25.1: Potential Excitation Signals and Currents in Voltammetry

In voltammetry we apply a time-dependent potential to an electrochemical cell and measure the resulting current as a function of that potential.

Potential Excitation Signals

As shown in Figure \(\PageIndex{1}\), the potential may consist of (a) a linear scan or (b) a series of pulses. For the linear scan in (a), the direction of the scan can be reversed and repeated for additional cycles. The series of pulses in (b) shows just one of several different pulsed potential excitation signals; we will consider other pulse trains in the section on polarography.

Figure \(\PageIndex{1}\): Two examples of potential excitation signals and three examples of current responses. In (a) the potential varies linearly with time and, as shown by the dashed line, the direction of the scan may be changed and repeated. In (b) the potential is changed by applying a series of pulses. The nature of the current response depends on both the potential excitation signal and other experimental conditions, such as if the solution is stirred.

Current

The current responses in Figure \(\PageIndex{1}\) show the three common types of signals. In (c) and (d) the current is monitored directly as the potential is changed. In (e) a change in current is recorded using the current immediately before and after the application of a potential pulse. The current itself has three components: faradic current from the oxidation or reduction of the analyte, a charging current, and residual currents.

Faradaic Current

Faradic current is the result of oxidation or reduction of the analyte at the working electrode. The ease with which electrons move between the electrode and the species that reacts at the electrode affects the faradiac current. When electron transfer kinetics are fast, the redox reaction is at equilibrium. Under these conditions the redox reaction is electrochemically reversible and the Nernst equation applies. If the electron transfer kinetics are sufficiently slow, the concentration of reactants and products at the electrode surface—and thus the magnitude of the faradaic current—are not what is predicted by the Nernst equation. In this case the system is electrochemically irreversible.

Charging Currents

In addition to the faradaic current from a redox reaction, the current in an electrochemical cell includes other, nonfaradaic sources. Suppose the charge on an electrode is zero and we suddenly change its potential so that the electrode’s surface acquires a positive charge. Cations near the electrode’s surface will respond to this positive charge by migrating away from the electrode; anions, on the other hand, will migrate toward the electrode. This migration of ions occurs until the electrode’s positive surface charge and the negative charge of the solution near the electrode are equal. Because the movement of ions and the movement of electrons are indistinguishable, the result is a small, short-lived nonfaradaic current that we call the charging current. Every time we change the electrode’s potential, a transient charging current flows.

The migration of ions in response to the electrode’s surface charge leads to the formation of a structured electrode-solution interface that we call the electrical double layer, or EDL. When we change an electrode’s potential, the charging current is the result of a restructuring of the EDL. The exact structure of the electrical double layer is not important in the context of this text, but you can consult this chapter’s additional resources for additional information. See Chapter 22.1 for additional details.

Residual Current

Even in the absence of analyte, a small, measurable current flows through an electrochemical cell. In addition to the charging current discussed above, the residual current includes a faradaic current from the oxidation or reduction of trace impurities in the sample. Methods for discriminating between the analyte’s faradaic current and the residual current are discussed later in this chapter.