Oxidation-reduction definition

The term electrochromism was apparently coined to describe absorption line shifts induced in dyes by strong electric fields (1). This definition of electrocbromism does not, however, fit within the modem sense of the word. Electrochromism is a reversible and visible change in transmittance and/or reflectance that is associated with an electrochemicaHy induced oxidation—reduction reaction. This optical change is effected by a small electric current at low d-c potential. The potential is usually on the order of 1 V, and the electrochromic material sometimes exhibits good open-circuit memory. Unlike the well-known electrolytic coloration in alkaU haUde crystals, the electrochromic optical density change is often appreciable at ordinary temperatures. 

The last definition has widespread use in the volumetric analysis of solutions. If a fixed amount of reagent is present in a solution, it can be diluted to any desired normality by application of the general dilution formula V,N, = V N. Here, subscripts 1 and 2 refer to the initial solution and the final (diluted) solution, respectively V denotes the solution volume (in milliliters) and N the solution normality. The product VjN, expresses the amount of the reagent in gram-milliequivalents present in a volume V, ml of a solution of normality N,. Numerically, it represents the volume of a one normal (IN) solution chemically equivalent to the original solution of volume V, and of normality N,. The same equation V N, = V N is also applicable in a different context, in problems involving acid-base neutralization, oxidation-reduction, precipitation, or other types of titration reactions. The justification for this formula relies on the fact that substances always react in titrations, in chemically equivalent amounts. 

In the actual oxidation-reduction process electrons are transferred from the reducing agent to the oxidising agent. This leads to the following definitions. 

When developed, this theory proved to be more general than the theory of Lewis, for it includes all the above acid-base definitions and also includes oxidation-reduction reactions. 

It is also sometimes simply referred to as the gram-equivalent . However, GEW has two distinct definitions for neutralization as well as as oxidation-reduction reactions as stated below ... 

The colors in rare earth glasses are caused by the ion being dissolved and they behave uniquely because the 4 f electrons are deeply buried. Their colors depend on transitions taking place in an inner electronic shell while in other elements such as the transition metals, the chemical forces are restricted to deformation and exchanges of electrons within the outer shell. Since the rare earth s sharp absorption spectra are insensitive to glass composition and oxidation-reduction conditions, it is easy to produce and maintain definite colors in the glass making process. ( )... 

The reaction of magnesium and oxygen is an example of an oxidation reaction. The combination of an element with oxygen was the traditional way to define an oxidation reaction. This definition of oxidation has been broadened by chemists to include reactions that do not involve oxygen. Our modern definition for oxidation is that oxidation takes place when a substance loses electrons. Anytime oxidation takes place and a substance loses one or more electrons, another substance must gain the electron(s). When a substance gains one or more electrons, the process is known as reduction. Reactions that involve the transfer of one or more electrons always involve both oxidation and reduction. These reactions are known as oxidation-reduction or redox reactions. 

Today, many reactions in aqueous solutions can be described as oxidation-reduction reactions (redox reactions). Oxidation is the process in which the oxidation number of atoms increases. Reduction is the process in which the oxidation number of atoms is decreased or made more negative. In another definition, oxidation is the loss of electrons by an atom, and reduction is the gain of electrons. Let us look at the following reaction ... 

Many half-reactions of interest to biochemists involve protons. As in the definition of AG °, biochemists define the standard state for oxidation-reduction reactions as pH 7 and express reduction potential as E °, the standard reduction potential at pH 7. The standard reduction potentials given in Table 13-7 and used throughout this book are values for E ° and are therefore valid only for systems at neutral pH Each value represents the potential difference when the conjugate redox pair, at 1 m concentrations and pH 7, is connected with the standard (pH 0) hydrogen electrode. Notice in Table 13-7 that when the conjugate pair 2ET/H2 at pH 7 is connected with the standard hydrogen electrode (pH 0), electrons tend to flow from the pH 7 cell to the standard (pH 0) cell the measured E ° for the 2ET/H2 pair is -0.414 V... 

COPPER (In Biological Systems), The activity of copper in plant metabolism manifests itself in two forms 11) synthesis of chlorophyll, and 12) activity of enzymes. In leaves, most of the copper occurs in close association with chlorophyll, but little is known of ns rale in chlorophyll synthesis, other than the presence of cupper is required. Copper is a definite constituent of several enzymes catalyzing oxidation-reduction reactions (oxidases), in which the activity is believed to be due to the shuttling of copper between the +1 and +2 oxidalicm states,... 

