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For a solution where many redox reactions occur (e.g. cytosol, soil solution), can we consider anything as 'overall' redox potential of a system? Is there a formula to quantify it in relation to the amount of each of the constituents?

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  • $\begingroup$ I think this is a good question, especially the environmental and biological chemistry aspects of it. One issue (in real life and in simulation): you have to draw the line somewhere for your system boundaries, otherwise the answer(s) is/are ill-defined. $\endgroup$ – Todd Minehardt Nov 16 '19 at 17:08
  • $\begingroup$ Related: chemistry.stackexchange.com/questions/124988/… $\endgroup$ – Karsten Theis Dec 16 '19 at 22:28
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Is it possible to calculate the 'overall' redox potential of a system?

No, it is impossible. You can have a situation where a cellular compartment contains a high ratio of $\ce{NAD+:NADH}$ (i.e. favoring oxidation) and a low ratio of $\ce{NADP+:NADPH}$ (i.e favoring reduction). This is advantageous for the cell because it can run oxidation of some metabolites in parallel with reduction of other metabolites, just by employing enzymes with distinct co-factor requirements.

This is different from pH. A solution is either acidic or basic, not both. The difference arises because acid/base reactions are usually fast, and water acts as acid or base, "communicating" acid/base status across the solution.

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  • $\begingroup$ related: chemistry.stackexchange.com/questions/124988/… $\endgroup$ – Karsten Theis Dec 17 '19 at 15:46
  • $\begingroup$ The question came to my mind while I was conducting a heme protein dithionite titration. I obtained a solution of photosynthetic bacteria chromatophores with redox mediators and constructed a cell using a platinum electrode. If nothing as 'redox system potential' exists, then what exactly is responsible for the resultant potential of such half-cell? In a solution of so many compounds as a solution of biological origin, it is probably impossible to pinpoint a single reaction that would solely determine it. $\endgroup$ – Dyaego Mar 5 at 2:19
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Before answering i would like to mention what I was able to interpret from your question: You want to know that is it possible to find the redox potential of a mixture basically (anything which has multiple non reacting constituents such as soil solution).

I think it is plausible to do so by using the same method which is employed to find out the redox potential of any cell i.e. to write down the cell reactions individually for each reacting species and then writing the overall cell reaction and then calculating the Cell potential keeping into consideration the individual changes in electron in each reaction.

At any instant only one cation and anion would be discharged so when one substance is used up then the lesser reacting substance will get a chance such as the case in aqueous solution of any salt where Hydrogen gas is liberated first and then the cation (if the are difficult to liberate as gases). Hence at different instances different cell reactions will be there and different values of redox potential will be obtained.

NOTE I have tried to answer the question as per my understanding of Electrochemistry but as i was fairly recently introduced to the subject in case of any mistakes in my arguments I would deeply appreciate any form of advise or correction.

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