Fig. 1 Three-electrode device. V, voltmeter

E. Protopopoff and P. Marcus, Potential Measurements with Reference Electrodes, Corrosion: Fundamentals,
Testing, and Protection, Vol 13A, ASM Handbook, ASM International, 2003, p 13–-16
Potential Measurements with Reference Electrodes
E. Protopopoff, Laboratoire de Physico-Chimie des Surfaces, CNRS, and P. Marcus, Ecole Nationale Supérieure de Chimie de Paris,
Université Pierre et Marie Curie

Electrode Selection Characteristics
A good reference electrode must reach its potential quickly, be reproducible, and remain stable with time. It
must have a practically nonpolarizable metal-solution interface; that is, its potential must not depart
significantly from the equilibrium value on the passage of a small current across the interface. The potential of
the junction between the electrolytes of the reference and test electrodes must be minimized. These criteria are
detailed subsequently.
Stable and Reproducible Potential. Electrodes used as references should rapidly achieve a stable and
reproducible potential that is free of significant fluctuations. To obtain these characteristics, it is advantageous,
whenever possible, to use reversible electrodes, which can easily be made.
The reference electrode arbitrarily chosen to establish a universal potential scale is the standard hydrogen
electrode (SHE). It consists of a platinized or black platinum wire or sheet immersed in an aqueous solution of
unit activity of protons saturated with hydrogen gas at a fugacity of one bar (14.5 psia). The half-cell reaction is
H
+
(aq) + e
-
↔ H
2
(g). Any non-standard reversible hydrogen electrode with well-controlled H
+
activity and H
2

particularly from oxygen; furthermore, the platinum electrode must be frequently replatinized, because it easily
gets “poisoned” by the adsorption of impurities present in the solution, which prevents the establishment of the
equilibrium potential. For these reasons, practical corrosion measurements are usually not performed with the
SHE but with secondary reference electrodes that are easier to construct and handle, less sensitive to impurities,
and whose potential is very stable and well- known with respect to the SHE (Ref 1).
The simpler reference electrodes are metal-ion (M
z+
/M) electrodes, also called metallic electrodes of the first
kind. The copper-copper sulfate (CuSO
4
/Cu) electrode is an excellent example of a good reversible electrode
and it is widely used as a reference electrode in the corrosion field. It can easily be made by immersing a copper
wire in a glass tube filled with a CuSO
4
aqueous solution and terminated by a porous plug (to allow ionic
conduction with the cell electrolyte), as shown in Fig. 2.

Fig. 2 Schematic of a copper/copper sulfate reference electrode
This electrode is reversible, because a small cathodic current produces the reduction reaction (Cu
2+
+ 2e
-
→
Cu), while an anodic current brings about the oxidation reaction (Cu → Cu
2+
+ 2e
-
). Copper is a semi-noble
metal and does not dissolve anodically in a solution of protons. In the case of the CuSO
4

For example, if silver chloride (AgCl), only slightly soluble in water, is present on the silver surface, then the
following equilibrium holds:
AgCl ↔ Ag
+
+ Cl
-(Eq 4)
The equilibrium mass law gives the solubility product as = K
s(AgCl)
. Expressing from this
relation and placing it in Eq 3 leads to the following expression of the equilibrium potential of the Ag
+
, Cl
-
/AgCl/Ag half-cell: (Eq 5)
corresponding to the global electrode equilibrium:
AgCl + e
-
↔ Ag + Cl
-(Eq 6)
The standard potential of this reversible silver-silver chloride (Ag/Cl) electrode is =
+ ln K

2
) which floats as a paste on the top of a liquid mercury drop and which, in
contact with a KCl solution, dissociates slightly into and Cl
-
ions. Using the same method as for the Ag-
AgCl electrode, the following relation is obtained: (Eq 7)
corresponding to the global electrode equilibrium:
Hg
2
Cl
2
+ 2e
-
↔ 2Hg + 2Cl
-(Eq 8)
Figure 3 shows a typical calomel electrode. A platinum wire connects the electrode to the rest of the circuit.
The most usual calomel electrodes are prepared with KCl solutions at a unit molarity of Cl
-
(normal calomel
electrode or NCE) or saturated calomel electrode or (SCE). At 25 °C (77 °F),
= 0.268 V/SHE, E
NCE
= 0.281 V, and E
SCE

, where E is
the potential, and E
eq
is the equilibrium potential, by (Ref 3): (Eq 9)
where i
0
is the exchange current density of the electrode reaction; and α is the charge transfer coefficient for the
anodic reaction (0 < α < 1), whose value is close to 0.5 for a single-step reaction (n = 1). (More detailed
information on polarization curves can be found in the articles “Kinetics of Aqueous Corrosion” and
“Electrochemical Methods of Corrosion Testing” in this Volume).
For potentials close to the equilibrium potential, such as η < RT/F (26 mV at room temperature), the relation
(Eq 9) can be approximated by a linear i versus η dependence: (Eq 10)
or (Eq 11)
The term RT/nFi
0
= (dη/di)
0
is called the polarization resistance, R
p
. A good reference electrode should have a
low polarization resistance, which implies high exchange current density. This happens for high rate constants

potential, which is included in the measured voltage, V, as expressed in: