*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 electrondeÜcient sulphur atom). In organic
reactions they tend to attack negatively charged parts of a molecule.
Compare nucleophile.
electrophilic addition An *addition reaction in which the Ürst step is
attack by an electrophile (e.g. a positive 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 compounds) in which a positive ion approaches the delocalized pi electrons
on the benzene ring.
electrophoresis (cataphoresis) A
technique for the analysis and separation of colloids, based on the movement of charged colloidal particles in
an electric Üeld. There are various experimental methods. In one the sample 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 applied, and the motion of the boundaries under the inÛuence of the Üeld
is observed. The rate of migration of
the particles depends on the Üeld,
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 electrodes and a voltage applied. Different components of the mixture
migrate at different rates, so the sample separates into zones. The components can be identiÜed by the rate
at which they move. In gel electrophoresis the medium is a gel, typically made of polyacrylamide (see
page), agarose, or starch.
Electrophoresis, which has also
been called electrochromatography,
is used extensively in studying mixtures of proteins, nucleic acids, carbohydrates, enzymes, etc. In clinical
medicine it is used for determining
the protein content of body Ûuids.
electrophoretic deposition A
technique for coating a material
making it an electrode in a bath containing a colloidal suspension of
charged particles. Under suitable conditions, the particles are attracted to,
and deposited on, the electrode. Electrophoretic deposition is used extensively in industry; for example, in
applying paint to metal components.
electrophoretic effect The effect
in which the mobility of ions in solution moving under the inÛuence of
an applied electric Üeld is affected by
the Ûow of ions of opposite charge in
the opposite direction.
electroplating A method of plating one metal with another by *electrodeposition. 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 expensive, or corrosion-resistant layer of
another metal.
electropositive Describing elements that tend to lose electrons
and form positive ions. The alkali
metals are typical electropositive elements.
electrospray ionization (ESI) A
technique for producing ions for
mass spectrometry, used especially
for obtaining ions from large molecules, which would be likely to pro201
electrospray ionization
e
www.AzShimi.ir www.AzShimi.com
(e.g. NO 2
+ ) or molecules that have a
positive charge on a particular atom
(e.g. SO 3 , which has an electrondeÜcient sulphur atom). In organic
reactions they tend to attack negatively charged parts of a molecule.
Compare nucleophile.
electrophilic addition An *addition reaction in which the Ürst step is
attack by an electrophile (e.g. a positive 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 compounds) in which a positive ion approaches the delocalized pi electrons
on the benzene ring.
electrophoresis (cataphoresis) A
technique for the analysis and separation of colloids, based on the movement of charged colloidal particles in
an electric Üeld. There are various experimental methods. In one the sample 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 applied, and the motion of the boundaries under the inÛuence of the Üeld
is observed. The rate of migration of
the particles depends on the Üeld,
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 electrodes and a voltage applied. Different components of the mixture
migrate at different rates, so the sample separates into zones. The components can be identiÜed by the rate
at which they move. In gel electrophoresis the medium is a gel, typically made of polyacrylamide (see
page), agarose, or starch.
Electrophoresis, which has also
been called electrochromatography,
is used extensively in studying mixtures of proteins, nucleic acids, carbohydrates, enzymes, etc. In clinical
medicine it is used for determining
the protein content of body Ûuids.
electrophoretic deposition A
technique for coating a material
making it an electrode in a bath containing a colloidal suspension of
charged particles. Under suitable conditions, the particles are attracted to,
and deposited on, the electrode. Electrophoretic deposition is used extensively in industry; for example, in
applying paint to metal components.
electrophoretic effect The effect
in which the mobility of ions in solution moving under the inÛuence of
an applied electric Üeld is affected by
the Ûow of ions of opposite charge in
the opposite direction.
electroplating A method of plating one metal with another by *electrodeposition. 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 expensive, or corrosion-resistant layer of
another metal.
electropositive Describing elements that tend to lose electrons
and form positive ions. The alkali
metals are typical electropositive elements.
electrospray ionization (ESI) A
technique for producing ions for
mass spectrometry, used especially
for obtaining ions from large molecules, which would be likely to pro201
electrospray ionization
e
www.AzShimi.ir www.AzShimi.com