Chemistry Reference
In-Depth Information
3.5
Alternative shapes of the measuring zone of glass-membrane
electrodes.
pH, and this pH can be determined after calibration of the electrode in
buffer solutions of known pH.
Finally, in Fig. 3.5, different possible shapes of the glass membrane that is
in contact with the solution to be analysed are proposed. Note that all of
them have a high degree of symmetry.
3.3.3
Limitations of pH electrodes
The main limitation of glass electrodes is the fact that they should always
be in contact with an aqueous solution, e.g. 3 mol l
-1
KCl. If these electrodes
become dry, their properties for pH measurement will drop. It is possible
to recover pH electrodes after they have been dry for some time. However,
it will be noted that these electrodes have longer reaction times, show a
decrease of the slope of the D
E
-pH calibration curve, and display increased
impedance of the membrane and a shift of the zero point.
Besides the fact that pH electrodes should always be kept in solution,
their lifetime can also be shortened by using the electrodes at high tem-
peratures (their lifetime ranges from 1 to 3 years at room temperature, up
to 6 months at 323 K and only a few weeks at 373 K) and/or in poisoning or
chemically aggressive solutions. Some cleaning procedures for the latter
sources of life-time shortening are proposed, but with limited success, and
are based on ultrasonic and/or electromagnetic treatment or treatment in
an etching solution
13
.
Another important source of errors for pH electrodes is the alkali error
14
.
At high pH (>11), the concentration of H
+
is smaller than 1 ¥ 10
-11
mol l
-1
,
which indicates that at these levels the influence of other components, such
as sodium and potassium ions, becomes important. Under such conditions,
positive ions in the glass membrane are not only exchanged with H
+
but
also with Na
+
and K
+
.The latter ions also contribute to
E
2
, giving rise to an
