Impedance electroanalytical techniques

If a resistance is placed in the feedback loop (Fig. 6.6b), the bias current ib will also create a difference between Ej and E0 by an amount ibRr. Even very inexpensive (< 1) OAs can have bias currents of less than 10 9 A, which means that the value of Rr will have to exceed 1 MO to create a 1-mV error. Amplifiers with bias currents of less than 0.1 pA (10 13 A) are available. Using the same criterion, Rr may then reach 1000 MQ, a value well beyond any resistance commonly encountered in dynamic electroanalytical techniques. Such amplifiers are, however, eminently useful for constructing pH meters and pH stats and measuring potentials in electrophysiology, where very small high-impedance electrodes are often used. 

The first group consists of conventional electroanalytical techniques such as cyclic voltammetry (CV), chronoamperometry, chronopotentiometry, coulometry, and electrochemical impedance spectroscopy (EIS), all of which provide general information about the doping process (see also Chapters 4 and 5). Below are listed some typical questions that can be answered using the above group of techniques ... 

The potential of electroanalytical techniques such as cyclic voltammetry and impedance spectroscopy is also not fully explored in photocatalyt-ic studies which is primarily used for the rapid screening and high throughput evaluation of photocatalysts. The use of in-situ analytical techniques used for heterogeneous catalysis is extensively reviewed elsewhere [213-218]. 

Electrochemistry involves the study of the relationship between electrical signals and chemical systems that are incorporated into an electrochemical cell. It plays a very important role in many areas of chemistry, including analysis, thermodynamic studies, synthesis, kinetic measurements, energy conversion, and biological electron transport [1]. Electroanalytical techniques such as conductivity, potentiometry, voltammetry, amperometric detection, co-ulometry, measurements of impedance, and chronopotentiometry have been developed for chemical analysis [2], Nowadays, most of the electroanalytical methods are computerized, not only in their instrumental and experimental aspects, but also in the use of powerful methods for data analysis. Chemo-metrics has become a routine method for data analysis in many fields of analytical chemistry that include electroanalytical chemistry [3,4]. 

Electroanalytical techniques, essentially similar to those employed in aqueous solutions, can be adapted for use in melts to provide data on solution equilibria by way of stability constant determinations, ion transport through diffusion coefficient measurements, as well as mechanistic analysis and product identification from mathematical data treatment. Indeed, techniques such as linear sweep voltammetry and chronopotentiometry may often be applied rapidly to assess or confirm general characteristics or overall stoichiometry of electrode processes in melts, prior to more detailed kinetic or mechanistic investigations requiring more elaborate instrumentation and equipment, e.g., as demanded by impedance studies. Thus, answers to such preliminary questions as... 

In spite of the instrumental simplicity of cyclic voltammetry, the method is surprisingly versatile and an experienced researcher can obtain a considerable amount of information on the basis of just recording a single CV. A large number of different electroanalytical techniques have been applied to the research on conducting polymers, from the chronoamperometric method to AC impedance methods, However, the methods rest only on the electrochemical characteristics of the polymer and naturally the information obtained is rather restricted. 

The two preceding electroanalytical techniques, one in which the measured value was the current during imposition of a potential scan and the other a potential response under an imposed constant current, owe their electrical response to the change in impedance at the electrode-electrolyte interface. A more direct technique for studying electrode processes is to measure the change in the electrical impedance of an electrode by electrochemical impedance spectroscopy (EIS). To relate the impedance of the electrode-electrolyte interface to electrochemical parameters, it is necessary to establish an equivalent circuit to represent the dynamic characteristics of the interface. 

Basically, experimental approaches to ion transfer kinetics rely on classical galvanostatic [152] or potentiostatic [146] techniques, such as chronopotentiometry [118, 138], chronocoulometry [124], cyclic voltammetry [146], convolution potential sweep voltammetry [147], phase selective ac voltammetry [142], or equilibrium impedance measurements [148]. These techniques were applied mostly to liquid-liquid interfaces with a macroscopic area (typically around 0.1 cm ). However, microelectrode methodology has been successfully introduced into liquid-liquid electrochemistry as a novel electroanalytical tool by Senda and coworkers [153] and... 

C. Deslouis and B. Tribollet, "Recent Developments in the Electro-Hydrodynamic (EHD) Impedance Technique," Journal of Electroanalytical Chemistry, 572 (2004) 389-398. 

Sapoval, B, J,-N, Chazalviel, and J. Peyridre, Electrical response of fractal and porous interfaces. Physical Review A, 1988. 38(11) pp. 5867-5887 Reiser, H, K.D, Beccu, and M.A. Gutjahr, Electrochimica Acta, 1976, 21 p. 539 Diard, J.R, B, Le Gorrec, and C. Montella, Linear diffusion impedance. General expression and applications. Journal of Electroanalytical Chemistry, 1999. 471 pp, 126-131 Deslouis, C, C, GabrieUi, M. Keddam, A, Khalil, R. Rosset, B. TriboUet, and M. Zidoune, Impedance techniques at partially blocked electrodes by scale deposition. Electrochimica Acta, 1997, 42(8) pp, 1219-1233... 

The application of Fourier transform techniques to the study of electroanalytical chemistry would not hve been feasible without the computer. Smith (3 has been responsible for most of the pioneering work in this area. The Fast Fourier Transform (FFT) can be used both for data acquisition and for data treatment. For example, the FFT approach in ac impedance measurements has the advantage that a range of frequencies can be superimposed during a single potential scan. 

Electroanalytical methods have been extensively applied in sensing and biosensing. Potentiometry, amperometry, cyclic voltammetry, linear voltammetry, differential pulse voltammetry, square-wave voltammetry, and electrochemical impedance spectroscopy (EIS) represent the most-used electrochemical techniques used for biosensor fabrication and detection. 

---