Chronoamperometry and Chronocoulometry

The potentials Ex and E2 should be chosen in such way that at Ex no electrode process occurs and at E2 the electrode reaction of an electroactive species takes place. If the rate of the electrode process is controlled only by diffusion, the Cottrell equation [Eq. (3.6)] can be applied. Therefore, the observed current should be a linear function of t m with the intercept at the origin (a test for diffusion control). The diffusion coefficient of the electroactive species is directly proportional to the slope of the curve. The heterogeneous rate constant of a kinetically limited electrode reaction (kc or k3) also can be evaluated. 

The chronocoulometry and chronoamperometry methods are most useful for the study of adsorption phenomena associated with electroactive species. Although less popular than cyclic voltammetry for the study of chemical reactions that are coupled with electrode reactions, these chrono- methods have merit for some situations. In all cases each step (diffusion, electron transfer, and chemical reactions) must be considered. For the simplification of the data analysis, conditions are chosen such that the electron-transfer process is controlled by the diffusion of an electroactive species. However, to obtain the kinetic parameters of chemical reactions, a reasonable mechanism must be available (often ascertained from cyclic voltammetry). A series of recent monographs provides details of useful applications for these methods.13,37,57 

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Having defined our near electrode region, we turn now to consider the various techniques that can be employed in the in situ investigation of the reactions that occur within it. The various methods that can be employed will each provide different types of information on the processes occurring there. As has already been discussed, cyclic voltammetry is the most common technique first employed in the investigation of a new electrochemical system. However, in contrast to the LSV and CV of adsorbed species, the voltammetry of electroactivc species in solution is complicated by the presence of an additional factor in the rate, the mass transport of species to the electrode. Thus, it may be more useful to consider first the conceptually more simple chronoamperometry and chronocoulometry techniques, in order to gain an initial picture of the role of mass transport. 

Thus, cyclic or linear sweep voltammetry can be used to indicate whether a reaction occurs, at what potential and may indicate, for reversible processes, the number of electrons taking part overall. In addition, for an irreversible reaction, the kinetic parameters na and (i can be obtained. However, LSV and CV are dynamic techniques and cannot give any information about the kinetics of a typical static electrochemical reaction at a given potential. This is possible in chronoamperometry and chronocoulometry over short periods by applying the Butler Volmer equations, i.e. while the reaction is still under diffusion control. However, after a very short time such factors as thermal... 

Scheme 5.1 Multipulse Chronoamperometry and Chronocoulometry. (a) Potential-time program (b) Current-time response (c) Charge-time response...

The general electrochemical behavior of surface-bound molecules is treated in Sect. 6.4. The response of a simple electron transfer reaction in Multipulse Chronoamperometry and Chronocoulometry, CSCV, CV, and Cyclic Staircase Voltcoulometry and Cyclic Voltcoulometry is also presented. Multielectronic processes and first- and second-order electrocatalytic reactions at modified electrodes are also discussed extensively. 

The different assumptions needed to make a statement of this problem will be presented in the following section. Then the general solution corresponding to the application of a sequence of potential pulses to attached molecules giving rise to simple charge transfer processes and particular solution corresponding to Multipulse Chronoamperometry and Chronocoulometry and Staircase Voltammetry will be deduced. Cyclic Voltammetry has a special status and will be discussed separately. Finally, some effects that cause deviation from the ideal behavior and more complex reaction schemes like multielectronic processes and chemical reactions in the solution coupled to the surface redox conversion will be discussed. 

Figure 3.15 Chronoamperometry and chronocoulometry (a) excitation potential step (b) chronoamperometric response (c) chionocoulometric response.

Andrieux, C.P, Hapiot, P, and Saveant, J.-M. 1984. Electron-transfer coupling of diffusional pathways Theory for potential step chronoamperometry and chronocoulometry. Journal of Electroanaytical Chemistry 1172, 49-65. 

In potential step experiments the potential of the working electrode is changed instantaneously, and either the current-time response or the charge-time response is recorded. These techniques are known respectively as chronoamperometry and chronocoulometry. First we will consider the type of chronoamperometric experiment that was briefly outlined in the first chapter. 

These cells are normally used for experiments (chronoamperometry and chronocoulometry) in which large-amplitude steps are applied in order to carry out an electrolysis in the diffusion region. For... 

