Part 2: Nitrogen Species


N2The main component of Earth’s atmosphere, nitrogen, prevents the rest (mostly oxygen) from being explosive. When lightning strikes, nitrogen and oxygen get converted into nitrate which fertilizes plants. A related reaction occurs in internal combustion engines, except that the product is not nitrate but the pollutant nitrogen dioxide gas. Specialized bacteria also have the ability to convert otherwise fairly inert nitrogen gas into ammonium, which plants can use to make protein, nucleic acids and other important biomolecules.

Nitrogen extends the shelf life of foods in bags. To get pure nitrogen it first has to be liquefied.  If you haven’t experienced liquid nitrogen, it is something to behold. If you pour some out of a Dewar flask into a styrofoam container, it makes the container squeak in a way that gives you goose bumps.  If you pour hot water on liquid nitrogen, within a dense cloud, what flies back at you is a showering cascade of  icy particles.


In a Youtube comment thread, a student pointed out that his teacher had looked at him as if he was crazy when he had asked how they could extract oxygen from the air. Of course the video he later found demonstrated how it’s perfectly feasible in a high school lab by using liquid nitrogen. You just get air to liquefy in a test tube by placing it in liquid nitrogen; wait a little and keep testing the gas above the liquid with a glowing splint until it relights. The key to the mystery is that the liquification point of a mixture of gases with different boiling points is between the boiling points of the pure gases. The boiling point of pure oxygen, O2, is -182 °C and the boiling point of nitrogen, N2, is lower at -196 °C. If air is cooled at atmospheric pressure, air will  completely liquefy at -194  °C , 2 °C above nitrogen’s boiling point. Not only does oxygen have the higher boiling point and thus will boil off after nitrogen, it is more dense than liquid nitrogen: 1141 kg/m3 for oxygen versus 808.5 kg/m3 . So an oxygen rich- liquid at the bottom of the test tube remains closer to the cold Dewar flask and oxygen gets less exposed to the warmer air.

Nitrous oxide (N2O)

nitrous oxideNitrous oxide, or laughing gas, was a recreational drug long before being used as an anesthetic. Industrially it’s made from the controlled melting of  potentially explosive ammonium nitrate. Mainly two different reactions occur between 190 and 250 °C. One produces laughing gas and water; the 2nd produces nitric acid and ammonia. After condensation, three different scrubbers are used to purify N2O.

Although its concentration in the atmosphere is 1250 times smaller than carbon dioxide’s, nitrous oxide has a greenhouse gas warming potential that’s 310 times stronger. In nature it forms as a byproduct from the bacterial denitrification of nitrate and as a byproduct from the microbial oxidation of ammonia, reasons for using fertilizer judiciously.

Nitrate ion (NO3)

nitratePlants can absorb nitrogen from the soil either in the form of nitrate (NO3) or ammonium (NH­­4+). Which one predominates around their roots depends on soil conditions. Either ion is used in the biosynthesis of a variety of life essential molecules. Interestingly, to use nitrate, plants first convert it to ammonium. The ammonium is attached to glutamate (1 amino group) to make glutamine (2 amino groups). Then the latter is converted into 2 glutamates, one of which goes back to the cycle while the other gets converted to different amino acids.

Nitrate is the most common form of nitrogen running off from terrestrial ecosystems to rivers, lakes and oceans. Excess levels of nitrates in water can create conditions that make it difficult for some aquatic organisms to survive. The nitrate ion is found in ammonium nitrate commercial fertilizer, which can also be a dangerous explosive since the compound contains both an oxidizing and reducing agent.

Nitrite ion (NO2)

nitiriteNitrite, like nitrate, is part of the natural nitrogen cycle, which helps reuse a vital element that is not as available as you would think, judging solely from the abundance of nitrogen gas in the atmosphere.

It’ s not a good idea to drink water from wells that have high levels of nitrite. The ion causes hemoglobin in the blood to change to methemoglobin by assisting the oxidation of hemoglobin’s Fe2+ to Fe3+. Methemoglobin reduces the amount of oxygen that
can be carried in the blood. Excess nitrites in food can also lead to the formation of nitrosamines, which are probable carcinogens. Legal limits for the addition of nitrates and nitrites have been set by several countries and EU.

Ammonium ion (NH4+)

ammonium2The ammonium ion is formed in the roots of leguminous plants, where Rhizobium bacteria convert nitrogen from the air into usable ammonium. Ammonium ion can also form when acid is added to ammonia. Smelling salts, which consist of ammonium carbonate, can be used to arouse consciousness, but a chemistry teacher should not use them on students who sleep in class. Breaking my own rule, I have discovered, however, that some people are far more sensitive to the smell of ammonia than others.

ammonia2Ammonia (NH)

It is the product of the Haber Process which converts hydrogen and nitrogen gases into NH3, which can then be oxidized to nitric acid, a source of nitrates for fertilizer.

Ammonia is naturally found in varying amounts in human sweat and reacts with HOCl in pools to produce a variety of chloramines, which give pools their characteristic smell, irritate eyes and sensitive lungs of asthmatics.  Urea from a cat’s litter or from any mammal’s pee will eventually decompose into pungent ammonia.

Can animals directly “pee out” ammonia the way we sweat some out? Given that animals start their lives as eggs and embryos and given the toxicity of ammonia, the answer to that question depends a lot on their immediate environment.

  • Eggs of freshwater fish and some amphibians have constant access to fresh water.    Those animals excrete much of their nitrogenous waste as ammonia.
  • If there’s less access to freshwater for the embryo, even though it requires an investment of energy, it is imperative to produce urea, which is less toxic than ammonia and does not have to be diluted as much. All mammals, sharks and many marine animals produce urea.
  • Urea is however water-soluble. If the embryo develops within an egg with a hard shell, excretion of urea would poison the food supply. The solution here is to produce uric acid which does not dissolve in water. Birds and reptiles excrete uric acid.





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