Chemistry Reference
In-Depth Information
Surface 1
Surface 2
Solution 1
Solution 2
E
1
D
E
E
2
3.1
Scheme of the potential difference over an ion-selective mem-
brane, with both surface sides in contact with a solution contain-
ing chemical species.
Typical for ion-selective electrodes is the selective exchange of one or
a limited number of species dissolved in solution at the surface of the
membrane. This can be obtained through different mechanisms, as will be
discussed in the next section of this chapter. The intensity of this exchange
determines the values of
E
1
and
E
2
and is therefore the value of D
E
.As a
consequence, the higher the activity of species i in the solution, the higher
the value of D
E
will be. This principle can be used for electroanalytical pur-
poses - hence the value of potentiometric sensors.
3.2.2
Types of ion-selective electrodes
A wide range of ion-selective electrodes were developed and can be clas-
sified in subgroups of the following main groups:
•
solid-membrane electrodes;
•
liquid-membrane electrodes.
Subgroups for the solid-membrane electrodes are mainly:
•
Glass-membrane electrodes, such as pH electrodes. In this type of elec-
trode, a glass body acts as a membrane and shows affinity for different
