small amounts of the ions NH 4 +and
OH–. The combustion of ammonia in
air yields nitrogen and water. In the
presence of catalysts NO, NO 2 , and
water are formed; this last reaction is
the basis for the industrial produc-
tion of nitric acid. Ammonia is a
good proton acceptor (i.e. it is a base)
and gives rise to a series of ammo-
nium salts, e.g.
NH 3 + HCl →NH 4 ++ Cl–.
It is also a reducing agent.
The participation of ammonia in
the *nitrogen cycle is a most impor-
tant natural process. Nitrogen-Üxing
bacteria are able to achieve similar
reactions to those of the Haber
process, but under normal conditions
of temperature and pressure. These
release ammonium ions, which are
converted by nitrifying bacteria into
nitrite and nitrate ions.ammoniacalDescribing a solution
in which the solvent is aqueous am-
monia.ammonia clockA form of atomic
clock in which the frequency of a
quartz oscillator is controlled by
the vibrations of excited ammonia
molecules. The ammonia molecule
(NH 3 ) consists of a pyramid with a ni-
trogen atom at the apex and one hy-
drogen atom at each corner of the
triangular base. When the molecule
is excited, once every 20.9 microsec-
onds the nitrogen atom passes
through the base and forms a pyra-
mid the other side: 20.9 microsec-
onds later it returns to its original
position. This vibration back and
forth has a frequency of 23 870 hertz
and ammonia gas will only absorb
excitation energy at exactly this fre-
quency. By using a crystal oscillator
to feed energy to the gas and a suit-
able feedback mechanism, the oscil-
lator can be locked to exactly this
frequency.ammonia–soda process See
solvay process.ammonium alumSee alums.ammonium bicarbonateSee am-
monium hydrogencarbonate.
ammonium carbonateA colour-
less or white crystalline solid,
(NH 4 ) 2 CO 3 , usually encountered as
the monohydrate. It is very soluble in
cold water. The compound decom-
poses slowly to give ammonia, water,
and carbon dioxide. Commercial ‘am-
monium carbonate’ is a double salt
of ammonium hydrogencarbonate
and ammonium aminomethanoate
(carbamate), NH 4 HCO 3 .NH 2 COONH 4.
This material is manufactured by
heating a mixture of ammonium
chloride and calcium carbonate and
recovering the product as a sublimed
solid. It readily releases ammonia
and is the basis of sal volatile. It is
also used in dyeing and wool prepa-
ration and in baking powders.ammonium chloride (sal ammo-
niac)A white or colourless cubic
solid, NH 4 Cl; r.d. 1.53; sublimes at
340 °C. It is very soluble in water and
slightly soluble in ethanol but insolu-
ble in ether. It may be prepared by
fractional crystallization from a solu-
tion containing ammonium sulphate
and sodium chloride or ammonium
carbonate and calcium chloride. Pure
samples may be made directly by the
gas-phase reaction of ammonia and
hydrogen chloride. Because of its
ease of preparation it can be manu-
factured industrially alongside any
plant that uses or produces ammo-
nia. The compound is used in dry
cells, metalÜnishing, and in the
preparation of cotton for dyeing and
printing.ammonium hydrogencarbonate
(ammonium bicarbonate)A white
crystalline compound, NH 4 HCO 3. It isammoniacal 32a