tion potentials. See also electromo-
tive series.
electrodialysis A method of ob-
taining pure water from water con-
taining a salt, as in desalination.
The water to be puriÜed is fed into a
cell containing two electrodes. Be-
tween the electrodes is placed an
array of semipermeable membranes
alternately semipermeable to posi-
tive ions and negative ions. The ions
tend to segregate between alternate
pairs of membranes, leaving pure
water in the other gaps between
membranes. In this way, the feed
water is separated into two streams:
one of pure water and the other of
more concentrated solution.
electroendosmosis See electro-
osmosis.
electroforming A method of form-
ing intricate metal articles or parts
by *electrodeposition of the metal on
a removable conductive mould.
electrokinetic potential (zeta po-
tential)Symbol ζ. The electric poten-
tial associated with an *electrical
double layer around a colloid at the
radius of shear, relative to the value
of the potential in the bulk of the so-
lution far from the colloid, where the
radius of shear is the radius of the
entity made up of the colloid and the
rigid layer of ions at the surface of
the colloid.
electroluminescence See lumines-
cence.
electrolysis The production of a
chemical reaction by passing an elec-
tric current through an electrolyte.
In electrolysis, positive ions migrate
to the cathode and negative ions to
the anode. The reactions occurring
depend on electron transfer at the
electrodes and are therefore redox re-
actions. At the anode, negative ions
in solution may lose electrons to
form neutral species. Alternatively,
atoms of the electrode can lose elec-
trons and go into solution as positive
ions. In either case the reaction is an
oxidation. At the cathode, positive
ions in solution can gain electrons to
form neutral species. Thus cathode
reactions are reductions.electrolyte A liquid that conducts
electricity as a result of the presence
of positive or negative ions. Elec-
trolytes are molten ionic compounds
or solutions containing ions, i.e. solu-
tions of ionic salts or of compounds
that ionize in solution. Liquid metals,
in which the conduction is by free
electrons, are not usually regarded as
electrolytes. Solid conductors of ions,
as in the sodium–sulphur cell, are
also known as electrolytes.electrolytic cell A cell in which
electrolysis occurs; i.e. one in which
current is passed through the elec-
trolyte from an external source.electrolytic corrosion Corrosion
that occurs through an electrochemi-
cal reaction. See rusting.
electrolytic gas (detonating gas)
The highly explosive gas formed by
the electrolysis of water. It consists of
two parts hydrogen and one part oxy-
gen by volume.electrolytic reÜningThe puriÜca-
tion of metals by electrolysis. It is
commonly applied to copper. A large
piece of impure copper is used as the
anode with a thin strip of pure cop-
per as the cathode. Copper(II) sul-
phate solution is the electrolyte.
Copper dissolves at the anode: Cu →
Cu2++ 2e, and is deposited at the
cathode. The net result is transfer of
pure copper from anode to cathode.
Gold and silver in the impure copper
form a so-called anode sludge at the
bottom of the cell, which is recov-
ered.195 electrolytic refining
e