L<1, the overall reaction rate is controlled by both mass transfer and electrochemical activation. For flooded diffusion electrodes, the current-voltage curves follow the Tafel equation with a slope of double the normal value. This is attributed to mass transfer control in agreement with previous work. Experimental results, obtained on the porous flow-through electrode, agreed well with the theoretical predictions. The calculations presented here enable a quantitative evaluation of the relative influence of the rate of any step on the overall behaviour of the electrode under the appropriate experimental conditions.     

Fabrication of organosilane-modified electrodes for metal ion detection at the molecular level

Organosilane-modified (island-type) electrodes of 5 and 10 microm were fabricated and used to detect trace amounts of metal ions using atomic force microscopy. The smaller electrode had a lower limit of detection due to the enhancement in electron tunneling through metal ions that are adsorbed between the conductive-tip (mobile) and the surface (fixed) electrode. To simulate the detection of metal ions at concentrations below 10(-3) mM, a sectional adsorption mechanism is proposed, which satisfactorily explains the adsorption of Cu(II) and Hg(II) to the amine and thiol moieties of the 5 microm-sized electrode.     

Catalysis of electrochemical reactions at redox polymer electrodes: Kinetic model for stationary voltammetric techniques

A kinetic model describing the catalytic activity of redox polymer film electrodes is presented and discussed for reaction schemes in which the primary reaction between the active form of the catalyst and the substrate is the rate-determining step. The derivations are given for stationary techniques, such as rotating disc electrode voltammetry. Besides the diffusion of the substrate from the bulk of the solution to the film—solution interface, three kinetic factors determine the magnitude of the catalytic current: “diffusion” of electrons from the electrode surface to the film—solution interface; diffusion of the substrate in the opposite direction; rate of the rate-determining step of the catalytic reaction. Correspondingly, the kinetics of the diffusion—reaction process can be entirely described by only three dimensionless parameters expressing the relative rates of the rate-controlling processes. Provision is made for the partition coefficient of the substrate between the film and the solution to be different from unity. A finite-difference resolution of the kinetics is developed, but attention is mainly focused on the search of characteristic behaviors depending upon a lesser number of parameters and expressed by simple closed-form equations which are particularly convenient to use in the processing of experimental data. These are essentially of four types.

• 1.(1) The electron and substrate diffusion of the film are so fast that the rate-controlling phenomenon in the film is the catalytic reaction.
• 2.(2) The cataytic reaction is so fast that the kinetics is controlled jointly by the two diffusion process.
• 3.(3) When “diffusion” of electron is faster than diffusion of substrate in the film a pure kinetic situation may arise by mutual compensation of the latter process and the catalytic reaction.
• 4.(4) In the opposite case a pure kinetic situation may again arise resulting this time from mutual compensation of electron “diffusion” and catalytic reaction.

The kinetics observed in intermediary situations is also described, as well as the variations of the parameters that are required to pass from one limiting behavior to the other. It is shown that, according to the kinetic situation prevailing in the film, a second wave way appear which features the direct reduction of the substrate at the electrode surface after it has diffused through the film. Diagnostic criteria of the various limiting situation are presented, being based on the variations of the height of the catalytic wave with the rotation speed and the solution concentration of the substrate, as well as on the existence and height of the second wave. The electron “diffusion” through the film can be characterized independently by means of experiments involving the oxido-reduction of the film in the absence of substrate. Conversely, the substrate diffusion in the film can be characterized using analogous system in wich catalysis is absent. With the help of these data, the catalytic reaction occuring in the film can be quantitatively characterized. It is shown that this does not require exact knowledge of the film thickness. Such application of the kinetic model to actual experimental systems are illustrated by the discussion of experimental data from the literature.     

Extension of the voltammetric theory to inert,permeable thin-film coated rotating disc electrodes: Part II. Behaviour of ferrocene-ferricinium ion and quinone-hydroquinone systems on polymer-coated electrodes

The voltammetric behaviour of ferrocene-ferricinium ion and quinone-hydroquinone systems is investigated on electrochemically thin film polymer coated electrodes. Ferrocene oxidation is studied in 0.1 M NBu₄ ClO₄-nitromethane on rotating disc platinum electrodes coated with poly(2-hydroxymethyl-1,4-phenylene) oxide (I), poly-(2,6-dimethyl-1,4-phenylene) oxide (II), and poly[4-(2-aminoethyl)-1,2-phenylene] oxide (III) films. The quinone-hydroquinone system is investigated in aqueous medium (1 M HClO₄) with (I), (II) and poly-(2-cyano-1,4-phenylene) oxide films. Experimental results are in good agreement with the previously calculated voltammetric curves in steady-state mass transfer conditions. The charge transfer and diffusion parameters on these polymer-coated electrodes are calculated The quinone-hydroquinone system is rendered reversible by coating a platinum electrode with I.     

