has either lost one or more electrons,
making it positively charged (a
cation), or gained one or more electrons, making it negatively charged
(an anion). See also ionization.
ion association The electrostatic
attraction between ions in solutions
of electrolytes that causes them to associate in pairs. As a result, complete
dissociation does not take place and
experimentally found electrical conductivities are lower than those predicted by the *Debye–Hückel theory.
In 1926, Neils Bjerrum (1879–1958)
proposed improving the Debye–
Hückel theory by taking ion association into account. He found that the
signiÜcance of ion association increases as the relative permittivity
of the solvent decreases.
ion exchange The exchange of
ions of the same charge between a
solution (usually aqueous) and a solid
in contact with it. The process occurs
widely in nature, especially in the
absorption and retention of watersoluble fertilizers by soil. For example, if a potassium salt is dissolved in
water and applied to soil, potassium
ions are absorbed by the soil and
sodium and calcium ions are released
from it.
The soil, in this case, is acting as
an ion exchanger. Synthetic ionexchange resins consist of various
copolymers having a cross-linked
three-dimensional structure to which
ionic groups have been attached. An
anionic resin has negative ions built
into its structure and therefore exchanges positive ions. A cationic resin
has positive ions built in and exchanges negative ions. Ion-exchange
resins, which are used in sugar reÜning to remove salts, are synthetic organic polymers containing side
groups that can be ionized. In anion
exchange, the side groups are ionized basic groups, such as –NH 3
+ to
which anions X
– are attached. The exchange reaction is one in which different anions in the solution displace
the X
– from the solid. Similarly,
cation exchange occurs with resins
that have ionized acidic side groups
such as –COO
– or –SO 2 O
– , with positive ions M
+ attached.
Ion exchange also occurs with inorganic polymers such as *zeolites, in
which positive ions are held at sites
in the silicate lattice. These are used
for water-softening, in which Ca
2+
ions in solution displace Na
+ ions in
the zeolite. The zeolite can be regenerated with sodium chloride solution.
Ion-exchange membranes are used as
separators in electrolytic cells to remove salts from sea water and in
producing deionized water. Ionexchange resins are also used as the
stationary phase in ion-exchange
chromatography.
ion-exchange chromatography
See ion exchange.
ionic bond See chemical bond.
ionic crystal See crystal.
ionic product The product of the
concentrations of ions present in a
given solution taking the stoichiometry into account. For a sodium chloride solution the ionic product is
[Na
+ ][Cl
–
]; for a calcium chloride solution it is [Ca
2+ ][Cl
–
]
2 . In pure water,
there is an equilibrium with a small
amount of self-ionization:
H 2 O ˆ H
+ + OH
–
The equilibrium constant of this dissociation is given by
K W = [H
+ ][OH
– ]
since the concentration [H 2 O] can
be taken as constant. K W is referred
to as the ionic product of water. It
has the value 10
–14 mol
2 dm
–6 at
25°C. In pure water (i.e. no added
acid or added alkali) [H
+ ] = [OH
– ] =
10
–7 mol dm
–3
. In this type of self289
ionic product
i
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making it positively charged (a
cation), or gained one or more electrons, making it negatively charged
(an anion). See also ionization.
ion association The electrostatic
attraction between ions in solutions
of electrolytes that causes them to associate in pairs. As a result, complete
dissociation does not take place and
experimentally found electrical conductivities are lower than those predicted by the *Debye–Hückel theory.
In 1926, Neils Bjerrum (1879–1958)
proposed improving the Debye–
Hückel theory by taking ion association into account. He found that the
signiÜcance of ion association increases as the relative permittivity
of the solvent decreases.
ion exchange The exchange of
ions of the same charge between a
solution (usually aqueous) and a solid
in contact with it. The process occurs
widely in nature, especially in the
absorption and retention of watersoluble fertilizers by soil. For example, if a potassium salt is dissolved in
water and applied to soil, potassium
ions are absorbed by the soil and
sodium and calcium ions are released
from it.
The soil, in this case, is acting as
an ion exchanger. Synthetic ionexchange resins consist of various
copolymers having a cross-linked
three-dimensional structure to which
ionic groups have been attached. An
anionic resin has negative ions built
into its structure and therefore exchanges positive ions. A cationic resin
has positive ions built in and exchanges negative ions. Ion-exchange
resins, which are used in sugar reÜning to remove salts, are synthetic organic polymers containing side
groups that can be ionized. In anion
exchange, the side groups are ionized basic groups, such as –NH 3
+ to
which anions X
– are attached. The exchange reaction is one in which different anions in the solution displace
the X
– from the solid. Similarly,
cation exchange occurs with resins
that have ionized acidic side groups
such as –COO
– or –SO 2 O
– , with positive ions M
+ attached.
Ion exchange also occurs with inorganic polymers such as *zeolites, in
which positive ions are held at sites
in the silicate lattice. These are used
for water-softening, in which Ca
2+
ions in solution displace Na
+ ions in
the zeolite. The zeolite can be regenerated with sodium chloride solution.
Ion-exchange membranes are used as
separators in electrolytic cells to remove salts from sea water and in
producing deionized water. Ionexchange resins are also used as the
stationary phase in ion-exchange
chromatography.
ion-exchange chromatography
See ion exchange.
ionic bond See chemical bond.
ionic crystal See crystal.
ionic product The product of the
concentrations of ions present in a
given solution taking the stoichiometry into account. For a sodium chloride solution the ionic product is
[Na
+ ][Cl
–
]; for a calcium chloride solution it is [Ca
2+ ][Cl
–
]
2 . In pure water,
there is an equilibrium with a small
amount of self-ionization:
H 2 O ˆ H
+ + OH
–
The equilibrium constant of this dissociation is given by
K W = [H
+ ][OH
– ]
since the concentration [H 2 O] can
be taken as constant. K W is referred
to as the ionic product of water. It
has the value 10
–14 mol
2 dm
–6 at
25°C. In pure water (i.e. no added
acid or added alkali) [H
+ ] = [OH
– ] =
10
–7 mol dm
–3
. In this type of self289
ionic product
i
www.AzShimi.ir www.AzShimi.com