Chemistry Reference
In-Depth Information
1
2
5
3
6
4
1.4
Scheme of a Ag|AgCl reference electrode with (1) connecting
tube, (2) silver wire, (3) silver wire coated with AgCl, (4) KCl solu-
tion, (5) cap for replacement of KCl solution and (6) glass frit.
However, this equilibrium is not obtained in practice because the Ag
+
ions
form immediately the sparingly soluble AgCl with the chloride ions in solu-
tion. Therefore the potential given in Equation 1.10 will be dependent of
the presence of Cl
-
ions. The unknown parameter is the activity of Ag
+
, but
this parameter can be calculated through the solubility constant, K
sp
:
K
sp
=
aa
[1.11]
+
-
Ag
Cl
Substitution of Equation 1.11 in Equation 1.10 results in the following
equation:
K
R
T
sp
0
EE
=
+
2.
log
[1.12]
+
Ag Ag
eq
n
F
a
-
Cl
which results in Equation 1.13 after reorganisation of the right-hand side of
Equation 1.12:
R
T
EE
=
0
-
2.
log
a
[1.13]
eq
Ag AgCl
-
n
F
Cl
The potential
E
0
Ag|AgCl
is equal to
E
0
Ag|Ag
+
+ log K
sp,AgCl
. In a saturated chlo-
ride solution, the activity of the chloride ions is equal to one, therefore
Equation 1.13 simplifies to Equation 1.14, which shows that the potential is
constant and not dependent on a dissolved species present in the electrolyte
solution:
EE
eq
=
0
[1.14]
Ag AgCl
