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INTRODUCTION TO CELLS: TERMINOLOGY AND BACKGROUND 301

For this reason, we say a battery or cell discharges during operation, with each
electron from the cell flowing from high energy to low.
All electrochemical cells (including batteries) have two poles:
Currents conduct
one relates to the half-cell that is positively charged and the other
through an electrode
relates to the negatively charged pole. Negatively charged electrons
by means of electrons.
are produced at the anode as one of the products of the electro-
chemical reaction occurring at there. But if the electrons are to
move then we need something through which they can conduct to and from the
terminals of the cell: we need an electrode.
The phenomenon we call electricity comprises a flow of charged electrons. Wood
is a poor conductor of electricity because electrons are inhibited from moving freely
through it: we say the wood has a high electrical ‘resistance’ R.By
contrast, most metals are good conductors of charge. We see how
A redox electrode acts
an electrode needs to be electrically conductive if the electrons are as a reagent as well as
to move. an electron conductor,
Most electrodes are metallic. Sometimes the metal of an electrode as the metal of an
can also be one component part of a redox couple. Good examples electrode can also be
include metallic iron, copper, zinc, lead or tin. A tin electrode forms one component part of
a couple when in contact with tin(IV) ions, etc. Such electrodes are aredox couple.
called redox electrodes (or non-passive). In effect, a redox electrode
has two roles: first, it acts as a reagent; and, secondly, it measures
the energy of the redox couple of which it forms one part when Metallic mercury is a
connected to a voltmeter. poor choice of inert
Some metals, such as aluminium or magnesium, cannot function electrode at positive
potentials because it
as redox electrodes because of a coating of passivating oxide. Oth-
oxidizes to form Hg(II)
ers, such as calcium or lithium, are simply too reactive, and would
ions.
dissolve if immersed in solution.
But it is also extremely common for both redox states of a redox
couple to be non-conductive. Simple examples might include dis- We require an inert
solving bromine in an aqueous solution of bromide ions, or the
electrode when both
oxidation of hydrogen gas to form protons, at the heart of a hydro-
parts of a redox couple
gen fuel cell; see Equation (7.13). In such cases, the energy of reside in solution, or
the couple must be determined through a different sort of elec- do not conduct: the
trode, which we call an inert electrode. Typical examples of inert electrode measures the
electrodes include platinum, gold, glassy carbon or (at negative energy of the couple.
potentials) mercury. The metal of an inert electrode itself does not
react in any chemical sense: such electrodes function merely as a
probe of the electrode potential for measurements at zero current, We denote an amal-
and as a source, or sink, of electrons for electrolysis processes if gam by writing the ‘Hg’
current is to flow.
in brackets after the
The final class of electrodes we encounter are amalgam elec- symbol of the dissolved
trodes, formed by ‘dissolving’ a metal in elemental (liquid) mer- element; cobalt amal-
cury, generally to yield a solid. We denote an amalgam with brack- gam is symbolized as
ets, so the amalgam of sodium in mercury is written as Na(Hg). The Co(Hg).
properties of such amalgams can be surprisingly different from their