Electrodes, oxidation-reduction chemical potential

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. 

ORP Oxidation reduction potential - the degree of completion of a chemical reaction by detecting the ratio of ions in the reduced form to those in the oxidized form as a variation in electrical potential measured by an ORP electrode assembly. OSHA The Williams-Steiger Occupational Safety and Health Act of 1970 (OSHA) is a law designed to protect the health and safety of industrial workers and treatment plant operators. It regulates the design, construction, operation and maintenance of industrial plants and wastewater treatment plants. The Act does not apply directly... 

The standard electrode potentials , or the standard chemical potentials /X , may be used to calculate the free energy decrease —AG and the equilibrium constant /T of a corrosion reaction (see Appendix 20.2). Any corrosion reaction in aqueous solution must involve oxidation of the metal and reduction of a species in solution (an electron acceptor) with consequent electron transfer between the two reactants. Thus the corrosion of zinc ( In +zzn = —0-76 V) in a reducing acid of pH = 4 (a = 10 ) may be represented by the reaction ... 

The principles outlined above are, of course, important in electro-synthetic reactions. The pH of the electrolysis medium, however, also affects the occurrence and rate of proton transfers which follow the primary electron transfer and hence determine the stability of electrode intermediates to chemical reactions of further oxidation or reduction. These factors are well illustrated by the reduction at a mercury cathode of aryl alkyl ketones (Zuman et al., 1968). In acidic solution the ketone is protonated and reduces readily to a radical which may be reduced further only at more negative potentials. 

A concentration cell contains the same electroactive material in both half-cells, but in different concentration (strictly, with different activities). The emf forms in response to differences in chemical potential /r between the two half-cells. Note that such a concentration cell does not usually involve different electrode reactions (other than, of course, that shorting causes one half-cell to undergo reduction while the other undergoes oxidation). 

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. 

Since natural waters are generally in a dynamic rather than an equilibrium condition, even the concept of a single oxidation-reduction potential characteristic of the aqueous system cannot be maintained. At best, measurement can reveal an Eh value applicable to a particular system or systems in partial chemical equilibrium and then only if the systems are electrochemically reversible at the electrode surface at a rate that is rapid compared with the electron drain or supply by way of the measuring electrode. Electrochemical reversibility can be characterized... 

The electrochemical analyzers are another important family of liquid analyzers. They include potentiometric, wherein an electric potential is measured and the solution remains unchanged conductive, in which a minute current is measured but the system is essentially unchanged and amperometric, in which a chemical reaction occurs during the course of the measurement. Potentiometric analyzers can measure the presence of dissolved ionized solids in a solution. These measurements include pH, oxidation-reduction potential (ORP), and ion-selective electrodes (ISEs) or probes. 

Equation 7.15 implies that, for an electrochemical reaction involving a redox reaction, there exists an electrode potential that is related to the chemical potentials of the reactants and the reaction products and is calculated by this equation. This electrochemical potential is called the redox potential . This potential is positive for an oxidation reaction, where a constituent involved will gain in valency, while it is negative for a reduction reaction, where the valency is reduced for the constituent. In the standard thermodynamic state (i.e., for an ideal condition where each of the species is 1 mol at standard temperature and pressure), the standard redox potential is... 

Figure 8. Chemiluminescence in redox-catalytic cycles. The metal-polypyridine complex M acts as light-emission sensitizer, LES [74, 266], D and A can be either a chemical reductant and oxidant, respectively, or an electrode polarized at appropriate potential.

Reactions that take place consecutive to the electrode process can be studied polarographioally only in those cases in which the electrode process is reversible. In these cases the wave-heights and the wave-shape remain unaffected by the chemical processes. However, the half-wave potentials are shifted relative to the equilibrium oxidation-reduction potential, determined e.g. potentiometrically. Hence, whereas in all above examples, limiting currents were measured to determine the rate constant, it is the shifts of half-wave potentials which are measured here. First- and second-order chemical reactions will be discussed in the following. 

General oxidation/reduction indicators are substances that change color on being oxidized or reduced. In contrast to specific indicators, the color changes of true redox indicators are largely independent of the chemical nature of the analyte and titrant and depend instead on the changes in the electrode potential of the system that occur as the titration progresses. 

Equivalence-point potential The electrode potential of the system in an oxidation/reduction titration when the amount of titrant that has been added is chemically equivalent to the amount of analyte in the sample. 

E > E (E less negative). The applied voltage is negative of Ej and as soon as B is produced at the electrode by the chemical reaction B is immediately reduced at a diffusion-controlled rate. This increases the observed height of peak A, for this wave now contains current from the reduction of B as well as from A. When the couple B/B is reversible, the wave for B can be seen if the reverse scan is sufficient to go positive of Eg. In a single CV scan, only the oxidation wave of B in the B/B couple will be seen, but if two successive triangular potentials are applied to the electrode, the full reversible wave of B will be observed (Fig. 2). 

Oxidation reduction reactions occur at two electrodes. The electrode at which oxidation occurs is called the anode the one at which reduction takes place is called the cathode. Electricity passes through a circuit under the influence of a potential or voltage, the driving force of the movement of charge. There are two different types of interaction of electricity and matter. Electrolysis is when an electric current causes a chemical reaction. Galvanic cell action is when a chemical reaction causes an electric current, as in the use of a battery. 

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