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 ammonium carbonate (which always
contains some hydrogencarbonate);
and the interaction of ammonia, carbon 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 ammonia (a Lewis base) with a hydrogen
ion. The ion has tetrahedral symmetry. The chemical properties of ammonium salts are frequently very
similar to those of equivalent alkalimetal 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 aqueous ammonia. Industrially, it is manufactured by the same reaction using
ammonia gas. Vast quantities of ammonium nitrate are used as fertilizers (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 soluble in water and insoluble in ethanol.
It occurs naturally as the mineral
mascagnite. Ammonium sulphate
was formerly manufactured from the
‘ammoniacal liquors’ produced during coal-gas manufacture but is now
produced by the direct reaction between ammonia gas and sulphuric
acid. It is decomposed by heating to
release ammonia (and ammonium
hydrogensulphate) and eventually
water, sulphur dioxide, and ammonia. Vast quantities of ammonium
sulphate are used as fertilizers.
ammonium thiocyanate A
colourless, soluble crystalline compound, NH 4 NCS. It is made by the
action of hydrogen cyanide on ammonium 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 employed as a test for ferric iron. Ammonium thiocyanate is used as a
rapid Üxative in photography and as
an ingredient in making explosives.
amorphous Describing a solid that
is not crystalline; i.e. one that has no
long-range order in its lattice. Many
powders that are described as ‘amorphous’ in fact are composed of
microscopic crystals, as can be
demonstrated by X-ray diffraction.
*Glasses are examples of true amorphous solids.
amount of substance Symbol 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 entities, the constant of proportionality
is the *Avogadro constant. The SI unit
of amount of substance is the *mole.
AMP See atp; cyclic amp.
ampere Symbol A. The SI unit of
electric current. The constant current
that, maintained in two straight parallel inÜnite conductors of negligible
cross section placed one metre apart
33
ampere
a
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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 ammonium carbonate (which always
contains some hydrogencarbonate);
and the interaction of ammonia, carbon 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 ammonia (a Lewis base) with a hydrogen
ion. The ion has tetrahedral symmetry. The chemical properties of ammonium salts are frequently very
similar to those of equivalent alkalimetal 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 aqueous ammonia. Industrially, it is manufactured by the same reaction using
ammonia gas. Vast quantities of ammonium nitrate are used as fertilizers (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 soluble in water and insoluble in ethanol.
It occurs naturally as the mineral
mascagnite. Ammonium sulphate
was formerly manufactured from the
‘ammoniacal liquors’ produced during coal-gas manufacture but is now
produced by the direct reaction between ammonia gas and sulphuric
acid. It is decomposed by heating to
release ammonia (and ammonium
hydrogensulphate) and eventually
water, sulphur dioxide, and ammonia. Vast quantities of ammonium
sulphate are used as fertilizers.
ammonium thiocyanate A
colourless, soluble crystalline compound, NH 4 NCS. It is made by the
action of hydrogen cyanide on ammonium 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 employed as a test for ferric iron. Ammonium thiocyanate is used as a
rapid Üxative in photography and as
an ingredient in making explosives.
amorphous Describing a solid that
is not crystalline; i.e. one that has no
long-range order in its lattice. Many
powders that are described as ‘amorphous’ in fact are composed of
microscopic crystals, as can be
demonstrated by X-ray diffraction.
*Glasses are examples of true amorphous solids.
amount of substance Symbol 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 entities, the constant of proportionality
is the *Avogadro constant. The SI unit
of amount of substance is the *mole.
AMP See atp; cyclic amp.
ampere Symbol A. The SI unit of
electric current. The constant current
that, maintained in two straight parallel inÜnite conductors of negligible
cross section placed one metre apart
33
ampere
a
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