Redox 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. The term redox comes from oxidation / reduction potential or ORP and is measured in units of volts, millivolts or Eh (1Eh = 1mV). Oxidation occurs when a species loses an electron and reduction occurs when a species gains an electron. There is no reduction without an accompanying oxidation, and vice versa.

The redox potential measures the tendency for a solution to either gain or lose electrons when it is subject to change by introduction of a new species. A solution with a higher redox potential will have a tendency to gain electrons from new species (i.e. oxidize them) and a solution with a lower redox potential will have a tendency to lose electrons to new species (i.e. reduce them).

pH and Eh together with the activity of dissolved chemical species characterise chemical reactions in aqueous solutions. Like pH, Eh represents an intensity factor. It does not characterise the capacity of the system for oxidation or reduction, in much the same way that pH does not characterise the buffering capacity.

Although measurement of the redox potential in aqueous samples is relatively straightforward, many factors limit its interpretation, such as irreversible reactions, slow electrode kinetics, non-equilibrium, presence of multiple redox couples, electrode poisoning, small exchange currents and inert redox couples. Consequently, practical measurements seldom correlate with calculated values. Nevertheless, redox measurement has proven useful as an analytical tool in monitoring changes in a system rather than determining their absolute value (e.g. process control and titrations).

Redox values are determined by measuring the potential difference between an inert indicator electrode in contact with the solution and a stable reference electrode connected to the solution by a salt bridge. The indicator electrode acts as a platform for electron transfer to or from the reference half cell. It is typically platinum, although gold and graphite can be 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 0.0 mV. However, it is fragile and impractical for routine laboratory use. Therefore, Ag/AgCl and saturated calomel (SCE) reference electrodes are commonly used. The voltages of the different reference electrodes can be interrelated using Table 3. The IJ64 uses a saturated KCl Ag/AgCl reference electrode.

Table 3: Potential Relationships of Several Reference Electrodes at 25^oC (mV)
SHE	SCE Saturated KCl	Ag/AgCl 1M KCl	Ag/AgCl 4M KCl	Ag/AgCl Saturated KCl (IJ reference)
0	+245	+236	+200	+199

For example: If you had a reading of 100mV using a saturated KCl Ag/AgCl reference and wanted to refer it back to an SHE you would add 199mV to obtain 299mV. Alternatively, if you took a reading in the same solution using an SCE, you would obtain 54mV (subtract 46mV from 100mV).