Chemistry Reference
In-Depth Information
At an early date, Nernst also introduced the concept of potentiometry
with polarised electrodes
5-7
, that went together with the many other spe-
cialised forms of potentiometric measurements for a wide range of chemi-
cal systems
8
.
2.2.2
Fundamentals of potentiometry
Potentiometric measurements are based on the Nernst equation, which was
developed from thermodynamic relationships and is therefore valid only
under equilibrium (read thermodynamic) conditions. As mentioned above,
the Nernst equation relates potential to the concentration of electroactive
species. For electroanalytical purposes, it is most appropriate to consider
the redox process that occurs at a single electrode, although two electrodes
are always essential for an electrochemical cell. However, by considering
each electrode individually, the two-electrode processes are easily com-
bined to obtain the entire cell process. Half reactions of electrode processes
should be written in a consistent manner. Here, they are always written as
reduction processes, with the oxidised species, O, reduced by
n
electrons to
give a reduced species, R:
ยจ
Oe R
+
n
-
[2.1]
For such a half reaction the free energy is given by the relation:
D
GG T
a
a
R
=
0
+
R
ln
[2.2]
O
or:
D
GGT
a
a
O
-=- +
0
R
ln
[2.3]
R
where -D
G
is the electromotive force indicating the tendency for the reac-
tion to go to the right; R is the gas constant (8.317 J mol
-1
K
-1
),
T
is the tem-
perature (K), and the quantity D
G
0
is the free energy of the half reaction
when the activities of the reactant and product have values of unity and is
directly proportional to the standard half-cell potential for the reaction as
written. The electromotoric force, also called the free energy of this half
reaction, is related to the electrode potential,
E
, by the expressions:
[2.4]
-=
D
Gr
F
-=
D
GrE
0
F
0
[2.5]
It also is a measure of the equilibrium constant for the half reaction assum-
ing the activity of electrons is unity and under this condition the following
equation is valid:
