2 pH units. In titration, the point at
which the reaction is complete is the
equivalence point (i.e. the point at
which equivalent quantities of acid
and base are added). The end point is
the point at which the indicator just
changes colour. For accuracy, the two
must be the same. During a titration
the pH changes sharply close to the
equivalence point, and the indicator
used must change colour over the
same range.
Other types of indicator can be
used for other reactions. Starch, for
example, is used in iodine titrations
because of the deep blue complex it
forms. Oxidation–reduction indica-
torsare substances that show a re-
versible colour change between
oxidized and reduced forms. See also
adsorption indicator.
indigo A blue vat dye, C 16 H 10 N 2 O 2.
It occurs as the glucoside indicanin
the leaves of plants of the genus In-
digofera, from which it was formerly
extracted. It is now made syntheti-
cally.
indium Symbol In. A soft silvery el-
ement belonging to group 13 (for-
merly IIIB) of the periodic table; a.n.
49; r.a.m. 114.82; r.d. 7.31 (20°C);
m.p. 156.6°C; b.p. 2080± 2 °C. It occurs
in zinc blende and some iron ores
and is obtained from zincÛue dust in
total quantities of about 40 tonnes
per annum. Naturally occurring in-
dium consists of 4.23% indium–113
(stable) and 95.77% indium–115 (half-
life 6 × 1014 years). There are a fur-
therÜve short-lived radioisotopes.
The uses of the metal are small –
some special-purpose electroplates
and some special fusible alloys. Sev-
eral semiconductor compounds are
used, such as InAs, InP, and InSb.
With only three electrons in its va-
lency shell, indium is an electron ac-
ceptor and is used to dope pure
germanium and silicon; it forms sta-
ble indium(I), indium(II), and in-
dium(III) compounds. The element
was discovered in 1863 by Ferdinand
Reich (1799–1882) and Hieronymus
Richter (1824–90).
A- Information from the WebElements site
indoleA yellow solid, C 8 H 7 N, m.p.
52 °C. Its molecules consist of a ben-
zene ring fused to a nitrogen-contain-
ingÜve-membered ring. It occurs in
some plants and in coal tar, and is
produced in faeces by bacterial ac-
tion. It is used in making perfumes.
Indole has the nitrogen atom posi-
tioned next to the fused benzene
ring. An isomer with the nitrogen
two atoms away from the fused ring
is called isoindole.
283 induced-fit model
i
Indoleinduced emission (stimulated
emission)The emission of a photon
by an excited atom or molecule in-
duced by an incident photon of suit-
able energy. The relation between
induced emission and *spontaneous
emission is given by the *Einstein co-
efÜcients. The process of induced
emission is essential for the opera-
tion of lasers and masers.
induced-Üt modelA proposed
mechanism of interaction between
an enzyme and a substrate. It postu-
lates that exposure of an enzyme to a
substrate causes the *active site of
the enzyme to change shape in order
to allow the enzyme and substrate
to bind (see enzyme–substrate com-
plex). This hypothesis is generally
preferred to the *lock-and-key mech-
anism.