Oxidation-reduction electrodes system

Another type of reversible electrode involves an unattackable metal, such as gold or platinum, immersed in a solution containing an appropriate oxidized and reduced form of an oxidation-reduction system, e.g., Sn++++ and Sn++, or Fe(CN)6 and Fe(CN)6 the metal merely acts as a conductor for making electrical contact, just as in the case of a gas electrode. The reaction at an oxidation-reduction electrode of this kind is either oxidation of the reduced state or reduction of the oxidized state, e.g.,... 

Determination of Standard Oxidation-Reduction Potentials.—In principle, the determination of the standard potential of an oxidation-reduction system involves setting up electrodes containing the oxidized and reduced states at known activities and measuring the potential B by combination with a suitable reference electrode insertion of the value of B in the appropriate form of equation (3) then permits B to be calculated. The inert metal employed in the oxidation-reduction electrode is frequently of smooth platinum, clthough platinized platinum, mercury and particularly gold are often used. 

The positive electrode may also be regarded as a simple oxidation-reduction electrode involving the plumbous-plumbic system thus... 

In any electrochemical system, there are two electrodes, an anode and a cathode. During electrolysis, electrons are consumed at the cathode while they are generated at the anode. The reactions occurring at the individual electrodes are usually called the component electrochemical reactions, the combination being termed a redox, or oxidation-reduction, reaction system. Let us now examine the component electrochemical reactions involved in the overall reaction. 

In reactions of this kind, the metal of the electrode forms part of the reacting system [see equations (4.66)-(4.68)]. A similar effect can be observed in the case of oxidation-reduction redox) systems in which both components are in solution [equations (4.69) and (4.70)]. If an inert electrode (platinum, gold, graphite, etc.) is immersed in such a solution, it will acquire a potential due to a balance between the two competing electrode processes. 

Precious metal electrodes are used either as reference electrodes or for making electrical contact in oxidation-reduction systems. Platinum, the more commonly used metal, may be used in the shiny form (oxidation-reduction electrodes), the black form (hydrogen electrode and conductance cells to reduce polarisation errors) or the grey form (conductance cells). 

More than three dozen elements have been shown to be reversible to their ions in molten solvents of one sort or another. Although this total encompasses a variety of solvents, it is generally true that a satisfactory metal-metal ion electrode couple in one solvent will also be reversible in other melts, provided, of course, that the metal or its ion do not react irreversibly with other components of the melt. Included in these electrode systems are not only halogen-halide electrodes (on carbon), but also the more unusual 02/0 , CO2 + 02/C03, and NO2 4- 02/N03 couples using Pt, C, etc. as the electrode material. In addition, inert conductors such as Pt, C, and Ta usually function very well as oxidation-reduction electrodes sensitive to two oxidation states of a given metal in the melt. 

Two methods are used to measure pH electrometric and chemical indicator (1 7). The most common is electrometric and uses the commercial pH meter with a glass electrode. This procedure is based on the measurement of the difference between the pH of an unknown or test solution and that of a standard solution. The instmment measures the emf developed between the glass electrode and a reference electrode of constant potential. The difference in emf when the electrodes are removed from the standard solution and placed in the test solution is converted to a difference in pH. Electrodes based on metal—metal oxides, eg, antimony—antimony oxide (see Antimony AND ANTIMONY ALLOYS Antimony COMPOUNDS), have also found use as pH sensors (8), especially for industrial appHcations where superior mechanical stabiUty is needed (see Sensors). However, because of the presence of the metallic element, these electrodes suffer from interferences by oxidation—reduction systems in the test solution. 

The reductant differs from the oxidant merely by n electrons, and together they form an oxidation-reduction system. Consider the reversible reduction of an oxidant to a reductant at a dropping mercury cathode. The electrode potential is given by ... 

The titrations so far discussed in this chapter have been concerned with the use of a reference electrode (usually S.C.E.), in conjunction with a polarised electrode (dropping mercury electrode or rotating platinum micro-electrode). Titrations may also be performed in a uniformly stirred solution by using two small but similar platinum electrodes to which a small e.m.f. (1-100 millivolts) is applied the end point is usually shown by either the disappearance or the appearance of a current flowing between the two electrodes. For the method to be applicable the only requirement is that a reversible oxidation-reduction system be present either before or after the end point. 

The DAC system consisted of computer, interface cards, meters, transmitters, and solid state relays (SSR). Electrodes of pH (Ingold), Oxidation-Reduction Potential (Cole-Parmer), and Dissolved Oxygen (Ingold) were installed and connected to individual meter. The status of reactor and the value of electrode signal were displayed in a computer monitor, and stored in data file. 

Fig. 5.12 Steady-state concentration distribution of the oxidized and of the reduced form of an oxidation-reduction system in the neighbourhood of the electrode...

Electroanalytical techniques are an extension of classical oxidation-reduction chemistry, and indeed oxidation and reduction processes occur at the surface of or within the two electrodes, oxidation at one and reduction at the other. Electrons are consumed by the reduction process at one electrode and generated by the oxidation process at the other. The electrode at which oxidation occurs is termed the anode. The electrode at which reduction occurs is termed the cathode. The complete system, with the anode connected to the cathode via an external conductor, is often called a cell. The individual oxidation and reduction reactions are called half-reactions. The individual electrodes with their half-reactions are called half-cells. As we shall see in this chapter, the half-cells are often in separate containers (mostly to prevent contamination) and are themselves often referred to as electrodes because they are housed in portable glass or plastic tubes. In any case, there must be contact between the half-cells to facilitate ionic diffusion. This contact is called the salt bridge and may take the form of an inverted U-shaped tube filled with an electrolyte solution, as shown in Figure 14.2, or, in most cases, a small fibrous plug at the tip of the portable unit, as we will see later in this chapter. 

The reduction-oxidation potential (typically expressed in volts) of a compound or molecular entity measured with an inert metallic electrode under standard conditions against a standard reference half-cell. Any oxidation-reduction reaction, or redox reaction, can be divided into two half-reactions, one in which a chemical species undergoes oxidation and one in which another chemical species undergoes reduction. In biological systems the standard redox potential is defined at pH 7.0 versus the hydrogen electrode and partial pressure of dihydrogen of 1 bar. 

The circuitry used for the breadboard testing of NO and NOp sensor cells was very similar to that shown in Figure 2 only the applied potential was changed. An applied potential of +1.30 V versus the SHE reference electrode was used for NO oxidation while a potential of 0.75 V versus the same reference electrode was used for N02 reduction. Current measurements were again made by measuring the voltage drop across resistor RA. Three electrode systems were used for both gases. 

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