Redox potential measurements, natural environments

If a system is not at equilibrium, which is common for natural systems, each reaction has its own Eh value and the observed electrode potential is a mixed potential depending on the kinetics of several reactions. A redox pair with relatively high ion activity and whose electron exchange process is fast tends to dominate the registered Eh. Thus, measurements in a natural environment may not reveal information about all redox reactions but only from those reactions that are active enough to create a measurable potential difference on the electrode surface. 

Metals are widely distributed in the aquatic environment and may be from either natural and/or anthropogenic sources. They are present in wide range of forms or species including freely dissolved metal, salts, organometalics, and bound to colloids and particulates. Factors such as pH and redox potential of the water and sediment, and presence of particulate material or organic matter, and microbial activity will determine in what form a particular metal is present in the environment. Since the different forms of a metal can have different bioavailabilities and toxicities, it is important to ensure that biologically relevant species are measured. Metals are generally bioavailable when present as free ions, bound to weak complexes or in a lipid soluble form (Hudson, 1998 Luider et al., 2004). Low concentrations of some metals, such as Cu and Zn,... 

The ability of any natural environment to bring about oxidation or reduction processes is measured by a quantity called its redox potential or Eh (see Box 5.4). 

Redox potential The ability of the natural environment to bring about any oxidation or reduction process. Is measured as the potential difference between a platinum electrode and an electrode of known potential (in millivolts). 

Morris, J.C. and W. Stumm. 1967. Redox equilibria and measurements of potentials in the aquatic environment. In W. Stumm, ed., Equilibrium Concepts in Natural Water Systems, pp. 270-285. American Chemical Society, Washington, DC. 

The measurement of redox conditions by means of a cell voltage, where one electrode has a fixed reference potential and the other is expected to react reversibly with natural systems, is attended by a number of problems. The platinum electrode works well only under certain conditions It is difficult to get the electrode into reducing environments without allowing some oxidation, and the method is restricted to ambient conditions except in research laboratories. We put up with these problems because there is little choice. 

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