Base quantity in the system of quantities upon which SI is based. It is the number of elementary
entities divided by the
Avogadro constant. Since it is proportional to the
number of entities, the proportionality constant being the reciprocal
Avogadro constant and the same for all substances, it has to be treated almost identically with the
number of entities. Thus the counted elementary entities must always be specified. The words '
of substance' may be replaced by the specification of the entity, for example: amount of chlorine
atoms,
, amount of chlorine molecules,
.
No specification of the entity might lead to ambiguities [amount of sulfur could stand
for
,
, etc.], but in many cases the implied entity is assumed to be known: for molecular
compounds it is usually the molecule [e.g. amount of benzene usually means
],
for ionic compounds the simplest formula unit [e.g. amount of sodium chloride usually
means
]
and for metals the atom [e.g. amount of silver usually stands for
].
In some derived quantities the words '
of substance' are also omitted, e.g.
amount concentration,
amount fraction. Thus in many cases the name of the
base quantity is shortened to amount and to avoid possible confusion with the general meaning of
the word the attribute chemical is added. The
chemical amount is hence the alternative name for amount of substance. In the field of clinical chemistry
the words '
of substance' should not be omitted and abbreviations such as
substance concentration (for amount of
substance concentration) and
substance fraction are in use. The quantity had no name prior to 1969 and was simply referred to as
the number of moles.
Source:
PAC, 1996, 68, 957
(Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations
1996))
on page 961
(
for gaseous mixtures)
