Pulse polarography: Part V. on the theory for the kinetic currents with an ece mechanism

We have derived an equation for the instantaneous limiting current in pulse polarography with an ECE mechanism. That equation has been derived for the expanding sphere electrode model (and also for the expanding plane electrode model) with the condition (k₁+k₂)>>1, k₁ and k₂ being the rate constants of the chemical reaction. We show that by an adequate choice of the time of application of the potential and time of the drop growth prior to potential application, it is possible — if the equilibrium constant for chemical reaction is known — to widen the interval of values for k₁ and k₂ that may be determined using this technique. At the same time, we also show that if the electrode sphericity is not taken into account, the values obtained for k₁ and k₂ are always lower than the real ones.     

The reduction of oxygen in molten lithium carbonate

Oxygen reduction on immersed gold electrodes has been studied in Li₂CO₃ melt under steady-state conditions and by the potential-sweep method. Reaction order measurements have established that the species being reduced is not molecular oxygen, but the peroxide ion. The latter is in chemical equilibrium with molecular oxygen and oxide ions. The rate-determining step is the primary charge transfer

where (O^?) is a transient species. The exchange current densities and activation energies have been determined. Under conditions where O₂^2? diffusion is not limiting (e.g. meniscus electrodes) the rate of neutralization of oxide by CO₂ at the electrode surface is probably rate-determining.     

Mechanistic studies with positive iodine: Kinetics of oxidation of thiocyanate ion by iodine monochloride and aqueous iodine in perchloric acid medium

Kinetics of oxidation of thiocyanate ion (NCS^?) by iodine monochloride and iodine has been studied in aqueous perchloric acid medium. The rates of oxidations followed the rate laws: Variations in ionic strength and dielectric constant of the medium had little effects on the rates of reactions with both the oxidants. Mechanisms consistent with the observed rate laws have been suggested. Rate limiting steps have been identified and the constants of some of these steps have been evaluated by varying [NCS^?] at each temperature. Activation parameters were computed from the Arrhenius plots. The rate constants predicted from the rate law as [NCS^?], and [H⁺], varied in iodine monochloride oxidation, are in good agreement with the experimental values providing support to the proposed mechanism.     

Theoretical treatment of intermediate electrolysis electrochemical detectors

A simple approach for deriving expressions for the limiting steady-state response of the open tubular electrode, under conditions of intermediate electrolytic yields, is described. This approach couples the principles of laminar fluid dynamics with the theory of the porous electrode reactor. The dependence of the limiting current, I₁, on flow rate, V_f, has the form of I₁ α V^m_f , with m ? $\text{1}\text{3}$, depending upon the electrolytic yield. The effects of electroactive species depletion upon the detector response are discussed. Experimental results are incorporated to support the theoretical conclusions. For a tubular electrode (4-cm long) the degree of conversion decreases from 0.21 to 0.03 and m decreases from 0.42 to 0.32 as the flow rate increases from 0.15 to 3.5 ml min^-1.     

Current-potential curves with an EE mechanism

The dc polarographic response for the EE mechanism both in the expanding plane electrode and in the expanding sphere electrode is examined. Explicit equations for the current-potential curves when all species are initially added to the solution and without using the steady-state approximation have been derived. The situation obtained in some particular cases (reversible steps, one reversible and the other irreversible, totally irreversible steps and well-separated steps) is also 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.     

Nickel pentacyanonitrosylferrate film modified aluminum electrode for electrocatalytic oxidation of hydrazine

The electrocatalytic oxidation of hydrazine at the aluminum electrode, modified by electroless deposition of nickel pentacyanonitrosylferrate (NiPCNF) on the surface of the electrode has been studied by cyclic voltammetry, chronoamperometry and rotating disk electrode voltammetry and the kinetics of the catalytic reaction were investigated. The results were explained using the theory of electrocatalytic reactions at chemically modified electrodes. It was found that a one-electron charge-transfer process is rate limiting and that the average values of the rate constant for the catalytic reaction and the diffusion coefficient, evaluated by different approaches, are 5.2×10³ M^–1s^–1 and 8.5×10^–6 cm²s^–1, respectively. Further examinations of the modified electrodes show that the modifying layers (NiPCNF) on the aluminum substrate have reproducible behavior and a high level of stability, after exposing them in air and hydrazine solutions for a long time. Electronic Publication     

Simulations of cyclic voltammetric and chronoamperometric electrode responses at a disk electrode using combinations of spherical and cylindrical electrode geometries

