Current reversal chronoamperometry

Sum and Skyllas-Kazacos [44] studied the deposition and dissolution of aluminum in an acidic cryolite melt. The graphite electrode was preconditioned (immersed in cryolite melt) to saturate the surface of the electrode in sodium before aluminum deposition could be observed. Current reversal chronoamperometry was used to measure the rate of aluminum dissolution in the acidic melt. Dissolution rate was mass transport controlled [45] and in the order of 0.8 10 7 and 1.8 10 7 molcm 2s 1 at 1030 °C and 980 °C respectively [44]. 

Chronopotentiometry closely resembles chronoamperometry with the exception that the role of current and potential are reversed. In chronopotentiometry the current is controlled and is the variable and the electrode potential is the observable. A single step of the current or a double step can be employed. The double step method called current reversal chronopotentiometry is more information-rich as in previous comparisons. Both the applied currenttime wave form and the potential-time response for a reversible electrode process are illustrated in Fig. 4. The current step from 0 to a predetermined value depending upon the experimental conditions is maintained until time... 

For constant current experiments on Ei-CiEr systems with > 0, the results are analogous to those found in chronoamperometry and linear scan voltammetry (10, 64). For small A in zone DP, as i oo, (8.2.14) applies with n = 1. In zone KP with large A as / 0, (8.2.14) again holds with n = 2. The overall dependence of on A involves a rather complicated expression the general trend is similar to that in Figure 12.3.33. On current reversal, only B is oxidized near the forward wave, and the ratio of transition times r /rf as a function of A is shown in Figure 12.3.37. 

A simple expression is also available to test the simulated current in chronoamperometry for non-reversible processes ... 

In yet another variation the current is recorded at the initial potential following a series of pulses increasing in amplitude from the initial potential toward more negative values (i.e., the same excitation shown in Fig. 3.31 A). When the pulses reach sufficiently negative potentials to cause reduction of the species, the product of the reduction is oxidized at Ej, and the sampled anodic current is recorded unless the couple is totally irreversible. This is equivalent to a series of potential pulse chronoamperometry experiments with samples taken on the reverse step. Techniques of this sort have been used occasionally in reversibility studies and for identification of unstable products of electrode pro-... 

A more elaborate version of the chronoamperometry experiment is the symmetrical double-potential-step chronoamperometry technique. Here the applied potential is returned to its initial value after a period of time, t, following the application of the forward potential step. The current-time response that is observed during such an experiment is shown in Figure 3.3(B). If the product produced during a reduction reaction is stable and if the initial potential to which the working electrode is returned after t is sufficient to cause the diffusion-controlled oxidation of the reduced species, then the current obtained on application of the reverse step, ir, is given by [63]... 

They are applicable to electrodes of any shape and size and are extensively employed in electroanalysis due to their high sensitivity, good definition of signals, and minimization of double layer and background currents. In these techniques, both the theoretical treatments and the interpretation of the experimental results are easier than those corresponding to the multipulse techniques treated in the following chapters. Four double potential pulse techniques are analyzed in this chapter Double Pulse Chronoamperometry (DPC), Reverse Pulse Voltammetry (RPV), Differential Double Pulse Voltammetry (DDPV), and a variant of this called Additive Differential Double Pulse Voltammetry (ADDPV). A brief introduction to two triple pulse techniques (Reverse Differential Pulse Voltammetry, RDPV, and Double Differential Triple Pulse Voltammetry, DDTPV) is also given in Sect. 4.6. 

It is of interest at this point to compare the study of Multipulse Chronoamperometry and Staircase Voltammetry with those corresponding to Single Pulse Chronoamperometry and Normal Pulse Voltammetry (NPV) developed in Chaps. 2 and 3 in order to understand how the same perturbation (i.e., a staircase potential) leads to a sigmoidal or a peak-shaped current-potential response as the equilibrium between two consecutive potential pulses is restored, or not. This different behavior is due to the fact that in SCV the current corresponding to a given potential pulse depends on the previous potential pulses, i.e., its history. In contrast, in NPV, since the equilibrium is restored, for a reversible process the current-potential curve is similar to a stationary one, because in this last technique the current corresponding to any potential pulse is independent of its history [8]. 

