Voltammetry solid electrode

A.C. Polarography 1 Coulometry - (Constant I) Voltammetry - Solid Electrode... 

In hydrodynamic voltammetry the solution is stirred either by using a magnetic stir bar or by rotating the electrode. Because the solution is stirred, a dropping mercury electrode cannot be used and is replaced with a solid electrode. Both linear potential scans or potential pulses can be applied. 

The difference between the various pulse voltammetric techniques is the excitation waveform and the current sampling regime. With both normal-pulse and differential-pulse voltammetry, one potential pulse is applied for each drop of mercury when the DME is used. (Both techniques can also be used at solid electrodes.) By controlling the drop time (with a mechanical knocker), the pulse is synchronized with the maximum growth of the mercury drop. At this point, near the end of the drop lifetime, the faradaic current reaches its maximum value, while the contribution of the charging current is minimal (based on the time dependence of the components). 

Sodium-silicate glass, 151 Sol-gel films, 120, 173 Solid electrodes, 110 Solid state devices, 160 Solvents, 102 Speciation, 84 Spectroelectrochenristry, 40 Spherical electrode, 6, 8, 9, 61 Square-wave voltammetry, 72, 92 Staircase voltammetry, 74 Standard potential, 3 Standard rate constant, 12, 18 Stripping analysis, 75, 79, 110 Supporting electrolyte, 102 Surface-active agents, 79... 

Data on the electrochemistry of the telluride ion in alkaline media are relatively limited. Mishra et al. [53] studied the oxidation of Te to Te° at solid electrodes, focusing on the intermediate step(s) of this process, and in particular, the possibility of detecting ditelluride Te via rotating ring disk electrode (RRDE) methodology. Oxidation beyond the elemental state to TeO and TeO was also studied using cyclic and hydrodynamic voltammetry. 

Early studies of ET dynamics at externally biased interfaces were based on conventional cyclic voltammetry employing four-electrode potentiostats [62,67 70,79]. The formal pseudo-first-order electron-transfer rate constants [ket(cms )] were measured on the basis of the Nicholson method [99] and convolution potential sweep voltammetry [79,100] in the presence of an excess of one of the reactant species. The constant composition approximation allows expression of the ET rate constant with the same units as in heterogeneous reaction on solid electrodes. However, any comparison with the expression described in Section II.B requires the transformation to bimolecular units, i.e., M cms . Values of of the order of 1-2 x lO cms (0.05 to O.IM cms ) were reported for Fe(CN)g in the aqueous phase and the redox species Lu(PC)2, Sn(PC)2, TCNQ, and RuTPP(Py)2 in DCE [62,70]. Despite the fact that large potential perturbations across the interface introduce interferences in kinetic analysis [101], these early estimations allowed some preliminary comparisons to established ET models in heterogeneous media. 

This is a dynamic electrochemical technique, which can be used to study electron transfer reactions with solid electrodes. A voltammo-gram is the electrical current response that is due to applied excitation potential. Chapter 18b describes the origin of the current in steady-state voltammetry, chronoamperometry, cyclic voltammetry, and square wave voltammetry and other pulse voltammetric techniques. 

Linear-Sweep and Cyclic Voltammetry at Solid Electrodes... 

In voltammetry, the electrode is a solid conductor. The surface of the electrode is not refreshed constantly as it is for a DME, so voltammograms do not have a sawtoothed shape, but are smooth. Rather than a current plateau. Id, voltammograms contain a peak current. Ip, with the magnitude of the peak being directly proportional to the bulk concentration of analyte, according to the Randles-Sev5ik equation (equation (6.13)). 

The oxidation processes of various anions, including halide ions, have been studied by voltammetry at solid electrodes [38 a]. For example, at a platinum electrode, iodide ion (I-) in various organic solvents is oxidized in two steps, i.e. 3I -> I3 + 2e and 2Ii-> 3I2+2e. From the difference in the standard potentials of the two steps, the dissociation constants of I3, p/f=-log ([yin/KD- have been determined as in Table 8.4. In aqueous solutions, p/f is 3 and I3 is not stable enough to use this method. The electrode oxidation of CN has been studied in several solvents using ESR spectroscopy [38b]. From the ultimate formation of the relatively stable tricyanomethylenimine radical anion, the following steps have been considered ... 

Electrochemical oxidation of 2-, 3-, and 4-aminopyridines as well as 2,6-diaminopyridines and aminopicolines was studied in CH3CN-LiC104 by means of RDE voltammetry.422 Also, the electrochemical oxidation of 3-aminopyridine, 2,3-diaminopyridine, and 2,6-diaminopyridine has been investigated in aqueous solutions in the pH range 0.7-13 at platinum and carbon paste solid electrodes.423 A reaction scheme for the oxidation of aminopyridines was proposed on the basis of the voltammetric results, but the products of the oxidations were not identified. 

