Alternative Measures of Oxidation Potential in Low Temperature Environments

Alternative Measures of Oxidation Potential in Low-Temperature Environments 

We have observed above that Eh measurements in natural systems may be difficult to interpret quantitatively because all possible redox couples might not have equilibrated with each other. In simple systems where dissolved Fe is the principal redox-active component, Eh can sometimes be used to predict the Fe /Fe activity ratio, but this should not be attempted if other redox couples are present. As an alternative to electrochemical measurements, the activity ratios of interest can often be analyzed directly. For example, there is often a problem with chromium, zinc and arsenic 

The problems at lower temperatures happen because reactions are slow and systems are often far out of equilibrium. (The thermodynamically stable state for all the flora and fauna in any well-aerated lake should be H2O and CO2, for example). Reaction rates increase exponentially with temperature however, and at temperatures of 350°C or higher, even reactions involving molecular oxygen can equilibrate in several days time. This is fortunate, because free energy data are sparse for aqueous species at elevated temperatures and it becomes difficult to measure or calculate Eh much above 100°C. Instead, we can use other reactions between redox-sensitive species to calculate redox-related parameters at higher temperatures. This is the subject of the following section. 

---