Other Electrodes

Figure 11.39 summarizes the reactions taking place in this amperometric sensor. FAD is the oxidized form of flavin adenine nucleotide (the active site of the enzyme glucose oxidase), and FAD1T2 is the active site s reduced form. Note that O2 serves as a mediator, carrying electrons to the electrode. Other mediators, such as Fe(CN)6 , can be used in place of O2. 

It would be preferable to incorporate both fluorescent and electron transport properties in the same material so as to dispense entirely with the need for electron-transport layers in LEDs. Raising the affinity of the polymer facilitates the use of metal electrodes other than calcium, thus avoiding the need to encapsulate the cathode. It has been shown computationally [76] that the presence of a cyano substituent on the aromatic ring or on the vinylene portion of PPV lowers both the HOMO and LUMO of the material. The barrier for electron injection in the material is lowered considerably as a result. However, the Wessling route is incompatible with strongly electron-withdrawing substituents, and an alternative synthetic route to this class of materials must be employed. The Knoevenagel condensation... 

Fig. 3. Steady state concentration profiles of catalyst and substrate species in the film and diffusion layer for for various cases of redox catalysis at polymer-modified electrodes. Explanation of layers see bottom case (S + E) f film d diffusion layer b bulk solution i, limiting current at the rotating disk electrode other symbols have the same meaning as in Fig. 2 (from ref.

Some electrodes are made of substances that participate in the redox reactions that transfer electrons. These are active electrodes. Other electrodes serve only to supply or accept electrons but are not part of the redox chemistry these are passive electrodes. In Figure 19-7. both metal strips are active electrodes. During the redox reaction, zinc metal dissolves from the anode while copper metal precipitates at the cathode. The reactions that take place at these active electrodes are conversions between the metals contained in the electrodes and their aqueous cations. 

Aromatic nitro and nitroso compounds are easily reduced at carbon and mercury electrodes. Other nitro compounds such as nitrate esters, nitramines, and nitrosamines are also typically easily reduced. The complete reduction of a nitro compound consists of three two-electron steps (nitro-nitroso-hydroxylamine-amine). Since most organic oxidations are only two-electron processes, higher sensitivity is typically found for nitro compounds. Several LCEC based determination of nitro compounds have been reported... 

Where a working electrode other than the dropping mercury electrode is used, the electrode material is indicated by brackets. 

Schematic diagram of a gas-sensing electrode. Selective ion electrode is shown as a glass electrode. The reference electrode is an Ag-AgCl electrode. Other electrode combinations are possible.

The SHE is experimentally inconvenient, so potentials are often measured and quoted with respect to reference electrodes other than the SHE. By far the most common reference is the saturated calomel electrode (SCE). We will usually make our choice of reference on the basis of experimental convenience. 

In addition, potentiometric titration methods exist in which an electrode other than an ion-selective electrode is used. A simple platinum wire surface can be used as the indicator electrode when an oxidation-reduction reaction occurs in the titration vessel. An example is the reaction of Ce(IV) with Fe(II) ... 

Another consequence of pore geometry is that for crystalline electrodes, other crystal planes are exposed to the electrolyte at the pore tip than at the pore walls. The dependence of pore growth on crystal orientation of the silicon electrode is discussed in Chapters 8 and 9. 

Pore Formation in Semiconductor Electrodes Other Than Si... 

We see that calculations with the SHE are so easy as to be almost trivial. In practice, however, the SHE is so difiicult to operate experimentally, and not particularly safe because it involves elemental hydrogen gas, that we avoid the SHE if it is at all possible. Instead, we employ a secondary reference electrode, where the word secondary implies a reference electrode other than the SHE, but for which the potential is known with respect to the SHE. 

In addition to solid-state electrodes, other ISEs operate with a polymer membrane, with a good example being the calcium electrode described below in Section 3.5.2.3. 

We then described how ion-selective electrodes (ISEs) function by means of generating an emf that is related to the activity of a single ion. The simplest ISEs are glass electrodes for pH determination, and the related pH combination electrodes. Other solid-state ISEs, such as the fluoride electrode, function in the same way. 

Therefore, the ISE potential depends on the CO2 partial pressure with Nernstian slope. Contemporary microporous hydrophobic membranes permitted the construction of a number of gas probes, developed mainly by the Orion Research Company (for a survey see [143]. The most important among these sensors is the ammonia electrode, in which ammonia diffusing through the membrane affects the pH at a glass electrode. Other electrodes based on similar principles respond to SO2, HCN, H2S (with an internal S ISE), etc. The ammonia probe has a better detection limit than the ammonium ion ISE based on the non-actin ionophore. The response time of gas probes depends mostly on the rate of diffusion of the test gas through the microporous medium [77,143]. 

How analytical methods deal with interferences is one of the more ad hoc aspects of method validation. There is a variety of approaches to studying interference, from adding arbitrary amounts of a single interferent in the absence of the analyte to establish the response of the instrument to that species, to multivariate methods in which several interferents are added in a statistical protocol to reveal both main and interaction effects. The first question that needs to be answered is to what extent interferences are expected and how likely they are to affect the measurement. In testing blood for glucose by an enzyme electrode, other electroactive species that may be present are ascorbic acid (vitamin C), uric acid, and paracetamol (if this drug has been taken). However, electroactive metals (e.g., copper and silver) are unlikely to be present in blood in great quantities. Potentiometric membrane electrode sensors (ion selective electrodes), of which the pH electrode is the... 

Some Ions Stuck to the Electrode, Others Scattered in Thermal Disarray The Stem Model... 

Voltammetry is a term used to include all the methods that measure current-potential curves (voltammograms) at small indicator electrodes other than the DME [6], There are various types of voltammetric indicator electrodes, but disk electrodes, as in Fig. 5.17, are popular. The materials used for disk electrodes are platinum, gold, graphite, glassy carbon (GC), boron-doped diamond8, carbon paste, etc. and they can be modified in various ways. For electrode materials other than mercury, the potential windows are much wider on the positive side than for mercury. However, electrodes of stationary mercury-drop, mercury-film, and mercury-pool are also... 

At a platinum electrode that is slightly activated by platinization, the dissolved hydrogen in various solvents is oxidized nearly reversibly by H2= 2H+ + 2c. We can determine by cyclic voltammetry the standard potential of this process. If the standard potentials in various solvents are compared using a common potential scale, the Gibbs energies of transfer of H+ can be obtained [44], With electrodes other than platinum, it is difficult to observe reversible oxidation of dissolved hydrogen [44 b],... 

Solid electrodes, including metals and semiconductors, have attracted attention since the 1950s and extensive research has been done by Vetter, Gerischer, Bockris, Conway, Parsons, Yeager, etc. The development of hydrodynamic electrodes other than the DME in the late fifties and sixties... 

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