There are several excellent articles which deal with the theory and practice of cyclic voltammetry.1-4 Foremost among these is the comprehensive treatise by Bard and Faulkner which gives a thorough description of the theory of controlled potential microelectrode techniques, including cyclic voltammetry.1 Particularly readable accounts of cyclic voltammetry and related techniques are given in Adams book, Electrochemistry at Solid Electrodes ,2 in Pletcher s review3 and in a series of articles which appeared in J. Chem. Educ.e>... 

In addition to the traditional SEV techniques discussed earlier, various pulse volt-ammetric techniques have been employed at solid electrodes in molten salts, especially in the room-temperature haloaluminate melts. Numerous pulse techniques have been devised, and some of the more common examples of this family of volt-ammetric methods are described in Chapters 3 and 5 of this volume. However, the application of these methods to molten salts is limited primarily to large amplitude pulse voltammetry (LAPV), differential-pulse voltammetry (DPV), and, more recently, reverse normal-pulse voltammetry (RNPV). The application of LAPV and... 

The most popular electroanalytical technique used at solid electrodes is Cyclic Voltammetry (CV). In this technique, the applied potential is linearly cycled between two potentials, one below the standard potential of the species of interest and one above it (Fig. 7.12). In one half of the cycle the oxidized form of the species is reduced in the other half, it is reoxidized to its original form. The resulting current-voltage relationship (cyclic voltammogram) has a characteristic shape that depends on the kinetics of the electrochemical process, on the coupled chemical reactions, and on diffusion. The one shown in Fig. 7.12 corresponds to the reversible reduction of a soluble redox couple taking place at an electrode modified with a thick porous layer (Hurrell and Abruna, 1988). The peak current ip is directly proportional to the concentration of the electroactive species C (mM), to the volume V (pL) of the accumulation layer, and to the sweep rate v (mVs 1). 

Electrochemical analytical methods, particularly polarography and voltammetry rise in the 1960s was caused by the demand in trace analysis and new technique of preliminary electrochemical concentration of the determined substance on the electrode surface [1,2]. The reason for the new renaissance is the use of screen-printed technologies, which resulted in creation of new electrodes so cheap that they can be easily disposed and there is no need of regenerating the solid electrode surface [3]. 

Although solid-electrode voltammetry had serious limitations because of the changing character of solid-electrode surfaces, it provides the ability to go to much more positive potentials than are possible with the mercury electrode. Contemporary solid electrodes, particularly glassy carbon, allow one to obtain reproducible results (see Chapter 5). 

Another limitation of solid electrodes has been their complex diffusion-current response relative to time with slow-sweep voltammetry. The development of a capillary hanging-mercury-drop electrode (HMDE) by Kemula and Kublik,4,5 together with modem electronic instrumentation, allowed the principles of voltage-sweep voltammetry and cyclic voltammetry to be established. The success has been such that this has become one of the most important research tools for electrochemists concerned with the kinetics and mechanisms of electrochemical processes. These important contributions by Nicholson and Shain6 7 rely, as have all electrochemical kinetic developments, on the pioneering work by Eyring et al.8... 

This relationship holds for any electrochemical process that involves semiinfinite linear diffusion and is the basis for a variety of electrochemical methods (e.g., polarography, voltammetry, and controlled-potential electrolysis). Equation (3.6) is the basic relationship used for solid-electrode voltammetry with a preset initial potential on a plateau region of the current-voltage curve. Its application requires that the electrode configuration be such that semiinfinite linear diffusion is the controlling condition for the mass-transfer process. 

The advantages of a solid electrode of fixed area that functions in the voltammetric experiment with a constant diffusion-layer thickness have led to the development of the rotated-disk and ring-disk electrodes.52-54 By rotation of a disk, the electrode diffusion layer becomes fixed such that the current is constant as a function of time and does not decay [in contrast to conventional voltammetry Eq. (3.6)]. Voltammetry with such an electrode system gives a current-potential wave that is analogous to a polarogram and follows the relationship... 

The use of pulse techniques for electroanalytical determinations has been much publicized, and is applicable to both solid electrodes and the HMDE/SMDE. The development in recent years of square wave voltammetry (SWV)39 widens the possibilities beause of its rapidity (Section 10.9) it is especially useful because the time necessary to do an experiment is only 2 s, which means that a SMDE drop in the dropping mode can also be used for micromolar determinations. Progress obtained with pulse techniques40,41 has meant that applications of a.c. voltammetry have been few, but there is no theoretical reason for this. The very low detection limits achieved in stripping voltammetry result not only from the pre-concentration step but also from the use of pulse waveforms in the determination step. 

---