Eh-pH diagrams, and their limitations

Platinum electrodes are sometimes utilized to obtain Eh measurements on water samples. However, they are often ineffective, especially with natural waters (Drever, 1997), 136, 180-182 (Lindberg and Runnells, 1984). Besides problems with chemical disequilibria in many natural waters, platinum electrodes usually fail to quickly respond to the kinetics of the couples that commonly control redox conditions in water samples, including O2-H2O, S02 -H2S, CO2-CH4, NO -N2, and N2-NH (Drever, 1997), 136. For arsenic, large discrepancies may exist between redox values calculated from the As(V)/As(III) couple and Eh measurements with platinum electrodes (Ryu et al., 2002), 2989-2990. Additionally, when collecting 

Eh-pH diagrams are sometimes used to predict or describe the major dissolved species and precipitates that should exist at equilibrium in aqueous solutions, including groundwaters, surface waters, laboratory solutions, and porewaters from soils, sediments, or rocks. However, as previously described, many natural aqueous systems are not at equilibrium and they often contain metastable species that are not predicted by Eh-pH diagrams. Metastable species refer to compounds, other substances, or ions that are present under redox, pH, pressure, temperature, or other conditions where chemical equilibrium indicates that they should be unstable and absent. Many metastable species (such as As(III) in oxygenated seawater) result from biological activity. 

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