Besides dissociation reactions, mainly redox processes control corrosion and scaling as well as nutrient and pollutant bioavailability in artificial and natural systems. For these processes, the redox potential Eh (sometimes expressed as pe, too) is of similar importance as the pH is for dissociation reactions.
But whilst the pH is a well understood parameter, the redox potential, as simple as its initial electrochemical definition by the Nernst equation is, remains unclear in practice. A lot of what we know about redox potential measurements in the environment is purely empirical and its use in environmental research often reflects this. Instead of being regarded as a resulting measure, which it is by definition, it is often perceived as a cause, or just noted for reference in field studies. Yet, obviously, it tells the researcher a lot about the investigated or monitored environment, may it be though, as stated until very recently, only by the means of a qualitative or semi-quantitative measure.
Attempts to achieve a better understanding of redox measurements in natural systems like in soils and groundwater at least date back to 1920, when Gillespie introduced the redox electrode to environmental science. Up until now, research on this matter brought up only more uncertainties and difficulties: Non-equilibrium systems, mixed potentials, reactions of the platinum electrode material itself, bad comparability, interferences with dissolved oxygen and last but not least bacteria taking part in redox processes blurred the picture until researchers concluded that the use of redox measurements even as an operational parameter is highly questionable. Only in a small number of publications, vague hints on possible quantitative relationships are given, even though modeled vs. measured potentials may disagree by a full 1000 mV in other cases.
This study shows, that quantitative studies using the redox potential electrode are actually possible in natural waters even at non-equilibrium. They depend on proper calculation of ferric iron and water dissociation besides a correction for the influence of dissolved oxygen on the platinum electrode. Given a proper assessment, measured potentials either indicate an oxidized state, give quantitative ferrous iron activity data or show the presence of sulfides. The influence of ferrous iron speciation and naturally occurring ligands is discussed. It is shown, that redox potentials can be used for iron clogging, corrosion and scaling diagnostics and monitoring.