The potential step methods are called chronoamperometry and like LSV and CV can deal with only the forward process (single step) or with the reverse process, involving the primary intermediate, as well. The latter is called double potential step chronoamperometry (DPSC) and is by far the most useful in kinetic studies. The applied potential-time wave form as well as the currenttime response for a reversible electrode process are illustrated in Fig. 3. The potential is stepped from a rest value where no current flows, usually 200-300 mV from the potential where the process of interest takes place, to one... 

Linear sweep voltammetry (LSV), also known as linear sweep chronoamperometry, is a potential sweep method where the applied potential (E) is ramped in a linear fashion while measming cnrrent (i). LSV is the simplest technique that uses this waveform. The potential range that is scanned begins at an initial or start potential and ends at a final potential. It is best to start the scan at rest potential, the potential of zero current. For a reversible couple, the peak potential can be calcnlated nsing equation (6). ... 

Figure 4-3. Electrochemical techniques and the redox-linked chemistries of an enzyme film on an electrode. Cyclic voltammetry provides an intuitive map of enzyme activities. A. The non-turnover signal at low scan rates (solid lines) provides thermodynamic information, while raising the scan rate leads to a peak separation (broken lines) the analysis of which gives the rate of interfacial electron exchange and additional information on how this is coupled to chemical reactions. B. Catalysis leads to a continual flow of electrons that amphfles the response and correlates activity with driving force under steady-state conditions here the catalytic current reports on the reduction of an enzyme substrate (sohd hne). Chronoamperometry ahows deconvolution of the potenhal and hme domains here an oxidoreductase is reversibly inactivated by apphcation of the most positive potential, an example is NiFe]-hydrogenase, and inhibition by agent X is shown to be essentially instantaneous.

Thus for a reversible wave, is independent of scan rate, and /p (as well as the current at any other point on the wave) is proportional to The latter property indicates diffusion control and is analogous to the variation of with t in chronoamperometry. A convenient constant in LSV is /p/u Co (sometimes called the current function), which depends on n and Z)q. This constant can be used to estimate n for an electrode... 

Potential-step chronoamperometry is a more convenient and accurate way of determining the rate of chemical reactions than cyclic voltammetry. The current responses for the transfer of FRTR+ shown in Figure 3 were analyzed quantitatively. The analysis of current -time curve for the E Ci mechanism is simplified when we examine the ratio of the current at in the reverse step to that at — x in the forward step, where t is the time of the potential reversal, as a function of t — r)/r [43]. The ratio for the Ej.C mechanism is given by [43]... 

Mercury electrodes are usually of spherical shape. In chronoamperometry of a simple, reversible electrode reaction Ox" +ne Red, using a hanging mercury drop electrode in an unstirred solution, the current density depends on time and electrode radius [5] ... 

Figure 19.3 Charging current and selection of step potential for chronoamperometry. (a) Potential waveform applied to the electrode in chronoamperometry. At i = 0, the potential is stepped from the initial value to a constant value (b) Dependence of faradaic current (if) and charging current (ij on time for a planar macroelectrode, (c) Dependence of if and on time for a UME. (d) Cyclic voltammogram at a UME showing and selection of Ef and E. (e) Cyclic voltammogram of a reversible redox couple at a macroelectrode showing and Ey and selection of and 3,. (J) Cyclic voltammogram of a quasi-reversible redox couple at a macroelectrode showing and and selection of E and...

Much more accurate measurements of diffusion coefficients can be obtained with LSV or CV using UMEs. These measurements are much less dependent on the electrochemical reversibility of the redox couple. Measurement of the diffusion-limited current from a voltammogram recorded at a microelectrode is demonstrated in Figure 19.3c. The concept is identical to that already discussed for chronoamperometry at UMEs at slow scan rates (i.e., long times) the current becomes steady state as long as the potential is well past Ey2-The dependence of D on the steady-state current is given by equation (19.3) for a hemispherical UME and equation (19.4) for a disk UME. The time considerations for CV are the same as those discussed above for chronoamperometry, except that the time is estimated from the scan rate and the difference between the final potential and Ey . 

The most useful equation in chronoamperometry is the Cottrell equation, which describes the observed current (planar electrode of infinite size) at any time following a large forward potential step in a reversible redox reaction (or to large overpotential) as a function of t. ... 

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