ORP THEORY

Just as the transfer of hydrogen ions between chemical species determines the pH of an aqueous solution, the transfer of electrons between chemical species determines the redox potential of an aqueous solution. ORP stands for oxidation reduction potential (also termed "Eh") is measured in mV. Ideal ORP obeys the Nernst equation:

Where [Ox] is the active concentration of an oxidized species such as ferric ion Fe³⁺and [Re] is the active concentration of the reduced form of that species namely ferrous ion Fe²⁺. 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 quione/hydroquinone are examples of reversible redox couples and are used for ORP standards. 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 way pH does not characterise the buffering capacity.

Redox values are determined by measuring the potential between an inert indicator electrode in cotact with the solution, and a stable reference electrode connected to the solution by a salt bridge. The indicator electrode acts as a surface for electron transfer to or from the refernce half cell. It is typically Platinum, however Gold and graphite are also used. The reference half cell consists of a redox standard of known potential. The standard hydrogen electrode (SHE) is the reference from which all standard redox potentials are determined and has been assigned an arbitrary half cell potential of 0.0mV. However it is impractical to use in most circumstances, and Ag/AgCl or calomel (SCE) are used instead.