31
30
Reducing pretreatment: immerse the electrode in
HI 7091
solution
for
a few minutes.
Oxidizing pretreatment: immerse the electrode in
HI 7092
solution
for
a few minutes.
If the pretreatment is not performed, the electrode will take signifi-
cantly longer to respond.
As with pH electrodes, gel-filled redox electrodes are more suitable for
industrial applications due to lesser maintenance requirements. How-
ever, 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 solutions contain-
ing sulfides or proteins, the cleaning of the diaphragm of the
reference electrode must be performed more often.
In order to have a correct functioning of the ORP electrode, immerse
it into
HI 7020
and measure the response; the obtained value should
be within 200 and 275 mV.
After this functional test, it is suggested to wash the electrode
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 electrode 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 electrode to
see if the problem is alleviated.
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 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 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 resistance than platinum
in conditions of strong oxidation such as cyanide, while platinum is
preferred for the measurements 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 absorbs hydrogen in the presence of reducing mediums. This
phenomenon is rough on the electrode.
To make correct redox measurements the following conditions must
prevail:
– The surface of the electrode must be cleaned and smooth.
– The surface of the electrode must undergo a pretreatment in
order to respond quickly.
Because the Pt/PtO system depends on the pH, the pretreatment of
the electrode may be determined by the pH and the redox potential
values of the solution to be measured.
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; otherwise a reducing pretreat-
ment is necessary:
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
TAKING REDOX MEASUREMENTS