*acids. They are either positive ions
(e.g. NO 2 +) or molecules that have a
positive charge on a particular atom
(e.g. SO 3 , which has an electron-
deÜcient sulphur atom). In organic
reactions they tend to attack nega-
tively charged parts of a molecule.
Compare nucleophile.
electrophilic addition An *addi-
tion reaction in which theÜrst step is
attack by an electrophile (e.g. a posi-
tive ion) on an electron-rich part of
the molecule. An example is addition
to the double bonds in alkenes.
electrophilic substitution A
*substitution reaction in which the
Ürst step is attack by an electrophile.
Electrophilic substitution is a feature
of reactions of benzene (and its com-
pounds) in which a positive ion ap-
proaches the delocalized pi electrons
on the benzene ring.
electrophoresis (cataphoresis) A
technique for the analysis and sepa-
ration of colloids, based on the move-
ment of charged colloidal particles in
an electricÜeld. There are various ex-
perimental methods. In one the sam-
ple is placed in a U-tube and a buffer
solution added to each arm, so that
there are sharp boundaries between
buffer and sample. An electrode is
placed in each arm, a voltage ap-
plied, and the motion of the bound-
aries under the inÛuence of theÜeld
is observed. The rate of migration of
the particles depends on theÜeld,
the charge on the particles, and on
other factors, such as the size and
shape of the particles. More simply,
electrophoresis can be carried out
using an adsorbent, such as a strip of
Ülter paper, soaked in a buffer with
two electrodes making contact. The
sample is placed between the elec-
trodes and a voltage applied. Differ-
ent components of the mixture
migrate at different rates, so the sam-
ple separates into zones. The compo-
nents can be identiÜed by the rate
at which they move. In gel elec-
trophoresis the medium is a gel, typi-
cally made of polyacrylamide (see
page), agarose, or starch.
Electrophoresis, which has also
been called electrochromatography,
is used extensively in studying mix-
tures of proteins, nucleic acids, car-
bohydrates, enzymes, etc. In clinical
medicine it is used for determining
the protein content of bodyÛuids.
electrophoretic deposition A
technique for coating a material
making it an electrode in a bath con-
taining a colloidal suspension of
charged particles. Under suitable con-
ditions, the particles are attracted to,
and deposited on, the electrode. Elec-
trophoretic deposition is used exten-
sively in industry; for example, in
applying paint to metal components.
electrophoretic effect The effect
in which the mobility of ions in solu-
tion moving under the inÛuence of
an applied electricÜeld is affected by
theÛow of ions of opposite charge in
the opposite direction.electroplating A method of plat-
ing one metal with another by *elec-
trodeposition. The articles to be
plated are made the cathode of an
electrolytic cell and a rod or bar of
the plating metal is made the anode.
Electroplating is used for covering
metal with a decorative, more expen-
sive, or corrosion-resistant layer of
another metal.electropositive Describing el-
ements that tend to lose electrons
and form positive ions. The alkali
metals are typical electropositive el-
ements.electrospray ionization (ESI)A
technique for producing ions for
mass spectrometry, used especially
for obtaining ions from large mol-
ecules, which would be likely to pro-201 electrospray ionization
e