A theoretical analysis and its application of the second order EC'''' reactions at microelectrodes under steady-state conditions

With a new approach,the general current expressions of two typical second ordercatalytic reactions are obtained for disk,hemispherical and spherical microelectrodes understeady-state conditions.This approach is based on the concept of reaction layer.For thesecond order EC' reactions,we also discussed how it is possible to observe pseudo-first orderor second order behavior.This consideration allows the study of the fast chemical reactionsand systems where the bulk concentrations of the reactants are nearly equal.Regenerationof Fe~(3+)(EDTA) with H_2O_2 was selected as an example of a rapid catalytic reaction.     

Effect of ion pairing on steady-state voltammetric limiting currents at microelectrodes Part I. Theoretical principles

Voltammetric studies in solutions of high resistivity are facilitated by the use of microelectrodes under steady-state conditions. Such solutions are encountered with solvents of low permittivity because of the very sparing solubility of electrolytes. Moreover, in such media the supporting electrolyte, as well as the electroactive ionic species, is usually extensively ion paired. Here we predict the limiting current that will flow in these circumstances, when a monovalent ion undergoes a one-electron transfer at a hemispherical microelectrode to form a neutral product. The ion pairing equilibria are assumed to be fast but all diffusion coefficients are treated as distinct. An analytical solution is elusive in the general case, but a simple numerical procedure allows the limiting current to be predicted for any combination of the system parameters. Several special cases are also discussed, some of which yield explicit formulae for the limiting current. In a companion paper, experimental data are compared with the theoretical predictions.     

超微盘、微半球电极上准稳态电流公式及其实验验证

经过严格的数学推导及合理的数学近似, 提出了超微盘及微半球电极上准稳态可逆波、准可逆波及不可逆波电流方程式。根据导出的准稳态电流方程, 提出了一个测定动力学参数的新方法。对理论公式也进行了实验验证。     

Cyclic voltammetry at microelectrodes. Influence of natural convection on diffusion layers as characterized by in situ mapping of concentration profiles

Cyclic voltammetry was performed at microelectrodes under experimental conditions where no forced convection participates to the mass transport. Mapping of the dynamic concentration profiles were achieved using a method previously described. According to the time scale of the experiments and size of the microelectrodes, the results show that several regimes may be achieved. To delineate respectively the involvement of planar diffusion, hemispherical diffusion and natural convection in the overall mass transport, the experimental voltammograms and concentration profiles were compared to simulated ones accounting for each contribution. Excellent agreement was observed between data and predictions establishing that convection influences cyclic voltammetry even at microelectrodes reputed to be immune owing to their micrometric sizes.     

Steady-state voltammetry

Steady-state voltammetry has been practised for decades but its use has recently been stimulated by the increasing popularity and availability of microelectrodes. This review concentrates on steady states obtained with the use of microelectrodes. After a general introduction, detailed discussions are given for steady-state voltammetry at hemispherical, inlaid disc, and ring electrodes, which are the most popular geometries that engender steady states. Ohmic polarization in steady-state voltammetry is also considered. Applications in analytical, kinetic and other studies are described.     

Analytical expression of non steady-state concentration for the CE mechanism at a planar electrode

The analytical solutions of the non-steady-state concentrations of species at a planar microelectrode are discussed. The analytical expression of the kinetics of CE mechanism under first or pseudo-first order conditions with equal diffusion coefficients at planar electrode under non-steady-state conditions are obtained by using Homotopy perturbation method. These simple new approximate expressions are valid for all values of time and possible values of rate constants. Analytical equations are given to describe the current when the homogeneous equilibrium position lies heavily in favour of the electroinactive species. Working surfaces are presented for the variation of limiting current with a homogeneous kinetic parameter and equilibrium constant. In this work we employ the Homotopy perturbation method to solve the boundary value problem. Furthermore, in this work the numerical simulation of the problem is also reported using Scilab program. The analytical results are found to be in excellent agreement with the numerical results.     

