Faradaic electrochemical methods

I) Faradaic electrochemical methods. From a general analytical point of view, electrochemical techniques are very sensitive methods for identifying and determining the electroactive species present in the sample and, in addition, they also are able to carry out speciation studies, providing a complete description of the states of oxidation in which the ionic species are present in the object. Other applications and improvements obtained by their hyphenation with other instrumental techniques, such as atomic force microscopy (AFM), will be described in the following chapters. 

U) Non-Faradaic electrochemical methods. Conductometric methods have been extensively used by scientists and conservators for monitoring the content of salts removed during water immersion treatments of ancient tiles and archaeological ceramic remains. In a different manner to IC, this technique provides the total ionic... 

The use of conventional electrochemical methods to study the effect of metal adatoms on the electrochemical oxidation of an organic adsorbate may be in some cases of limited value. Very often, in the potential region of interest the current due to the oxidation of an organic residue is masked by faradaic or capacitive responses of the cocatalyst itself. The use of on-line mass spectroscopy overcomes this problem by allowing the observation of the mass signal-potential response for the C02 produced during the oxidation of the adsorbed organic residue. 

How electron transfer kinetics may be investigated by means of an electrochemical method such as cyclic voltammetry is the question we address now, starting with the case where the reactants are immobilized on the electrode surface, as in the beginning of Section 1.2. The key equations are those that relate the surface concentrations rA and rB to the current. The first of these expresses the Faradaic consumption of A and production of B as the current flows ... 

Refs. [i] Delahay P (1961) Fast electrode processes by relaxation methods. In Delahay P (ed) Advances in electrochemistry and electrochemical engineering, vol. 1. Interscience, New York, Chap. 5 [ii] Reinmuth WH (1964) Anal Chem 36 211R [Hi] Bond AM (1980) Modernpolarographic methods in analytical chemistry. Marcel Dekker, New York, pp 296,298, 361 [iv] Agarwal HP (1974) Faradaic rectification method and its applications in the study of electrode processes. In Bard A (ed) Electroanalyt-ical chemistry, vol. 7. Marcel Dekker, New York, pp 161... 

Potential step methods have emerged as valuable electrochemical methods due to the highly sensitive nature of the technique. The waveform employed in potential step methods, also referred to as pulsed methods, have some advantages over potential sweep methods. The main advantage is that the steplike waveform can discriminate and separate the capacitive current versus the faradaic current, the current due to the reduction or oxidation undergone by the analyte, increasing signal to noise. Capacitive versus faradaic current discrimination is the basis for all of the pulsed techniques. The rate of decay of the capacitive current and the faradaic current is not the same. The capacitive current has an exponential decay whereas the faradaic current decays as a function of t Since the rate of decay of the capacitive current is much... 

The double-layer capacitance is another useful electrochemical method for probing the integrity of molecular assemblies in contact with electrolyte solutions. If a cyclic voltammogram is recorded in a solution of electrolyte only, a non-Faradaic current results, and the double-layer capacitance, Cji (or differential capacitance), is measured as half the width of the voltammogram charging envelope given by [30, 48]... 

Two developmental difficulties existed in the application of electrochemical methods to CE detection. The first involved the cross-talk or interference that resulted from the high voltages used in CE separations. These dc voltages may be as high as 50 kV, and the resulting electroosmotic currents can be up to six orders of magnitude greater than the faradaic currents measured at an amperometric detector poised... 

This operation determines the values of R and C that, in series, behave as the cell does at the measurement frequency. The impedance is measured as a function of the frequency of the ac source. The technique where the cell or electrode impedance is plotted V5. frequency is called electrochemical impedance spectroscopy (EIS). In modem practice, the impedance is usually measured with lock-in amplifiers or frequency-response analyzers, which are faster and more convenient than impedance bridges. Such approaches are introduced in Section 10.8. The job of theory is to interpret the equivalent resistance and capacitance values in terms of interfacial phenomena. The mean potential of the working electrode (the dc potential ) is simply the equilibrium potential determined by the ratio of oxidized and reduced forms of the couple. Measurements can be made at other potentials by preparing additional solutions with different concentration ratios. The faradaic impedance method, including EIS, is capable of high precision and is frequently used for the evaluation of heterogeneous charge-transfer parameters and for studies of double-layer structure. 

