Chemistry Reference
In-Depth Information
3.3.2
Practice
Practically, D
E
is calculated by measuring the potential over two reference
electrodes (e.g. Ag|AgCl electrodes), each electrode being positioned at one
side of the glass membrane (Fig. 3.4a). The potential of both reference elec-
trodes is the same because they are identical, so if a difference in potential
is measured over these electrodes, this difference should be related to the
presence of the glass membrane. As explained above, the contribution of
E
1
is constant, so the potential difference measured over the entire cell
system is related to
E
2
, or to the hydrogen-ion activity of the solution of
unknown pH.
In order to make this type of measurement practically possible in a rela-
tively easy way, pH electrodes were developed (Fig. 3.4b), which consist of
a glass tube (1) ending in a ball or other symmetrical shape (2) and filled
with a buffer solution (3). It is the ball-shaped part of the tube that acts as
the measuring zone for hydrogen-ion activity-monitoring and thus will be
in direct contact with the solution of unknown pH. Inside this compartment
and immersed in the buffer solution, a Ag|AgCl electrode is positioned (4).
The glass tube (1) is surrounded by a second tube (5), filled with a Cl
-
solu-
tion (6) of identical concentration as the one used for the inner part (3) of
the electrode. Also in this tube, a Ag|AgCl reference electrode (7) is
immersed, which is in contact with the solution of unknown pH through a
diaphragm (8). The potential difference measured over the two reference
electrodes is related to the hydrogen-ion activity of the solution of unknown
Reference electrode 1
Reference electrode 2
Solution 1
Solution 2
H
+
H
+
H
+
H
+
6
Known pH
Unknown pH
7
H
+
5
H
+
H
+
H
+
H
+
H
+
8
4
H
+
H
+
H
+
H
+
H
+
H
+
H
+
1
(a)
2
3
(b)
3.4
(a) Scheme of the setup for measurement of pH with a glass
membrane and (b) scheme of (a) incorporated into a suitable elec-
trode system that allows simple and user-friendly handling.
