Chronocoulometry is a transient electrochemical technique in which a single or double pulse of potential is applied to an electrolytic cell (Figure 12.1). The resulting response curve is the cumulative charge (Q) passing through the cell measured vs. time. As in voltammetry, cells contain reference and counter electrodes and a working electrode. The reference electrode is designed to hold a constant potential, and a potentiostatic circuit is used so that a variation in the potential applied to the cell changes the oxidizing or reducing power of the working electrode. With this basic arrangement, electrochemical events occurring at the working electrode are reflected in the charge vs. time data obtained.
For the simple electrode reaction
the typical form of single and double potential steps (Figure 12.1) shows that an intial potential (£,-) is chosen to be positive of E°', and the final potential of the first pulse is far enough negative of E°' that rapid, diffusion-controlled electrolysis takes place . In a double potential step experiment, the second pulse often returns to the initial potential.
The charge passing through an electrochemical cell is the integral of the current vs. time response. That is,
This integral may be obtained electronically or computed by numerical integration of the current-time data. The input potential waveform in chronocoulometry is identical to the that in chronoamperometry. The only difference between the techniques is that chronocoulometry outputs the charge, whereas chronoamperometry outputs current. Models for chrono-
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