Modern inorganic chemistry

208 GROUP V

plants from nitrogen compounds in the soil, usually with the help

of bacteria although some plants can absorb and utilise free gaseous

nitrogen. The replacement of nitrogen compounds in the soil is

essential for continued growth of crops; hence the manufacture of

fertilisers such as ammonium or nitrate salts is a major industry

since, because they are water soluble, inorganic nitrogen compounds

are only rarely found in nature. Deposits of sodium nitrate are

found in Chile and a few other regions which have a dry climate.

By far the greatest and most important source of nitrogen is the

atmosphere, which consists of about 78 % nitrogen by volume and,

therefore, acts as a reservoir.

Industrially, elemental nitrogen is extracted from the air by the

fractional distillation of liquid air from which carbon dioxide and

water have been removed. The major fractions are nitrogen, b.p.

77 K and oxygen, b.p. 90 K, together with smaller quantities of the

noble gases.

In the laboratory nitrogen can be made by the oxidation of the

ammonium ion (p. 221).

PHOSPHORUS

Phosphorus, like nitrogen, is an essential constituent of living matter
where it may be partly in combination (as phosphate groups) with
organic groups, for example in lecithin and egg yolk, or mainly in
inorganic form, as calcium phosphate(V), in bones and teeth.
A number of phosphorus-containing minerals occur in nature;
these are almost always salts of phosphoric(V) acid, notably the
calcium salts, for example phosphorite or hydroxy-apatite
3Ca 3 (PO 4 ) 2 .Ca(OH) 2 , apatite 3Ca 3 (PO 4 ) 2 .CaF 2. Other minerals
are vivianite Fe 3 (PO 4 ) 2. 8H 2 O and aluminium phosphate. Ele-
mental phosphorus is manufactured on a large scale, the world
production exceeding 1 million tons annually. A phosphorus-
containing rock, usually apatite, is mixed with sand, SiO 2 , and coke
and the mixture is heated in an electric furnace at about 1700K.
At this temperature the non-volatile silica displaces the more
volatile phosphorus(V) oxide from the phosphate:

2Ca 3 (PO 4 ) 2 + 6SiO 2 -> 6CaSiO 3 + P 4 O 10

The phosphorus(V) oxide is then reduced by coke, and phosphorus
vapour and carbon monoxide are produced:

P 4 O 10 + loc ->loco! + P 4 T