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REDOX MEASUREMENTREDOX MEASUREMENT
REDOX MEASUREMENTREDOX MEASUREMENT
REDOX MEASUREMENT
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 a plati-
num surface. 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 accurate redox measurements the surface of the electrode
must be clean and smooth. At certain mV and pH values, especially
those found in pools, the ORP electrode requires a considerable
amount of time before it reads the proper value. This is due to the
fact that it is moving from a reducing to an oxidizing state. Once it
reaches a stable condition though, it reacts rapidly to changes.
Hence when the process is first set up allow sufficient time for the ORP
electrode to adapt itself to the sample stream.
As with pH electrodes, gel-filled redox electrodes are more suitable for
industrial applications due to less maintenance requirements.
In order to test that the ORP electrode is functioning properly,
immerse it into a HI 7020 solution. The measured value should be
between 200 and 250 mV.
When not in use, the electrode tip should be kept moist in order for
the reference junction, especially Teflon models, to respond quickly.
Otherwise, soak the electrode overnight in a HI 70300 storage
solution or allow more time upon installation for its stabilization. Also
keep the electrode far from any type of mechanical stress which might
cause damage.
Install the electrode in such a way that it is constantly in a well filled
with the sample (stream or tank) and does not dry up.
