Chemistry Reference
In-Depth Information
consist of a metal immersed in a solution which contains ions of the same
metal as the electrode, for example, a copper electrode immersed in a
Cu(II) solution. These electrode systems provide a direct response to the
ion or species to be measured:
¨
Mn M
n
+
+
e
-
[2.17]
R
F
T
EE
=+
0
ln
a
[2.18]
M
n
+
n
Therefore, the primary electrode reaction includes the sensed species. Such
electrodes give a direct response according to the Nernst equation for the
logarithm of the activity of the species.
Electrodes classified in the second group of electrode systems are those
in which the metal electrode is coated with a layer of a sparingly soluble
salt of the electroactive species and the metal ion of the metal electrode,
such that the potentiometric response is indicative of the concentration
of the inactive anion species. Thus the silver/silver-chloride electrode
system, which is representative of this class of electrodes, gives a potential
response that is directly related to the logarithm of the chloride ion activ-
ity (see also Chapter 1, section 1.5), even though it is not the electroactive
species:
¨
AgCl
+
e
-
Ag
+
Cl
-
[2.19]
R
T
EE
=-
0
ln
a
[2.20]
-
n
F
Cl
This is true because the chloride ion concentration, through the solubility
product, controls the activity of the silver ion, which is measured directly
by the potentiometric silver-electrode system.
Finally, there is a third group of electrodes, which are a more specialised
case of the electrodes belonging to the second group. They consist of the
metal being in direct contact with a sparingly soluble salt of the metal, which
is then used to monitor the activity of an electroinactive metal ion in
equilibrium with a more soluble salt that includes the same anion as the
electrode-salt system. For example, the concentration of calcium ions
in equilibrium with solid calcium oxalate may be monitored using a
silver/silver oxalate electrode system. The concentration of calcium ion
affects the concentration of oxalate ion, which in turn controls the concen-
tration of silver ion; the latter is monitored by the potentiometric silver-
electrode system. A rather more important example of this type of electrode
is the Hg|Hg(II)-EDTA electrode system, which is used as a sensing system
for the potentiometric titration of electroinactive metal ions with EDTA
9-10
.
The stability constant of the Hg(II)-EDTA complex is so high that only a
