Editing Nitrate (section)

== Chemical and biochemical properties ==
In the {{Chem2|NO3-}} anion, the [[oxidation state]] of the central nitrogen atom is V (+5). This corresponds to the highest possible [[oxidation number]] of nitrogen. Nitrate is a potentially powerful [[Oxidizing agent|oxidizer]] as evidenced by its [[explosive]] behaviour at high temperature when it is [[Detonation|detonated]] in [[ammonium nitrate]] ({{Chem2|NH4NO3}}), or [[Gunpowder|black powder]], ignited by the [[shock wave]] of a [[Explosive#Primary|primary explosive]]. In contrast to [[red fuming nitric acid]] ({{Chem2|HNO3/N2O4}}), or concentrated [[nitric acid]] ({{Chem2|HNO3}}), nitrate in [[aqueous solution]] at neutral or high [[pH]] is only a weak [[oxidizing agent]] in [[redox]] reactions in which the [[reductant]] does not produce hydrogen ions (such as mercury going to [[calomel]]). However it is still a strong oxidizer when the reductant does produce hydrogen ions, such as in the oxidation of hydrogen itself. Nitrate is stable in the absence of [[microorganism]]s or reductants such as organic matter. In fact, nitrogen gas is thermodynamically stable in the presence of 1 [[atmosphere (unit)|atm]] of oxygen only in very acidic conditions, and otherwise should combine with the oxygen to form nitrate. This is shown by subtracting the two oxidation reactions:<ref>{{cite book |last1=[[Marcel Pourbaix]] and N. [[Zubov|de Zoubov]] |editor1-last=Marcel Pourbaix |title=Atlas of Electrochemical Equilibria in Aqueous Solutions |date=1974 |pages=49, 497, 500 |url=http://sunlight.caltech.edu/aic/pourbaix.pdf |chapter=Nitrogen}}</ref>

: {{chem2|N2 + 6 H2O → 2 NO3- + 12 H+ + 10 e-}} <math>\qquad E_0 = 1.246-0.0709 \text{ pH } + \frac {0.0591} {10} \log\frac{(NO_3^-)^2}{P_{N_2}}</math>

: {{chem2|2 H2O → O2 + 4 H+ + 4 e-}} <math>\qquad\qquad\qquad E_0 = 1.228-0.0591 \text{ pH } + \frac {0.0591} 4 \log{P_{O_2}}</math>

giving:

: {{chem2|2 N2 + 5 O2 + 2 H2O → 4 NO3- + 4 H+}} <math>\qquad0=0.018-0.0118 \text{ pH } + \frac {0.0591} {10} \log\frac{(NO_3^-)^2}{P_{N_2}}-\frac {0.0591} 4 \log{P_{O_2}}</math>

Dividing by 0.0118 and rearranging gives the equilibrium relation:

: <math>\log\frac{(NO_3^-)}{P_{N_2}^{1/2}P_{O_2}^{5/4}}=\text{ pH }-1.5</math>

However, in reality, nitrogen, oxygen, and water do not combine directly to form nitrate. Rather, a reductant such as hydrogen reacts with nitrogen to produce "fixed nitrogen" such as [[ammonia]], which is then oxidized, eventually becoming nitrate. Nitrate does not accumulate to high levels in nature because it reacts with reductants in the process called [[denitrification]] (see [[Nitrogen cycle]]).

Nitrate is used as a powerful terminal [[electron acceptor]] by [[denitrifying bacteria]] to deliver the energy they need to thrive. Under [[Hypoxia (environmental)|anaerobic conditions]], nitrate is the strongest electron acceptor used by [[prokaryote]] [[microorganism]]s ([[bacteria]] and [[archaea]]) to respirate. The [[redox]] couple {{Chem2|NO3-}}/{{Chem2|N2}} is at the top of the [[Redox gradient|redox scale]] for the [[anaerobic respiration]], just below the couple oxygen ({{O2}}/{{H2O}}), but above the couples Mn(IV)/Mn(II), Fe(III)/Fe(II), {{Chem2|SO4(2-)}}/{{Chem2|HS-}}, {{CO2}}/{{CH4}}. In natural waters, inevitably contaminated by microorganisms, nitrate is a quite unstable and labile dissolved chemical species because it is [[metabolism|metabolised]] by denitrifying bacteria. Water samples for nitrate/nitrite analyses need to be kept at 4 °C in a refrigerated room and analysed as quick as possible to limit the loss of nitrate.

In the first step of the denitrification process, dissolved nitrate ({{Chem2|NO3-}}) is [[Catalysis|catalytically]] [[Redox|reduced]] into nitrite ({{Chem2|NO2-}}) by the [[Enzyme catalysis|enzymatic activity]] of bacteria. In aqueous solution, dissolved nitrite, N(III), is a more powerful oxidizer that nitrate, N(V), because it has to accept less [[electron]]s and its [[Redox|reduction]] is less [[Chemical kinetics|kinetically]] hindered than that of nitrate.

During the biological denitrification process, further nitrite reduction also gives rise to another powerful oxidizing agent: [[nitric oxide]] (NO). NO can fix on [[myoglobin]], accentuating its red coloration. NO is an important biological [[Cell signaling|signaling molecule]] and intervenes in the [[vasodilation]] process. Still, it can also produce [[Radical (chemistry)|free radicals]] in [[Tissue (biology)|biological tissues]], accelerating their degradation and aging process. The [[reactive oxygen species]] (ROS) generated by NO contribute to the [[oxidative stress]], a condition involved in vascular dysfunction and [[Atherosclerosis|atherogenesis]].<ref name="Lubos2008">{{cite journal | vauthors = Lubos E, Handy DE, Loscalzo J | title = Role of oxidative stress and nitric oxide in atherothrombosis | journal = Frontiers in Bioscience | volume = 13 | issue = 13 | pages = 5323–5344 | date = May 2008 | pmid = 18508590 | pmc = 2617738 | doi = 10.2741/3084 | publisher = IMR Press }}</ref>