|As nitrogen is present in such large quantity in the atmosphere (more than 4000 billion tons), it is not surprising to find that at the present day a large proportion of the nitrogen used in manufactures is produced by the fractional evaporation of liquid air. A number of other methods have been utilised for obtaining nitrogen from air, and it will be obvious that these methods will all produce "atmospheric nitrogen," different from pure nitrogen in that it contains small amounts of argon and the other inert gases. Thus there are two main sources of nitrogen: |
- The atmosphere.
- Chemical compounds.
Preparation of Nitrogen from the Atmosphere
- Physical Processes:
- Liquefaction - The first liquefaction of air by Linde in 1889 was followed by other processes by Hampson, Pictet, and Claude. In England a combination of these systems is used by which nitrogen of 99.5 per cent, purity is obtained.
- Absorption in Charcoal - The greater absorption of oxygen by coconut charcoal at low temperatures allows of a partial separation of nitrogen.
- Transfusion - Different rates of diffusion of oxygen and nitrogen through caoutchouc gives a rough separation of these gases.
- Chemical Processes:
The absorption of oxygen by chemical means offers a large variety of methods for preparing nitrogen.
- Alkaline Pyrogallol. - Air shaken with excess of alkaline pyrogallol has its oxygen rapidly and completely absorbed, and this experiment may be used to demonstrate the composition of air by volume.
- Phosphorus. - The formation of phosphorus oxides by passing air over smouldering or burning phosphorus results in a complete removal of oxygen. Lavoisier made use of this reaction to prove that nitrogen is a separate entity.
- Copper. - The absorption of atmospheric oxygen by passing air over heated copper, either as turnings or finely divided metal, can be utilised for the isolation of nitrogen. Water-vapour and carbon dioxide are previously removed by means of concentrated sulphuric acid and caustic potash. A number of methods have been devised for the absorption of oxygen at ordinary temperatures by using copper in conjunction with a mixture of air and ammonia. Lupton proposed using ammonia gas, while Berthelot used aqueous ammonia. A modification of this latter method consists in passing a mixture of air and "ammonia liquid" in equal volumes over copper chips. The so-called ammonia liquid is a saturated ammonium-carbonate solution mixed with an equal volume of aqueous ammonia. The air-liquid mixture is completely deoxygenated after passing over the copper chips, and the residual nitrogen is freed from excess of ammonia by dilute sulphuric acid and then dried.
- Other Processes. - Reference should also be made to some of the numerous other methods which have been proposed for the preparation of nitrogen by absorption of oxygen from air.
- Reduced iron.
- Platinum sponge (air mixed with hydrogen).
- Sulphur burnt in a special furnace.
- Carbonaceous matter, such as coal or petroleum.
- Fused alkali cyanides.
- Mixture of alkali plumbate and manganate (alternate air and steam).
- Coke-oven gases.
As air is usually a constituent of the gases derived from combustion, coke-oven distillation, etc., a number of patents have been taken out for the preparation of nitrogen from these gases. A mixture of copper and copper oxide is generally used, the former to remove oxygen, and the latter to convert any carbon monoxide into carbon dioxide, and also to remove any traces of hydrogen. Some of the processes involve the addition of small quantities of reducing gases to prevent complete oxidation of the copper, and thus save the extra cost of subsequent reduction of copper oxide. The nitrogen is freed from carbon dioxide by absorption of the latter in caustic soda,
The burning of electrolytic hydrogen is the principle of the Casale process for removing atmospheric oxygen, and the nitrogen, after purification, is used for the manufacture of synthetic ammonia.
Preparation of Nitrogen from Chemical Compounds
|Ammonium nitrite, NH4NO2, readily decomposes into nitrogen and water when its solution is gently heated: |
NH4NO2 = N2+2H2O.
Actually, in practice, it is found more convenient to warm a solution of sodium nitrite with one of ammonium chloride or sulphate, when the ammonium nitrite, formed by double decomposition, decomposes as above. Overheating should be avoided, otherwise turbulent ebullition occurs. Equal parts of sodium nitrite and ammonium sulphate dissolved in 5 parts of water and heated will give a stream of nitrogen which can be purified by passing through a dilute solution of sulphuric acid (to absorb ammonia) and then over heated copper (to remove oxides of nitrogen and oxygen). Purification can also be effected by passing the gas through a saturated solution of potassium bichromate (5 vols.) and concentrated sulphuric acid (1 vol.).
The decomposition by heat of ammonium bichromate is a convenient method for obtaining nitrogen,
(NH4)2Cr2O7 = Cr2O3+N2+4H2O.
As this reaction may be rather violent, a modification is adopted by heating a mixture of potassium bichromate and ammonium chloride in a retort, when the ammonium bichromate is formed by double decomposition.
