226Detection
Free nitrate ions in solution can be detected by a nitrate ion selective electrode. Such electroders
analogously to the pH selective electrode. This response is partially described by the Nernst
equation.
Toxicity/Toxicosis
Nitrate toxicosis can occur through enterohepatic metabolism of nitrate to 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. Although nitrite converts to ammonia,
if there is more nitrite than can be converted, the animal slowly suffers from a lack of oxygen.
Human Health Effects
Humans are subject to nitrate toxicity, with infants being especially vulnerable to
methemoglobinemia due to nitrate metabolizing triglycerides present at higher concentrations than
at other stages of development. Methemoglobinemia in infants is known as blue baby syndrome.
Although nitrates in drinking water were once thought to be a contributing factor, there are now
significant scientific doubts as to whether there is a causal link.
Blue baby syndrome is now thought to be the product of a number of factors, which can include
any factor which causes gastric upset, such as diarrheal infection, protein intolerance, heavy metal
toxicity etc., with nitrates playing a minor role. Nitrates, if a factor in a specific case, would most
often be ingested by infants in high nitrate drinking water. However, nitrate exposure may also
occur if eating, for instance, vegetables containing high levels of nitrate. Lettuce may contain
elevated nitrate 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 nitrate than others. The methemoglobin
reductase enzyme may be under-produced or absent in certain people that have an inherited
mutation. Such individuals cannot break down methemoglobin as rapidly as those that 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. It is the increased consumption of green, leafy vegetables that typically
accompany 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 nitrate.
Methemoglobinemia can be treated with methylene blue, which reduces ferric iron (3+) in affected
blood cells back to ferrous iron (2+). Nitrate also is a by-product of septic systems. To be specific,
it is a naturally occurring chemical that is left after the breakdown 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 nearby bodies of water. Lakes that rely on ground water are often affected by
nitrification through this process.
Nitrate in drinking water at levels above the national standard poses an immediate threat to young
children. Excessive levels can result in a condition known as "blue baby syndrome". If untreated,
the condition can be fatal. Boiling water contaminated with nitrate increases the nitrate
concentration and the potential risk.