Chemistry Reference
In-Depth Information
Solution 1
Solution 2
H
+
H
+
Known pH
Unknown pH
H
+
H
+
H
+
H
+
H
+
H
+
H
+
H
+
H
+
H
+
H
+
H
+
H
+
H
+
H
+
D
E
3.3
Potential variation over a glass membrane in contact with two
solutions of different composition, one being of constant and
known hydrogen-ion activity.
pH
=-
log
a
[3.4]
In practice, the trick to obtain the pH is to use a membrane that is on one
side exposed to a solution of known and constant hydrogen-ion activity (e.g.
a buffer solution) and on the other side is exposed to the solution of
unknown pH (Fig. 3.3). On the first side of the membrane, an exchange of
H
+
occurs, but because of the constant composition of the buffer solution
1, this will result in a constant potential,
E
1
. On the other side of the mem-
brane, a potential,
E
2
, is established which is determined by the activity of
hydrogen ions in the solution of unknown pH. Therefore, D
E
is dependent
only on
E
2
, and thus the pH of this solution can be determined using a glass
membrane.
Of course, before such a glass membrane can be used, the measured
potential, D
E
must be linked to the pH. This can be done through calibra-
tion by measuring the potential differences when the membrane surface is
immersed in buffer solutions of known pH. In that case,
E
2
is linked to the
pH of the buffer and a calibration curve. Finally, a solution of unknown pH
can be analysed by measuring D
E
and calculating the pH using the cali-
bration curve.
+
H
