Coulostatic techniques

For controlled-current DC polarography, especially its current density mode, see under Chronopotentiometry at a dme (p. 172). For charge-step polarography, i.e., a controlled charge of coulostatic technique, see ref. 9, pp. 424-429, and ref. 3, pp. 270-276. 

There are two advantages of the coulostatic method in the study of kinetics of electrode reactions. First, the ohmic drop is not of importance, therefore the measurements can be carried out in highly resistive media. Second, since Ic = IF, Q does not interfere in the measurement. By the help of this technique jo values up to about 0.1 A cm-2 and - standard rate constants up to 0.4cms 1 can be determined. A detailed discussion of coulostatic techniques can be found in Ref. [vi]. 

For decades the electrochemical techniques, i.e., potential, current, or charge step methods such as - chronoamperometry, - chronocoulometry, - chrono-potentiometry, coulostatic techniques were consid-... 

Transient technique — A technique whose response is time dependent and whose time dependence is of primary interest, e.g., -> chronoamperometry, -> cyclic voltammetry (where current is the transient), -> chronopotentiometry and -> coulostatic techniques (where voltage is the transient). A transient technique contrasts with steady-state techniques where the response is time independent [i]. Some good examples are cyclic voltammetry [i, ii] (fast scan cyclic voltammetry), the indirect-laser-induced-temperature-jump (ILIT) method [iii], coulostatics [i]. The faster the transient technique, the more susceptible it is to distortion by -> adsorption of the redox moiety. 

The detection method resembles that of the coulostatic technique (Section 8.7), in that one observes the potential shift caused by charge ejection and the resulting relaxation back to the original state. Figure 8.9.2 offers some typical data for the N2O system, and Problem 8.10 deals with its interpretation. 

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. 

The combination of three transient techniques - coulostatic, modified coulostatic, and galvanostatic methods was applied for the study of the electrode reaction Cd(II)/Cd(Hg) in aqueous solution of 1 M Na2S04, pH 4 [28]. 

The variable to be perturbed is either the potential, E, or the Current density, j. The response on a potential perturbation, logically, will be the resulting current j (t), but it is additionally useful to measure the integral of j(t), i.e. the charge q (0 that has passed the interface. As a counterpart, a technique is known where the perturbation is a current pulse of infinitely small time duration (delta function) comprising a certain amount of charge the coulostatic impulse technique. 

With respect to chemical steps prior to the electron-transfer step, chrono-potentiometiy offers a convenient technique. The methods of measurement and the quantitative relationships are outlined in Chapter 4. Post-electron-transfer reactions to the electron-transfer step are most conveniently characterized by cyclic voltammetry (see Chapter 3). Although the techniques of cyclic voltammetry and chronopotentiometiy both provide a means for the qualitative detection of adsorption processes at an electrode, the coulostatic method and chrono-coulometry are the methods of choice for quantitative measurements of adsorption. 

The coulostatic method was applied to in situ measurement of the polarization resistance i p (Eq. (25)). Thus, the rate of electroless plating was determined by Suzuki et al. [21-23]. Two advantages of this technique have been cited by the inventors (1) measured values are not influenced by the solution resistance, and (2) measurements can be finished very rapidly, within a few tens of milliseconds. The principle is briefly explained below. 

We will now examine the E-t behavior following a coulostatic impulse for several cases of interest. Details of the theoretical treatments have been given by Delahay (29, 30) and Reinmuth (31, 32) and their coworkers, who first described the application of this technique. 

The technique of controlled-potential cathodic deposition followed by anodic stripping with a linear potential sweep has been applied to the determinations of a number of metals (e.g., Bi, Cd, Cu, In, Pb, and Zn) either alone or in mixtures (Figure 11.8.5). An increase in sensitivity can be obtained by using pulse polarographic, square wave, or coulostatic stripping techniques. Other variants, such as stripping by a potential step, current step, or more elaborate programs (e.g., an anodic potential step for a short time followed by a cathodic sweep) have also been proposed (68-74). 

Several coulometric and pulse techniques are used in electroanalytical chemistry. Rather low detection limits can be achieved, and kinetic and transport parameters can be deduced with the help of these fast and reliable techniques. Since nowadays the pulse sequences are controlled and the data are collected and analyzed using computers, different pulse programs can easily be realized. Details of a wide variety of coulometric and pulse techniques, instrumentation and applications can be found in the following literature controlled current coulometry [6], techniques, apparatus and analytical applications of controlled potential coulometry [7], coulostatic pulse techniques [8], normal pulse voltammetry [9], differential pulse voltammetry [9], and square-wave voltammetry [10]. 

Voltammetric Methods for the Study of Adsorbed Species, Etienne Laviron Coulostatic Pulse Techniques, Herman P. van Leeuwen... 

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