The electrochemical techniques usually utilized to obtain the spectroscopic properties of redox generated species are those based on potentiometric measurements at potential step, like chronoamperometry and chronocoulometry. Their basic principles are briefly illustrated in the following (for a more detailed discussion see [16]). 

In the chronoamperometry and chronocoulometry experiments a step potential difference is imposed (Fig. 9.8) from a starting Ei value, at which no redox process occurs, to a E2, at which the oxidized or reduced species is mainly present. 

Two electrochemical techniques are directly based on the expression for the faradaic current density jF, namely chronoamperometry and normal pulse polarography. A third technique, named chronocoulometry, deals with the integral of jF, giving the charge transferred per unit area via the faradaic process as a function of time. The general expression obtained... 

Many membrane proteins are electrogenic, that is, translocate a net charge across a membrane. Consequently, it is possible to monitor their function directly by measuring the current flowing along an external electrical circuit upon their activahon. The techniques of choice for these measurements are EIS and potenhal-step chronoamperometry or chronocoulometry, because the limited volume of the ionic reservoir created by a hydrophilic spacer in solid-supported biomimetic membranes cannot sustain a steady-state current. 

Potential-Step chronoamperometry (CA), chronocoulometry (CC), and sampled current voltammetry were used to probe the nature of the charge transport process close to the electrode surface. By manipulating the time scale of the current decay response at sufficiently short times, semi-infinite diffusion is observed. The resulting current response conforms to the Cottrell equation ... 

For decades the electrochemical techniques, i.e., potential, current, or charge step methods such as chronoamperometry, -r chronocoulometry, chrono-potentiometry, coulostatic techniques were considered as fast techniques, and only with other pulse techniques such as temperature jump (T-jump) introduced by Eigen [i] or flash-photolysis method invented by Norrish and Porter [ii], much shorter time ranges became accessible. (For these achievements Eigen, Norrish, and Porter shared the 1964 Nobel Prize.) The advanced versions of flash-photolysis allow to study fast homogeneous reactions, even in the picosecond and femtosecond ranges [hi] (Zewail, A.H., Nobel Prize in Chemistry, 1999). Several other techniques have been elaborated for the study of rapid reactions, e.g., flow techniques (stopped-flow method), ultrasorhc methods, pressure jump, pH-jump, NMR methods. 

Electrochemical methods such as Chronoamperometry (CA) (monitoring of current decay with time upon application of a specified potential) and Chronocoulometry... 

Appropriate electroanalytical procedures to verify the one or other case have been given in the references of this section. The main techniques are cyclic voltammetry, chronoamperometry, chronocoulometry, and rotating disk voltammetry. The last one appears to be best suited since constant mass transport in the film is a very important feature as outlined aixive Table 2 gives examples for... 

R.W. Murray, Chronoamperometry, Chronocoulometry and Chronopoten-tiometry, Physical Methods of Chemistry. Electrochemical Methods. A. Weissberger and B.W. Rossiter eds, Vol. 2. Wiley Interscience, New York, 1986. 

It is important to recognize that the Nernst equation is valid only at the equilibrium condition, determined by specifying i, = 0. Most electrochemical techniques (chronoamperometry, chronocoulometry, voltammetry, etc.) involve nonequilibrium conditions and therefore cannot be expected to exhibit a Nernstian response unless the rates are very fast and equilibrium is quickly reestablished at the surface. 

The electroanalytical techniques chronoamperometry, chronocoulometry, and chronoabsorptometry are all based on the same excitation function of one or more potential steps that are applied to an electrode immersed in a nonstirred solution. The system response is thus identical for all three techniques. They differ only in the data domain of the monitored response. Consequently, the common excitation aspect is dealt with here, whereas each monitored response is considered individually in subsequent sections. 

Many of the electroanalytical techniques that are routinely employed in conventional solvents, such as, chronoamperometry, chronocoulometry, chronopotentiometry, coulometry, cyclic (stationary electrode) voltammetry, rotating electrode voltammetry, and pulse voltammetry, have also been applied to molten salts. Some of these techniques are discussed next with special attention to their employment in molten salts. References to noteworthy examples appearing in the literature are included. Background information about these techniques is available elsewhere in this book. 

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