User Guide

17
16
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 un-
dergo a preventive treatment depend-
ing whether the solution to be mea-
sured has oxidizing or reductive char-
acteristics.
Because the Pt/PtO system depends on the
pH, the pretreatment of the electrode may
be determined by the pH and the redox
potential of the solution to be measured.
As a general rule, 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; other-
wise a reducing pre-treatment 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
Reducing pre-treatment: immerse the elec-
trode for a few minutes in HI 7091.
Oxidizing pre-treatment: immerse the elec-
trode for a few minutes in HI 7092.
If pre-treatment is not performed, the elec-
trode will take a significantly longer time to
respond.
When working with electrodes of the refill-
ing type, the electrolyte used for filling must
be constantly kept at an adequate level (no
less than 2½ centimeters from the filling
hole) and topped up if necessary with HI 7071
refilling solution.
TAKING REDOX MEASUREMENTS
Redox measurements allow the quantifica-
tion 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 oxi-
dized, the other automatically is reduced,
therefore the term oxidation-reduction is fre-
quently used.
Redox potentials are measured by an elec-
trode capable of absorbing or releasing elec-
trons 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 plati-
num in conditions of strong oxidation, while
platinum is preferred for the measurements
of oxidizing solutions containing halides and
for more general uses.
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 phe-
nomenon is rough on the electrode.