Coulostatic pulses

Small step and without creating significant concentration gradients Equation (10.59) gives 

Large step, sufficient to reach the plateau of the voltammetric wave and with Cd independent of potential  

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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]. 

Fig. 5 Simultaneous excitation of HTC and Eu(III)-l-NCS at oxide-coated aluminum electrode [68], Conditions Oxide-covered Al-strip working electrode, Pt-wire counter electrode, coulostatic pulse generator, appUed pulse voltage -40 V, pulse frequency 20 Hz, pulse chaige 120 pC 0 2 M boric acid buffer at pH 9.2, 1x10 M KzSzOg. Time-resolved spectra were measured with an instrument having relatively good sensitivity but poor resolution [79]...

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

CS = coulostatic method, CV = cyclic voltamogram, FD = Faradaic distortion method, FR = Faradaic rectification, GD = galvanostatic double pulse method, IP = impedance method, PS = potential step method. See also list of... 

To overcome these problems, most voltammetric detectors have used pulsed waveforms such as staircase , squarewaveand differential pulseThe current is sampled at the end of the pulse after the charging current has decayed. In addition, because the charging current is typically the major current source, iR problems are not as severe. Last has described a coulostatic detector based on charge pulses instead of potential pulses which eliminates iR and charging current... 

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. 

Barker later described some work that involved apparatus like that shown in Figure 28.11. Light was supplied to a continuously renewed mercury pool electrode by a Q-switched, frequency-doubled ruby laser with a pulse width of — 15 ns. The electrode was set initially at any desired potential by a simple polarizing circuit, the response of which was slow enough that the electrode s reaction to the flash could be monitored as a coulostatic transient, AE (measured with respect to the initial potential) versus time. The difference in charge with respect to the initial condition is straightforwardly related to AE,... 

Consider the application of a very short current pulse to the interphase. The charge = i t injected during the pulse changes the potential across the double-layer capacitance by AE = qp/C, - Starting from the equilibrium potential, this will be equal to the overpotential 1j, as seen in Fig. 7K. We use the subscript zero in this figure, since this is the initial overpotential (corresponding to t = 0) in the decay transient studied in a coulostatic experiment. 

Table 2K The effect of solution resistance on the minimum pulse duration and the maximum value of the exchange current density measurable in a coulostatic experiment.

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). 

J.W. Park, D.W. Lee, Pulse electrochemical polishing for microrecesses based on a coulostatic analysis, Int. 

Many electrochemical analytical combined methods allow for simultaneous calculation of DC, exchange current (EC), and transfer coefficients. For example, in a method called the coulostatic or charge-step method [28] a current pulse of 0.1-1 ps is applied to the electrode, and the variation of the electrode potential with time after the pulse (that is, at open circuit) is recorded. The method is very similar to GSPM however, the instrumentation is different, namely, the charge is injected by discharging a small capacitor across the electrode. The results are not affected by the double-layer capacity and electrode resistance. 

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. 

Since the charge is injected in a very short time (preferably 1 is or less), measurement can often be completed before diffusion limitation has become significant In this respect the charge-injection (coulostatic) method is similar to the double-pulse galvano static method, except that one has more freedom in the choice of the parameters of the pulse, since there is no need to match it to the second pulse. 

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