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Oxidation and Reduction

The phrase "oxidation and reduction" refers to both a class of chemical reactions and chemical conditions in a water body, soil or sediment. In an oxidation reaction, an element loses one or more electrons and its oxidation state increases. In a reduction reaction, an element gains one or more electrons and its oxidation state decreases. Oxidation and reduction reactions occur simultaneously. The substance (element, compound, or ion) that gets oxidized is called the reductant and the substance that gets reduced is called the oxidant. For example, in the first stage of nitrification the net reaction (leaving out the intermediate steps) is

The oxidant is O2 and the reductant is NH4+.

Oxidation-reduction conditions refer to the presence/absence or relative concentrations of the oxidized and reduced forms of one or more elements in a water body, soil, or sediment. In an oxic system, oxygen is present in measureable amounts, while an anoxic system has no detectable O2. The term sub-oxic is sometimes used to describe an anoxic system that is not sulfate-reducing. Table 1 shows the oxidants and reduced species present under various oxidation-reduction conditions. Organic matter is usually the reductant and is not shown in the table. NO3- is usually the predominant form of inorganic N present under oxic conditions, while ammonia is usually the predominant inorganic N species under sulfate-reducing and methane-fermenting conditions. Organic-N may be found under all oxidation-reduction conditions.

Table 1. Oxidation-reduction conditions in natural waters
Condition Oxidants Present Reduced Species Present
O2 NO3- Fe,Mn Oxides SO42- Fe2+, Mn2+ NH4+ H2S H2, CH4
Sulfate Reducing          
Methane Fermenting        

"Redox" is sometimes used in place of "oxidation-reduction."