2. REFERENCE ELECTRODE
An ideal reference electrode is one that maintains a
constant potential irrespective
of the amount of current (if any) that is passed
through it.
3. Voltammetric methods are those in which current
passing in an electrochemical cell is measured as a
function of the potential applied to the working
electrode. Potential, by definition, is not something
that can be directly measured. Rather, the
measurement of applied potential requires that a
reference point first be established, and individual
potentials be measured relative to that reference
point. This is accomplished by placing a second
electrode, called the reference electrode, in the cell
and measuring potential as the energy difference
between the two electrodes, called the reference
electrode.
4. Reference electrodes should be constructed using
half-cell components that are stable over time and
with changing temperature, present at well-defined
values of activity. They should possess fixed,
reproducible electrode potentials.
5. The reference half-cell with which most of us
are familiar is the standard hydrogen
electrode (SHE), composed of an inert solid
like platinum on which hydrogen gas is
adsorbed, immersed in a solution containing
hydrogen ions at unit activity. The half-cell
reaction for the SHE is given by
H+ + e-→ ½ H2 (g) ( 1atm)
6. The potential of the standard
hydrogen electrode is arbitrarily
assigned the value of zero (0.000)
volts at 250 C. When standard
hydrogen electrode is coupled with
another electrode, the EMF of the cell
thus formed will evidently be equal
to the potential of that electrode to
which it is coupled. The potentials
obtained in this way are called
relative potentials, or electrode
potentials on the hydrogen scale.
7. Practical application of the SHE is limited by the
difficulties in preparing solutions containing H+ at
unit activity and maintaining unit activity for H2
(g) in the half-cell. Most experiments carried out
in aqueous solutions utilize one of two other
common reference half cells – the saturated
calomel electrode (SCE) or the silver-silver
chloride electrode (Ag/AgCl).
8. The Calomel Electrode
Calomel electrode is most widely used as secondary reference
electrode. The standard calomel electrode (SCE) comprises of
wide glass – tube fitted with narrow side tubes. Pure mercury is
placed at the bottom of the wide tube and covered with a paste
of mercurous chloride (calomel). A solution of KCl is introduced
above the paste. A saturated KCl solution may also be used. A
Pt wire sealed into a glass tube is used to make electrical
contact. The side tube containing KCl solution provides the salt
bridge, which connects the electrode to any other electrode.
The calomel electrode is represented as
Cl– (1 M) Hg2Cl2 , Hg
And the half-cell reaction is written as
Hg2Cl2 + 2e– 2Hg + 2Cl–
10. Silver/Silver Chloride Electrode
(Ag/AgCl)
The silver/silver chloride reference electrode is
composed of a silver wire (Ag) that has been
coated with a layer of solid silver chloride
(AgCl), immersed in a solution that is saturated
with KCl and AgCl. The half cell reaction is
AgCl (s) + e - ↔Ag (s) + Cl-(sat’d)
11. Both the SCE and the Ag/AgCl reference
electrodes offer stable half-cell potentials that
do not change over time
