referred to as gas-phase electro-
phoresis.ionophore A relatively small hy-
drophobic molecule that facilitates
the transport of ions across lipid
membranes. Most ionophores are
produced by microorganisms. There
are two types of ionophore: channel
formers, which combine to form a
channel in the membrane through
which ions canÛow; and mobile ion
carriers, which transport ions across
a membrane by forming a complex
with the ion.ion pairA pair of oppositely
charged ions produced as a result of
a single ionization; e.g.
HCl →H++ Cl–.
Sometimes a positive ion and an elec-
tron are referred to as an ion pair, as
in
A →A++ e–.ion pumpA type of *vacuum
pump that can reduce the pressure
in a container to about 1 nanopascal
by passing a beam of electrons
through the residual gas. The gas is
ionized and the positive ions formed
are attracted to a cathode within
the container where they remain
trapped. The pump is only useful at
very low pressures, i.e. below about
1 micropascal. The pump has a lim-
ited capacity because the absorbed
ions eventually saturate the surface
of the cathode. A more effective
pump can be made by simultane-
ously producing aÜlm of metal by
ion impact (sputtering), so that
fresh surface is continuously pro-
duced. The device is then known as
a sputter-ion pump.IPSee ionization potential.IRSee infrared radiation.
iridiumSymbol Ir. A silvery metal-
lic *transition element (see also plat-inum metals); a.n. 77; r.a.m. 192.20;
r.d. 22.42; m.p. 2410°C; b.p. 4130°C.
It occurs with platinum and is
mainly used in alloys with platinum
and osmium. The element forms a
range of iridium(III) and iridium(IV)
complexes. It was discovered in 1804
by Smithson Tennant (1761–1815).
A- Information from the WebElements site
iron Symbol Fe. A silvery malleable
and ductile metallic *transition el-
ement; a.n. 26; r.a.m. 55.847; r.d.
7.87; m.p. 1535°C; b.p. 2750°C. The
main sources are the ores *haematite
(Fe 2 O 3 ), *magnetite (Fe 3 O 4 ), limonite
(FeO(OH)nH 2 O), ilmenite (FeTiO 3 ),
siderite (FeCO 3 ), and pyrite (FeS 2 ).
The metal is smelted in a *blast fur-
nace to give impure *pig iron, which
is further processed to give *cast
iron, *wrought iron, and various
types of *steel. The pure element has
three crystal forms: alpha-iron, stable
below 906°C with a body-centred-
cubic structure; gamma-iron, stable
between 906°C and 1403°C with a
nonmagnetic face-centred-cubic
structure; and delta-iron, which is
the body-centred-cubic form above
1403 °C. Alpha-iron is ferromagnetic
up to its Curie point (768°C). The el-
ement has nine isotopes (mass num-
bers 52–60), and is the fourth most
abundant in the earth’s crust. It is re-
quired as a trace element (see essen-
tial element) by living organisms.
Iron is quite reactive, being oxidized
by moist air, displacing hydrogen
from dilute acids, and combining
with nonmetallic elements. It forms
ionic salts and numerous complexes
with the metal in the +2 or +3 oxida-
tion states. Iron(VI) also exists in the
ferrate ion FeO 4 2–, and the element
also forms complexes in which its ox-
idation number is zero (e.g. Fe(CO) 5 ).
A- Information from the WebElements site
ionophore 292i