NITROGEN, FIXATION OF. Compared with the enormous quantity of nitrogen contained in the atmosphere, esti mated at about 5.8 metric tons over each square yard of the earth's surface, the amount circulating in animals and plants is small, perhaps 1 in soo,000. Continual interchange takes place between atmospheric and combined nitrogen; some is returned to the soil as nitric acid in rain, some is fixed by the bacilli in the nodules formed on the roots of leguminous plants. Much of this nitrogen finds its way into the bodies of animals and plants; on the decay of such bodies ammonia is produced, and some of this is acted on by denitrifying bacteria which return the nitrogen to the air in an elementary condition. Until recent years the only artificial nitrogenous fertilizers used on an extensive scale were sodium nitrate from Chile and ammonium sulphate re covered from the ammoniacal liquors of gas-works and coke ovens.
In ordinary circumstances, nitrogen is somewhat inert, but Henry Cavendish (1785) discovered that it unites with oxygen when electric sparks are passed through a mixture of the gases, and W. F. Donkin later obtained ammonia by the action of a silent electric discharge on a mixture of nitrogen and hydrogen. Any idea of using these reactions technically was abandoned for many years and more attention was paid to the possibility of synthesizing cyanides. As early as 1828 Desfosses found that a cyanide was produced when nitrogen was passed over a heated mixture of carbon and an alkali or alkaline carbonate, N2+4C+ If baryta be used in place of an alkali, barium cyanide is formed, and since this compound gives am monia on heating strongly in steam, leaving a residue from which the baryta may be recovered, the reactions afford a possible technical process for the fixation of nitrogen (Margueritte and Sourdeval, 186o).
The establishment of the calcium carbide industry has made it possible to combine nitrogen with carbon commercially, the calcium cyanamide produced giving calcium carbonate and am monia as ultimate products of hydrolysis (A. R. Frank and N. Caro, 1895-8), CaC2+ N2 = C and CaCN2+31120=CaCO3+ 2NH3. This method underwent rapid development after the dis covery that calcium cyanamide might be used directly as a nitro genous fertilizer (1901). Certain metals form compounds with nitrogen which yield hydroxides and ammonia on treatment with water, e.g., (a) (b) =3Mg(OH)2+2NH3; (c) The price of the metals precludes their employment, but the discovery of 0. Serpek, that aluminium nitride is formed when aluminium
oxide (or bauxite) and carbon are heated in an atmosphere of nitrogen was actually used on a technical scale.
The development of nitrogen fixation during the present cen tury has been made possible by a better understanding of the thermodynamics of chemical change and increased engineering resources. The manufacture of cyanamide was established rapidly as the necessary carbide was available and its production thor oughly understood. The actual reaction between carbide and nitrogen is exothermic (heat is evolved), and when initiated proceeds to completion. Nitrates and nitrites were manufactured on a large scale after 1903 by combining nitrogen and oxygen to nitric oxide at the temperature of the electric arc, and sub sequently oxidizing the nitric oxide to nitric acid in the presence of water. Although the nitric oxide formation is endothermic (heat is absorbed), the large supply of energy was obtained at a cheap rate where water-power was available, notably in Norway. The exothermic synthesis of ammonia was the last fixation process to be developed technically; this was due to the slowness of the reaction Production.—Until the outbreak of the World War, the pro duction of nitrates from ammonia had no apparent technical ad vantage. W. Ostwald had succeeded in greatly improving the yield of nitric oxide by the combustion of ammonia, 4NH3+502=4N0 and, with the shortage of nitrates, the process found adoption. The production of nitric acid from ammonia has now attained great importance, for whilst both ammonium salts and nitrates are used as fertilizers, the latter act more readily and hence enjoy considerable demand. It seems now to be more economical to use a given amount of electrical energy in the synthesis of ammonia and to burn this to nitric oxide rather than to use it in the direct formation of the latter compound. Thus in 1913 the greater portion of the combined nitrogen used in agri culture and the arts was derived either from Chilean nitrate or from ammonia compounds recovered from coke-ovens or gas works, whilst at the present time synthetic nitrogen compounds predominate. The following figures for production of combined nitrogen are given on the authority of J. H. Lucas, L'Industrie Chimique (Jan. 1928, p.