Nitric oxide release during evoked neuronal activity in cerebellum slices: detection with platinized carbon-fiber microelectrodes.

Nitric oxide is an important biological messenger that particularly induces the relaxation of smooth muscle cells surrounding vessels, and, hence, controls the flow of blood. This mechanism is essential for brain function, and its fine control, termed functional hyperemia, is supposed to be realized by certain neurons that may release bursts of NO*. The aim of the present study is to examine the advantages of platinized carbon-fiber microelectrodes (5-7 microm tip diameter) for the direct and in situ electrochemical detection of NO* released by neurons into ex vivo cerebellum slices. After establishing the different analytical properties of the platinized carbon-fiber microelectrodes in vitro on NO* solutions at 50 nM to 1 mM concentration, they were characterized using DEA-NONOate solutions that chemically decompose into NO*, and therefore mimic the measurement of transient variations of NO* concentration in biological samples. This validated the present approach, so that direct, in situ ex vivo measurements of nitric oxide released by neurons in a rat cerebellar slice are presented and discussed.     

Pre-equilibrium approximation in chemical and photophysical kinetics

For most mechanisms of chemical reactions and molecular photophysical processes the time evolution of the concentration of the intervening species cannot be obtained analytically. The pre-equilibrium approximation is one of several useful approximation methods that allow the derivation of explicit solutions and simplify numerical solutions. In this work, a general view of the pre-equilibrium approximation is presented, along with the respective analytical solution. It is also shown that the kinetic behavior of systems subject to pre-equilibration can be obtained by the application of perturbation theory. Several photophysical systems are discussed, including excimer formation, thermally activated delayed fluorescence, and external-heavy atom quenching of luminescence.     

超微电极上半微分电分析法

介绍了超微半球电极上具有简单反应时的半微分循环伏安理论。理论表明,超微电极上半微分电流与电活性物质的浓度成正比,并据此提出了一种利用超微半球电极上半微分曲线进行电化学分析测量的新方法。该法具有灵敏度高、分辨率好等特点。     

Staircase,cyclic and differential voltammetries of the nine-member square scheme at microelectrodes of any geometry with arbitrary chemical stabilization of the three redox states

Simple analytical expressions are deduced for the cyclic (staircase) and differential cyclic (staircase) voltammetric responses of the nine-member square scheme at the most widely used microelectrode geometries: disc, (hemi)spheres, cylinders and bands. The generality of the reaction scheme considered enables us to tackle many different and common electrochemical situations where the oxidized, intermediate and/or reduced species of a two-electron transfer also take part in homogeneous chemical equilibria such as protonations, complexations, or ion pairings. The effect of the coupled chemical processes on the voltammograms is analyzed at electrodes of different size and shape under a variety of conditions depending on which redox state is chemically stabilized most significantly. Also, working curves for the determination of the formal potentials and the chemical equilibrium constants are given for macroelectrodes, microelectrodes and ultramicroelectrodes in cyclic voltammetry and differential cyclic voltammetry.     

Mercury-coated platinum microelectrodes for steady-state voltammetry in aqueous solutions at high temperature

Sphere-cap mercury microelectrodes fabricated on a platinum disk substrate were tested in aqueous solutions over the temperature range 295–353 K. The performance of these electrodes was assessed in solutions containing Ru(NH₃)₆Cl₃ and LiClO₄ as supporting electrolyte. From the steady-state limiting current obtained at three different mercury microelectrodes, the diffusion coefficient of the electroactive species at different temperatures was determined. It was found that the diffusion coefficient values were consistent with the Stokes–Einstein and activation energy models. The results obtained also allowed us to conclude that sphere-cap mercury microelectrodes can be useful for elevated temperature electroanalysis in aqueous solutions.     