Alternatively, a concentrated solution of sodium nitrite is run slowly into a mixture of a solution saturated both with ammonium chloride and potassium dichromate. This solution is heated, and the gas evolved is washed (a) with dilute sulphuric acid, (b) with ferrous sulphate solution, and is then passed over (c) heated copper. The gas is dried by phosphorus pentoxide and then passed through a spiral cooled by liquid air. It is then frozen by liquid oxygen under reduced pressure and the solid nitrogen allowed to melt. The first fractions boiling from this are pure nitrogen, and this portion was used in the determination of physical constants by Onnes and v. Urk.
Decomposition of ammonium nitrate occurs when it is heated with glycerol, giving an almost quantitative yield of nitrogen. It would seem that nitrous oxide (from the ammonium nitrate) is completely reduced by the glycerol, the latter being converted into glyceric acid. Thus:
2NH4NO3+C3H8O3 = 2N2+C3H6O4+5H2O.
In the laboratory 10 grams of ammonium nitrate are dissolved in 20 grams of glycerol, to which 3 drops of concentrated sulphuric acid have been added, Nitrogen begins to come off at 100° C., and at 165° C. a steady stream is obtained. It is not advisable to raise the temperature above 170° C. Traces of carbon dioxide are removed by passing through alkali, and moisture and minute amounts of pyridine bases by concentrated sulphuric acid.
Pure nitrogen may be obtained by the action of chlorine gas upon ammonia:
8NH3+3Cl2 = 6NH4Cl+N2.
A concentrated ammonia solution is placed in a triple-necked Woulfe's bottle and chlorine is led in from a separate generator. An open glass tube passes down into the liquid through the centre neck, and a delivery tube from the third tubulure carries off the nitrogen which may be collected over water. Ammonia must always be kept in excess in order to prevent the formation of explosive nitrogen trichloride, NCl3.
A convenient laboratory method for preparing nitrogen consists in the decomposition of concentrated ammonia solution with a thin paste of bleaching powder, which latter is added slowly. On warming the mixture a copious supply of nitrogen is obtained, in accordance with the equation
3Ca(OCl)2+4NH3 = 3CaCl2+2N2+6H2O.
Hypobromites (bromine and caustic soda) may be used similarly to decompose ammonia:
3NaOBr+2NH3 = 3NaBr+N2+3H2O.
Hypobromites and urea (or other acid amides) react to give a mixture of nitrogen and carbon dioxide:
3NaOBr+CO(NH2)2 = 3NaBr+N2+CO2+2H2O.
This quantitative reaction is used as a standard method for the estimation of urea.
Pure nitrogen is obtained when a mixture of nitric oxide (or nitrous oxide) and ammonia is passed over heated copper gauze or platinised asbestos. The nitric oxide (from nitric acid and copper turnings) is passed through strong ammonia solution, and the mixed gases led through a heated combustion tube containing either rolls of copper gauze or platinised asbestos. Purification is effected by passing the gas successively through dilute sulphuric acid, fused caustic potash, concentrated sulphuric acid, and finally red-hot copper gauze.
|In laboratory nitrogen may be prepared by decomposition of Ammonium nitrate:|
NH4NO2 => N2^ + H2O
It is an exothermic reaction which releases 80 kcal or 335 J of thermal energy, so cooling of the reservoir is required, however Ammonium nitrate has to be heated to initialize the reaction.
Practically in this case saturated solution of sodium nitrite is dripped into heated saturated ammonium sulphate solution. This reaction yields ammonium nitrite which is instantly decomposed. Disengaged gas is contaminated by ammonia, nitrogen oxide (I), and O, and is purified on the next stage by passing it through diluted sulfuric acid, ferrous sulphate (II), and glowing copper. After that nitrogen is dried.
Another laboratory method of preparation is heating of mixture of potassium dichromate and ammonium sulfate with mass ratio 2:1. The reaction equations are:
K2Cr2O7 + (NH4)2SO4 = (NH4)2Cr2O7 + K2SO4
(NH4)2Cr2O7 =>(t) Cr2O3 + N2^ + 4H2O
Most pure nitrogen may be produced by decomposition of metals azides:
2NaN3 =>(t) 2Na + 3N2^
So-called air or atmosphere nitrogen, which is the mixture of nitrogen with Noble Gass, may be yielded by the reaction between air and glowing coke:
O2 + 4N2 + 2C => 2CO + 4N2
The product of this reaction is so-called generator gas or air-blast gas which is used as a fuel and as a feedstock for chemical synthesis. The nitrogen may be separated, if necessary, by carbon monoxide capturing.
The industrial way of obtaining molecular nitrogen is the fractional distillation oh liquid air. This is also a suitable way to produce atmosphere nitrogen.
The laboratory way of preparation is passing ammonia over copper oxide (II) at temperature approximately 700°C:
2NH3 + 3CuO => N2^ + 3H2O + 3Cu
Ammonia may be obtained by heating its saturated solution. Before applying nitrogen is purified by passing over copper and its oxide at the same temperature approximately 700°C, and then dried by concentrated sulfuric acid and dry alkali. This is a slow process, but it worth it as high grade purity gas is evolved.