Reversible systems stationary electrodes

In Section 7.2, we looked at electroanalytical systems where the electrode rotates while the bulk of the solution remained still. In this present section, we will reverse this experimental concept by considering the case where it is the solution which flows - this time past a stationary electrode. Here, we shall be looking at flow ceils and channel electrodes. The principal mode of mass transport in both cases is convection, since the solution moves relative to the electrode. 

Figure 3.31 illustrates the simplest example of LAPV at a stationary electrode. In this case we assume that the time delay td between pulses is such that the initial condition is restored. This implies that all concentration profiles are completely relaxed before the next pulse is applied and every pulse initiates a new chronoamperometric experiment. The length of td required to accomplish this will depend on the chemical reversibility of the system, being shorter for reversible reactions and longer for irreversible reactions. 

Linear and cyclic sweep stationary electrode voltammetry (SEV) play preeminent diagnostic roles in molten salt electrochemistry as they do in conventional solvents. An introduction to the theory and the myriad applications of these techniques is given in Chapter 3 of this volume. Examples of the linear and cyclic sweep SEV current-potential responses expected for a reversible, uncomplicated electrode reaction are shown in Figures 3.19 and 3.22, respectively. The important equation of SEV, which relates the peak current, ip, to the potential sweep rate, v, is the Randles-Sevcik equation [67]. For a reversible system at some temperature, T, this equation is... 

DPV can be carried out quite successfully at a stationary electrode, such as a Pt disk or an HMDE, even though such systems do not allow physical renewal of the solution near the electrode with each measurement cycle. As we have seen above, the DPV method is based on the concept of using the preelectrolysis at potential E to establish apparent bulk concentrations, which are then interrogated with the pulse. If the system is kineti-cally reversible, the preelectrolysis can establish those conditions as well as at a renewed electrode, despite the fact that the effects of prior cycles are not erased from the diffusion layer. In fact, because the changes in potential from cycle to cycle are small, the cumulative effect of successive cycles is gradually to thicken the diffusion layer in a manner that supports the assumptions used in the treatment of wave shape and peak height given in Section 7.3.4(a). 

The previous two sections have dealt generally with ac voltammetry as recorded by the application of successive steps and with a renewal of the diffusion layer between each step. The DME permits the most straightforward application of that technique, but other electrodes can be used if there is a means for stirring the solution between steps so that the diffusion layer is renewed. On the other hand, this requirement for periodic renewal is inconvenient when one wishes to use stationary electrodes, such as metal or carbon disks, or a hanging mercury drop. Then one prefers to apply as a ramp and to renew the diffusion layer only between scans. In this section, we will examine the expected ac voltammograms for reversible and quasireversible systems when is imposed as a linear sweep and we will compare them with the results obtained above for effectively constant... 

This conclusion is very important because it implies that all relations and all qualitative conclusions presented in Section 10.5.1 also hold for linear sweep ac voltammetry of reversible systems at a stationary electrode. 

Nicholson RS, Shain I. Theory of stationary electrode polarogr hy. Single Scan and Cyclic Methods Applied to Reversible, Irreversible, and Kinetic Systems. Anal Chem 1964 37 706-723. 

Recently, stationary electrode polarography has been used to study the electrode kinetics of several metal-ion systems on mercury in DMSO solutions. In cadmium discharge, the rate-determining step was believed to be a chemical reaction, the loss of molecules of solvation from the reactant ion, preceding a reversible charge transfer. The sensitivity of the rate towards the electrolyte cation suggests that the slow step is essentially confined to the double layer. 

R. S. Nicholson and 1. Shain. Theory of stationary electrode polarography. Single scan and cychc methods applied to reversible, irreversible, and kinetic systems. Anal. Chem. 36, 706-23 (1964). 

Reinmuth has examined chronopotentiometric potential-time curves and proposed diagnostic criteria for their interpretation. His treatment applies to the very limited cases with conditions of semi-infinite linear diffusion to a plane electrode, where only one electrode process is possible and where both oxidized and reduced forms of the electroactive species are soluble in solution. This approach is further restricted in application, in many cases, to electrode processes whose rates are mass-transport controlled. Nicholson and Shain have examined in some detail the theory of stationary electrode polarography for single-scan and cyclic methods applied to reversible and irreversible systems. However, since in kinetic studies it is preferable to avoid diffusion control which obscures the reaction kinetics, such methods are not well suited for the general study of the mechanism of electrochemical organic oxidation. The relatively few studies which have attempted to analyze the mechanisms of electrochemical organic oxidation reactions will be discussed in detail in a following section. 

Two identical stationary micro-electrodes (usually platinum) across which a potential of 0.01-0.1 V is applied can be used in place of either the DME or the rotating platinum micro-electrode. The equivalence point is marked by a sudden rise in current from zero, a decrease to zero, or a minimum at or near zero (Figures 6.16(a), (b) and (c)). The shape of the curve depends on the reversibility of the redox reactions involved. The two platinum electrodes assume the roles of anode and cathode, and in all cases a current flows in the cell only if there is a significant concentration of both the oxidized and reduced forms of one of the reactants. In general, two types of system can be envisaged ... 

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