Chapter 16 Reference
16.2 Measuring mV (oxidation-reduction potential [ORP])
F-52, 53, 54, 55 221
In this respect, the value of ORP must be understood to be only one indicator of the property
of a solution.
The measurement of ORP is widely used, however, as an important index in the analysis of
solutions (potentiometric titration) and in the disposal and treatment of solutions.
Recently, there have appeared various claims regarding this matter, such as that a high degree
of ORP is effective in sterilization or that drinking water that has a low ORP reduces the
chance of illness by reacting with the activated oxygen in the cells of the body. ORP is used as
an index for alkaline drinking water.
● 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 measure
-
ment, the ORP value of the same solution may appear to be different.
HORIBA uses Ag/AgCl with 3.33 mol/L KCl as the standard solution for reference elec-
trodes. 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
=-3.024 V VS N.H.E.
However, in some literature, the “+” and “-” signs are reversed.
B Li
=+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, today, is consis
-
tent in its use of the signs as they are used in A, above. For this reason, HORIBA, too, uses
signs concerning ORP that are consistent with A, above.
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