Hanna Instruments bl 7917 User Manual
30
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:
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
– 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
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
TAKING REDOX MEASUREMENTS
TAKING REDOX MEASUREMENTS
TAKING REDOX MEASUREMENTS
TAKING REDOX MEASUREMENTS