4) Give the description of properties, technology of production and analysis of Nitrogen.

Properties of Nitrogen. Nitrogen in its elemental form is a non-metallic gas that makes up 78 percent of Earth's atmosphere. For the first time nitrogen (latium nitrogenium) was studied by the Scottish chemist D. Rutherford in 1772. In nature, nitrogen occurs in free (in air) and bound (minerals, living organisms). The approximate nitrogen content in the earth's crust is 0.05%. Nitrogen in a free state is a gas without color and odor, little soluble in water. It is somewhat lighter than air, at -196 ° C it condenses, and at -220 ° C it freezes. The molecule of nitrogen is diatomic. Nitrogen is used to synthesize ammonia and further nitric acid, as an inert medium in many industries. In the laboratory, nitrogen is obtained by decomposition of ammonium nitrite upon heating: NH₄NO₂ = N₂ + ₂H₂O. Or the interaction of a mixture of aqueous solutions of sodium nitrite and ammonium chloride: NaNO₂ + NH₄Cl = N₂ + NaCl + 2H₂O. In industry, nitrogen is obtained from air using a difference in the boiling points of nitrogen and oxygen. Chemically, nitrogen is quite an inert gas because of a strong covalent bond, atomic nitrogen is chemically very active. Of metals, free nitrogen reacts under ordinary conditions only with lithium, forming a nitride: 6Li + N₂ = 2Li₃N. As the temperature increases, the activity of molecular nitrogen increases. When nitrogen is reacted with hydrogen under heating, increased pressure and the presence of a catalyst, ammonia is formed: N₂ + 3H₂ = 2NH₃ With oxygen, nitrogen is connected only in an electric arc with the formation of nitric oxide (II): N₂ + O₂ = 2NO. Nitrogen is capable of forming chemical compounds, being in all degrees of oxidation from +5 to -3. Compounds in positive degrees of oxidation, nitrogen forms with fluorine and oxygen, and in degrees of oxidation more than +3 nitrogen can be found only in compounds with oxygen. Compounds in which the degree of oxidation is 3 are called nitrides.

Production. Nitrogen gas is an industrial gas produced by the fractional distillation of liquid air, or by mechanical means using gaseous air (pressurised reverse osmosis membrane or pressure swing adsorption). Nitrogen gas generators using membranes or pressure swing adsorption (PSA) are typically more cost and energy efficient than bulk delivered nitrogen.[61] Commercial nitrogen is often a byproduct of air-processing for industrial concentration of oxygen for steelmaking and other purposes. When supplied compressed in cylinders it is often called OFN (oxygen-free nitrogen). Commercial-grade nitrogen already contains at most 20 ppm oxygen, and specially purified grades containing at most 2 ppm oxygen and 10 ppm argon are also available. In a chemical laboratory, it is prepared by treating an aqueous solution of ammonium chloride with sodium nitrite. NH₄Cl + NaNO₂ → N₂ + NaCl + 2 H₂O. Small amounts of the impurities NO and HNO₃ are also formed in this reaction. The impurities can be removed by passing the gas through aqueous sulfuric acid containing potassium dichromate. Very pure nitrogen can be prepared by the thermal decomposition of barium azide or sodium azide. 2NaN₃ → 2Na + 3N₂.

Determination of nitrogen content in organic substances according to Kjeldahl. The Kjeldahl method or Kjeldahl digestion (Danish pronunciation: in analytical chemistry is a method for the quantitative determination of organic nitrogen in chemical substances like ammonia. This method was developed by Johan Kjeldahl in 1883. The method consists of heating a substance with sulphuric acid, which decomposes the organic substance by oxidation to liberate the reduced nitrogen as ammonium sulphate. In this step potassium sulphate is added to increase the boiling point of the medium (from 337 °C to 373 °C) . Chemical decomposition of the sample is complete when the initially very dark-coloured medium has become clear and colourless. The solution is then distilled with a small quantity of sodium hydroxide, which converts the ammonium salt to ammonia. The amount of ammonia present, and thus the amount of nitrogen present in the sample, is determined by back titration. The end of the condenser is dipped into a solution of boric acid. The ammonia reacts with the acid and the remainder of the acid is then titrated with a sodium carbonate solution by way of a methyl orange pH indicator. Degradation: Sample + H₂SO₄ → (NH₄)₂SO₄(aq) + CO₂(g) + SO₂(g) + H₂O(g). Liberation of ammonia: (NH₄)₂SO₄(aq) + 2NaOH → Na₂SO₄(aq) + 2H₂O(l) + 2NH₃(g). Capture of ammonia: B(OH)₃ + H₂O + NH₃ → NH⁴⁺ + B(OH)⁴⁻. Back-titration: B(OH)3 + H2O + Na₂CO₃ → NaHCO₃(aq) + NaB(OH)₄(aq) + CO₂(g) + H₂O. In practice, this analysis is largely automated; specific catalysts accelerate the decomposition. Originally, the catalyst of choice was mercuric oxide. However, while it was very effective, health concerns resulted in it being replaced by cupric sulfate. Cupric sulfate was not as efficient as mercuric oxide, and yielded lower protein results. It was soon supplemented with titanium dioxide, which is currently the approved catalyst in all of the methods of analysis for protein in the Official Methods and Recommended Practices of AOAC International.