Using geometric models based on one-dimensional transport at spheres and cylinders, three methods for improving the simulation of voltammetric behavior of a disk electrode have been explored. One method is based on the common assumption of equivalency between the limiting currents for a disk and a hemisphere under steady-state diffusion conditions. The second method involves the use of a partial-sphere geometry which is a better approximation that is suitable at the extreme diffusional limits achievable at a disk electrode of fully planar and steady-state transport. The third method, which is generally applicable, is a further refinement that uses a combination of appropriate one-dimensional spherical and cylindrical geometries. The results demonstrate that reasonably accurate approximations of disk behavior for several reaction mechanisms can be achieved in a fraction of the time required to compute the more rigorous two-dimensional model. We propose that the approximation serve primarily as a fast way to explore system behavior and establish approximate values of the relevant parameters. More accurate computations can then be performed using the two-dimensional model.     

A study of the electrocatalytic oxidation of aspirin on a nickel hydroxide-modified nickel electrode

The electrocatalytic oxidation of aspirin has been investigated on a nickel oxide-modified nickel electrode in alkaline solution. The process of oxidation and its kinetics have been investigated by using cyclic voltammetry, chronoamperometry, and electrochemical impedance spectroscopy techniques and also steady-state polarization measurements. Voltammetric studies have indicated that in the presence of aspirin, the anodic peak current of low-valence nickel species increases, followed by a decrease in the corresponding cathodic current. This indicates that aspirin was oxidized on the redox mediator immobilized on the electrode surface via an electrocatalytic mechanism. The rate constant of the catalytic oxidation of aspirin and the electron transfer coefficient have been found to be 1.15×10⁵ cm³ mol⁻¹s⁻¹ and 0.49, respectively. Impedance measurements show that aspirin is diffused into the bulk of the modifier film, and the oxidation process of aspirin occurs in the bulk of nickel oxide film. It has been shown that by using this modified electrode, aspirin can be determined with a detection limit of 4.8×10⁻⁵ and successfully applied for determination of aspirin in tablet.     

The role of foulant–foulant electrostatic interaction on limiting flux for RO and NF membranes during humic acid fouling—Theoretical basis,experimental evidence,and AFM interaction force measurement

A limiting flux model has been recently developed for predicting the fouling behavior of reverse osmosis and nanofiltration membranes by organic macromolecules [C.Y. Tang, J.O. Leckie, Membrane independent limiting flux for RO and NF membranes fouled by humic acid, Environmental Science and Technology 41 (2007) 4767–4773]. Several interesting results have been observed: (a) there was a maximum pseudo-stable flux (the limiting flux) beyond which further increase in applied pressure did not translate to a greater stable flux; (b) all membrane samples attained the limiting flux under constant pressure conditions as long as their initial flux was greater than the limiting flux; (c) the limiting flux did not depend on the properties of membranes; (d) the limiting flux had strong dependence on the feedwater composition, such as pH, ionic strength, and divalent ion concentration. The current study investigates the dependence of limiting flux on intermolecular interaction between foulant molecules. It was observed that the limiting flux was directly proportional to the intermolecular electrostatic repulsive force and that conditions enhancing foulant-deposited-foulant repulsion resulted in greater limiting flux values. Such observations agree well with a theoretical model capturing both hydrodynamic and DLVO interactions. Interaction force measurements by atomic force microscopy (AFM) were also performed. The limiting flux correlated reasonably well with AFM interaction force between the model foulant and the fouled membrane surface.     

Molecular-modulation spectrometry: CH3ONO photolysis and detection of nitroxyl

The method of molecular-modulation spectrometry for studying photochemical reactions has been applied to methyl nitrite photolysis. The infrared absorption of the nitroxyl radical HNO has been observed in the gas phase at 3300 cm^?1. Under the present experimental conditions the steady-state concentration of HNO under steady illumination was 1.1 × 10¹² particles/cc, and the observed modulation amplitude was 4.5 × 10¹⁰ particles/cc. At 25°C and 1 atm of nitrogen, the cross section for infrared absorption by HNO at 3300 cm^?1 is 1.7 × 10^?19 cm². The rate constant ratio b/c was found to be 8.0. From the literature value of the rate constant d , the observed rate constant for the reaction is e = (5 ± 1) × 10^?11 cc/particle sec.     

