The basis of all controlled-potential techniques is the measurement of the current response to an applied potential. A multitude of potential excitations (including a ramp, potential steps, pulse trains, a sine wave, and various com-binations thereof) exists. The present chapter reviews those techniques that are widely used.
1.CHRONOAMPEROMETRY
Chronoamperometry involves stepping the potential of the working electrode from a value at which no faradaic reaction occurs to a potential at which the surface concentration of the electroactive species is effectively zero (Fig. 1a). A stationary working electrode and unstirred (quiescent) solution are used. The resulting current–time dependence is monitored. As mass transport under these conditions is solely by diffusion, the current–time curve reflects the change in the concentration gradient in the vicinity of the surface . This involves a gradual expansion of the diffusion layer associ-ated with the depletion of the reactant, and hence decreased slope of the con-centration profile as time progresses (see Fig. 1b). Accordingly, the current (at a planar electrode) decays with time (Fig. 1c), as given by the Cottrell equation
Figure 1 Chronoamperometric experiment: (a) potential–time waveform; (b) change in concentration profiles as time progresses; (c) the resulting current–time response.
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where n, F, A, C, D, and t are the number of electrons, Faraday’s constant, the surface area, the concentration, the diffusion coefficient, and time, respectively. Such an it1/2 constancy is often termed a “Cottrell behavior.” Deviations from such behavior occur at long times (usually over 100 s) as a result of natural convection effects, due to coupled chemical reactions, and when using non-planar electrodes or microelectrodes with high perimeter : area ratio . In the latter case, a time-independent current (proportional to the concentration) is obtained for t > 0.1 s, due to a large radial diffusion con-tribution. Similar considerations apply to spherical electrodes whose current response following potential step contains time-dependent and time-independent terms . Recall also that for short values of t (t < 50 ms), the chronoamperometric signal contains an additional background contribu-tion of the charging current . This exponentially decaying charging current represents the main contribution to the response in the absence of an electroactive species. Chronoamperometry is often used for measuring the diffusion coefficient of electroactive species or the surface area of the working electrode. Some analytical applications of chronoamperometry rely on pulsing of the potential of the working electrode repetitively at fixed time intervals. Some popular test strips for blood glucose involve potential-step measurements of an enzymatically liberated product (in connection with a preceding incubation reaction). Chronoamperometry can also be applied to the study of mechanisms of electrode processes. Particularly attractive for this task are reversal double-step chronoamperometric experi-ments (where the second step is used to probe the fate of a species generated in the first one). 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 (Qdl ) and reaction of the adsorbed species (Qi ): 1-2
Such a charge measurement procedure, known as chronocoulometry , is par-ticularly useful for measuring the quantity of adsorbed reactants (because of the ability to separate the charges produced by the adsorbed and solution species). A plot of the charge ( Q) versus t 1/2 , known as an Anson plot , yields an intercept at t = 0 that corresponds to the sum of Qdl and Qi . The former can be estimated by subtracting the intercept obtained in an identical experiment carried out in the blank solution.
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