Application of hot platinum microelectrodes for determination of flavonoids in flow injection analysis and capillary electrophoresis

The determination of quercetin and rutin by flow injection analysis (FIA) and capillary electrophoresis (CE) using electrochemical detection was described. These flavonoids were determined at normal (unheated) and hot platinum microelectrodes using cyclic voltammetry. When quercetin or rutin is reaching the platinum electrode, a change of the current in the region of the platinum oxide formation is observed. Integration of the current changes in this in this region creates analytical signals in the form of peaks. An increase of temperature to about 76 ?C in a small zone adjacent to the microelectrode causes an increase of the analytical signal by more than 6 times under FIA conditions. This method enables the use of hot microelectrodes as detectors in HPLC or CE. In CE the improvement of the analytical signal at hot microelectrodes is smaller than in FIA and increase only 1.3–3.4 times. Heated microelectrodes were used for analysis of the flavonoids in natural samples of the plant (extract of sea buckthorn) and a pharmaceutical preparation (Cerutin).     

Electrochemical treatment of tumours: a simplified mathematical model

Electrochemical treatment of tumours implies that tumour tissue is treated with a direct current. During electrolysis, electrical energy is converted to chemical energy through electrochemical reactions at the electrodes. The anode is preferably placed in the tumour and the cathode in a blood vessel or in fresh surrounding tissue. The main electrochemical reactions are chlorine and oxygen evolution, at the anode, if platinum is used. Hydrogen evolution takes place at the cathode. The aim of this paper is to show how mathematical modelling can be used as a tool for defining and optimising the operating conditions of electrochemical treatment (ET) of tumours. A simplified mathematical model is presented for direct current treatment of tumours, focusing on tissue surrounding a spherical platinum anode. The tissue is treated as an aqueous solution of sodium chloride and only the major electrochemical reactions are considered. The model is based on transport equations of ionic species in dilute solutions. Kinetic expressions for the electrochemical reactions, at the anode surface, are introduced. Inputs to the model are the applied current density, and sizes of the anode and electrolyte domain. Concentration profiles of the ionic species and potential distribution, as a function of time, are calculated. In addition, current yields of the anode reactions are obtained from the model.     

Redox sorption in metal–ion-exchanger nanocomposites upon electrochemical polarization

A conjugated macrokinetic problem is solved for two moving boundaries of chemical reactions during redox sorption in metal–ion-exchange nanocomposites under conditions of current flow. Numerical solutions to the multipoint boundary value problem indicate that the impact of the current includes a slowing of front migration associated with distinct stages of the chemical reaction between metal nanoparticles and oxygen due to electrochemical reduction, a reduced surface concentration of the active sorbate (oxygen), and an increased degree of redox sorption. An increase in the contribution from the electrochemical component and a transition to external diffusion control are observed as the current density grows.     

Density Functional Theory Study of Reaction Equilibria in Signal Amplification by Reversible Exchange

An in-depth theoretical analysis of key chemical equilibria in Signal Amplification by Reversible Exchange (SABRE) is provided, employing density functional theory calculations to characterize the likely reaction network. For all reactions in the network, the potential energy surface is probed to identify minimum energy pathways. Energy barriers and transition states are calculated, and harmonic transition state theory is applied to calculate exchange rates that approximate experimental values. The reaction network energy surface can be modulated by chemical potentials that account for the dependence on concentration, temperature, and partial pressure of molecular constituents (hydrogen, methanol, pyridine) supplied to the experiment under equilibrium conditions. We show that, under typical experimental conditions, the Gibbs free energies of the two key states involved in pyridine-hydrogen exchange at the common Ir-IMes catalyst system in methanol are essentially the same, i. e., nearly optimal for SABRE. We also show that a methanol-containing intermediate is plausible as a transient species in the process.     

