Diffusion-controlled oxidation

The current flowing through the electrode due to diffusion-controlled oxidation can be easily deduced from Eq. (37) ... 

The important concept in these dynamic electrochemical methods is diffusion-controlled oxidation or reduction. Consider a planar electrode that is immersed in a quiescent solution containing O as the only electroactive species. This situation is illustrated in Figure 3.1 A, where the vertical axis represents concentration and the horizontal axis represents distance from the electrodesolution interface. This interface or boundary between electrode and solution is indicated by the vertical line. The dashed line is the initial concentration of O, which is homogeneous in the solution the initial concentration of R is zero. The excitation function that is impressed across the electrode-solution interface consists of a potential step from an initial value E , at which there is no current due to a redox process, to a second potential Es, as shown in Figure 3.2. The value of this second potential is such that essentially all of O at the electrode surface is instantly reduced to R as in the generalized system of Reaction 3.1 ... 

To impose the diffusion-controlled conversion of O to R as described earlier, the potential E impressed across the electrode-solution interface must be a value such that the ratio Cr/Cq is large. Table 3.1 shows the potentials that must be applied to the electrode to achieve various ratios of C /Cq for the case in which Eq R = 0. For practical purposes, C /C = 1000 is equivalent to reducing the concentration of O to zero at the electrode surface. According to Table 3.1, an applied potential of -177 mV (vs. E° ) for n = 1 (or -88.5 mV for n = 2) will achieve this ratio. Similar arguments apply to the selection of the final potential. On the reverse step, a small C /Cq is desired to cause diffusion-controlled oxidation of R. Impressed potentials of +177 mV beyond the E° for n = 1 (and +88.5 mV for n = 2) correspond to Cr/Cq = 10"3. These calculations are valid only for reversible systems. Larger potential excursions from E° are necessary for irreversible systems. Also, the effects of iR drop in both the electrode and solution must be considered and compensated for as described in Chapter 6. 

A more elaborate version of the chronoamperometry experiment is the symmetrical double-potential-step chronoamperometry technique. Here the applied potential is returned to its initial value after a period of time, t, following the application of the forward potential step. The current-time response that is observed during such an experiment is shown in Figure 3.3(B). If the product produced during a reduction reaction is stable and if the initial potential to which the working electrode is returned after t is sufficient to cause the diffusion-controlled oxidation of the reduced species, then the current obtained on application of the reverse step, ir, is given by [63]... 

Because of the appearance of equation 65, B is called the parabolic rate constant. This limiting case is the diffusion-controlled oxidation regime that occurs when oxidant availability at the Si-Si02 interface is limited by transport through the oxide (thick-oxide case). 

For measurements in the feedback mode, the working solution contains one redox form of a quasi-reversible redox couple (R-> O+rce ). For the discussion of the working principle, it is assumed that initially only the reduced form R is present. This compound serves as electron mediator and is added typically in millimolar concentrations to an excess of an inert electrolyte2. The UME is poised at a potential sufficiently large to cause the diffusion-controlled oxidation of R. In solution bulk the... 

In our hands, EQCM studies of this system have confirmed previous reports of y-FeOOH deposition kinetics and the chemical reaction of ferrous ions with this film after a current interruption step. Figure 12.1 depicts the simultaneous transients of anodic current (Fig. 12.1(b)) and frequency shift (A/) when a potential step (Fig. 12.1(c)) is applied from a potential where there is no reaction on gold to a potential where diffusion controlled oxidation of ferrous ions takes place. The current transient shown in Fig. 12.1(b) can be described by a diffusion process since a linear dependence of the anodic current density with t 1/2 was found as predicted by the Cottrell equation ... 

Oxide type Valency of alloying element compared with parent metal Effect Diffusion-controlled oxidation rate... 

Cyclic voltammetry was carried out in the presence of penta- and hexacyano-ferrate complexes in order to probe the homogeneity and conductivity of the TRPyPz/CuTSPc films (125), (Fig. 36). When the potentials are scanned from 0.40 to 1.2 V in the presence of [Fe (CN)6] and [Fe CN)5(NH3)] complexes, no electrochemical response was observed at their normal redox potentials (i.e., 0.42 and 0.33 V), respectively. However, a rather sharp and intense anodic peak appears at the onset of the broad oxidation wave, 0.70 V. The current intensity of this electrochemical process is proportional to the square root of the scan rate, as expected for a diffusion-controlled oxidation reaction at the modified electrode surface. The results are consistent with an electrochemical process mediated by the porphyrazine film, which act as a physical barrier for the approach of the cyanoferrate complexes from the glassy carbon electrode surface. 

Chromia is thought to grow via outward diffusion of cahons from the metal to the oxide/ gas interface. When the scale formation proceeds by diffusion along short-circuit paths such as grain boundaries in the oxide layer, Eq. 1 describes the usual diffusion controlled oxidation kinetics ... 

