Chemistry Reference
In-Depth Information
(
)
EEEE
cell
=-
cell
being positive
[2.14]
1
2
Insertion of the appropriate forms of Equation 2.7 gives an overall expres-
sion for the cell potential:
a
b
aa
aa
R
T
O
R
0
0
EEE
=-+
1
ln
1
2
[2.15]
cell
2
n
F
a
b
O
R
1
2
The equilibrium constant for the chemical reaction expressed by Equation
2.15 is related to the difference of the standard half-cell potentials by the
relation:
F
R
n
T
(
)
ln
K
=
EE
1
0
-
2
0
[2.16]
Equation 2.16 shows that potentiometry is a valuable method for the
determination of equilibrium constants. However, it should be borne in
mind that the system should be in equilibrium. Some other conditions,
which are described below, also need to be fulfilled for use of potentiome-
try in any application. The basic measurement system must include an indi-
cator electrode that is capable of monitoring the activity of the species of
interest, and a reference electrode that gives a constant, known half-cell
potential to which the measured indicator electrode potential can be
referred. The voltage resulting from the combination of these two elec-
trodes must be measured in a manner that minimises the amount of current
drawn by the measuring system. This condition includes that the impedance
of the measuring device should be much higher than that of the electrode.
For low-impedance electrode systems, a conventional potentiometer is
satisfactory. However, electrochemical measurements with high-impedance
electrode systems, and in particular the glass-membrane electrode, require
the use of an exceedingly high-input impedance-measuring instrument
(usually an electrometer amplifier with a current drain of less than 1 pA).
Because of the logarithmic nature of the Nernst equation, the measuring
instrumentation must have considerable sensitivity. Another important con-
dition is that the potential response is directly dependent on the tempera-
ture of the measuring system. Thus, if the correct temperature is not used
in the Nernst expression, large absolute errors can be introduced in the
measurement of the activity for an electroactive species. In addition, tem-
perature indirectly has an influence through the activity coefficients, ionic
strength and dilution of the solution.
2.2.3
Electrode systems in potentiometry
The indicator electrodes for potentiometric measurements have tradition-
ally been categorised into three separate groups. A first group of electrodes
