5-(5-氯-2-吡啶偶氮)-2,4-二氨基甲苯的电分析特性研究

本文采用多种电化学方法研究了5-Cl-PADAT的电分析特性,求得了该化合物的电极反应电子转移数,扩散系数及电极反应标准速率常数,研究了其在滴汞电极上的吸附性质,提出了可能的电极还原过程。并且应用光度法测定了化合物的酸离解常数。     

Kinetic considerations on the electrogenerated luminescence of luminol at platinum electrode in the presence of hydrogen peroxide and oxygen

The ECL behavior of the luminol/H2O2 and luminol/O2 systems was evaluated at Pt electrode by using different electroanalytical techniques such as chronoamperometry, cyclic and rotating disk electrode (RDE) voltammetry. Diffusive and kinetic parameters such as the diffusion coefficient of luminol, D, the number of exchanged electrons, n, and the apparent heterogeneous rate constant, kap, were determined in the maximum light emission conditions achieved at pH 11, at an electrode potential of 750 mV vs. SCE. The experimental order of reaction were determined from the relation between the reactant concentrations and the emitted light intensity.     

Electrochemical behaviour of 3,8-difluorobenzo[c]cinnoline at mercury electrode

Voltametric and coulometric methods have been used to study the electroanalytical behaviour of 3,8-difluorobenzo[c]cinnoline. The number of electrons transferred, the wave characteristics, diffusion coefficient and reversibility of the reactions have been investigated in ethyl alcohol-BR. Adsorption of the molecule on the surface of the mercury drop electrode was analyzed and this phenomena was exploited to calculate the diffusion coefficient of 3,8-difluorobenzo[c]cinnoline. A mechanism for the electrode reaction was proposed.     

Analysis of charge transport in gels containing polyoxometallates using methods of different sensitivity to migration

Two methods have been used for examination of transport of charge in gels soaked with DMF and containing dissolved polyoxometallates. The first method is based on the analysis of both Cottrellian and steady-state currents and therefore is capable of giving the concentration of the electroactive redox centres and their transport (diffusion-type) coefficient. The second method provides the real diffusion coefficients, i.e. transport coefficients free of migrational influence, for both the substrate and the product of the electrode reaction. Several gels based on poly(methyl methacrylate), with charged (addition of 1-acrylamido-2-methyl-2-propanesulphonic acid to the polymerization mixture) and uncharged chains, have been used in the investigation. The ratio obtained for the diffusion coefficient (second method) and transport coefficient (first method) was smaller for the gels containing charged polymer chains than for the gels with uncharged chains. In part these changes could be explained by the contribution of migration to the transport of polyoxomatallates in the gels. However, the impact of the changes in the polymer-channel capacity at the electrode surface while the electrode process proceeds was also considered. These structural changes should affect differently the methods based on different time domains.     

Chemometrics assisted resolving of net faradaic current contribution from total current in potential step and staircase cyclic voltammetry

Total current in the electroanalytical data is assumed to be consisting of three main constituents: faradaic current, step charging current and induced charging current. Both charging currents can cause an interfering effect on precise determination of faradaic currents, and hence insert direct effects on sensitivity and detection limit of the electroanalytical techniques. Despite the widespread techniques introduced until now, the extraction of the net faradaic current from total current still remains a challenge. In this work, by using multivariate curve resolution-alternating least square (MCR-ALS) as a powerful curve resolution-based chemometrics method, a straightforward method has been introduced for resolving faradaic current from the two types of charging currents (step charging current and induced charging current) in single potential step and staircase cyclic voltammetric methods. By simultaneous analyses of the current data matrices for different electrochemical systems, the three sources of current were successfully identified and their contributions in the total signal were easily calculated. Also, in this manner, the cell time constant can be obtained easily. Contrary to the previously reported methods, the present method does not need any pre-determined mathematical method; particularly there is no need to know the cell time constant.     

Investigation of interparticle interactions of larger (4.63 nm) monolayer protected gold clusters during quantized double layer charging

In this article, the effect of interparticle interactions of 4.63 nm sized monolayer protected gold clusters (Au MPCs) during quantized double layer (QDL) charging has been investigated using electrochemical techniques. Voltammetry and scanning tunneling microscopy have been used to compare their electron transfer behavior. Furthermore, since the QDL process is diffusion controlled, the diffusion coefficient values have been estimated at various charge steps using two independent electroanalytical techniques, viz. chronoamperometry and impedance. These results show that higher core charge facilitates higher diffusion coefficient values, and indicate that repulsive interactions dominate for charged MPCs compared to those of its neutral analogue, which are mainly attractive in nature. Additionally, the electron transfer rate constants at various charge steps have been estimated from the impedance results, showing comparatively faster electron transfer rate at higher charge states.     

