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REDOX MEASUREMENT (HI9920 only)
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 accurate redox measurements the surface of the electrode must
be clean and smooth. At certain mV and pH values, the ORP electrode
requires a considerable amount of time before it reads the proper value.
This is at times 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 the event that measurements are made in solutions containing heavy
doses of sulfide or protein, the diaphragm of the reference electrode
must be cleaned more often.
In order to test if the ORP electrode is functioning properly, immerse
it into a
HI7021
solution. The measured value should be between
240 ±20 mV.