Surface excesses of electroactive species are often examined by methods sensitive to the faradaic reactions of the adsorbed species. Cyclic voltammetry, chronocoulometry, polarography, and thin layer methods are all useful in this regard. Discussions of their application to this type of problem are provided in Section 14.3. In addition to these electrochemical methods for studying the solid electrode/electrolyte interface, there has been intense activity in the utilization of spectroscopic and microscopic methods (e.g., surface enhanced Raman spectroscopy, infrared spectroscopy, scanning tunneling microscopy) as probes of the electrode surface region these are discussed in Chapters 16 and 17. 

Three broad classifications of electrochemical methods are used in this chapter. Po-tentiometric methods include zero-current potentiometry and methods in which current of controlled magnitude is apphed to the working electrode, such as in potentiometric stripping analysis (PSA). Amperometric methods consider all techniques in which current is measured these include constant-potential amper-ometry and amperometric measurements made in response to a variety of applied potential waveforms in voltammetric methods. Impedimetric methods comprise a final classification in these methods, faradaic currents are generally absent, and impedance, conductance, or capacitance is the measured property. 

Impedimetric measurements are based on the nonfaradaic response observed between two electrodes immersed in a sample solution when a high frequency (1 to 300 kHz) alternating potential is applied. The amplitude of the applied waveform is small, and this, combined with the high frequency relative to other electrochemical methods, effectively prevents faradaic reactions from occurring to any significant extent. In contrast to potentiometric and amperometric methods, impedimetric methods are not analyte-selective instead, bulk properties of the solution and the electrode-solution interfaces are monitored. 

Electrochemical impedance spectroscopy (EIS including Faradaic impedance in the presence of a redox probe and non-Faradaic-capacitance methods) is considered as a rapid technique for the characterization of the structure and functional operation of biomaterial-functionalized electrodes (Yang and Bashir, 2008). The immobilization of biomaterials on electrodes produces changes in the capacitance and interfacial electron transfer resistance of electrodes, causing changes in the impedance. Flence, interfacial... 

Residual Current Even in the absence of analyte, a small current inevitably flows through an electrochemical cell. This current, which is called the residual current, consists of two components a faradaic current due to the oxidation or reduction of trace impurities, and the charging current. Methods for discriminating between the faradaic current due to the analyte and the residual current are discussed later in this chapter. 

Electrochemical impedance spectroscopy (EIS) in a sufficiently broad frequency range is a method well suited for the determination of equilibrium and kinetic parameters (faradaic or nonfaradaic) at a given applied potential.268,269 EIS has been used to study polycrystalline Au, Cd, Ag, Bi, Sb, and other electrodes.152249 270-273... 

Transient measnrements (relaxation measurements) are made before transitory processes have ended, hence the current in the system consists of faradaic and non-faradaic components. Such measurements are made to determine the kinetic parameters of fast electrochemical reactions (by measuring the kinetic currents under conditions when the contribution of concentration polarization still is small) and also to determine the properties of electrode surfaces, in particular the EDL capacitance (by measuring the nonfaradaic current). In 1940, A. N. Frumkin, B. V. Ershler, and P. I. Dolin were the first to use a relaxation method for the study of fast kinetics when they used impedance measurements to study the kinetics of the hydrogen discharge on a platinum electrode. 

Analytical methods based upon oxidation/reduction reactions include oxidation/reduction titrimetry, potentiometry, coulometry, electrogravimetry and voltammetry. Faradaic oxidation/reduction equilibria are conveniently studied by measuring the potentials of electrochemical cells in which the two half-reactions making up the equilibrium are participants. Electrochemical cells, which are galvanic or electrolytic, reversible or irreversible, consist of two conductors called electrodes, each of which is immersed in an electrolyte solution. In most of the cells, the two electrodes are different and must be separated (by a salt bridge) to avoid direct reaction between the reactants. 

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