Upstream electrode

Electrophoresis and electro osmosis can be used to enhance conventional cake filtration. Electrodes of suitable polarity are placed on either side of the filter medium so that the incoming particles move toward the upstream electrode, away from the medium. As most particles carry negative charge, the electrode upstream of the medium is usuaHy positive. The electric field can cause the suspended particles to form a more open cake or, in the extreme, to prevent cake formation altogether by keeping aH particles away from the medium. 

Both cell designs permit positioning of the second electrode downstream of the first working electrode (Fig. 11), which is known as the series configuration. This electrochemical transducer is used in the same manner as the classic ring-disk electrode. Products generated at the upstream electrode are detected (or collected) at the downstream electrode Selectivity is enhanced when the products of the upstream... 

Isocratic HPLC system with electrochemical detection (5010 Electrode, ESA, Chelmsford, USA). Upstream Electrode, 50 mV, downstream electrode, 450 mV. 

Double electrodes are particularly useful in kinetic studies. Intermediates produced on the generator (upstream) electrode are transported to the downstream electrode where they react further. This is useful for the study of short-lived species, the quantity reaching the second... 

Various circuits have been described to measure collection efficiencies based on galvanostatic control of the upstream electrode with the downstream electrode being held at the limiting current for the reaction taking place there. It is also possible to measure N0 by a potentiostatic shielding experiment. For, an irreversible electrode reaction, measurement of N0 in these two different ways will, in principle, give different results if the upstream electrode is not uniformly accessible. 

Figure 27.12 Timing diagram for voltammetric-amperometric detection. Wl, upstream electrode W2, downstream electrode Eu, upstream (Wl) potential Iu, upstream (Wl) current Id, downstream (W2) current t, time delay between electrodes. [Reprinted with permission from Ref. 25.]...

The power of electrochemical detection can be improved by using more than one working electrode (89). Different strategies, based primarily on dualelectrode detection, can thus be employed. For example, in the series mode (Fig. 3.30, top) the first upstream electrode can be used to generate an electroactive species that is then more easily detected at the downstream electrode. Discrimination against compounds with irreversible redox chemistry can also be achieved. Significantly improved qualitative information can be... 

In this last case the use of a double hydrodynamic electrode, generating R on the upstream electrode and detecting it on the downstream electrode, may be easier and more sensitive. The rotating disc electrode has also been used with success to distinguish similar mechanisms with coupled homogeneous reactions (ECE, DISP1, and DISP2)5. 

In this technique the upstream electrode (generator) is galvanostati-cally controlled to generate a species that reacts with species X in solution in a second-order homogeneous reaction, the detector electrode being used to quantify the fraction of the electrogenerated species that did not react ... 

A unique voltammetric detector has been developed using a series dual-electrode cell. In the detector, the upstream electrode s potential is scanned while the downstream electrode is used to monitor the redox reaction occurring at the upstream electrode without the charging current contributions. In essence, the upstream electrode is operated voltammetrically and the downstream electrode operated amperometrically so that the detector has been named a voltammetric-amperometiic detector. Detection limits of 10 M have been reported using this detection scheme. 

The use of a second downstream electrode to monitor chemical fluxes at the working electrode is proving to be an important technique for the investigation of electrode mechanisms. This is particularly true for electrodes which have a more complicated structure than a simple metallic surface. Examples are modified electrodes, oxide electrodes, or enzyme electrodes. For these more complex systems, the separate measurement of the fluxes at the electrolyte-electrode interface provides unique and valuable information. Double electrodes can be constructed for all three hydrodynamic systems. A crucial parameter for such a double electrode is the collection efficiency, N, which, in the steady state, relates the flux of material detected as a limiting current on the downstream electrode to the flux of material generated on the upstream electrode. The collection efficiency is a function of the geometry of the electrode and is given for all three systems by [4, 9]... 

Measuring from the upstream edge of the upstream electrode, the gap lies between and l2 and the downstream edge of the collecting electrode is at l3. 

Fig. 3. (a) A rapid transition in the number of electrons transferred (ntS) is observed at the uniformly accessible rotating disc electrode when the diffusion layer becomes sufficiently thin for the intermediate (shown stippled) to survive long enough to cross it (b) A more gradual transition is seen at the channel electrode, where some intermediate escapes (upstream electrode edge) and some is always trapped (downstream electrode edge), due to the shape of the diffusion layer. 

Implict in the above discussion is that, whereas for a reversible electrode reaction a plot of Idet vs. Igen will be linear as the voltammetric wave on the upstream electrode is scanned, the behaviour for an irreversible process will result in a slight curve as N changes from the value predicted by the Cook theory to the (slightly lower) value expressed by the Matsuda-Braun theory. 

Mechanism Upstream electrode Solution Downstream electrode Current density on upstream electrode... 

The function g(t) employed in the description of the current density on the upstream electrode for the Erev C process is given by... 

Fig. 40. Transport-limited current-flow rate behaviour for the ADMA (EC) system at (a) the upstream electrode (due to the oxidation of ADMA), (b) the downstream electrode (total cathodic current), and (c) the downstream electrode (reduction of oxidized ADMA). Electrode geometry is defined by x1 = 0.105, x2 = 0.129, x3 - 0.237 cm. From ref. 125.

Fig. 42. Investigation of anodic metal dissolution by application of a galvanostatic step to the upstream electrode. When the dissolution process is complicated by adsorption of the intermediate, the current transient at the detector electrode has a time lag longer than that without adsorption.

The extent to which the surface of the upstream electrode is covered by the adsorbed species can be evaluated by comparing the current-time curve at the detector electrode with that predicted under the condition where adsorption does not take place. The area between the two curves represents the total charge, AQ, of the adsorbed intermediate. Hence, the number of ions, Nt, adsorbed on the metal surface is... 

Tsuru et al. [129-131] applied the double channel electrode transient technique to the investigation of the anodic dissolution of iron in sulphate and chloride aqueous media, in the pH range 1-3. The upstream electrode served as the generator electrode at which the anodic dissolution reaction occurred. Overall this involves... 

The upstream electrode reaction was monitored at either a gold or glassy carbon detector electrode, which oxidized the incoming ferrous ions to ferric... 

Series dual electrodes are set up such that one electrode is in oxidative and the other is in reductive mode. The downstream electrode measures the products of the upstream electrode. The second electrode only responds to compounds which are converted reversibly. The redox product is more selectively detected. 

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