Redox reactions in nature

It is not practical to galvanize large objects, such as a ships or pipes. Instead a block of a reactive metal, such as magnesium (or zinc), is attached to the large object and, again, preferentially loses electrons to oxygen. This method of protecting the metal is known as sacrificial protection. 

Atmospheric nitrogen is particularly unreactive however, it is oxidized to NO when lightning flashes. Oxygen in the atmosphere oxidizes NO to NO2, which reacts with rain water to form the acids HNO2 and HNO3. These acids react with metal oxides and carbonates in the soil, to form nitrate and nitrite salts. 

If a substance is suspected to contain a nitrate ion (NO3), the presence of the ion can be detected by heating the nitrate with sodium hydroxide soiution and Devarda s alloy. Devarda s alloy is a roughly equal mixture of aluminium and copper, to which a little zinc is added. If a nitrate is present it is reduced and ammonia is evolved. Ammonia can be detected by its choking smell or, because it is an alkaline gas, it turns damp red litmus paper blue. The detection of ammonia confirms that a nitrate was present. 

The availability of nitrogen to plants (and therefore to animals) is vital for producing food. This is why fertilizers, ammonium and nitrate salts, are produced industrially to help supply nitrogen to crops. 

The Haber-Bosch process is an industrial process whereby nitrogen gas is reduced to ammonia using an iron catalyst and is the first step in the production of fertilizers  

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The equilibrium model, despite its limitations, in many ways provides a useful if occasionally abstract description of the chemical states of natural waters. However, if used to predict the state of redox reactions, especially at low temperature, the model is likely to fail. This shortcoming does not result from any error in formulating the thermodynamic model. Instead, it arises from the fact that redox reactions in natural waters proceed at such slow rates that they commonly remain far from equilibrium. 

In conclusion we should stress that quantification of rates of redox reactions in natural systems is difficult. Numerous compound- and system-specific factors may influence the overall reaction rate. Evaluation of the relative reactivities of a series of structurally related compounds that are likely to react by the same reaction mechanism(s), may, however, provide important insight into the processes determining a given reaction in a given system. Such information may allow at least order-of-magnitude estimates of how fast a given compound will undergo oxidation or reduction in that system. 

Some pH-Controlling Redox Reactions in Natural Waters... 

Together with acid-base reactions, where a proton transfer occurs (pH-dependent dissolution/ precipitation, sorption, complexation) redox reactions play an important role for all interaction processes in aqueous systems. Redox reactions consist of two partial reactions, oxidation and reduction, and can be characterized by oxygen or electron transfer. Many redox reactions in natural aqueous systems can actually not be described by thermodynamic equilibrium equations, since they have slow kinetics. If a redox reaction is considered as a transfer of electrons, the following general reaction can be derived ... 

In summary, because many redox reactions in natural waters do not couple with one another readily, different apparent redox levels exist in the same locale thus an electrode or any other indicator system cannot measure a unique h or pc- If tho electrode (or the indicators) reached equilibrium with one of the redox couples, it would indicate the redox intensity of that couple only. A few conditions are necessary to obtain meaningful operational values ... 

To illustrate how we may now calculate more precisely to what extent a given organic compound may be reduced or oxidized in a given system,.we consider a dilute solution (e.g., 0.1 mM) of 2-hydroxy naphthoquinone [trivial name lawson (LAW), reaction 13 in Table 5] in 5 mAf aqueous hydrogen sulfide at various pH values. As we will see later, since such quinoid compounds undergo reversible redox reactions in natural systems, they may play a pivotal role in the transformation of organic pollutants. Hence, we consider the reversible reaction... 

A discussion of specific catalytic effects of mineral surfaces must be based on thermodynamic and structural information of the reactive surface species. The first part includes a brief outline of the emerging picture of mineral surfaces as two-dimensional arrays of surface complexes. This part assembles also the kinetic tools that are useful for comparison of the reactivity of aqueous metal ions and their adsorbed surface complexes. The second part presents a reevaluation of the most extensively studied inorganic redox reactions in natural waters the oxygenation of VO2 +, Mn2+, Fe2 + and Cu + in homogeneous and heterogeneous systems. 

Bostrom, K. (1967). Some pH-controlling redox reactions in natural waters. Am. Chem. Soc. Adv. Chem. Ser. 67, 286-311. 

Burkhard DIM, Ulmer GC, Redhammer G, Myer GH (1999) Dynamic electrochemicwal assessment of redox reactions in natural micas between 613 and 1373 K at 10 Pa. Am Mineral 84 493-505 Bums RG, Solberg TC (1988) Fe-bearing oxide, silicate, and aluminosilicate minerals. In Spectroscopic Characterization of Minerals and Their Surfaces 415 263-282. LM Coyne, SWS McKeever, DF Blake (eds) Am Chem Soc, Los Angeles, California... 

The purpose of this section is to expand the discussion on the chemistry of iron and to use it as an example of redox reactions in natural waters. The reaction kinetics of redox reactions will be discussed using ferrous iron oxidation as an example. 

The oxidation-reduction reactions in the environment have been reviewed [34]. Although the conditions leading to oxidation or reduction are created by hving organisms, the redox reactions in natural systems may proceed without further mediation by organisms. A few QSARs are hsted for oxidation and reduction reactions, but they are only for narrow chemical classes (phenols, nitrobenzenes, halocarbons) [34]. 

I) Pecher, K. Haderlein, S.B. Schwarzenbach, R.P. In 213th ACS National Meetings Symposium on Redox Reactions in Natural and Engineered Aqueous Systems, ACS, Division of Environmental Chemistry, Ed. Preprints of Papers 37 1997,185-187. 

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