A number of copper -containing protein compounds are enzymes with an oxidase function (ascorbic acid oxidase, urease, etc 1 and these play an important role in Ihe biological oxidation-reduction system. There is a definite relationship of copper with iron in connection with utilization of iron in hemoglobin function. 

This is a very unsatisfactory definition because many oxidation-reduction or redox reactions do not involve changes in hydrogen or oxygen content, as the following example illustrates ... 

In oxidation-reduction equilibria, the common reagent is the electron e", and matters are simplified considerably if one treats the electron like any other reagent, H+, Ag+, Cl", etc. (Note that neither of these exists free and unhydrated in aqueous solutions). Clark has suggested (6) using e, and J0rgensen (8) has suggested the definition ... 

Ronald Breslow and his collaborators have given some attention to the problem of estimating the degree of destabilization of cyclobutadiene with respect to nonconjugated models. They have concluded from electrochemical measurements of oxidation-reduction potentials of the system 37 38, of which only the quinone 38 has the cyclobutadiene fragment, that the C4H4 ring is destabilized by some 12-16 kcal mole-1 and so is definitely antiaromatic.15... 

Before we review the methods used to determine surface acidity, we wish to define the type of acidity that should be measured. An acid is an electron-pair acceptor. In our opinion, the term acid should be limited to this definition rather than broadening the term to include oxidizing agents as well. We agree with Flockhart and Pink (10) who suggest a clear distinction be made between Lewis acid-Lewis base reactions (which involve coordinate bond formation) and oxidation-reduction reactions (which involve complete transfer of one or more electrons). 

The measurements of water quality parameters (oxidation-reduction potential, pH, temperature, conductivity, dissolved oxygen, and turbidity) and the collection of field screening data with field portable instruments and test kits constitute a substantial portion of field work. Field measurements, such as pH, stand on their own as definitive data used for the calculations of solubility of chemical species and chemical equilibrium in water, whereas others serve as indicators of well stabilization or guide our decision-making in the field. Table 3.8 shows the diversity of field measurement... 

These laws (determined by Michael Faraday over a half century before the discovery of the electron) can now be shown to be simple consequences of the electrical nature of matter. In any electrolysis, an oxidation must occur at the anode to supply the electrons that leave this electrode. Also, a reduction must occur at the cathode removing electrons coming into the system from an outside source (battery or other DC source). By the principle of continuity of current, electrons must be discharged at the cathode at exactly the same rate at which they are supplied to the anode. By definition of the equivalent mass for oxidation-reduction reactions, the number of equivalents of electrode reaction must be proportional to the amount of charge transported into or out of the electrolytic cell. Further, the number of equivalents is equal to the number of moles of electrons transported in the circuit. The Faraday constant (F) is equal to the charge of one mole of electrons, as shown in this equation ... 

The oxidation of a molecule involves the loss of electrons. The reduction of a molecule involves the gain of electrons. Since electrons are not created or destroyed in a chemical reaction, if one molecule is oxidized, another must be reduced (i.e. it is an oxidation-reduction reaction). Thus, by definition, oxidation-reduction reactions involve the transfer of electrons. In the oxidation-reduction reaction ... 

Anomalous response to the NADH oxidase in animal cells, such as amiloride-insensitive proton transport, may be based on activation of the H+-ATPase or direct electron transport-linked proton transfer. Further definition of the components of the NADH oxidase and the characteristics of electron transport are needed. In addition, the presence of a poorly characterized glutathione oxidase in the plasma membrane opens an alternative for oxidation-reduction control of proton transport. At this stage no evidence has been found for control of HCOj/Cl" exchange or organic acid transport by the plasma membrane oxidase. 

All the oxidation-reduction reactions used in examples (a) to (e) proceed in one definite direction e.g. Fe3+ can be reduced by Sn2+, but the opposite process, the oxidation of Fe2+ by Sn4+ will not take place. That is why the single arrow was used in all the reactions, including the half-cell processes as well. If however we examine one half-cell reaction on its own, we can say that normally it is reversible. Thus, while Fe3+ can be reduced (e.g. by Sn2+) to Fe2+, it is also true that with a suitable agent (e.g. MnO ) Fe2+ can be oxidized to Fe3+. It is quite logical to express these half-cell reactions as chemical equilibria, which also involve electrons, as... 

The definition of oxidation and reduction given in Chapter 10 expands the concept to reactions that you might not have considered to be oxidations or reductions. As you learn new reactions, try to classify them as oxidations, reductions or neither. 

---