Nitrate
Nitrate is a polyatomic ion with the chemical formula. Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are soluble in water. An example of an insoluble nitrate is bismuth oxynitrate.
In nature, nitrates are produced by a number of species of nitrifying bacteria in the natural environment using ammonia or urea as a source of nitrogen and source of free energy. Nitrate compounds for gunpowder were historically produced, in the absence of mineral nitrate sources, by means of various fermentation processes using urine and dung. Modern nitrate production is mostly focused on creation for fertilizer and chemical manufacturing for various applications, such as medicine synthesis, ceramics and preservation of meat. Annually, about 195 million metric tons of synthetic nitrogen fertilizers are used worldwide, with nitrates constituting a significant portion of this amount.
Because nitrates are soluble and easily can be swept away from the soil because of precipitation, excessive agricultural use has been associated with nutrient runoff, water pollution, and the proliferation of aquatic dead zones. Direct exposure of nitrates for humans can have direct health consequences: the excess consumption of nitrates in cured meats is associated with intestinal cancers.
Chemical structure
The nitrate anion is the conjugate base of nitric acid, consisting of one central nitrogen atom surrounded by three identically bonded oxygen atoms in a trigonal planar arrangement. The nitrate ion carries a formal charge of −1. This charge results from a combination formal charge in which each of the three oxygens carries a − charge, whereas the nitrogen carries a +1 charge, all these adding up to formal charge of the polyatomic nitrate ion. This arrangement is commonly used as an example of resonance. Like the isoelectronic carbonate ion, the nitrate ion can be represented by three resonance structures:Image:Nitrate-ion-resonance-2D.png|400px|Canonical resonance structures for the nitrate ion
Chemical and biochemical properties
In the anion, the oxidation state of the central nitrogen atom is V. This corresponds to the highest possible oxidation number of nitrogen. Nitrate is a potentially powerful oxidizer as evidenced by its explosive behaviour at high temperature when it is detonated in ammonium nitrate, or black powder, ignited by the shock wave of a primary explosive. In contrast to red fuming nitric acid, or concentrated nitric acid, 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. 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 microorganisms, or reductants such as organic matter. In fact, nitrogen gas is thermodynamically stable in the presence of of oxygen only in very acidic conditions, and otherwise would combine with it to form nitrate. This is shown by subtracting the two oxidation reactions:giving:
Dividing by 0.0118 and rearranging gives the equilibrium relation:
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.
Nitrate is used as a powerful terminal electron acceptor by denitrifying bacteria to deliver the energy they need to thrive. Under anaerobic conditions, nitrate is the strongest electron acceptor used by prokaryote microorganisms to respirate. The redox couple is at the top of the redox scale for the anaerobic respiration, just below the couple oxygen, but above the couples Mn/Mn, Fe/Fe, /, /. In natural waters inevitably contaminated by microorganisms, nitrate is a quite unstable and labile dissolved chemical species because it is 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 is catalytically reduced into nitrite by the enzymatic activity of bacteria. In aqueous solution, dissolved nitrite, N, is a more powerful oxidizer that nitrate, N, because it has to accept less electrons and its reduction is less kinetically hindered than that of nitrate.
Electrochemical reduction of nitrate is also well-known, although its use for energy storage and denitrification remains underdeveloped.
During the biological denitrification process, further nitrite reduction also gives rise to another powerful oxidizing agent: nitric oxide. NO can fix on myoglobin, accentuating its red coloration. NO is an important biological signaling molecule and intervenes in the vasodilation process. Still, it can also produce free radicals in biological tissues, accelerating their degradation and aging process. The reactive oxygen species generated by NO contribute to the oxidative stress, a condition involved in vascular dysfunction and atherogenesis.
