Nitrogen

The accepted value for the atomic weight of nitrogen is 14.008 (1927, 0=i6) ; according to Aston the element has no isotopes and is an exception to the whole-number rule. Rutherford has shown that the nitrogen atom is disintegrated by the impact of swift a-particles, and hydrogen atoms are expelled from its nucleus.

Nitrogen gas is somewhat inert, but it can unite directly under certain conditions with several elements : oxygen, hydrogen, boron, silicon, tungsten, titanium, manganese, vanadium, calcium, ba rium, magnesium, lithium, tantalum and (probably) carbon. The compounds with metals (nitrides) are generally decomposed by water with evolution of ammonia or some other gas.

The Nitrogen Cycle.

Nitrogen is an essential constituent of animal and vegetable tissues. Animals derive the nitrogen of their tissue proteins partly from other animal proteins and partly (sometimes wholly) from plant proteins. Plants build up their proteins from inorganic nitrogen compounds in the soil and to some extent from free nitrogen in the atmosphere. Berthelot found that sterilized soils do not take up nitrogen from the air, whilst unsterilized soils do, and he concluded that assimilation is due to micro-organisms. These were found (Winogradsky, 1895; Beijerinck, 1901) to be a Clostridium (C. Pasteurianum), an aerobic, i.e., incapable of functioniag in presence of oxygen unless protected by certain bacteria, and several varieties of bacteria of which the most active is Azotobacter chrodcoccum. The amount of nitrogen fixed by these bacteria increases by 3o% if certain protozoa are present in the soil, although protozoa feed on bacteria.

Atmospheric nitrogen is also fixed by the co-operative action of bacteria and certain plants, chiefly Leguminosae, such as peas, beans and clover. The parasitic bacteria are present in small nodules on the roots of the plants, which are grown on a large scale in order to restore the nitrogen content of soils.

Large quantities of nitrogen are also fixed as oxides by electrical discharges in the atmosphere, and conveyed to the soil as nitric and nitrous acids by rain, nitrates and nitrites being formed in the soil. Altogether about 250,000 tons of nitric acid are said to be formed in this way in 24 hours. Nitrites and ammonium corn pounds are frequently of little use as plant foods and hence they must first be oxidized to nitrates by special soil bacteria, the pro cess being known as "nitrification." Various other kinds of bac teria bring about denitrification in the soil and by their agency considerable quantities of gaseous nitrogen are returned to the air.

The combined nitrogen content of cultivated soil is generally enriched and renewed by means of nitrogenous fertilizers, such as nitrates and ammonium salts.

Compounds of Nitrogen and Hydrogen.—Nitrogen com bines with hydrogen to form three compounds—two bases, am monia (q.v.), and hydrazine, ; and an acid, hydrazoic acid or azoimide (q.v.), Secondary compounds of ammonia and hydrazine with hydrazoic acid are known, such as N.H. and Hydrazine (diamide), N,H., was originally obtained by Curtius (1887) from organic compounds containing two nitrogen atoms combined together. It is prepared commercially by a process due to Raschig (19o7). Sodium hypochlorite solution is mixed with a small quantity of glue and warmed with an excess of concen trated ammonia. After addition of sulphuric acid and cooling, hydrazine sulphate, crystallizes out. When the sul phate is distilled under reduced pressure with concentrated potash solution, a colourless fuming liquid is obtained, called hydrazine hydrate. Anhydrous hydrazine is obtained by distilling the hydrate with solid caustic soda or barium oxide under reduced pressure. It is a colourless liquid, b.p. 113.5° C, which solidifies to white crystals, m.p. is poisonous, decomposes on heating and reacts violently with the halogens. In solution hydrazine acts as a weak base and forms two series of salts, e.g., Hydrazine and its salts are the most powerful reducing agents known, precipitating many metals from solutions of their salts. It rapidly destroys rubber and cork and when hot attacks glass.

Hydrazoic acid, also discovered by Curtius (189o), is formed by the action of an oxidizing agent, e.g., nitric acid, on hydrazine; its sodium salt is formed by passing nitrous oxide over heated sodamide, when pure it is a colour less liquid (b.p. 37°, f.p. —80°) with an exceedingly irritating odour. It is very poisonous and dangerously explosive, decom posing with a blue flash on heating. The solution is strongly acid and dissolves many metals with evolution of hydrogen and ammonia, forming salts called azides. These salts, especially those of the heavy metals, are explosive, and lead azide (q.v.) is sometimes used as a detonator. The compounds N.H. and are colourless explosive crystals obtained by neutralizing hydrazoic acid with ammonia and hydrazine respectively.