An
electrostatic potential map of the nitrate ion. Areas colored red are lower in energy than areas coloured yellow. The oxygen atoms carry the majority of the negative charge.
The structure and bonding of the nitrate ion. The N−O bonds are intermediate in length and strength.
In
inorganic chemistry, a
nitrate is a
salt of
nitric acid with an
ion composed of one
nitrogen and three
oxygen atoms (NO). In
organic chemistry the
esters of nitric acid and various
alcohols are called nitrates. Nitrogen trioxide is an alternative name for nitrate.
Chemical properties
The
nitrate ion is a
polyatomic ion with the
molecular formula NO and a
molecular mass of 62.0049. It is the
conjugate base of nitric acid, consisting of one central nitrogen
atom surrounded by three identical oxygen atoms in a
trigonal planar arrangement. The nitrate ion carries a
formal charge of negative one, where each oxygen carries a − charge while the nitrogen carries a +1 charge, and is commonly used as an example of
resonance. Like the
isoelectronic carbonate ion, the nitrate ion can be represented by resonance structures:
Canonical forms of the nitrate ion resonating
Almost all inorganic nitrate salts are
soluble in
water at
standard temperature and pressure.
In
organic chemistry a nitrate (not to be confused with nitro) is a
functional group with general chemical formula RONO
2 where R stands for any organic residue. They are the
esters of nitric acid and
alcohols formed by
nitroxylation. Examples are
methyl nitrate formed by reaction of
methanol and nitric acid, the nitrate of
tartaric acid, and the inappropriately named
nitroglycerin.
Related materials
Nitrates should not be confused with
nitrites, (NO) the salts of
nitrous acid.
Organic compounds containing the nitro
functional group (which has the same formula and structure as the nitrate ion save that one of the O
− atoms is replaced by the R group) are known as
nitro compounds.
Human toxicity
Nitrate toxicosis in humans occurs through
enterohepatic metabolism of nitrates to
ammonia, with nitrite being an intermediate. Nitrites
oxidize the
iron atoms in
hemoglobin from
ferrous iron (2+) to
ferric iron (3+), rendering it unable to carry oxygen
. This process can lead to generalized
lack of oxygen in organ tissue and a dangerous condition called
methemoglobinemia.
Methemoglobinemia can be treated with
methylene blue, which reduces ferric iron (3+) in affected blood cells back to ferrous iron (2+).
Infants in particular are especially vulnerable to methemoglobinemia due to nitrate metabolizing
triglycerides present at higher concentrations than at other stages of development. Methemoglobinemia in infants is colloquially know as "blue baby syndrome". Initial exposure is most often caused by high levels of nitrates in drinking water. However nitrate exposure may also occur if eating for instance vegetables containing high levels of nitrate. Lettuce may contain elevated nitrates under growth conditions such as reduced sunlight, undersupply of the essential micronutrients molybdenum (Mo) and iron (Fe), or high concentrations of nitrate due to reduced assimilation of nitrate in the plant. High levels of nitrate fertilization also contribute to elevated levels of nitrate in the harvested plant .
Some adults can be more susceptible to the effects of nitrates than others. The
methemoglobin reductase enzyme may be under-produced or absent in certain people who have an inherited mutation. Such individuals cannot break down methemoglobin as rapidly as those who do have the enzyme, leading to increased circulating levels of methemoglobin (the implication being that their blood is not as oxygen-rich). Those with insufficient stomach acid (including some vegetarians and vegans) may also be at risk. Ironically, the increased consumption of green, leafy vegetables that typically accompanies these types of diets may lead to increased nitrate intake. A wide variety of medical conditions, including food allergies, asthma, hepatitis, and gallstones may be linked with low stomach acid; these individuals may also be highly sensitive to the effects of nitrates.
Marine toxicity
right|thumb|Sea surface nitrate from the World Ocean Atlas.
In
freshwater or
estuarine systems close to land, nitrate can reach high levels that can potentially cause the death of fish. While nitrate is much less toxic than ammonia or nitrite, levels over 30 ppm of nitrate can inhibit growth, impair the immune system and cause stress in some aquatic species. However, in light of inherent problems with past protocols on acute nitrate toxicity experiments, the extent of nitrate toxicity has been the subject of recent debate.
In most cases of excess nitrate concentrations in aquatic systems, the primary source is
surface runoff from agricultural or
landscaped areas which have received excess nitrate fertilizer. These levels of nitrate can also lead to algae blooms, and when nutrients become limiting (such as potassium, phosphate or nitrate) then
eutrophication can occur. As well as leading to water
anoxia and
dead zones, these blooms may cause other changes to
ecosystem function, favouring some groups of organisms over others. Consequently, as nitrates form a component of
total dissolved solids, they are widely used as an indicator of
water quality.
Nitrates are also a by-product of
septic systems. Specifically, they are a naturally occurring chemical that is left after the break down or decomposition of animal or human waste. Water quality may also be affected through ground water resources that have a high number of septic systems in a watershed. Septics leach down into ground water resources or aquifers and supply near by bodies of water. Lakes that rely on ground water are often affected by nitrification through this process.
See also
