formed naturally as a decay product
of nitrogenous matter and is made
commercially by various methods:
the action of carbon dioxide and
steam on a solution of ammonium
carbonate; heating commercial am-
monium carbonate (which always
contains some hydrogencarbonate);
and the interaction of ammonia, car-
bon dioxide, and water vapour. It is
used in some *baking powders and
medicines.
ammonium ion The monovalent
cation NH 4 +. It may be regarded as
the product of the reaction of ammo-
nia (a Lewis base) with a hydrogen
ion. The ion has tetrahedral symme-
try. The chemical properties of am-
monium salts are frequently very
similar to those of equivalent alkali-
metal salts.
ammonium nitrate A colourless
crystalline solid, NH 4 NO 3 ; r.d. 1.72;
m.p. 169.6°C; b.p. 210°C. It is very
soluble in water and soluble in
ethanol. The crystals are rhombic
when obtained below 32°C and
monoclinic above 32°C. It may be
readily prepared in the laboratory by
the reaction of nitric acid with aque-
ous ammonia. Industrially, it is man-
ufactured by the same reaction using
ammonia gas. Vast quantities of am-
monium nitrate are used as fertiliz-
ers (over 20 million tonnes per year)
and it is also a component of some
explosives.
ammonium sulphate A white
rhombic solid, (NH 4 ) 2 SO 4 ; r.d. 1.77;
decomposes at 235°C. It is very solu-
ble in water and insoluble in ethanol.
It occurs naturally as the mineral
mascagnite. Ammonium sulphate
was formerly manufactured from the
‘ammoniacal liquors’ produced dur-
ing coal-gas manufacture but is now
produced by the direct reaction be-
tween ammonia gas and sulphuric
acid. It is decomposed by heating to
release ammonia (and ammonium
hydrogensulphate) and eventually
water, sulphur dioxide, and ammo-
nia. Vast quantities of ammonium
sulphate are used as fertilizers.ammonium thiocyanateA
colourless, soluble crystalline com-
pound, NH 4 NCS. It is made by the
action of hydrogen cyanide on am-
monium sulphide or from ammonia
and carbon disulphide in ethanol. On
heating, it turns into its isomer
thiourea, SC(NH 2 ) 2. Its solutions give
a characteristic blood-red colour with
iron(III) compounds and so are em-
ployed as a test for ferric iron. Am-
monium thiocyanate is used as a
rapidÜxative in photography and as
an ingredient in making explosives.amorphousDescribing a solid that
is not crystalline; i.e. one that has no
long-range order in its lattice. Many
powders that are described as ‘amor-
phous’ in fact are composed of
microscopic crystals, as can be
demonstrated by X-ray diffraction.
*Glasses are examples of true amor-
phous solids.amount of substanceSymbol n.
A measure of the number of entities
present in a substance. The speciÜed
entity may be an atom, molecule,
ion, electron, photon, etc., or any
speciÜed group of such entities. The
amount of substance of an element,
for example, is proportional to the
number of atoms present. For all en-
tities, the constant of proportionality
is the *Avogadro constant. The SI unit
of amount of substance is the *mole.AMPSee atp; cyclic amp.ampere Symbol A. The SI unit of
electric current. The constant current
that, maintained in two straight par-
allel inÜnite conductors of negligible
cross section placed one metre apart33 ampere
a