Double potential step experiments

Figure 6. A plot of the logarithm of the square root of the initial slope of the current time transients determined in the double step experiments against the final potential for the three single crystal surfaces investigated.

The simplest controlled potential experiment is the potential step [34] illustrated in Fig. 15. Such experiments are sometimes termed chrono-amperometry , signifying that the current (-ampero-) is measured (-metry) as a function of time (chrono-). Sometimes, two steps, as in a double-step experiment [34] [Fig. 16(a)], or a sequence of small steps, as in staircase voltammetry [35—37] [Fig. 16(b)], are applied. When the potential of the working electrode is changed by a step for only a brief period of time before being returned to its original (or near to its original) value, we speak of a pulse . There are many varieties of pulse voltammetry [38—41], some of which are discussed in Chap. 4. 

The shape of the response function and the height of the peak can be treated quantitatively in a straightforward manner. Note that the events during each drop s lifetime actually comprise a double-step experiment. From the birth of the drop at r = 0 until the application of the pulse at = r, the base potential E is enforced. At later times, the potential is E + A ", where A " is the pulse height. Each drop is bom into a solution of the bulk composition, but generally electrolysis occurs during the period before r and the pulse operates on the concentration profiles that prior electrolysis creates. This situation is analogous to that considered in Section 5.7, and it can be treated by the techniques developed there. Even so, we will not take that approach, because the essential simplicity of the problem is obscured. 

Chronoamperometry is often used for measuring the diffusion coefficient of electroactive species or the surface area of the working electrode. Analytical applications of chronoamperometry (e.g., in-vivo bioanalysis) rely on pulsing of the potential of the working electrode repetitively at fixed tune intervals. Chronoamperometry can also be applied to the study of mechanisms of electrode processes. Particularly attractive for this task are reversal double-step chronoamperometric experiments (where the second step is used to probe the fate of a species generated in the first step). 

Figure 48 Chronoamperometric double potential step experiment for the process Ox + ne Red inversion time t = 0.2 s. The top part shows the perturbation of the potential applied to the working electrode with time...

Figure 49 The variation of the current ratio in a double potential step experiment for a simple reduction process (t > t)...

Chronopotentiometry is a transient constant-current technique in which the potential of the electrode is followed, as a function of time, in a quiet solution (Figure 6). Double-step applications [30], as well as programmed current experiments [31] have been described. 

Several detailed studies of electrochemiluminescence emission under controlled potentials have been conducted.11 13,63"67 These have been double potential step experiments where one of the ion radicals was first generated at a potential slightly more negative (or positive, for cation generation) than its half-wave potential for a short time period, and then the potential at the electrode was switched to some... 

Double potential step experiments have provided useful information on some details of the electrochemiluminescence phenomenon. As expected, they have shown that emission is not generated on potential reversal when the initially generated ion is not sufficiently stable to be detected with cyclic voltammetry. Furthermore, current reversal with... 

Equation (171) would allow rD to be determined if Qd) were known. An approximate estimate of Qdl could be obtained from a blank experiment, stepping from Ei to E in the absence of O. However, since qM ( ,) will be a function of T0, this procedure is not rigorous. A much better, or even rigorous, correction for Qdl can be performed by means of the double potential step chronocoulogram [47, 137]. In this method, the potential is stepped back from Ef to E after a period r and the charge Q t > r) is measured as a function of time. It needs some algebraic manipulation [137] to derive the fact that a plot of Q (t > r) vs. 6 = r1/2 + (t — t)v2 — f)/2 is a straight line and has an intercept equal to... 

Potential step experiments (chronoamperometry and double potential step chronoamperometry)... 

Figure 5 Concentration-distance profiles for reactants generated in double potential step experiments involving initial oxidation of reagent A followed by reduction of A. (From Ref. 36.)...

The potential-step experiment can also be used to record the charge-time dependence. This is accomplished by integrating the current resulting from the potential step and adding corrections for the charge due to the double-layer charging (0) and reaction of the adsorbed species (Qi) ... 

Figure 6 shows that the amount of reduction taking place depends upon the amount of oxidation that occurred initially at Ei= 1.000 volt. To help evaluate electron transfer, double potential step chronoamperometry was employed (Figure 10). In each instance the potential was stepped from to E2 +1.000 volt at the instant the cell was engaged. Initial oxidation proceeded for 25 seconds after which the potential stepped back to the starting potential E. In the short time of the experiment it was assumed that diffusion effects were minimal and the reduction at Ef acted on the oxidized product formed at E2 until it was consumed.

Figure 5.7.1 General waveform for a double potential step experiment.

The control error in a fast experiment may be a transient problem existing only during brief periods of high current flow. Consider a step experiment on the equivalent circuit shown in Figure 15.6.1a, in which the working interface has only a capacitance representing the double layer. Even if an ideal control circuit exists so that e f is instantaneously stepped (from e.g., 0 V), there will be a lag in the true potential, because iR is nonzero while the double layer is charging. The actual relation (see Problem 15.8) is... 

Figure 17.1.3 Responses at a 2000 wire-per-inch gold minigrid during a double potential step experiment. The solution contained 0.8 mM o-tolidine in 1 M HCIO4-O.5 M acetic acid. In the forward step a-tohdine was oxidized in a diffusion-controlled, two-electron process. The stable product was...

In chronoamperometric (CA) experiments (see Fig. 3) the working electrode potential is changed instantaneously from the initial potential to the first step potential, and it is held at this value for the first step time. This is a single potential step experiment. In a double potential step experiment, the potential is changed to the second step potential after the first step time, and it is then held at this value for the second step time. The current is monitored as a function of time. 

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