Chapter 7 Reference
7.2 mV (oxidation-reduction potential [ORP]) measurement
96
HORIBA
●
Standard electrode (reference electrode) types and ORP
The ORP of a solution that is obtained through measurement is a value
that corresponds to the reference electrode employed. If different kinds
of reference electrodes are used for measurement, the ORP value of
the same solution may appear to be different.
HORIBA uses Ag/AgCl with 3.33 mol/L KCl as the reference solution for
reference electrodes. According to general technical literature, standard
hydrogen electrodes (N.H.E.) are often used as the standard electrode.
The relationship between N.H.E. and the ORP that is measured using
an Ag/AgCl with 3.33 mol/L KCl electrode is expressed by the following
equation.
E
N.H.E.
= E+206-0.7(t-25) mV t = 0 – 60ºC
E
N.H.E.
:Measured ORP value using N.H.E. as the reference
electrode
E : Measured ORP value using Ag/AgCl with 3.33 mol/L
KCl as the reference electrode
●
Potential sign
Standard ORP is expressed in the following way, in literature related to
electrochemistry and analytical chemistry.
A Li
+
+e
-
→
Li
E
0
=-3.024 V VS N.H.E.
However, in some literature, the “+” and “-” signs are reversed.
B Li
→
Li
+
+e
-
E
0
=+3.024 V VS N.H.E.
In expressions like B, above, the reaction is just reversed and there is
no essential difference. But this kind of expression does invite
confusion. The majority of the world is consistent in its use of the signs
as they are used in A, above. For this reason, HORIBA also uses signs
concerning ORP that are consistent with A.
