Submitted to: Encyclopedia of Water
Publication Type: Book / Chapter
Publication Acceptance Date: 11/23/2004
Publication Date: 5/31/2005
Citation: Gao, S., Tanji, K. 2005. Eh (redox potential). Water Encyclopedia Vol 2: Water Quality and Resource Management. Jay H. Lehr and Jack Keeley (eds.). Wiley-Interscience, A John Wiley & Sons, Inc. Publication, pp 464-469. Interpretive Summary: Biogeochemical transformations controlling the fate of chemicals or elements often occur under a variety of redox conditions and thus categorizing these conditions becomes critical. A general parameter, redox potential (Eh), has been long used for this purpose. Eh is an intensity measure of the reducing or oxidizing conditions in a system and reflects the tendency of ions or molecules in a solution to donate or accept electrons. However, there is a large discrepancy between theoretical definition and measured Eh values leading to difficulties in correlating the measured Eh with specific redox reactions or redox species in water. This is mainly because redox systems in the natural environment are seldom at equilibrium and the presence of multiple redox components and many redox species are not electroactive. Thus, Eh measurement only gives a qualitative indication of the electron richness or poorness of a system. By following proper procedures, Eh data can be used to indicate the progress of a system towards reduction or oxidation, delineate redox gradients in a stratified system, and assist in further determination of redox reactions. The Eh chapter reviews and summarizes theoretical predictions, measurement, and the applications of Eh in aqueous chemistry.
Technical Abstract: Redox potential (Eh) is often measured to evaluate reducing or oxidizing conditions of an aqueous system. Theoretical Eh values indicate the electron activity or the tendency of ions or molecules in a solution to donate or accept electrons. Measured Eh values using Pt and reference electrodes, however, only reflects electron activity qualitatively. This is mainly because redox systems in natural environment are seldom at equilibrium, many redox reactions are not at equilibrium with each other, and many redox species are not electroactive. Eh data can be used to indicate the progress of a system towards reduction or oxidation, delineate redox gradients in a stratified system, and assist in further determination of redox reactions.