Voltammetric peaks, separation between

The first effect that one notices in the voltammetric response is that quasireversibility induces a separation between the forward and the reverse peaks i.e. the peak-to-peak separation A p) much greater than that of a reversible process (to determine this parameter correctly one must use instrumentation able to compensate for the solution resistance). 

The spatial uniformity of temperature in the cell is difficult to determine, and we are not aware of a careful study of this problem. In most experiments, it is the temperature of the electrode-solution interface or that of the diffusion layer that is relevant. A possible internal thermometer could be created by measuring a temperature-sensitive voltammetric function, for example, the peak separation in the cyclic voltammogram of a reversible reaction, which is 2.22RT/ F. The resolution is not likely to be outstanding, but such a technique would probably allow detection of serious differences between the thermocouple reading and the actual temperature of the electrode-solution interface. 

The cyclic voltammetric experiment can give a great deal of information about the redox activity of a compound and the stability and accessibility of its reduced or oxidised forms. For a fully chemically reversible process, ipa must equal rpc, i.e. all of the material oxidised at the electrode surface on the forward scan must be re-reduced on the reverse scan (or vice versa). If this condition does not hold true, then the process may be partially reversible (rpc < ipa) or irreversible (rpc = 0). Observation of processes that are not fully reversible implies decomposition or chemical reaction of the reduced or oxidised species and the ratio of ipa to /p(. will show a strong dependence on scan rate since the reverse current is related to the lifetime of the redox-generated material. Note that processes that are chemically reversible (in the sense that the reduced and oxidised species are both stable) may not be electrochemically reversible (a term that relates to the relative rates of forward and back electron transfer). Electrochemically reversible processes are characterised by a separation between the forward and reverse potential peaks of exactly 59 mV. 

Figure 8.4 illustrates the voltammograms with activated platinum electrodes for dissolved H2 in five solvents. The peak potential shifts with the basicity of the solvent and the separation between the potentials for the anodic and cathodic peaks reflects the unbuffered solvent matrix and the system s conformity to Eq. (8.2). The voltammogram for dissolved H2 in MeCN indicates severe adsorption effects, which precludes this solvent system for quantitative determinations via peak-current measurements. The other solvents yield anodic peak currents that are proportional to the concentration of dissolved H2 and its diffusion coefficient (DH2). The H2 concentration, in turn, is dependent on the partial pressure of dissolved H2 (PH2) and its solubility in a particular solvent. Figure 8.5 summarizes the voltammetric peak currents (tp a) as a function of PH2 in H20, Me2SO, DMF, and py. The slopes of the linear curves are proportional... 

Figure 8(a) shows a cyclic voltammetric curve obtained at BDD electrode in 0.5 M H2SO4. The fact that the separation between the cathodic and the anodic peaks (AEp) is very high (about 0.9 V) indicates that the Q/H2Q system is irreversible at the boron-doped diamond electrode. Furthermore, the apparent equilibrium redox potential of the couple Q/H2Q(Eo = 0.65 V) is much closer to the anodic peak potential than to the cathodic one. 

The kinetic barrier of the interface for electron-transfer between the species in solution and electrode was tested using the electroactive species such as the [Fe(CN)6]3" 4" couple [56], As expected, the [Fe(CN)6]3" 4" couple exhibits reversible behavior at the plain GC electrode. A quasi-reversible voltammetric response with very low peak currents and large peak-to-peak separation was observed when the plain GC electrode was modified with silicate sol-gel (MTMOS(SG)) [52a]. The introduction of gold nanoparticles in the MTMOS silicate sol-gel... 

Electrodes that featured a submonolayer (-15% of a monolayer) of 12 nm diameter Au colloids exhibited reversible cyt c voltammetry at scan rates of 50, 100, 200, and 500 mV s (Figure 13.3). Au particles in the submonolayer films can be thought of as electron antennae , efficiently funnel-ing electrons between the electrode and the electrolyte. However, the authors also reported a wide variation in voltammetric responses, with peak-to-peak separations ranging from 60 to 115 mV, among several similarly prepared electrodes. The heterogeneous rate constant calculated from the dependence of the peak-to-peak separation on scan rate was 7 x 10 cm s", which is in agreement with previously reported measurements obtained with bulk material electrodes. ... 

The need to measure concentrations in very small volumes is not restricted to biological systems. For example, open tubular columns for liquid chromatographic separations offer the advantage of increased resolution, but because their internal diameters may be as small as 15 pm, the amount of material in the eluted peaks is very small. Thus, the use of these columns requires detectors that can be used with low concentrations in small volumes. Jorgenson and co-workers showed that this could be accomplished by the insertion of a 10-pm-diameter, cylindrical electrode made from a carbon fiber into the end of the column [4]. The close fit between the column wall and the fiber ensured that a large fraction of the eluting molecules were electrolyzed. When the electrochemical data were collected in a voltammetric mode, the resolved compounds could be classi-... 

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