Chemistry

How Nitrogen was Discovered in the 18th Century



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Since normal air is primarily nitrogen, with a little bit of oxygen and traces of some other gases, it might appear that nitrogen does not need discovery. After all, this is what people are submerged in throughout their lives. But science needs to identify and explain things in the proper way. In this way nitrogen came to be discovered in the late 18th century through the efforts of a group of investigators working independently in various countries. The official joint credit now goes to Daniel Rutherford from Scotland and Carl Wilhelm Scheele from Sweden.

Modern chemistry can said to begin with the publication of Robert Boyle’s The Sceptical Chymist in 1661. But early chemists had very little idea of what they were looking for beyond the properties of substances. Boyle had studied the physical properties of air, and this air was thought, from antiquity, to be an element. However, chemists in the 18th century were building on the list of the four classical elements of earth, air, fire and water. It was also beginning to appear that these were not elements after all, with the spotlight upon the case of air.

It was generally suspected that air was a mixture, since all kinds of fumes just vanished into it. But chemists still did not know the difference between a mixture and a compound, and this fact colored many of the early investigations. For example, Joseph Priestley called his sample of nitrogen “phlogisticated air,” suggesting that it is a product of phlogistication or burning. Henry Cavendish went straight ahead and called his own sample “burnt air.”

Nitrogen is indeed what is left behind after the oxygen in normal air is spent through burning. Such clues were evident as far back as 1674, when English physician John Mayow demonstrated that only part of the air burns. However, chemists continued with the notion of burning air to obtain its product. Scottish chemist Joseph Black “burnt” air using phosphorus for better results. He, along with Priestley and Cavendish, obtained good samples of nitrogen in this way.

However, the credit goes to Rutherford for having a better understanding of the process and his more systematic approach. He was a doctoral student of Black who was able to carry the research into burnt air to its conclusion. Rutherford carried out the simple candle-in-a-bottle experiment regularly performed by school children. In it a candle is made to burn in a bottle of air upturned in water. When the candle is extinguished, water has risen to occupy one fifth of the volume of the bottle demonstrating that one fifth or the air (oxygen) has been spent.

Rutherford took further steps to remove all the oxygen and also the carbon dioxide. He demonstrated that the remaining gas does not support life and neither dissolves in water nor alkali solutions, as does carbon dioxide. He called it “noxious gas” (thereby mistaking it for being poisonous instead of neutral). He published his findings in 1772.

It is believed that Scheele obtained nitrogen following a very similar approach in the same year, working independently in Sweden. Apart from burning phosphorus he also used sulfur and iron fillings to remove oxygen. Scheele published his findings in 1777, but is now given joint credit with Rutherford for the discovery.

It devolved on the great French chemist Antoine Lavoisier to name the new gas, and his name was “azote,” implying absence of life. The name still survives in many languages such as French, Italian, Russian and Polish, as well as in the names of certain nitrogen compounds, such as hydrazine and azotic acid. However, the more common name was supplied by another French chemist Jean-Antoine Chaptal. Taking note that the gas is obtained from nitric acid, in turn derived from the mineral nitre, he called it nitrogen.

Of course, the nitrogen obtained by the 18th century investigators was not entirely pure, containing other trace gases of the atmosphere. Now there are other standard ways of obtaining nitrogen in the laboratory, for example, through heating an ammonium nitrate solution. Industrial production commonly uses the fractional distillation of liquid air.

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