Ionode IJ pH V5.1 Bedienungsanleitung PDF herunterladen (Seite 31)

Ionode IJ Instruction Manual 31

5. ORP

5.1. Theory

ORP stands for oxidation reduction potential.

Ideal ORP obeys the Nernst equation:















][

Where [Ox] is the active concentration of an

oxidized species such as ferric ion Fe

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

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