Taking redox measurements – Hanna Instruments HI 8711 User Manual
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Redox measurements allow the quantification of the solution oxidizing/
reducing power, and are commonly expressed in mV.
Oxidation may be defined as the process during which a molecule (or
an ion) loses electrons and reduction as the process by which electrons
are gained.
Oxidation is always coupled together with reduction, so that as one
element gets oxidized, the other is automatically reduced, therefore
the term oxidation-reduction is frequently used.
Redox potentials are measured by an electrode capable of absorbing
or releasing electrons without causing any chemical reaction.
The most common ORP electrodes are provided with gold or platinum
surfaces; gold features a higher resistance than platinum in conditions
of strong oxidation, while platinum is preferred for measuring oxidizing
solutions containing halides, and for general purposes.
When a platinum electrode is immersed in an oxidizing solution, a
monomolecular layer of oxygen is developed on its surface. This layer
does not prevent the electrode from functioning, but it increases the
response time. The opposite effect is obtained when the platinum
surface absorbs hydrogen in the presence of reducing mediums. This
phenomenon is rough on the electrode.
To make correct redox measurements, it is necessary that the surface
of the electrode is clean and smooth, and that a preventive treatment
is performed.
Because the Pt/PtO system depends on the solution pH level, the
electrode pre-treatment may be determined by the pH and the redox
potential of the solution to be measured.
Generally, if the ORP (mV) reading corresponding to the pH solution
value is higher than the value in the table below, an oxidizing
pre-treatment is necessary; otherwise a reducing pre-treatment is necessary:
Reducing pre-treatment: immerse the electrode for some minutes in
HI 7091 solution.
TAKING REDOX MEASUREMENTS
TAKING REDOX MEASUREMENTS
TAKING REDOX MEASUREMENTS
TAKING REDOX MEASUREMENTS
TAKING REDOX MEASUREMENTS
H
p
V
m
H
p
V
m
H
p
V
m
H
p
V
m
H
p
V
m
0
0
9
9
1
0
2
9
2
0
6
8
3
0
0
8
4
0
4
7
5
0
8
6
6
0
4
6
7
0
8
5
8
0
2
5
9
0
6
4
0
1
0
0
4
1
1
0
4
3
2
1
0
8
2
3
1
0
2
2
4
1
0
6
1