Anodes, reduction process

Figure 21. Angular movement of the fee end of a bilayer during the flow of a cathodic current using the conducting polymer as cathode. A platinum sheet (left side of the picture) is used as anode. The reference electrode is observed at the bottom, a to e Movement during the reduction process e to a Movement under flow of an anodic current. The movement is stopped at any intermediate point (a, b, c, d, or e) by stopping the current flow, and this position is maintained for a long time without polarization.

Polymeric chains, and oxidation and reduction processes, a schematic, 344 Polymerization anodic, 555-556 chemical, 329... 

Figure 26 shows the redox potential of 40 monolayers of cytochrome P450scc on ITO glass plate in 0.1 KCl containing 10 mM phosphate buffer. It can be seen that when the cholesterol dissolved in X-triton 100 was added 50 pi at a time, the redox peaks were well distinguishable, and the cathodic peak at -90 mV was developed in addition to the anodic peak at 16 mV. When the potential was scanned from 400 to 400 mV, there could have been reaction of cholesterol. It is possible that the electrochemical process donated electrons to the cytochrome P450scc that reacted with the cholesterol. The kinetics of adsorption and the reduction process could have been the ion-diffusion-controlled process. 

Three general reaction types compare the activation-control reduction processes. In Fig. 25-12, in Case I, the single reversible corrosion potential (anode/cathode intersection) is in the active region. A wide range of corrosion rates is possible. In Case 2, the cathodic curve intersects the anodic curve at three potentials, one active and two passive. If the middle active/passive intersection is not stable, the lower and upper... 

The waste materials produced during the primary production of aluminum are fluoride compounds. Fluoride compounds are principally produced during the reduction process. One reason that prebaked anodes are favored is that the closure of the pots during smelting facilitates the capture of fluoride emissions, although many modern smelters use other methods to capture and recycle fluorides and other emissions. 

The fourth factor is the current density. At an inert anode and for 100% Faradaic efficiency for water oxidation, the density of the current controls the flux of H+ ions. The cathodic current density and the species available in its vicinity establish the efficiency of the reduction processes (Pb2+ —> Pb). These vary to a greater extent than the anode process, because the pH and the species reaching the cathode vary with processing time. Thus, control of the current density is critical to ensure optimal EO efficiency and contaminant removal. 

Electroanalytical techniques are an extension of classical oxidation-reduction chemistry, and indeed oxidation and reduction processes occur at the surface of or within the two electrodes, oxidation at one and reduction at the other. Electrons are consumed by the reduction process at one electrode and generated by the oxidation process at the other. The electrode at which oxidation occurs is termed the anode. The electrode at which reduction occurs is termed the cathode. The complete system, with the anode connected to the cathode via an external conductor, is often called a cell. The individual oxidation and reduction reactions are called half-reactions. The individual electrodes with their half-reactions are called half-cells. As we shall see in this chapter, the half-cells are often in separate containers (mostly to prevent contamination) and are themselves often referred to as electrodes because they are housed in portable glass or plastic tubes. In any case, there must be contact between the half-cells to facilitate ionic diffusion. This contact is called the salt bridge and may take the form of an inverted U-shaped tube filled with an electrolyte solution, as shown in Figure 14.2, or, in most cases, a small fibrous plug at the tip of the portable unit, as we will see later in this chapter. 

It is evident that at very negative potential values (compared to Eeq) the anodic component is zero, so that the current is due only to the reduction process. The inverse effect occurs for very positive potentials (compared to Eeq). On the other hand, moving away from the equilibrium potential in either direction, even only slightly, the current rises rapidly as a consequence of the exponential terms in the equation. However, at high values of q the current reaches a limiting value (yV), beyond which it can rise no more. This happens because the current is limited by the rate of the mass transport of the species Ox or Red from the bulk of the solution to the electrode surface, rather than from the rate of the heterogeneous electron transfer. Hence, one can say that the effect of the exponential factors in the equation is restrained by the ratios COx(0,t)/Cox and CRed(0,t)/C ed, which... 

When a disproportionation reaction occurs, the potential of the forward peak Ep shifts (for a reduction process) towards more cathodic values by 20/n (mV) for every ten-fold increase in the scan rate, whereas it shifts to more anodic values by 20/n (mV) for every ten-fold increase in the concentration of the Ox species... 

The high current intensity associated with the return peak relative to the second reduction process is characteristic of the so-called anodic stripping, which originates from the fast reoxidation of the metallic copper deposited on the electrode surface during the Cu+/Cu° reduction. 

A more useful chronoamperometric method involves a double potential step (somewhat reminiscent of the previously discussed reversal electrolysis). In this case, for instance for a reduction process, a first cathodic potential step is applied according to the preceding criteria (such that instantaneously COx(0,0 — 0), followed at a time x by the application of a second anodic potential step which causes the species Red previously generated at times < x to be instantaneously reoxidised (such that CRed(0,0 — 0). 

Generally, the reduction to the metallic state implies decomplexation of the bound ligands (hence, irreversible reduction), electrodeposition of metallic zinc on the electrode surface and consequently the anodic stripping process in the backscan. 

As shown, this highly distorted octahedral complex191 in MeCN solution displays a two-electron reduction (Ep = —1.37 V) that in the backscan exhibits the typical anodic stripping of the electrodeposed zinc metal. As the Zn(II) ion in MeCN solution (i.e. [Zn(MeCN)4]2+ ion) undergoes a two-electron reduction at Ep = -1.10 V, it is evident that complexation renders more difficult the reduction process.190... 

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