Flow electrodes

Amongst the greatest advantages of channel-flow electrodes is the possibility of controlling the rate of mass... 

The chaimel-flow electrode has often been employed for analytical or detection purposes as it can easily be inserted in a flow cell, but it has also found use in the investigation of the kinetics of complex electrode reactions. In addition, chaimel-flow cells are immediately compatible with spectroelectrochemical methods, such as UV/VIS and ESR spectroscopy, pennitting detection of intennediates and products of electrolytic reactions. UV-VIS and infrared measurements have, for example, been made possible by constructing the cell from optically transparent materials. 

The exchange current is directiy related to the reaction rate constant, to the activities of reactants and products, and to the potential drop across the double layer. The larger the more reversible the reaction and, hence, the lower the polarization for a given net current flow. Electrode reactions having high exchange currents are favored for use in battery apphcations. 

Alternatively, the solution can be moved, rather than the electrode, by a solution flow passing a stationary electrode. Flow electrodes are described in different setups and shapes. The main reasons for using these electrodes are that, depending on the flow rate, a steady state will be obtained (time-independent current signal), and that these electrodes are very useful for analysis of continuous flow of solution. A typical scheme for such an electrode setup is shown in Fig. 1.8. A very important condition that needs to be fulfilled in flow electrodes is that there is no turbulent behaviour because this would seriously disturb the measured signal. 

Historically, various sign conventions have been adopted for charge flow, electrode potential, and reaction direction. Benjamin Franklin arbitrarily called the charge... 

Flow electrodes. Rather than move the electrode past the solution, the sample solution can be flowed past a stationary electrode. The tubular platinum electrode (Figure 5.34) and the gold micromesh flow-through electrode (Figure 5.35) are both ingenious attempts to produce electrodes that are useful for the measurement of electroactive materials in a continuously flowing stream. Ap-... 

The flow electrode of Figure 5.36 is designed for external generation of a coulometric intermediate. This is most useful in situations where the sample contains some species that would interfere with the electrode reaction that generates the coulometric reactant. A substantial flow is required through the electrode to sweep the products of the reaction into the sample, and a design... 

Fig. 49. Current vs. time curves for two different experiments in which a single pit growing under potentiostatic control [9.8 V (Ag/AgCl)] was subjected to flow. Electrode, 304 stainless steel electrolyte, 0.1 m Na2S04, 0.2 M NaCl (pH = 3.5) [98]. Pe = Peclet number. Reproduced from Corros. Scl. 29, 31 (1989) by permission of the Editor.

Critical design parameters for these systems can be cast into idealized, quantitative design equations in order to define such factors as reactor volume-flow, electrode overpotential, and hold-up time, as functions of reactor design (40). 

E. B. Schalscha, T. Schirado, and I. Vergara, Flow Injection Analysis of Nitrate in Soil Extracts—Evaluation of a Nitrate-Selective Flow Electrode Method. J. Soil Sci. Soc. Amer., 45 (1981) 446. 

FIGURE 25-3 Some common types of commercial voltammetric electrodes (a) a disk electrode (b) a hanging mercury drop electrode (HMDE) (c) a microelectrode (d) a sandwich-type flow electrode. (Electrodes [a], [c]. and [d) courtesy of Bioanalyticai Systems. Inc.. West Lafayette. IN. with permission.)... 

In a galvanic cell, there is a reversal of the anode and the cathode in relation to electrolysis. In this case, the anode is the negative electrode, while the cathode is the positive electrode. The reason is that here electrons are freed on the anode, from where they move up in the external metallic circuit and then move down onto the cathode where they are taken up by chemical species in the electrolyte. In common with electrolysis, an oxidation half reaction occurs at the anode and a reduction half reaction occurs at the cathode. Figure 8.9 shows an electrolytic cell which draws electric current from a galvanic cell. Electron flow, electrode polarity, anodes and cathodes, and the half reactions (oxidations and reactions) that take place are shown. 

Fig. 1. Concentration changes near electrode surfaces during electrolysis. Quiet—unstirred solution flowing—electrode in flowing stream or well-stirred solution 81,2, etc.—diffusion layer thickness t—duration of electrolysis.

Many enzyme-catalyzed reactions involve a proton exchange with the medium so that a pH sensor is the simplest measuring system since it does not require a coupled indicator reaction. A flow electrode constructed on this basis indicated a linear relationship between pH and the substrate concentration [348]. 

Anderson JL, Moldoveanu S (1984) Numerical simulation of convective diffusion at a rectangular channel flow electrode. 

The cell voltage may be influenced by many process parameters including temperature, electrolyte composition, electrolyte flow, electrode material, form and surface condition. 

Fig. 2.2] Difitnbutipn of electrode potential and normalized current in a porous electrode. (a) and (b) Thin bed electrode, (c) and

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