Oxygen reduction potential (ORP/Redox)

Lauri Solismaa, Geological Survey of Finland, PO Box 1237, 70211 Kuopio, Finland; lauri.solismaa(at)gtk.fi

Introduction

Measurement of oxygen reduction potential (ORP) or just redox potential in monitoring of mine influenced sediments or water is a way of measuring the electrochemical potential or electron availability in these systems.  Most geologists express hydrogen ion activity as a number (pH) and electron activity as voltage (Eh). Electron activity can also be presented as pE = -log [e–]. This is similar to the definition of pH and allows the use of numbers instead of voltages but conversion offers no advantage when dealing with measured voltages (Nordström 2005).

Description of the methodology

Measuring of the oxidation-reduction potential of chemical system is usually done with inert metal (platinum) electrode. To form a complete cell the inert platinum electrode is used in conjuction with a reference electrode (DeLaune  & Reddy 2005). Measuring is done by placing a probe in to water or sediment to measure.

Picture 1: Measuring pH, redox and electrical conductivity at study site: Photo: Lauri Solismaa, GTK.

Appropriate applications

Mesasuring the electrochemical potential or electron availability in water or sediments

Performance

After the collection of a sample chemical, physical, and biological reactions can cause the Eh of water to change significantly. This can happen within minutes or even seconds. The practicality of Eh measurements is limited to iron in acidic mine waters and sulfide in waters undergoing sulfate reduction because other redox species are not sufficiently electroactive to establish an equilibrium potential at the surface of the conducting electrode. (Nordström 2005).

From measurements an overview of the conditions can be made. Reducing (-300-0 mV) quite reducing (0-400 mV) or oxidizing (400-700 mV). (DeLaune  & Reddy 2005)

References

DeLaune, R.D., Reddy K.R., 2005. Redox Potential. In Encyclopedia of Soils in the Environment. D.Hillel (ed). Academic Press. Pp. 366-371.

Nordstrom, D. Wilde, F. 2005. Reduction-oxidation potential (electrode method). In National Field Manual for the Collection of Water-Quality Data (TWRI Book 9) http://water.usgs.gov/owq/FieldManual/Chapter6/6.5_contents.html (Accessed May 2015)