Taking Redox Measurements - Hanna Instruments HI21 Series Instruction Manual

TAKING REDOX MEASUREMENTS

Redox measurements allow the quantification of the oxidiz-
ing 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 automati-
cally 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 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
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 ab-
sorbs 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;
otherwise a reducing pretreatment is necessary:
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