Orp 5.1. theory – Ionode IJ pH V5.1 User Manual
Page 31
Ionode IJ Instruction Manual
31
5. ORP
5.1. Theory
ORP stands for oxidation reduction potential.
Ideal ORP obeys the Nernst equation:
]
[
]
[
ln
0
Re
Ox
nF
RT
E
E
Where [Ox] is the active concentration of an
oxidized species such as ferric ion Fe
3+
and
[Re] is the active concentration of the reduced
form of that species namely ferrous ion Fe
2+.
The pair provides a redox couple.
The following is required for a stable ORP
reading:
1) Significant concentrations of both species of
the couple
2) Both species must be capable of readily
transferring electrons to or from each other
(reversible redox couple) and readily
accepting or removing electrons from an
inert metal surface.
Ferric/ferrous,
iodine/iodide
and
quinone/hydroquinone
are
examples
of
reversible redox couples and are used for ORP
standards (see
Table 7 - Table 9
)
Generally, most practical ORP measurements
are made on samples which do not meet the
above
criteria.
This
results
in
poor
reproducibility, drift, stirring rate dependence
and non-Nernstian behaviour. Nevertheless,
ORP is useful for measuring changes in a
system rather than absolute values (e.g.
process control to an ORP set-point and
titrations).
Like pH, ORP represents an intensity factor. It
does not characterize the capacity of the
system for oxidation or reduction, in the same