Chapter 7 Reference
7.2 mV (oxidation-reduction potential [ORP]) measurement
94
HORIBA
7.2
mV (oxidation-reduction potential [ORP]) measurement
●
ORP principles
ORP is an abbreviation for oxidation-reduction potential. ORP is the
energy level (potential) determined according to the state of equilibrium
between the oxidants (M
z+
) and reductants (M
(z-n)+
) that coexist within a
solution.
For one type of equilibrium in a solution:
M
z+
+ne
-
⇔
M
(z-n)+
••••••
①
If only
①
exists within a solution, a metal electrode (platinum, gold, etc.)
and a reference electrode are inserted into the solution, forming the
ORP measuring system shown in Fig. 1. Measuring the potential (ORP)
that exists between the two electrodes enables the potential to generally
be expressed by the following equation.
••••••
②
E: Electric potential E
0
: Coefficient R: Gas coefficient
T: Absolute temperature n: Electron count
F: Faraday constant a: Activity
Fig. 1 ORP measuring system
For example, for a solution in which trivalent iron ions coexist with
bivalent iron ions, equations
①
and
②
would be as follows.
Fe
3+
+e
-
⇔
Fe
2+
••••••
①
'
••••••
②
'
When only one type of equilibrium state
①
exists in the solution, the
ORP of the solution can only be determined by equation
②
. What is
important here is that ORP is determined by the ratio of activity between
the oxidant (Fe
3+
) and the reductant (Fe
2+
)) (using the equation aFe
2+
/
aFe
3+
). In actuality, however, many kinds of states of equilibrium exist
simultaneously between various kinds of ions, in most solutions. This
means that under actual conditions, ORP cannot be expressed using
E
E0
RT
nF
‑‑‑‑
aM
z
n
–
(
)
+
aM
z +
‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑
ln
–
=
Potentiometer
E
E0
RT
F
‑‑‑‑
aFe
2 +
aFe
3 +
‑‑‑‑‑‑‑‑‑‑‑‑‑‑
ln
–
=