Electrocatalysis of the reduction of nitrate ion at cadmium electrodes by electrodeposited copper

The irreversible reduction of nitrate to nitrite at a cadmium electrode in slightly alkaline solution is electrocatalyzed by copper deposited on the electrode surface. Mass transport-limited currents were observed at a Cu—Cd disc electrode to rotational velocities of 1050 rad s^-1 (10000 rev. min^-1). Electron mugraphs revealed that the copper is plated as closely packed muspheres less than 1 μm in diameter.     

Amperometric detection of simple alcohols in aqueous solutions by application of a triple-pulse potential waveform at platinum electrodes

Application of a triple-pulse waveform is described for the anodic detection of methanol, ethanol, and ethylene glycol. The execution of the waveform incorporates the cleaning and reactivation of the platinum electrode, by alternate anodic and cathodic polarization, with measurement of the faradaic signal for the analyte at the reduced electrode surface. Some results for formic acid are also presented. The waveform is completed within approximately 2 s and the faradaic signal exhibits no decay with time as would be the case for amperometric detection at a constant applied potential. Calibration curves made by plotting —1/I vs. 1/C are linear. This is consistent with a reaction mechanism in which the analyte is adsorbed prior to anodic detection. The technique is applicable for detection in chromatographic and flow-injection systems.     

Amperometric detection of simple carbohydrates at platinum electrodes in alkaline solutions by application of a triple-pulse potential waveform

The response of ten simple carbohydrates was investigated voltammetrically at platinum electrodes in 0.10 M sodium hydroxide by application of a conventional linear sweep waveform and a triple-pulse waveform. Linear-sweep data were interpreted to suggest that electrochemical reactions of the carbohydrates involve oxidation of adsorbed hydrogen atoms produced by surface-catalyzed dehydrogenation of the adsorbed carbohydrate. The triple-pulse measurement technique was evaluated for a flow-injection system by introducing 100-μl samples into a stream of 0.1 M NaOH with a flow rate of 0.375 ml min^-1, and measuring the peak current. Peak currents for ten carbohydrates at 0.5 mM ranged from 17 to 42 μa and a detection limit of 0.01 mM was evaluated for dextrose. Calibration plots of reciprocal peak current (I^-1_p) vs. reciprocal of concentration (C^-1) were linear for dextrose and sorbitol concentrations between 0.1 and 1.0 mM.     

Use of poly(ester-sulfonic acid) film-coated electrodes as amperometric detectors in flow injection analysis

Within the past several years significant advances have been made towards the development and incorporation of chemically modified electrodes as selective detectors for high performance liquid chromatography and flow injection analysis. In many cases the chemically modified electrode systems closely approach the “ideal” detector specifications of chemical and mechanical stability along with a significant linear response region. This paper will discuss the characterization and incorporation of ionomeric poly(ester-sulfonic acid) coated electrodes as nonaqueous electrochemical detectors. The orientation of the electrodes in the detector system as well as the increased sensitivity levels to 10⁻¹⁰ g ml⁻¹ for cationic species and 10⁻⁹ g ml⁻¹ for neutral species will be presented. Also the applicability of the ionomer coated electrodes as nonelectrolyte detectors achieved a reproducible response with detection limits to 10⁻⁶ g ml⁻¹. Overall this system performed as well as, or better than, more specialized and expensive thin layer electrochemical detectors.     

Reduction of complexes in unbuffered solutions a unified treatment of the reduction of complexes at a stationary planar electrode or at an expanding mercury electrode

A general treatment is given of an electrode reaction connected with complexation at low ligand concentrations in unbuffered media. The reduction of the simple or complex metal ions occurs at an electrode expanding in accord with some power law; the stationary (solid or mercury) electrode and the dropping mercury electrode are special cases. The ligand is added to the solution as a j-protic acid. Linear semi-infinite diffusion is regarded as the sole mode of transport of all dissolved substances.The solution found, being of a very wide generality, was applied to the case of a potentiostatic regime of electrolysis on a dropping mercury electrode, as well as to the case of a galvanostatic regime on a stationary electrode. The voltammetric relationships obtained embrace all known equations of polarographic and chronopotentiometric reduction of simple and complex metal ions.The theoretical polarographic I—E curves in buffered and unbuffered solutions are presented graphically and compared. Apparently, the changes in the surface proton concentration cause a stretching of the wave.