Oxidation-Reduction Processes in Nature

Therefore, oxidation-reduction processes in nature control the behavior of elements or substances. During oxidation-reduction, the potential for reactions to take effect changes because the redox status of elements changes. A summary of soil-water mineral-ion properties known to be affected by redox chemistry is listed below ... 

While there are very many good examples of oxidation-reduction processes in nature, perhaps the two most classic examples are aerobic respiration and alcohol metabolism by the liver, each of which is considered briefly in turn here. 

Quinones, which play an important part in oxidation-reduction processes in nature, have very low solubility in water but their one-electron reduction can be readily investigated in methanol by pulse radiolysis [12], In this way, the semiquinone radicals of 9,10-anthraquinone [12a] and quinizarin [12b], generated by es , CH2OH and CH20 , have been characterized. 

Response of water might be probable initial event of an organism s response to mild exposure this is probably connected to high sensitivity of oxidation-reduction processes in water media to action of external factors. Such features as electronic work function, zero charge potential, electrode potential, etc. are connected to concept of electrochemical processes. For this reason, the structure of near-electrode layer will depend on nature of electrodes material and specific nature of its interaction with solvent [8]. 

Combustion. Combustion is a chemical reaction between a material and oxygen. The reaction is an oxidation-reduction process in which one material becomes chemically oxidized and the other becomes chemically reduced. In the context of fossil fuels, the material that becomes oxidized is coal, fuel liquids, or natural gas. Combustion of these carbonaceous materials converts each atom of carbon in the fuel molecules to a molecule of carbon dioxide, according to the general equation ... 

It should be noted that all terms concerning the electrons in the metals as well as those connected with the metals not directly participating in the cell reaction (Pt) have disappeared from the final Eq. (3.1.49). This result is of general significance, i.e. the EMFs of cell reactions involving oxidation-reduction processes do not depend on the nature of the metals where those reactions take place. The situation is, of course, different in the case of a metal directly participating in the cell reaction (for example, silver in the above case). 

Besides playing a vital role in the oxidation-reduction processes of living organisms, quinones occur widely as natural pigments found mainly in plants, fungi, lichens, marine organisms, and insects (see alizarin, Section 28-4A, as representative of a natural anthraquinone-type dye). 

The need for biological mediation of most redox processes encountered in natural waters means that approaches to equilibrium depend strongly on the activities of the biota. Moreover, quite different oxidation-reduction levels may be established within biotic microenvironments than those prevalent in the over-all environment diffusion or dispersion of products from the microenvironment into the macroenvironment may give an erroneous view of redox conditions in the latter. Also, because many redox processes do not couple with one another readily, it is possible to have several different apparent oxidation-reduction levels in the same locale, depending upon the system that is being used as reference. 

Precipitation refers to dissolved species (such as As(V) oxyanions) in water or other liquids reacting with other dissolved species (such as Ca2+, Fe3+, or manganese cations) to form solid insoluble reaction products. Precipitation may result from evaporation, oxidation, reduction, changes in pH, or the mixing of chemicals into an aqueous solution. For example, As(V) oxyanions in acid mine drainage could flow into a nearby pond and react with Ca2+ to precipitate calcium arsenates. The resulting precipitates may settle out of the host liquid, remain suspended, or possibly form colloids. Like sorption, precipitation is an important process that affects the movement of arsenic in natural environments and in removing arsenic from contaminated water (Chapters 3 and 7). 

Pfeifer, H.-R., Gueye-Girardet, A., Reymond, D. et al. (2004) Dispersion of natural arsenic in the Malcantone watershed, southern Switzerland Field evidence for repeated sorption-desorption and oxidation-reduction processes. Geoderma, 122(2-4 SPEC. IIS.), 205-34. 

Stumm, W., and B. Sulzberger, The cycling of iron in natural environments Considerations based on laboratory studies of heterogeneous redox processes, Geochim. Cosmochim. Acta 56 3233 (1992). A comprehensive review of surface-oxidation-reduction processes on iron oxyhydroxide solids. 

The data in Figure 7.13 show reductive-dissolution kinetics of various Mn-oxide minerals as discussed above. These data obey pseudo first-order reaction kinetics and the various manganese-oxides exhibit different stability. Mechanistic interpretation of the pseudo first-order plots is difficult because reductive dissolution is a complex process. It involves many elementary reactions, including formation of a Mn-oxide-H202 complex, a surface electron-transfer process, and a dissolution process. Therefore, the fact that such reactions appear to obey pseudo first-order reaction kinetics reveals little about the mechanisms of the process. In nature, reductive dissolution of manganese is most likely catalyzed by microbes and may need a few minutes to hours to reach completion. The abiotic reductive-dissolution data presented in Figure 7.13 may have relative meaning with respect to nature, but this would need experimental verification. 

The transfer of a single electron between two chemical entities is the simplest of oxidation-reduction processes, but it is of central importance in vast areas of chemistry. Electron transfer processes constitute the fundamental steps in biological utilization of oxygen, in electrical conductivity, in oxidation reduction reactions of organic and inorganic substrates, in many catalytic processes, in the transduction of the sun s energy by plants and by synthetic solar cells, and so on. The breadth and complexity of the subject is evident from the five volume handbook Electron Transfer in Chemistry (V. Balzani, Ed.), published in 2001. The most fimdamental principles that govern the efficiencies, the yields or the rates of electron-transfer processes are independent of the nature of the substrates. The properties of the substrates do dictate the conditions for apphcability of those fimdamental... 

There are several disadvantages to potential sweep methods. First, it is difficult to measure multiple, closely spaced redox couples. This lack of resolution is due to the broad asymmetric nature of the oxidation/reduction waves. In addition, the analyte must be relatively concentrated as compared to other electrochemical techniques to obtain measurable data with good signal to noise. This decreased sensitivity is due to a relatively high capacitance current which is a result of ramping the potential linearly with time. Potential sweep methods are easy to perform and provide valuable insight into the electron transfer processes. They are excellent for providing a preliminary evalnation, bnt are best combined with other complementary electrochemical techniqnes. 

The results obtained at the cathode in the iodine coulometer show that Faraday s laws hold for the reduction of iodine to iodide ions the laws apply, in fact, to all types of electrolytic reduction occurring at the cathode, e.g., reduction of ferric to ferrous ions, ferricyanide to ferro-cyanide, quinone to hydroquinone, etc. The laws are applicable similarly to the reverse process of electrolytic oxidation at the anode. The equivalent weight in these cases is based, of course, on the nature of the oxidation-reduction process. 

H.R. Pfeifer, A. Gueye-Girardet, D. Reymond et al.. Dispersion of Natural Arsenic in the Malcantone Watershed, Southern Switzerland Field Evidence for Repeated Sorption-Desorption and Oxidation-Reduction Processes, Geoderma. 122(2-4), 205-234, Oct. (2004). 

As Hueper was busy with his dog experiments, other widely used materials began to emerge as carcinogens. One was the element chromium. This metal, which in nature is almost always found in its trivalent chemical form, is extracted by converting it into the hexavalent form, which dissolves easily in water. The main commercial products are purified chromate or bichromate salts, usually sold in powder form. Because the element is extracted from the ore by oxidation—the opposite, from a chemist s point of view, of the reduction process in a copper or iron smelter—and the final product is a salt, chromium production was part of the chemical industry. 

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