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 oxygen as a 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
Nitrates are mainly produced for use as fertilizers in agriculture because of their high
solubility
and
biodegradability.
The
main
nitrate
fertilizers
are ammonium, sodium, potassium, calcium, and magnesium salts. Several million
kilograms are produced annually for this purpose.
The second major application of nitrates is as oxidizing agents,
most notably in
explosives where the rapid oxidation of carbon compounds liberates large volumes
of gases (see gunpowder for an example). Sodium nitrate is used to remove air
bubbles from molten glass and some ceramics. Mixtures of the molten salt are used
to harden some metals.
Nitrate was also used as a film stock through nitrocellulose. Due to its high
combustibility, the studios swapped to acetate safety film in 1950.
Detection
Almost all methods for detection of nitrate rely on its conversion to nitrite followed
by nitrite-specific tests. The reduction of nitrate to nitrite
is effected by copper-
cadmium material. The sample is introduced with a flow injection analyzer, and the
resulting nitrite-containing effluent is then combined with a reagent for colorimetric
or electrochemical detection. The most popular of these assays is the Griess test,
whereby nitrite is converted to a deeply colored azo dye suited for UV-vis
spectroscopic analysis. The method exploits the reactivity
of nitrous acid derived
from 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.
Safety
The acute toxicity of nitrate is low. "Substantial disagreement" exists about the long-
term risks of nitrate exposure. The two areas of possible concern are that (i) nitrate
could be a precursor to nitrite in the lower gut, and
nitrite is a precursor to
nitrosamines, which are implicated in carcinogenesis, and (ii) nitrate is implicated
in methemoglobinemia, a disorder of hemoglobin in red blood cells.
Methemoglobinemia
Nitrates do not affect infants and pregnant women. Blue baby syndrome is caused
by a number of other factors such as gastric upset, such as diarrheal infection, protein
intolerance, heavy metal toxicity etc., with nitrates playing a minor role.
Drinking water standards
Through the Safe Drinking Water Act, the United States Environmental Protection
Agency has set a maximum contaminant level of 10 mg/L or 10 ppm of nitrate in
drinking water.
An acceptable daily intake (ADI) for nitrate ions was established in the range of 0–
3.7 mg (kg body weight)−1 day−1 by the Joint FAO/WHO Expert Committee on
Food Additives (JEFCA).
Aquatic toxicity
Sea surface nitrate from the World Ocean Atlas
In freshwater or estuarine systems close to land, nitrate can reach concentrations that
are lethal to fish. While nitrate is much less toxic than ammonia, levels over 30 ppm
of nitrate can inhibit growth, impair the immune system
and cause stress in some
aquatic species. Nitrate toxicity remains a subject of debate.
In most cases of excess nitrate concentrations in aquatic systems, the primary
sources
are wastewater discharges, as well as surface runoff from agricultural
or landscaped areas that have received excess nitrate fertilizer. The
resulting eutrophication and algae blooms result in anoxia and dead zones. As a
consequence, as nitrate forms a component of total dissolved solids, they are widely
used as an indicator of water quality.
Domestic animal feed
Symptoms of nitrate poisoning in domestic animals include increased heart rate and
respiration; in advanced cases blood and tissue may turn a blue or brown color. Feed
can be tested for nitrate; treatment consists of supplementing or substituting existing
supplies with lower nitrate material. Safe levels of nitrate for various types of
livestock are as follows:
Category %NO3
%NO3–
N
%KNO3
Effects
1
< 0.5
< 0.12
< 0.81
Generally safe for beef cattle and sheep
2
0.5–
1.0
0.12–
0.23
0.81–
1.63
Caution: some subclinical symptoms may
appear in pregnant horses,
sheep and beef
cattle
3
1.0
0.23
1.63
High nitrate problems: death losses and
abortions can occur in beef cattle and sheep
4
< 1.23 < 0.28
< 2.00
Maximum safe level for horses. Do not feed
high nitrate forages to pregnant mares
The values above are on a dry (moisture-free) basis.