Combined use of electroanalytical methods to derive calibration plots for species difficult to standardize

A simple method is described for the derivation of calibration graphs for electroactive active analytes which are difficult to standardize from a calibration for an easily standardized reference species dissolved in the same medium. The method is based on evaluation of the ratios between the current-signals recorded for the analyte and for the reference solution by rotating-disk-electrode voltammetry and by an electroanalytical technique based on stationary electrodes which provides purely diffusion-controlled responses (e.g., chronoamperometry or normal-pulse voltammetry). This approach can be applied to all electroactive species which undergo diffusion-controlled processes, regardless of the degree of reversibility involved. Reliability tests, done with electroactive organic species dissolved in dimethyl sulphoxide, show that both accuracy and precision are within ±5%. The method is applied to the determination of hydrogen and oxygen solubilities in dimethyl sulphoxide and acetonitrile, respectively, as an example of quantitative evaluations of species which are difficult to standardize.     

Simple and direct interpretation of phase angles or derivation degrees in term of energy conservation vs. dissipation with Formal Graphs

With the help of a new theory using the notion of path in a graph, a physical interpretation and meaning can be given to electroanalytical measurements, without recourse to mathematical treatments. The frequency dependence of impedances measured by ac techniques, or the scan rate dependence of current vs. potential characterizations in large signal techniques (cyclic voltammetry), can be interpreted through this approach as a determination of the proportion of conserved energy vs. total energy involved in a mechanism. This total energy includes the energy which is lost by dissipation, i.e., converted into heat. This new way of thinking sheds an interesting light on the difference between normal and anomalous or fractal diffusion and provides to the analyst a simple and immediate tool for interpreting experimental data.     

Fabrication and characterization of boron doped diamond microelectrode arrays of varied geometry

Boron doped diamond (BDD) is a well-known electrode material that exhibits an excellent electrochemical potential window with very low background current. With this, microelectrodes and microelectrode arrays (MEAs) have been found to even further lower background currents without compromising sensitivity. As such, BDD MEAs are excellent electrode materials for a variety of electroanalytical applications, capable of multi-mode detection. We fabricated BDD MEAs adapting traditional semiconductor microfabrication processes; the resulting MEAs were patterned in different geometries to find an optimum electrochemical response, depending on the application. This is demonstrated using 4 different MEA geometries of different size and spacing using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), where the charge transfer resistance (R_ct) increases as the electrodes are farther spaced from one another. Excellent sigmoidal voltammogram shape in CV was obtained for each BDD MEA geometry. BDD MEAs spaced farther from one another were found to give better resolution from the background in fast scan cyclic voltammetric measurements of dopamine due to the decrease in the double layer capacitance (C_dl) as verified with EIS. This work furthers the understanding of BDD MEAs and their pertinence to sensitive electroanalytical techniques.     

Diffusion coefficients at rotated microelectrodes

The limiting current for several electrode reactions at a rotated microelectrode is observed to vary with the two-thirds power of the diffusion coefficient at infinite dilution in accordance with the theories of TACHI, EISBNBERG, LIN, and others. However, the uncertainties involved in estimating diffusion coefficients at given ionic strength require, that for accurate work, the limiting currents for different electrode reactions be compared on the basis of electrode sensivity, k_ion= i₁/C, and without regard to the role of diffusion in the transport process. The method of LAITINEN AND KOLTHOFF using an electrode reaction with only linear diffusion is recommended for measuring diffusion coefficients at given ionic strength under conditions similar to those extant in voltammetry.     