An enzyme electrode based on immobilized arylsulfatase for the selective assay of sulfate ion

An enzyme electrode has been constructed for the assay of sulfate ion based on inhibition of the reaction

The steady-state current arising from oxidation of the product, 4-nitrocatechol, is measured at +0.8 V vs. S. C. E. The competitive inhibition of this reaction by added sulfate ion causes a decrease in this steady-state current in a linear relationship to pSO₄ in the range 2–4. The enzyme arylsulfatase (arylsulfate sulfohydrolase, EC 3.1·6.1) is chemically immobilized in a layer on a platinum electrode. This enzyme electrode also gives linear calibration plots for phosphate ion (10^-2–10^-4 M) based on its competitive inhibition of the above reaction, and for fluoride ion (10^-2–10^-4 M) based on its activation of the reaction. The assay of 4-nitrocatechol sulfate (NCS) in the range 10^-6–10^-4 M is possible. By proper control of the NCS concentration the electrode can be made almost completely specific for sulfate: only molybdate interferes. To establish the best operating conditions for the electrode, the effect of pH on the V_m and K_m were determined.     

Maximum Current Density in the Reduction of the Bromate Anion on a Rotating Disk Electrode: Asymptotic Behavior at Large Thicknesses of the Diffusion Layer

The reduction of the bromate anion on a rotating disk electrode (RDE) in a steady-state mode due to the catalytic cycle consisting of a reversible bromine/bromide redox pair and irreversible counter-proportionation reaction was studied theoretically. As the cycle is autocatalytic (EC″ mechanism: Electrochim. Acta, 2015, vol. 173, p. 779), at high volume concentrations of bromate the passing current can reach huge values limited by the ultimate diffusion current of bromate through the diffusion layer even at very low volume concentrations of bromine. In contrast to previous theoretical studies of this process, for numerical solution of the set of nonlinear equations with boundary conditions for concentrations we used the COMSOL Multiphysics program package, with which the solution can be found for both the galvanostatic mode (at a given current density) and the maximum current density. This allowed us to study the behavior of the maximum current density for the case of very high thickness of the diffusion layer and very high reaction rate constant. In this mode, the ratio of the maximum current to the limiting diffusion current of the reduction of the bromate anion to bromine was found to exceed not only intuitively anticipated unity, but also the “critical” value of 1.2, which formally corresponds to the limiting diffusion current of its reduction to the bromide anion (though the real end product is bromine), and this limiting value depends on the volume concentrations of both bromate and bromine.     

旋转玻碳电极上二茂铁的电化学阻抗行为及其与DNA的相互作用

应用旋转圆盘电极和电化学阻抗法研究了二茂铁在Tris-NaC l(pH=7.2)缓冲溶液中于旋转玻碳电极上的电化学阻抗行为及其与DNA的相互作用.结果表明,二茂铁于旋转电极的伏安曲线呈现明显的极限电流平阶,而其交流阻抗谱则出现两个电容弧.二茂铁与DNA的作用,若受扩散过程控制则其极限扩散电流随DNA浓度增大而减小,而在电化学控制过程中则表现为电化学反应电阻随DNA浓度增大而增大.根据旋转圆盘电极和电化学阻抗谱测试,表明由这两种方法数据拟合求得的二茂铁条件电位速率常数能够很好地相互吻合,但如存在DNA时,则其条件电位速率常数有一定程度的减小.     

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.     

Electrocatalytic oxidation of aspirin and acetaminophen on a cobalt hydroxide nanoparticles modified glassy carbon electrode

The electrocatalytic oxidation of aspirin and acetaminophen on nanoparticles of cobalt hydroxide electrodeposited on the surface of a glassy carbon electrode in alkaline solution was investigated. The process of oxidation and the kinetics have been investigated using cyclic voltammetry, chronoamperometry, and steady-state polarization measurements. Voltammetric studies have indicated that in the presence of drugs, the anodic peak current of low valence cobalt species increases, followed by a decrease in the corresponding cathodic current. This indicates that drugs are oxidized on the redox mediator which is immobilized on the electrode surface via an electrocatalytic mechanism. With the use of Laviron’s equation, the values of anodic and cathodic electron-transfer coefficients and charge-transfer rate constant for the immobilized redox species were determined as α _s,a = 0.72, α _s,c = 0.30, and k _s = 0.22 s⁻¹. The rate constant, the electron transfer coefficient, and the diffusion coefficient involved in the electrocatalytic oxidation of drugs were reported. It was shown that by using the modified electrode, aspirin and acetaminophen can be determined by amperometric technique with detection limits of 1.88 × 10⁻⁶ and 1.83 × 10⁻⁶ M, respectively. By analyzing the content of acetaminophen and aspirin in bulk forms using chronoamperometric and amperometric techniques, the analytical utility of the modified electrode was achieved. The method was also proven to be valid for analyzing these drugs in urine samples.     

