General Characteristics of Voltammetry

Voltammetric methods are a collection of electroanalytical techniques in which the potential of an electrolytic cell is varied toward negative or positive values and the current is measured [1,2]. The output of the experiment is the current vs. the potential applied to the cell. Typical cells consist of 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. One example of voltammetry was discussed in Section 3.B.I.

Different types of voltammetry are characterized by the different types of potential waveforms applied to the cell. For example, if the applied potential is varied linearly and the current measured continuously, the method is called linear sweep voltammetry. If a series of pulses of increasing heights is applied to the cell and the current measured at the end of each pulse, the technique is called normal pulse voltammetry. There are a wide variety of voltammetric methods [1]. Characteristics of some of those discussed in this chapter are summarized in Table 11.1.

A typical input potential waveform for a steady state voltammetric method such as rotating disk voltammetry is shown in Figure 11.1. The slope of this line in mV s"1 is called the scan rate. For a reversible, fast electron transfer reaction from reductant R to the electrode to yield oxidant O:

The current vs. potential ouput has the shape in Figure 11.2. E°', called the formal potential, is the apparent standard potential of the redox couple O/R measured in the solution of interest. Figure 11.2 points out the position

Table 11.1 Characteristics of Voltammetric Methods

Potential input

Current

Type of voltammetry

to the cell

measurement

Comments

Linear sweep

Linear variation, one direction

Continuous

Stationary electrode

Rotating disk

Linear variation, one direction

Continuous

Rotating electrode

Normal pulse

Increasing pulses

End of each pulse

Decreased charging current

Polarography"

Linear variation, one direction

Continuous

Dropping mercury electrode

Cyclic

Linear variation, one direction; then reverses scan

Continuous

Stationary electrode

Square wave

Forward-reverse square pulse on top of a staircase

End of each pulse

Best signal to noise and resolution

Differential pulse

Forward-reverse square pulse on top of a ramp

End of each pulse

Good signal to noise and resolution

Ultramicroelectrodefe

Any of the preceding

Either of preceding

Electrode has dimension in ¡1.m range

" Any voltammetric method using a dropping mercury electrode as the working electrode is a type of polarography; for example, normal pulse polarography or square wave polarog-raphy.

b Discussed in detail in Section B.2.

Figure 11.1 Typical potential ramp applied to an electrolytic cell for techniques such as rotating disk voltammetry and linear sweep voltammetry.

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Figure 11.2 Current vs. potential output for a reversible oxidation in a steady state volta-metric method.

of the half-wave potential Em as well as the definition of the so-called limiting current ih which is proportional to the amount of R dissolved in solution.

Note that the convention used for plotting voltammograms in this book has cathodic current as positive and anodic current as negative, and a potential scale that is negative to the right and positive to left [1, 2], This convention is

Figure 11.3 shows the input potential waveform for normal pulse voltammetry. The current is measured at the end of each pulse of applied potential. The shape of a reversible normal pulse current vs. potential ouput is the same as in Figure 11.2, except that the data are discrete points from one current measurement at the end of each pulse. Figure 11.4 shows the typical output from a reversible normal pulse voltammogram (NPV). In general, all pulse volammetric methods have a discrete digital output.

Sign convention used for plotting voltammograms cathodic current (reductions)

(oxidations) anodic current

W" 200

Figure 11.3 Typical pulsed potential input to an electrolytic cell for normal pulse voltamme-try (NPV).

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