Experimental techniques

The kind of output from linear-sweep voltammetry is illustrated in Fig. 25.43. Initially, the absolute value of the potential is low, and the cathodic current is due to the migration of ions in the solution. However, as the potential approaches the reduction potential of the reducible solute, the cathodic current grows. Soon after the potential exceeds the reduction potential the current rises and reaches a maximum value (as specified in eqn 25.57). This maximum current is proportional to the molar concentration of the species, so that concentration can be determined from the peak height after subtraction of an extrapolated baseline. In differential pulse voltammetry the current is monitored before and after a pulse of potential is applied, and the processed output is the slope of a curve like that obtained by linear-sweep voltammetry (Fig. 25.44). The area under the curve (in effect, the integral of the derivative displayed in the illustration) is proportional to the concentration of the species. In cyclic voltammetry the potential is applied in a sawtooth manner to the work ing electrode and the current is monitored. A typical cyclic voltammogram is shown in Fig. 25.45. The shape of the curve is initially like that of a linear sweep experiment, but after reversal of the sweep there is a rapid change in current on account of the high concentration of oxidizable species close to the electrode that were generated on the reductive sweep. When the potential is close to the value required to oxidize the reduced species, there is a substantial anodic current until all the oxidation is complete, and the current returns to zero. When the reduction reaction at the electrode can be reversed, as in the case of the [Fe(CN)6]3−/[Fe(CN)6]4−couple, the cyclic voltammogram is broadly symmetric about the standard potential of the couple (as in Fig. 25.45b). The scan is initiated with [Fe(CN)6]3− present in solution and, as the potential approaches E7 for the couple, the [Fe(CN)6]3− near the electrode is reduced and current begins to flow. As the potential continues to change, the cathodic current begins to decline again because all

Fig. 25.43 The change of potential with time and the resulting current/potential curve in a voltammetry experiment. The peak value of the current density is proportional to the concentration of electroactive species (for instance, [Ox]) in solution.

Fig. 25.44 A differential pulse voltammetry experiment. (a) The potential is swept linearly as a mercury droplet grows on the end of a capillary dipping into the sample and then pulsed as shown by the purple line. The resulting current is shown as the blue line and is sampled at the two points shown. (b) The data output is obtained as the difference of the currents at the two sampled points.

Fig. 25.45 (a) The change of potential with time and (b) the resulting current/potential curve in a cyclic voltammetry experiment.

the [Fe (CN)6]³⁻ near the electrode has been reduced and the current reaches its limiting value. The potential is now returned linearly to its initial value, and the reverse series of events occurs with the [Fe (CN)6]⁴⁻ produced during the forward scan now undergoing oxidation. The peak of current lies on the other side of E7, so the species present and its standard potential can be identified, as indicated in the illustration, by noting the locations of the two peaks. The overall shape of the curve gives details of the kinetics of the electrode process and the change in shape as the rate of change of potential is altered gives information on the rates of the processes involved. For example, the matching peak on the return phase of the sawtooth change of potential may be missing, which indicates that the oxidation (or reduction) is irreversible. The appearance of the curve may also depend on the timescale of the sweep for, if the sweep is too fast, some processes might not have time to occur. This style of analysis is illustrated in the following example.

Example 25.5 Analysing a cyclic voltammetry experiment

The electroreduction of p-bromonitrobenzene in liquid ammonia is believed to occur by the following mechanism:

BrC₆H₄NO₂+e⁻→BrC₆H4NO₂ ^–

 BrC6H4NO₂ ⁻ →·C₆H₄NO₂+Br⁻ ·

C₆H₄NO₂+e⁻→C₆H₄NO^{2 –}

 C₆H₄NO₂ − +H+→C₆H₅NO₂

Suggest the likely form of the cyclic voltammogram expected on the basis of this mechanism. Method Decide which steps are likely to be reversible on the timescale of the potential sweep: such processes will give symmetrical voltammograms. Irreversible processes will give unsymmetrical shapes because reduction (or oxidation) might not occur. However, at fast sweep rates, an intermediate might not have time to react, and a reversible shape will be observed. Answer At slow sweep rates, the second reaction has time to occur, and a curve typical of a two-electron reduction will be observed, but there will be no oxidation peak on the second half of the cycle because the product, C6H5NO2, cannot be oxidized (Fig. 25.46a). At fast sweep rates, the second reaction does not have time to take place before oxidation of the BrC₆H₄NO₂⁻ intermediate starts to occur during the reverse scan, so the voltammogram will be typical of a reversible one electron reduction (Fig. 25.46b).

Self-test 25.9 Suggest an interpretation of the cyclic voltammogram shown in Fig. 25.47. The electroactive material is ClC6H4CN in acid solution; after reduction to ClC6H4CN−, the radical anion may form C6H5CN irreversibly.

[ClC6H4CN+e−⇌ ClC₆H₄CN−, ClC6H4CN⁻+H⁺+e⁻→C₆H₅CN+Cl⁻, C₆H₅CN+e−⇌ C₆H₅CN⁻]

Fig. 25.46 (a) When a non-reversible step in a reaction mechanism has time to occur, the cyclic voltammogram may not show the reverse oxidation or reduction peak. (b) However, if the rate of sweep is increased, the return step may be caused to occur before the irreversible step has had time to intervene, and a typical ‘reversible’ voltammogram is obtained.

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Units

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Working galvanic cells

Electrolysis

The rate laws

The electric potential at the interface

The structure of the interface

Catalytic activity at surfaces

Mechanisms of heterogeneous catalysis

Mobility on surfaces

The rate of desorption

The rate of adsorption