A quantitative study of alkyl radical reactions by kinetic spectroscopy. II. Combination of the methyl radical with the oxygen molecule

The kinetics of the reaction have been studied, using the technique of flash photolysis and kinetic spectroscopy to follow the methyl radical concentration. The order of the reaction lies between 2 and 3 throughout the range of pressure from 25 to 380 torr at 22°C, and the results are consistent with a single reaction sequence: The limiting values of the third-order rate coefficients at low pressures are (3.6±0.3) × 10¹¹ 1.² mole^?2 sec^?1 when M is neopentane, and (0.94 ± 0.03) × 10¹¹ 1.² mole^?2 sec^?1 when M is nitrogen. The limiting value of the second-order rate coefficient at high pressures is (3.1 ± 0.3) × 10⁸ 1. mole^?1 sec^?1. The rate constant for the independent second-order reaction is shown to be not much greater than 2 × 10⁵ 1. mole^?1 sec^?1, so that this reaction does not complete significantly with the combination reaction. This new interpretation is contrary to currently accepted views.     

The behaviour of an electrochemical detector used in liquid chromatography and continuous flow voltammetry: Part 1. Mass transport-limited current

A simple expression relating the current at a rectangular channel-type flow-through electrode to the volume flow rate of solution, cell dimensions and physical constants is derived. The expression for steady-state current is valid for laminar flow with neglect of longitudinal depolarizer diffusion. The equation is derived to extract information on the analytical utility of these cells, i.e. signal-to-noise ratio (SNR). For standard amperometric detection the optimum cell design is one in which the width of the cell is equal to the width of the electrode and the length and thickness of the cell are as small as possible, the limit of these dimensions either being physical or dictated by amplifier noise in the output. There is no optimum shape for an electrode of a given size. For standard amperometric detection with a constant cell volume, V, the optimum dimensions are given by b = ($\text{VD}\text{0.42}$U)^{$\text{1}\text{2}$}, (b = thickness, D = diffusion coefficient, -U= average volume flow rate). For flow rate-modulated operation, the optimum thickness is vanishingly small, and the electrode area (shape not critical) is given by A = 0.52 UbD^-1.     

Polarographic studies on the electrode kinetics of the nickel(II) complexes with glycine and some of its derivatives: I. glycine complexes

D.C. polarograms of the Ni(II)-glycine complexes were measured with varying pH values at constant total glycine concentration (0.05 M) and conversely with varying total glycine concentration at constant pH values (3.5, 4.5, 7.5 and 9.5). The current—voltage curves obtained were analysed to determine the limiting currents, the transfer coefficients, and the half-wave potentials, by using the non-linear least squares method. From the dependence of the half-wave potentials on the free glycine concentration, the mechanism of the electrode processes was elucidated. It was shown that the first wave corresponded to the reduction of the Ni²⁺ aquo-complex, the second to the NiG⁺ complex, the third to the NiG₂ complex and the fourth to the NiG^?₃ complex. The kinetic parameters for the four charge transfer processes were determined.The second and third waves were kinetic in character and the rate constants of the dissociation and association reactions between NiG⁺ and NiG₂ and between NiG₂ and NiG^?₃ were determined by analysing the dependence of the kinetic limiting currents on the free glycine concentration.     

Caffeic acid modified glassy carbon electrode for electrocatalytic oxidation of reduced nicotinamide adenine dinucleotide (NADH)

A new modified electrode was prepared by electrodeposition of caffeic acid (CFA) at the surface of an activated glassy carbon electrode. Cyclic voltammetry was used to investigate the redox properties of this electrode at various solution pH values and at various scan rates. The pH dependence of the electrode response was found to be 58.5 mV/pH, which is very close to the expected Nernstian value. The electrode was also employed to study electrocatalytic oxidation of reduced nicotinamide adenine dinucleotide (NADH), using cyclic voltammetry, chronoamperometry and rotating disk voltammetry as diagnostic techniques. It was found that the modified electrode exhibits potent and persistent electrocatalytic properties toward NADH oxidation in phosphate buffer solution (pH 7.0) with a diminution of the overpotential of about 450 mV compared to the process at an unmodified electrode. The electrocatalytic current increases linearly with NADH concentration in the range tested from 0.05 to 1.0 mM. The apparent charge transfer rate constant and transfer coefficient for electron transfer between the electrode surface and immobilized CFA were calculated as 11.2 s⁻¹ and 0.43, respectively. The heterogeneous rate constant for oxidation of NADH at the CFA-modified electrode surface was also determined and found to be about 3 × 10³ M⁻¹ s⁻¹. Finally, the diffusion coefficient of NADH was calculated as 3.24 × 10⁻⁶ cm² s⁻¹ for the experimental conditions, using chronoamperometric results. Received: 6 January 1999 / Accepted: 11 May 1999