Chemometric tools in electroanalytical chemistry: Methods for optimization based on factorial design and response surface methodology

The aim of this paper is to give a brief overview of chemometric techniques based on factorial designs and response surface methodologies used in the optimization of electroanalytical methods. Chemometric techniques have several important advantages over one-way optimization for analytical applications, including a relatively low cost, a reduced number of experiments, and possibilities to evaluate interactions among variables. These techniques also enable the selection of optimal experimental conditions, helping to avoid trivial mistakes during optimization. Despite these facts, chemometric techniques have rarely been applied to electroanalytical data, especially in comparison with their use in spectroscopy. The application of chemometric methods in electroanalytical chemistry has been mostly used for solving overlapping signals, multivariate calibration methods, model identification and optimization of analytical procedures. This review is focused on the latter applications and overviews the role of full or fractional factorial designs (first-order designs), as well as second-order designs, such as central composite, Doehlert and Box-Behnken designs, for optimization of electroanalytical methods. A discussion of chemometric-related advantages is also given for stripping analyses, flow injection systems with amperometric detection, differential pulse voltammetry, square wave voltammetry and electrochemical sensor preparation.     

A survey of the information on co-ordination compound features which can be gained from electroanalytical methods

The information provided by modern electroanalytical techniques on co-ordination compounds is surveyed. The problem of the interaction between the electrode and intermediates or products is also briefly considered; it is pointed out that the electroanalytical approach can be successfully employed to provide new insight into chemical properties of metal complexes only when weakly interacting species are involved. The information obtainable is considered under the following headings: (i) mechanistic studies on metal complexes and electroanalytical evidence for their reactivity and stability; (ii) structural features of co-ordination compounds in solution; (iii) feasibility of electrochemical syntheses; and (iv) stability of intermediate oxidation states with reference to the nature of the ligands co-ordinated to the metal.     

Synthetic diamond, a new electrode material for electroanalysis

Depending on the doping level, diamond exhibits properties of either a semiconductor or a semimetal. Heavily doped “metallic” diamond was found to be a corrosion-resistant electrode, suitable for electrochemical syntheses and analyses. The advantages of synthetic diamond in electroanalytical chemistry are its corrosion resistance, good reproducibility of electrochemical properties, low background currents, and selectivity to a number of reactions used to develop electroanalytical methods. Presented at the V All-Russian Conference with the Participation of CIS Countries on Electrochemical Methods of Analysis (EMA-99), Moscow, December 6–8, 1999.     

Hydrogen peroxide in the marine environment: cycling and methods of analysis

Hydrogen peroxide (H₂O₂) is a key intermediate in many biological and environmental processes and there are a range of analytical methods available for its quantification, e.g. spectrophotometry, fluorimetry, chemiluminescence and electroanalytical techniques. This article focuses on the determination of H₂O₂ in seawater, wherein it is believed to play a significant role in redox reactions and the photochemical oxidation of dissolved organic matter. The cycling of H₂O₂ in seawater and potential approaches to in-situ monitoring are discussed, with particular reference to chemiluminescence techniques.     

Transport properties of solid polymer electrolytes prepared from oligomeric fluorosulfonimide lithium salts dissolved in high molecular weight poly(ethylene oxide)

Transport properties such as ionic conductivity, lithium transference number, and apparent salt diffusion coefficient are reported for solid polymer electrolytes (SPEs) prepared using several oligomeric bis[(perfluoroalkyl)sulfonyl]imide (fluorosulfonimide) lithium salts dissolved in high molecular weight poly(ethylene oxide) (PEO). The salt series consists of polyanions in which two discrete fluorosulfonimide anions are linked together by [(perfluorobutylene)disulfonyl]imide linker chains. The restricted diffusion technique was used to measure the apparent salt diffusion coefficients in SPEs, and cationic transference numbers were determined using both potentiostatic polarization and electrochemical impedance spectroscopy methods. A general trend of diminished salt diffusion coefficient with increasing anion size was observed and is opposite to the trend observed in ionic conductivity. This unexpected finding is rationalized in terms of the cumulative effects of charge carrier concentration, anion mobility, ion pairing, host plasticization by the anions, and salt phase segregation on the conductivity.     

Solid-state electrochemical behavior of Keggin-type borotungstic acid single crystal

We demonstrate the electrochemical characteristics of the, poorly described in the literature, mixed-valence proton conducting solid borotungstic acid single crystal, H₅BW₁₂O₄₀ xH₂O, in the absence of liquid electrolyte phase. We performed electrochemical measurements in an all-solid cell with a gold fiber ultramicrodisk (diameter 10, 25, and 40 μm) working electrode, a silver semi-reference disk electrode, and a glassy carbon ring counter electrode. Diagnostic experiments at different scan rates aimed at probing the model of mass transport and potential kinetic limitations. Such bulk parameters as the effective diffusion coefficient of charge propagation and the concentration of mixed-valence redox centers were determined by two methods. The first method is based on the analysis of both Cottrellian and steady-state currents (the mixed-regimes method), and the second method provides the true diffusion coefficient (transport coefficient free of the migration influence) for both the substrate and the product of the electrode reaction. Together, these methods constitute a double potential step chronoamperometry experiment. The data obtained with these electrochemical experiments (effective diffusion coefficients, concentration of mixed-valence redox centers, etc.) can support the results obtained with other techniques (XRD, FTIR, and TGA).     

