Taking Redox Measurements - Hanna Instruments HI 22 Series Instruction Manual

• No Slope:
- Check the electrode for cracks in glass stem or bulb
(replace the electrode if cracks are found).
- Make sure cable and connections are not damaged nor
lying in a pool of water or solution.
• Slow Response/Excessive Drift: Soak the tip in Hanna solu-
tion HI 7061 for 30 minutes, rinse thoroughly in distilled
water and then follow the Cleaning Procedure above.
• For ORP Electrodes: polish the metal tip with a lightly abra-
sive paper (paying attention not to scratch the surface)
and wash thoroughly with water.
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 (and recalibrate the controller) to see if the problem
is alleviated.
60

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 automatically re-
duced, therefore the term oxidation-reduction is frequently
used.
Redox potentials are measured by an electrode capable of ab-
sorbing 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 measurements
of oxidizing solutions containing halides and for general use.
When a platinum electrode is immersed in an oxidizing solu-
tion a monomolecular layer of oxygen is developed on its
surface. This layer does not prevent the electrode from func-
tioning, 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 condi-
tions 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 pretreat-
ment 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; other-
wise a reducing pretreatment is necessary:
61