Organic materials are generally removed by addition of powdered activated carbon. The carbon may be added at any point in the plant, although it is advantageous to have as much contact as possible. The adsorption reaction is slow at room temperature, since it is diffusion-controlled. Oxidation with chlorine, potassium permanganate, or ozone may destroy tastes and odors or it may intensify them, depending upon the particular compounds involved. For example, chlorination of phenolic compounds leads to gready increased tastes and odors. For this reason, the system must be studied in the laboratory prior to water treatment. 

Alex is an -> aluminum powder formed by explosion of electrically heated aluminum wires in inert atmospheres with particle sizes between 50 and 200 nm. Due to a passivation layer of a thickness between 2 to 4 nm a substantial amount of the particles is already converted to alumina the formation of which should be avoided by in-situ coating. In addition to the diffusion controlled oxidation at lower temperatures, a partial oxidation of the particles can occur by a fast chemically controlled reaction. Alex can increase the burning rate of solid composite rocket propellants up to a factor of 2. An increase of detonation velocity is not confirmed but Alex might improve -> air blast or fragment velocities of some high explosives. 

CVs for the oxidation of ferrate ions in alkaline solution are shown in Figure 10.. 5 (Licht et al., 2001), where an apparently irreversible diffusion-controlled oxidation process is recorded. A primary ferrate(VI) battery contains a Fe(VI) cathode and can use a zinc anode and an alkaline electrolyte such as a conventional alkaline battery. For Ag2FeO4, the general discharge reaction would be ... 

The normalized steady-state approach curve of tip current versus tip/sub-strate separation for the reduction of III(BF4) is shown in Figure 14. This was obtained with a 25 pm diameter Pt UME, biased at —0.8 V versus AgQRE to affect the diffusion-controlled reduction of III, as the probe was translated towards a 1 mm diameter Pt disk substrate, biased at 0.0 V versus AgQRE to promote the diffusion-controlled oxidation of IV. When analyzed in terms of EQ theory, the approach curve yielded a value k, = 145 s-1, which was in excellent agreement with that determined by cyclic voltammetry at sweep rates between 10 and 50 V s-1. 

Typical steady-state tip and substrate electrode responses for TG/SC voltammetric measurements, on the reduction of 28.2 mM fumaronitrile (FN), with a tip (a = 5 pm) - substrate (rs = 50 pm) separation of 1.8 pm are shown in Figure 19. These were obtained by scanning the tip (generator) potential at 100 mV/s through the FN/FN - wave while holding the substrate (collector) electrode at a potential to detect FN by diffusion-controlled oxidation to FN. The voltammograms provide a clear illustration of the ability of the SECM TG/SC mode to pick up relatively unstable intermediates with good sensitivity. 

FIG. 28 Dissolution rate image of the (010) surface of potassium ferrocyanide trihydrate recorded in the same area of the crystal as the topographic image shown in Figure 27. The tip was held at a potential to establish the diffusion-controlled oxidation of ferrocyanide and scanned at a speed of 50 gm s... 

Figure 35 shows a typical approach curve of long-time current, obtained in an SECM configuration, for the diffusion-controlled oxidation of Br to Bri as a function of tip-crystal separation for a 5 /xm diameter UME. The measured current has been normalized with respect to the steady-state current that flowed when the UME was placed far from the crystal surface. The... 

Simplified treatment of diffusion-controlled oxidation Metal Scale Gas... 

Figure 3.9 Simplified model for diffusion-controlled oxidation.

In the time period 70-180 min, the zone boundary moves in towards the center of the filament following a square root of time relationship, which corresponds to a diffusion-controlled oxidation of the cycled pol5mer. The original outer zone darkens further to black. 

The kinetics and mechanism of the oxidation of [Fe(phen)2(CN)2] by HNO have been reported. At low acidities ( 0.1M) in H2SO4, the rate equation is of the form Rate= [H+][HN02]IFe ]2, and the reaction proceeds via diffusion-controlled oxidation of the complex by NO. Under more concentrated conditions ([H2S04] 6M) protonation of the iron(ii) species to yield [Fe(phen)2-(CNH)2] is invoked in the interpretation of the rate variations. 

The electrode potential value, Ef, must be chosen so that the diffusion-controlled oxidation of Red is ensured. The duration of the potential step (time interval i of Ef) can be varied. The minimum time is limited by the ability of the potentiostat to charge the electrode. As is mentioned above, the measured current response, I, consists of Faradaic current and charging current, 1, which is undesirable for analytical purposes. However, 1 decays more rapidly than the Faradaic current as shown by the dashed curve in Fig. 4 and a suitable choice of the sampling time tg allows the sampling of the current free from 1. ... 

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