53
52
pH
mV
pH
mV
pH
mV
pH
mV
pH
mV
0
990
1
920
2
860
3
800
4
740
5
680
6
640
7
580
8
520
9
460
10
400
11
340
12
280
13
220
14
160
Reducing pretreatment: immerse the electrode for a few min-
utes in
HI 7091
.
Oxidizing pretreatment: immerse the electrode for a few min-
utes in
HI 7092
.
If the pretreatment is not performed, the electrode will take
significantly longer to respond.
As with pH electrodes, gel-filled redox electrodes are more
suitable for industrial applications due to lesser mainte-
nance requirements. However, if working with refillable
electrodes, the electrolyte level should not fall more than
2½ cm (1") below the fill hole and topped up if necessary.
Use
HI 7071
Refill Solution for single junction and
HI 7082
for double junction electrodes.
In the event that measurements are performed with solu-
tions containing sulfides or proteins, the cleaning of the
diaphragm of the reference electrode must be performed
more often to maintain the proper functioning of the ORP
electrode. Therefore, immerse it into
HI 7020
and mea-
sure the response; the obtained value should be within
200 and 275 mV.
After this functional test, it is suggested to wash the elec-
trode thoroughly with water and proceed to the oxidizing
or reducing pretreatment before taking measurements.
When not in use, the electrode tip should be kept moist
and far from any type of mechanical stress which might
cause damage. This can be achieved by installing the elec-
trode in such a way that it is constantly in a well filled with
the sample (stream or tank). The protective cap can also
be filled with
HI 70300
Storage Solution if the electrode is
not being used at all.
Note
With industrial applications, it is always recommended to
keep at least one spare electrode handy. When anomalies
are not resolved with a simple maintenance, change the elec-
trode to see if the problem is alleviated.
TAKING REDOX MEASUREMENTS
Redox measurements allow the quantification of the oxidizing
or reducing power of a solution, 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 automati-
cally reduced, therefore the term oxidation-reduction is
frequently used.
Redox potentials are measured by an electrode capable of
absorbing or releasing electrons without causing a chemical
reaction with the elements with which it comes into contact.
The electrodes most usually available for this purpose have
gold or platinum surfaces; gold possesses a higher resis-
tance than platinum in conditions of strong oxidation such
as cyanide, while platinum is preferred for the measure-
ments of oxidizing solutions containing halides and for
general use.
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 ab-
sorbs hydrogen in the presence of reducing mediums. This
phenomenon is rough on the electrode.
To make correct redox measurements the following condi-
tions must prevail:
– The surface of the electrode must be cleaned and smooth.
– The surface of the electrode must undergo a pretreat-
ment in order to respond quickly.
Because the Pt/PtO system depends on the pH, the pre-
treatment of the electrode may be determined by the pH
and the redox potential values of the solution to be mea-
sured.
As a general rule, if the ORP mV reading corresponding to
the pH value of the solution is higher than the values in the
table below, an oxidizing pretreatment is necessary; other-
wise a reducing pretreatment is necessary:
