Having defined the half-reaction and a potential associated with
the reduction depicted by the half-reaction, we need to determine
what the potential is.
There need to be two half-cells to make up the whole cell, just
as there are two half reactions which make up the full redox reaction.
In defining potentials, they are made relative to the H+/H2
redox couple, which has a reduction potential of zero. A reference
electrode for determining the potential of other couples is therefore
the standard hydrogen electrode.
This has been (arbitrarily) chosen for reporting electrode potentials,
which are determined for the half-cell when measured against it.
The standard hydrogen is being used to measure the potential of the Fe2+/Fe3+ couple.
Some important features of the standard hydrogen
In the standard hydrogen electrode, the pressure
of the hydrogen gas is fixed at one atmosphere, and the concentration
of the hydrochloric acid is 1.18M. This corresponds to a proton
activity of one.
The potential between the two electrodes is measured
using a digital voltmeter. This draws a negligible current,
and so no electrolysis occurs during the potential measurement,
and there is no change in the concentrations of H+
and H2, or of Fe2+ and Fe3+.
The reference electrode is made from platinized
platinum, rather than a smooth platinum surface. This is to
ensure the presence of catalytic, defect sites, and thus fast
electrode kinetics, where the equilibrium, between electrons
at the metal surface and aqueous hydrogen ions with the hydrogen
gas produced, is rapidly established.
When the hydrogen electrode is operating under
standard conditions, and the activities of all the ions present
are one, the value of the potential measured is known as the
standard electrode potential of
the Fe2+/Fe3+ couple.
An important feature of the structure of the cell
used with the standard electrode is that the two half-cells
are separated by a salt bridge.
The salt bridge is a tube containing an aqueous solution of
potassium chloride, and its purpose is to keep the two half
cells in electrical contact, whilst preventing the solutions
from the two different half cells from mixing. Any mixing would
result in the potential being recorded being disturbed.
The Calomel reference electrode
The standard hydrogen electrode is that against
which the potentials of all other half-cells are made. However,
the standard hydrogen electrode is often inconvenient to use as
a reference electrode experimentally. It has to have freshly made
platinum electrodes, a solution of HCl with a very accurately
determined concentration corresponding to unit activity of hydrogen
ions, and the safe provision of a hydrogen gas source at 1 atmosphere
It is generally much more convenient to use a calomel
electrode as the reference electrode.
|The saturated Calomel reference electrode
A calomel electrode used as a reference electrode
eliminates liquid junction potentials
The reduction reaction occurring at the calomel
electrode corresponds to the reduction of mercury (I).
The potential of the calomel electrode depends
on the chloride ion activity, but this is kept constant, and
saturated, due to the present of excess Cl- in the
undissolved KCl crystals. The electrode is in constant with
the test solution by means of the porous frit in the bottom
of the electrode, which acts as the equivalent of a salt bridge.
To calculate the reduction potential for the test
solution, it is measured in a cell as above. To adjust this
so that it is compared to the standard hydrogen electrode, we
need to know the standard electrode potential of the saturated
calomel electrode: this is 0.242 V at 25 oC. The
standard electrode potential for the test solution is then given