solubility product Ksp. Therefore, if we put insoluble silver chloride, AgCl, in
contact with a silver rod,
a (Ag+) =Ksp/(a(Cl-))so that the concentration of silver ions is controlled by the silver chloride
solubility.E(AgCl/Ag ) =En(AgCl/Ag) -RT/Fln (a(Cl-))
Since Ksp(AgCl) ~ 1.6¥ 10 -^10 at 25∞C,En(AgCl/Ag) =0.22 V
This electrode is reversible to chloride ions.
Another important electrode of this class is the calomel electrode,which is
discussed below.
In order to make accurate potentiometric measurements, a cell must be
constructed that is reproducible and reliable. The emf should depend chiefly on
the particular species in the sample that it is intended to measure, and if neces-
sary the system must be calibrated.
As noted in Topic C2, electrode potentials are conventionally referred to the
standard hydrogen electrode (SHE), for which the standard electrode potential,
En=0.000V. Another electrode, for example the silver electrode, may be com-
bined with the SHE to form a cell. Table 2gives a representative list of standard
electrode potentials at 25∞C.68 Section C – Analytical reactions in solution
Table 2. Standard reduction electrode potentials at 25∞C
Electrode reaction En/V
Li+ +e-=Li -3.04
K+ +e-=K -2.92
Ca^2 + +2e-=Ca -2.87
Na+ +e-=Na -2.71
Mg^2 + +2e-=Mg -2.37
Al^3 + +3e-=Al -1.66
Mn^2 + +2e-=Mn -1.18
Zn^2 + +2e-=Zn -0.76
Fe^2 + +2e-=Fe -0.44
Sn^2 + +2e-=Sn -0.14
Pb^2 + +2e-=Pb -0.13
H+ +e-=^1 ⁄ 2 H 0.0000 exactly
AgBr +e-=Ag +Br- +0.10
Sn^4 + +2e-=Sn^2 + +0.15
AgCl +e-=Ag +Cl- +0.22
Cu^2 + +2e-=Cu +0.34
Hg 2 Cl 2 +2e-=2Hg +2Cl- +0.27(^1) ⁄ 2 I 2 +e-=I- +0.54
Fe^3 + +e-=Fe^2 + +0.76
Ag+ +e-=Ag +0.80
IO 3 - +6H++5e-=^1 ⁄ 2 I 2 +3H 2 O +1.19
(^1) ⁄ 2 O 2 +2H++2e-=H 2 O +1.23
(^1) ⁄ 2 Cr 2 O 72 - +7H++3e-=Cr^3 ++7/2 H 2 O +1.33
(^1) ⁄ 2 Cl 2 +e-=Cl- +1.36
Ce^4 + +e-=Ce^3 + +1.44
MnO 4 - +8H++ 5e-=Mn^2 ++4H 2 O +1.52