Detection in chemical analysis
The nitrate anion is commonly analysed in water by ion chromatography along with other anions also present in the solution. The main advantage of IC is its ease and the simultaneous analysis of all the anions present in the aqueous sample. Since the emergence of IC instruments in the 1980s, this separation technique, coupled with many detectors, has become commonplace in the chemical analysis laboratory and is the preferred and most widely used method for nitrate and nitrite analyses.Previously, nitrate determination relied on spectrophotometric and colorimetric measurements after a specific reagent is added to the solution to reveal a characteristic color. Because of interferences with the brown color of dissolved organic matter often present in soil pore water, artefacts can easily affect the absorbance values. In case of weak interference, a blank measurement with only a naturally brown-colored water sample can be sufficient to subtract the undesired background from the measured sample absorbance. If the DOM brown color is too intense, the water samples must be pretreated, and inorganic nitrogen species must be separated before measurement. Meanwhile, for clear water samples, colorimetric instruments retain the advantage of being less expensive and sometimes portable, making them an affordable option for fast routine controls or field measurements.
Colorimetric methods for the specific detection of nitrate often rely on its conversion to nitrite followed by nitrite-specific tests. The reduction of nitrate to nitrite can be effected by a copper-cadmium alloy, metallic zinc, or hydrazine. The most popular of these assays is the Griess test, whereby nitrite is converted to a deeply red colored azo dye suited for UV–vis spectrophotometry analysis. The method exploits the reactivity of nitrous acid derived from the acidification of nitrite. Nitrous acid selectively reacts with aromatic amines to give diazonium salts, which in turn couple with a second reagent to give the azo dye. The detection limit is 0.02 to 2 μM. Such methods have been highly adapted to biological samples and soil samples.
In the dimethylphenol method, 1 mL of concentrated sulfuric acid is added to 200 μL of the solution being tested for nitrate. Under strongly acidic conditions, nitrate ions react with 2,6-dimethylphenol, forming a yellow compound, 4-nitro-2,6-dimethylphenol. This occurs through electrophilic aromatic substitution where the intermediate nitronium ions attack the aromatic ring of dimethylphenol. The resulting product is analyzed using UV-vis spectrophotometry at 345 nm according to the Lambert-Beer law.
Another colorimetric method based on the chromotropic acid was also developed by West and Lyles in 1960 for the direct spectrophotometric determination of nitrate anions.
If formic acid is added to a mixture of brucine and potassium nitrate, its color instantly turns red. This reaction has been used for the direct colorimetric detection of nitrates.
For direct online chemical analysis using a flow-through system, the water sample is introduced by a peristaltic pump in a flow injection analyzer, and the nitrate or resulting nitrite-containing effluent is then combined with a reagent for its colorimetric detection.
Occurrence and production
Nitrate salts are found naturally on earth in arid environments as large deposits, particularly of nitratine, a major source of sodium nitrate.Nitrates are produced by a number of species of nitrifying bacteria in the natural environment using ammonia or urea as a source of nitrogen and source of free energy. Nitrate compounds for gunpowder were historically produced, in the absence of mineral nitrate sources, by means of various fermentation processes using urine and dung.
Lightning strikes in earth's nitrogen- and oxygen-rich atmosphere produce a mixture of oxides of nitrogen, which form nitrous ions and nitrate ions, which are washed from the atmosphere by rain or in occult deposition.
Nitrates are produced industrially from nitric acid.
Uses
Agriculture
Nitrate is a chemical compound that serves as a primary form of nitrogen for many plants. This essential nutrient is used by plants to synthesize proteins, nucleic acids, and other vital organic molecules. The transformation of atmospheric nitrogen into nitrate is facilitated by certain bacteria and lightning in the nitrogen cycle, which exemplifies nature's ability to convert a relatively inert molecule into a form that is crucial for biological productivity.Nitrates are used as fertilizers in agriculture because of their high solubility and biodegradability. The main nitrate fertilizers are ammonium, sodium, potassium, calcium, and magnesium salts. Several billion kilograms are produced annually for this purpose. The significance of nitrate extends beyond its role as a nutrient since it acts as a signaling molecule in plants, regulating processes such as root growth, flowering, and leaf development.
While nitrate is beneficial for agriculture since it enhances soil fertility and crop yields, its excessive use can lead to nutrient runoff, water pollution, and the proliferation of aquatic dead zones. Therefore, sustainable agricultural practices that balance productivity with environmental stewardship are necessary. Nitrate's importance in ecosystems is evident since it supports the growth and development of plants, contributing to biodiversity and ecological balance.