Electrodes poisoning

Electrochemically active compounds can be evaluated using a potentiometer to generate a cyclic voltammogram for the analyte. Cyclic voltammetry will allow the analyst to determine whether the compound can be oxidized or reduced, to choose the appropriate potential to use in the electrochemical detector, and to establish whether oxidation or reduction is irreversible. Irreversible oxidation or reduction of the analyte could be predictive of problems with electrode poisoning and reduced sensitivity of the electrochemical detector over time. Turberg et al. used EC detection at an applied potential of -1-600 mV to analyze for ractopamine. 

The reduction of C02 can be driven electrochemically at metallic cathodes, however, it requires large overpotentials (<—1.5 V) and electrode poisoning often occurs.65 Those problems can be addressed by adding catalysts. Metal complexes are a priori good candidates as electrocatalysts. It is expected that their reduction will be accompanied by the appearance of a vacant coordination site able to bind C02 and thus activate its reduction in the metal coordination sphere.1... 

From hereafter, we will neglect the electrochemical aspects of redox processes of adsorbed proteins, in order to consider in more detail redox processes governed by diffusion of the protein in solution. As mentioned in Chapter 2, Section 1.6, the case of adsorbed species is in some ways easier to treat, given the absence of the mathematical laws of mass transport, but in some cases it may be complicated by anomalous currents or electrode poisoning. 

However, if one writes the reaction in this way, a point is missed—the presence in the incompletely re-formed gas of the powerful electrode poison, CO. Thus, the re-forming reaction proceeds via the mechanism ... 

The derivation of the equation for the diffusion-limited current at the dropping mercury electrode differs from that of the other electrodes described above owing to its cyclic operation. Mercury flowing down through a fine capillary forms a drop at the bottom end of the capillary. This drop, the electrode, increases in size until it falls by the force of gravity the electrode is then renewed by formation of another drop. There are thus virtually no problems of electrode poisoning. The mode of... 

In order to make an electrode more selective we can arrange for only certain species to reach its surface, with the additional advantage of reducing electrode poisoning. This can be done by modifying the electrode surface (see next section), by a porous membrane touching the electrode or separated from it by a thin film of electrolyte, or by using a metallized membrane as indicator electrode. 

The benefits of electrode erosion in preserving electrode activity have been seen in the cases of electrogenerated polymers (Madigan et al., 1994), where the ultrasound punches holes through the film, the deposition of reduced methylviologen, which can otherwise passivate an electrode surface under silent conditions (Benahcene et al., 1995), and the oxidation of Cr(CO)6, where insonation counters electrode poisoning (Compton et al., 1994). 

A major difficulty with contact potential work is the provision of a completely inert reference electrode. Poisoned nickel has been used, but aged gold or platinum is more widely applicable. Bewig and Zisman have described reference electrodes of gold and platinum coated with Teflon resin. These electrodes were more stable than the bare metals in wet and dry oxygen, nitrogen and in carbon dioxide, hydrogen and helium. 

The available electrode area decreases with increasing polarization if a potential-dependent adsorption of a species occurs that completely inhibits the reaction and the extent of the adsorption increases with increasing overpotential for the reaction. The most prominent example of such an electrode poisoning is the formation of oxide layers in many metal dissolution reactions. 

Deterioration of the sensor dynamics changes the control state, especially if the response times lean/rich and rich/lean are asymmetrically increased. Adsorption of CO and HC and plugging of the protective layer will slow down the rich/lean response, while electrode poisoning (lead poisoning), which leads to reduced catalytic activity, does the same with the lean/rich response. Symmetrically delayed response times have no effect on the control state, but on the emissions by increasing the control amplitude, which diminishes the conversion rate of the catalyst [1]. 

Designed and tested advanced membrane technology that separates pure hydrogen from reformate, thereby enabling higher fuel cell power densities and eliminating potential for electrode poisoning. 

Obviously, electroanalytical techniques possess some disadvantages such as electrode poisoning, particle deposition on the sensing surface and problems arising from the chemical Irreversibility of some processes, all of which require a careful choice of the particular technique to be applied. 

On the other hand, amperometrlc measurements are confronted with the risk of electrode poisoning during the relatively long periods of time over which the electrodes are In contact with the unknown solution. Such a risk Is even greater with organic compounds and precipitation tltrlmetry. 

Electrode poisoning by the sample matrix and unwanted surface reactions may cause serious problems. 

Interpreted in terms of reduced electrode poisoning. Treatment of 3-0 -raethyl-D-glucose with lead hydroxide in water gave the isomeric 2-deoxy-3-Q-methyl-D-arabino-hexonic acid in the presence of sodium hydroxide, however, the products were a- and 3-gluco-meta-saccharinlc acld.- ... 

Most importantly, all the DAFCs described above are fully regenerated (same OCV and galvanostatic performance) upon replacement of the cell exhausts with fresh 2 M KOH solutions of methanol, ethanol or glycerol. This procedure was repeated several times with no apparent performance decay. In view of these results, Bianchini and cowoikers ascribed the polarizations shown in Fig. 27 to other factors than catalyst or electrode poisoning, i. e. the increasing viscosity of the solutions, the decreased OH" concentration and the competitive adsorption of substrate/partial oxidation product on the catalyst surface. 

Membrane Cells. Membrane cells are not subject to the electrode poisoning suffered by mercury cells. They are in this respect similar to diaphragm cells, but the membranes themselves are exceptionally sensitive to brine impurities [77], and brine specifications for membrane cells are more onerous. Section 4.8 discussed the structure and performance of membranes and explained the reasons for this sensitivity. Certain impurities can affect cell performance and the service life of the membranes even when present at ppb levels. Their concentrations in the brine must be rigidly controlled. When this is done successfully and ultra-pure brine is consistently available, service life can be quite long, and test cells have operated well for up to 9 years [78]. 

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