IM 12B03D02-01E
9-9
9
.
Technical Information
F0911.ai
Potentiometer
Test solution
Reference electrode
Indicator electrode (Pt)
E
Figure 9.11 Measurement System of ORP Meter
9.9 Reference Electrode
The oxidation-reduction potential in measurement is a value relative to the reference
electrode. If different types of reference electrodes are used, the oxidation-reduction
potentials of identical solutions are apparently different. In the electrochemical field,
the hydrogen electrode is generally used as a reference electrode. However, it has
complicated construction and is impractical. Therefore, in Yokogawa's ORP sensors,
an Ag/AgCl electrode filled with a 3.3 mol/L KCl solution is used as a reference
electrode.
The relationship between the Ag/AgCl electrode and the standard hydrogen electrode
can be derived from the Nernst equation based on the following cell scheme.
Electrode (Pt) H
2
Electrolyte solution
Liquid junction or salt bridge
KCl(m) AgCl Ag
The reaction formula is
AgCl + e Ag + Cl
-
-
, If
a
represents the activity,
from equation 9.6
E : oxidation-reduction potential when potential of
standard hydrogen electrode* is 0
E : standard electrode potential when [Ox] = [Red]
R : gas constant
F : Faraday constant
n : number of electrons
T : absolute temperature
[Ox] : activity of oxidized form of substance
[Red] : activity of reduced form of substance
E = E +
ln
R T
n F
[Ox]
[Red]
*
°
°
Standard Hydrogen Electrode (SHE)
Where:
(9.6)
E'
AgCl
can be obtained using the average ion activity coefficient,
g
±KCl
, of the KCl (m)
solution instead of the activity coefficient of Cl
–
(actual measurement is impossible).
E = E -
ln m
γ
R T
F
°
'
AgCl
KCl ±KCl
(9.7)