Kinetics and exchange behaviour of certain sulphur anions on strongly basic anion exchangers in aqueous KCl medium

The kinetics of the exchange of sulphate and chloride ions on the strongly basic anion exchanger Dowex 1X8, of different particle diameters is investigated. The diffusion coefficient values are calculated by two diferent methods, discussed in the light of the processes governing the exchange reactions encountered, and found to be mainly controlled by particle diffusion. The distribution behaviour of sulphate, sulphite, thiosulphate and sulphide between aqueous KCl solutions of different concentrations and a number of selected anion exchange resins is studied. The data are explained on the premises of the different interactions involved in both aqueous and resineous phases. The effect of the alkali metal chloride molarities in the aqueous phase (LiCl, NaCl and KCl) on the exchange behaviour of the different S-anions is also investigated. The results are interpreted in the light of the water—water interaction and the competition for hydration between the alkali metal cations and the exchanged S-anions.     

Defining the electroanalytically measured species in a natural water sample

The possible definition, using electroanalytical techniques, of the chemical species disolved in natural waters is discussed. Emphasis is place on the formulation of concepts rather than completely rigorous mathematical solutions. The time of measurement is identified as the critical parameter for determining which species in solution will be measured when using transient techniques. When measurements are made in stirred solutions, as is the case in anodic stripping voltammetry and conventional polarography the effective diffusion layer thickness becomes the critical parameter. It may be regarded as being analogous to the time of mesurement and for most experimental systems it is simple to measure. It is recommended that whenever polarographic or voltammetric measurements are made on complex natural systems either the effective diffusion layer thickness or the time of measurement should be reported. Evidence in the literature suggests that a better appreciation of exactly what species are measured would greatly increase our understanding of natural systems.     

Anomalous dependence on the diffusion coefficients of the ionic relaxation time in electrolytes

We show, by using a numerical analysis, that the dynamic toward equilibrium for an electrolytic cell subject to a step-like external electric field is a multirelaxation process when the diffusion coefficients of positive and negative ions are different. By assuming that the diffusion coefficient of positive ions is constant, we observe that the number of involved relaxation processes increases when the diffusion coefficient of the negative ions diminishes. Furthermore, two of the relaxation times depend nonmonotonically on the ratio of the diffusion coefficients. This result is unexpected, because the ionic drift velocity, by means of which the ions move to reach the equilibrium distribution, increases with increasing ionic mobility.     

Comparison of Voltammetric Techniques for Ammonia Sensing in Ionic Liquids

In electroanalytical chemistry, it is often observed that square wave voltammetry (SWV) and differential pulse voltammetry (DPV) are more sensitive techniques compared to linear sweep voltammetry (LSV), due to their method of sampling which minimises the charging current (non‐faradaic processes). In this work, a comparison of the three techniques (LSV, DPV and SWV) is performed for ammonia (NH₃) gas oxidation (a chemically and electrochemically irreversible redox process) in an ionic liquid over a concentration range of 10–100 ppm. Four different platinum electrodes are employed: a screen‐printed electrode (SPE), a thin‐film electrode (TFE), a microarray thin‐film electrode (MATFE) and a Pt microdisk electrode (μ‐disk). Calibration plots (current vs concentration) for all three different electrochemical techniques on all four surfaces showed excellent linearity with increased concentrations of NH₃ gas and relatively low limits of detection (LODs). On the larger mm‐sized surfaces (SPE and TFE), the current responses for LSV and SWV were quite similar, but DPV gave the lowest currents. Whereas for the smaller micron sized electrodes (MATFE and μ‐disk), currents were of the order LSV>SWV>DPV, with LSV being far superior to the pulse techniques. These findings suggest that the pulse techniques of SWV and DPV may not be the optimum methods, particularly on microelectrodes, for the detection of analytes such as ammonia in RTILs.