Nitrogen | Research & Encyclopedia Articles

Nitrogen

According to the definition of an organic chemical, nitrogen need not apply; carbon is the essential ingredient. Yet nitrogen is found in all proteins and in many other important organic substances such as protoplasm, the living material in plant and animal cells. All life depends on the cycling of nitrogen from the atmosphere to the soil and into the food chain.

Although nitrogen was one of the earliest gases to be discovered, chemists did not understand its true nature for many years. The atmosphere contains 78 percent nitrogen by volume, but because nitrogen is chemically inactive, it does not support combustion or respiration. It is the oxygen in the air that takes part in these processes. Without nitrogen to dilute the oxygen, however, substances would burn and explode much more easily.

When scientists began to identify individual gases during the 1700s, they believed that a substance called phlogiston is released during combustion and absorbed by the surrounding air. In 1772, Daniel Rutherford was studying the portion of air (mainly nitrogen and some carbon dioxide) that does not support combustion. When Rutherford removed all of the carbon dioxide from his sample, the gas that was left over would still not support combustion. He called it "phlogisticated" air because he believed it had absorbed all of the phlogiston it could hold. Today this gas is known as nitrogen.

Around the same time, several other chemists, including Karl Wilhelm Scheele, Joseph Priestley, and Henry Cavendish, were also studying the portion of air that does not support combustion. However, Rutherford's report was the first to distinguish nitrogen from carbon dioxide. A few years later, Antoine-Laurent Lavoisier developed the modern theory of combustion, which explained the behavior of oxygen and other gases. Lavoisier recognized that nitrogen is an element, which he called "azote" (without life).

In 1790, nitrogen's modern name was introduced by French chemist Jean Antoine Claude Chaptal (1756-1832). The name was chosen to indicate that the gas is a constituent of "nitre" (potassium nitrate, or saltpeter). Nitrogen (atomic number 7) has no color, odor, or taste. The gas is abundant, occurring "free" (as an element, N2) in the atmosphere. However, in order to be useful to plant and animal life, free nitrogen must be combined with other elements into compounds, such as nitrates, which are found in fertile soils. These nitrogen compounds can be restored to depleted soils by the addition of manure and other natural fertilizers.

In the mid-1800s, Jean Boussingault showed that plants could also flourish in soil that had been treated with chemical fertilizers such as ammonium salts. Boussingault also found that certain plants called legumes (which include beans, peas, clover, and alfalfa) can replenish the soil's nitrogen by extracting, or assimilating, it from the air. Boussingault's agricultural research marked the beginning of our modern understanding of the nitrogen cycle.

Since then, scientists have learned that nitrogen is assimilated not by the plant itself, but by bacteria that live on the plant's roots. These nitrogen-fixing bacteria convert free nitrogen into ammonia (NH3), which is then converted by nitrifying bacteria into compounds that can be used by plants to make proteins, which are essential nutrients for all life. When plants and animals die and decay, their nitrogen compounds are recycled to the soil. Some nitrogen is also recycled during thunderstorms, which produce nitrogen oxides that are carried to the earth by the rain.

Many types of nitrogenous fertilizers are used in agriculture. Natural fertilizers include garbage, sewage, fish scraps, meat-processing wastes, guano (seabird waste), and animal manure. During decay, bacteria decompose these organic materials and release ammonia, which then enters the nitrogen cycle in the soil. Farmers also use manufactured chemical fertilizers such as sodium and calcium nitrates. When an excess of nitrogen fertilizer is applied to the soil, however, it can contribute to water pollution. In addition to fertilizers, nitrogen is used to manufacture explosives such as TNT (trinitrotoluene) and nitroglycerin. These compounds are made by converting ammonia into nitric acid.

Until the turn of the twentieth century, most nitrogen compounds were produced from Chile saltpeter (sodium nitrate), but William Crookes predicted that the world would soon run short of this raw material. Scientists feared that, without fertilizers to increase crop yields, the growing world population would go hungry. This began the search for a way to "fix" free nitrogen from the air into usable compounds such as ammonia.

In the early 1900s, Norwegian physicist Kristian Olaf Bernhard Birkeland (1867-1917) and his co-worker Samuel Eyde (1866-1940) developed an electric-arc method of combining atmospheric nitrogen and oxygen into nitrogen oxides, but the process required large amounts of electricity. Another nitrogen-fixation method called the cyanamide process was invented around the same time by Adolf Frank (1834-1916) and Heinrich Caro (1834-1910), but it also used too much power to be practical. Then in 1913, German chemists Fritz Haber (1868-1934) and Carl Bosch (1874-1940) developed a commercial process for synthesizing ammonia from atmospheric nitrogen. During World War I, Germany was able to continue manufacturing ammunition from synthetic ammonia after its supplies of raw materials had been cut off by Allied ships.

Nitrogen is one of the more difficult gases to liquefy. By the mid-1800s, most gases had been liquefied, but a few so-called permanent gases resisted all attempts at liquefaction. Then in 1877, French physicist Louis Paul Cailletet (1832-1913) succeeded in liquefying small amounts of nitrogen. Liquid gases were still a laboratory curiosity rather than a commercially available product, however.

In 1895, German engineer Carl von Linde (1842-1934) invented a continuous process for producing large quantities of liquid air, which is mainly nitrogen and oxygen. Linde's process immediately became a commercial success. In an improved version, it is still used today to produce liquid air, and liquid nitrogen has found many practical uses in research and industry. In biological research, for example, liquid nitrogen is used to freeze blood cells, sperm, tissues, and even whole small organisms. When frozen, the cell stops its normal activities, allowing scientists to examine a "freeze-frame" of cell life. In industry, nitrogen is used for refrigeration, food processing, and metal heat treating. The electronics industry uses nitrogen as a blanketing medium in the manufacture of components such as transistors. Nitrogen is also used to make the anesthetic "laughing gas," or nitrous oxide.

Although nitrogen is very valuable to humankind, the proliferation of fertilizers and fossil fuels today has created too much of a good thing. As of the late 1990s, human activities had doubled the natural rate at which nitrogen was made available on land. This new nitrogen in the system is due mainly to manmade fertilizers. Other major sources include the increased cultivation of legumes and other crops that harbor nitrogen-fixing bacteria and the burning of fossil fuels. Researchers think that this excess nitrogen may be decreasing biological diversity in some areas. In addition, nitrous oxide is building up in the atmosphere, where it can eat away at the ozone layer. Other nitrogen compounds contribute to smog and acid rain.