Aluminium speciation in environmental samples: a review

Because of its toxic effects on living beings, Al may represent an environmental hazard, particularly under increased acidic conditions. Growing environmental concern over the presence of increased Al concentrations in soil solutions and fresh waters resulted in the development of numerous analytical techniques for the determination of Al species. Al has a very complex chemistry that is significantly influenced by pH. Different Al species are present in environmental solutions, and many of them are unstable. Contamination of samples and reagents by extraneous Al represents an additional problem in speciation of Al at trace concentrations. Due to these reasons quantitative determination of particular chemical forms of Al is still a very difficult task for analytical chemists. The most important analytical methodologies of the last decade and new trends for the speciation of Al in environmental samples are comprehensively reviewed here.     

A critical evaluation of a long pathlength cell for flow-based spectrophotometric measurements

Coupling of a flow cell based on a liquid core waveguide (LCW) to flow systems for spectrophotometric measurements was critically evaluated. Flow-based systems with and without chemical reactions were exploited to estimate the increase in analytical signal in comparison to those obtained with a conventional 1-cm cell under different experimental conditions. The Schlieren effect associated to intense concentration gradients in the sample zone was investigated with model solutions that do not absorb visible electromagnetic radiation. The effect of radiation scattering was lower than the expected by considering the increase in the optical path, being the magnitude of the perturbation up to 40% higher for the 100-cm LCW cell. Several alternatives for compensation of the Schlieren effect were experimentally investigated. The potentiality of the LCW for turbidimetric measurements and coupling to monosegmented flow analysis was also evaluated.     

A Genetic Algorithm‐Based Method for the Automatic Reduction of Reaction Mechanisms

An automatic method for the reduction of chemical kinetic mechanisms under specific physical or thermodynamic conditions is presented. The method relies on the genetic algorithm search logic to gradually reduce the number of reactions from the detailed mechanism while still preserving its ability to describe the overall chemistry at an acceptable error. Accuracy of the reduced mechanism is determined by comparing its solution to the solution obtained with the full mechanism under the same initial and/or physical conditions. However, not only the chemical accuracy and the size of the mechanism are considered but also the time for its solution which helps to avoid stiff and slow converging mechanisms, thus preferring the fast solutions. The reduction method is demonstrated for a detailed mechanism for methane combustion, GRI‐Mech 3.0, which was reduced from 325 reactions and 53 species to 58 reactions and 26 species, and for an iron oxide formation mechanism from iron pentacarbonyl doped flames by Wlokas et al. (Int J Chem Kinet 2013, 45(8), 487–498),  originally consisting of 144 reactions and 34 species, which was reduced to 37 reactions and 24 species. The performance of the reduced mechanisms is shown for homogeneous constant pressure reactors and for burner‐stabilized flames. The results show a good agreement between reduced and full mechanisms for both the reactor and flame cases. The presented method is flexible and can be easily adjusted to either yield more accurate (but bigger) or smaller (but less accurate) reduced mechanisms, depending on the user's preference.     

Coupled pervaporation/mass spectrometry for investigating membrane mass transport phenomena

The integration of organophilic pervaporation into processes of varying feed concentration, such as bioconversions, chemical reactions, or analytical sample preparation, requires not only the understanding of mass transport phenomena across the membrane under steady-state conditions, but also the insight into the transient response of the pervaporation membrane to changes as they occur in practice. For this purpose, a laboratory-scale pervaporation unit was coupled to a mass spectrometer for on-line permeate analysis, maintaining the overall pervaporation operating conditions controllable independently, and without introducing any inert gases for sample transfer.The experimental set-up was employed for investigating the transport of aroma compounds across a POMS–PEI composite membrane, focusing in particular on the so-called “membrane conditioning”; the possible synergetic effect of ethanol on the flux of one model aroma compound, ethyl hexanoate; the application of the system proposed to the rapid screening of the effect of the hydrodynamic upstream conditions on the degree of concentration polarisation.The method proposed proved to be robust and flexible, not only allowing insights into transient mass transport phenomena otherwise not attainable, but also reducing experimental workload significantly when characterising the effect of varying operating conditions on the pervaporation performance.