While redox and pH are clearly related in that most redox processes consume or release hydrogen ions, it would be impossible to reliably predict redox potential from soil pH. There are many other factors affecting soil pH, such as the dominant buffering phases present (e.g. carbonates), replacement of major cations by H+ during chemical weathering and/or leaching, external acid inputs, and many others.
While redox and pH are clearly related in that most redox processes consume or release hydrogen ions, it would be impossible to reliably predict redox potential from soil pH. There are many other factors affecting soil pH, such as the dominant buffering phases present (e.g. carbonates), replacement of major cations by H+ during chemical weathering and/or leaching, external acid inputs, and many others.
Some authors suggest to add a correction to the Eh value at varying pH values (59mV). The relationship is there, but not as you suggest. The pH will influence the redox potential and the microbes creating a redox potential, but one cannot infer the Eh from pH on itself.
According to Sparks,"The Nernst Equation is the basis for the measurement of redox potential in soils and sediments. In aqueous systems the bounds of Eh is dictated by the stability of water. In oxidizing systems the oxidation of water to yield oxygen and protons via the following reaction is the upper boundary for Eh. Since the reaction yields protons, the stability is a function of pH". I believe you will find your answer in the document attached or the book; Environmental Soil Chemistry
By Donald L. Sparks
And then he continues with the formula 11 to show how that works in water, linking pH and Eh. But, the question here is about the relationship of Eh and pH in soil. In reality, H2O plays some role there, but (as in natural waters by the way) there are a lot of other substances in there that dictate the Eh.
Yes Michel you're right about Eh to be affected by other substances in soil, but if there is a basic start to consider a relationship between two, I find the information helping. As a whole, soil is a live complicated system and every part of the solution even the interaction with sediments, MO activities, compounds, or chemical species can affect Eh as well as pH.
Ok. So let's go back to the original question: can one infer Eh from the pH. The answer to that is no. The point raised here, is the correction of Eh to pH. Do you need to correct the Eh (=Em+Eref) to standard pH (7). If you want to do so, you follow the Nernst, like Clark says, and adjust with 59mV.
Dr Rate is very right, and correct.
So if you measure Eh, and report it, you should always report pH as well, and describe if you corrected for the pH of the soil/medium.
Some basic reading on redox potentials: search for DeLaune or Patrick, or look in the Wetland books by Mitsch (I won't refer to our own papers here ;) ).
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