Voltammogram solution phase

The potential of the working electrode is ramped at a scan rate of v. The resultant trace of current against potential is termed a voltamnu ram. In linear-sweep voltammetry (LSV), the potential of the working electrode is ramped from an initial potential Ei to a final potential Ef (cf. Figure 6.2). Figure 6.12 shows a linear-sweep voltammogram for the reduction of a solution-phase analyte, depicted as a function of scan rate. Note that the jc-axis is drawn as a function of overpotential (equation (6.1)), and that the peak occurs just after = 0. 

Figure 6.13 shows a voltammogram for a simple solution-phase couple such a plot is known as a cyclic voltammogram (CV). The adjective cyclic arises from the closed loop drawn within the plot. The shape of the CV shown in Figure 6.13 is typical for a couple that is wholly reversible in the thermodynamic sense other simple diagnostic tests for electro-reversibility are listed in Table 6.3. 

For example, for two well-separated waves (AE < 0), is small, and reaction (12.3.39) lies to the left (i.e., the comproportionation of A and C dominates the disproportionation of B). Thus, at potentials of the second wave, C diffusing away from the electrode can reduce A diffusing towards it, so that the concentration profiles of A, B, and C are perturbed from those that would exist if the solution phase reaction did not occur. However, for the Ej-Ej. reaction scheme, the observed voltammogram is independent of the rates of the forward and back reactions in (12.3.39), because, at any given potential, the average oxidation state in any layer of solution near the electrode remains the same (53). At potentials of the second wave, species A, which would take two electrons, is removed by the comproportionation reaction, but two B molecules are produced, and each of these would take one electron for no net change. This is not true, however, if the heterogeneous rate constants for the electron transfer are slow (Section 12.3.7) or in ECE reactions (Section 12.3.8). 

Figure 11.1.4 compares the characteristic features of (a) a steady-state process, (b) a potential step experiment, and (c) a cyclic voltammogram. The steady-state experiment is independent of time and gives a sigmoidally shaped response. Most important is the extent to which the concentration profile penetrates into the solution phase. For a steady-state process, there is no time dependence and the diffusion layer thickness, 5, remains constant. In a chronoamperometric or potential step... 

Cyclic voltammetry can be used directly to establish the initial redox state of a compound if data analysis is applied in a careful manner [70]. In Fig. II.1.15, simulated and experimental cyclic voltammograms are shown as a function of the ratio of Fe(CN)g and Fe(CN) present in the solution phase. It can clearly be seen that the current at the switching potential, ix,a or fyc is affected by the mole fraction /Hred- Employing multi-cycle voltammograms at slow scan rates is recommended. Quantitative analysis of mixed redox systems with this method may be based on the plot shown in Fig. II. 1.15d. 

In order to improve the detection of short-lived intermediates, the potential step or chronoamperometric experiment can be replaced by a cyclic voltammet-ric experiment, which involves applying a triangular potential ramp. With a fast UVA is spectrometer, e.g. a diode array system, additional UVWis/NIR spectroscopic information as a function of the potential can be recorded simultaneously to the voltammetric data. However, recording cyclic voltammograms with the simple cell shown in Fig. II.6.4 is complicated by the presence of ohmic drop in the solution phase, which is amplified by poor cell design. In this kind of cell, the peak-to-peak separation in cyclic voltammograms of a reversible redox couple may increase by several hundreds of millivolts. Voltammetric data (and simultaneously recorded spectroscopic data) are therefore very difficult to interpret quantitatively. 

While the voltammograms can be quantitatively analyzed to yield permeant transport parameters - for example, via numerical simulations - we have instead utilized rotating disk electrode (RDE) voltammetry for this purpose. Sequential solution and film transport problems are generally well described by a variant of the Koutecky-Levich analysis [11]. This analysis represents the series process Idnetically as a reciprocal sum of pure solution-phase convection/dilFusion (cd) and pure film permeation (p) rates or fluxes. Since detection is electrochemical, the fluxes can be written in terms of currents, where ilim is the observed overall, mass-transport limited, current [12] ... 

Chronoamperometric studies show that the reductive desorption of adsorbed alka-nethiolates from a metal surface is an event that takes place in less than a second and, typically, in much shorter time [67]. Within this timescale, a nucleation-and-growth type treatment [67, 68, 69], and possibly, the ion penetration into the desorbed SAM [1] are needed to be considered. However, in conventional voltammetry of the scan rate that is less than 100 mV an equilibrium approach assuming an adsorption isotherm to correlate the amount of adsorbed species and its concentration in the adjacent solution phase is useful for interpreting the voltammograms [87, 70]. 

Cyclic voltammetric inspection of the permeation of electroactive species in thin layer immobilized on an electrode illustrated by the permeation of FcMeOH in PGMA brushes. Typical voltammograms of 3 mM FcMeOH solution at v = 20 mV s" on 0.03 cm Au surfaces showing (a) permeation in a 12 nm thick brush controlled by diffusive transport (here diffusion in the solution phase) or (b) permeation in a 110 nm thick brush steady-state transport controlled by the partition reaction at the brush-solution interface, (c) Interpretation of FcMeOH permeation into (from top to bottom) e = 12, 70, 110, and 170 nm thick PGMA brushes. Comparison with theoretical prediction of the different kinetic regimes observed for permeation in thin layers (transition from control by (i) solution diffusion to (ii) partition to (iii) diffusion in the layer). (Adapted from Matrab, T., et al., ChemPhysChem, 11, 670-682,2010.)... 

LCEC is a special case of hydrodynamic chronoamperometry (measuring current as a function of time at a fixed electrode potential in a flowing or stirred solution). In order to fully understand the operation of electrochemical detectors, it is necessary to also appreciate hydrodynamic voltammetry. Hydrodynamic voltammetry, from which amperometry is derived, is a steady-state technique in which the electrode potential is scanned while the solution is stirred and the current is plotted as a function of the potential. Idealized hydrodynamic voltammograms (HDVs) for the case of electrolyte solution (mobile phase) alone and with an oxidizable species added are shown in Fig. 9. The HDV of a compound begins at a potential where the compound is not electroactive and therefore no faradaic current occurs, goes through a region... 

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