NAVI D-25, Instruction Manual

Page: 193 / 227

7 Reference
7.2 mV (oxidation-reduction potential [ORP]) measurement
182 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.