# Most understandable notation for reduction and oxidation potentials

Nowadays a lot of electrochemistry is done using just electrode potentials (i.e. reduction potentials), so if in doubt the notation $$E^o$$ tends to imply a reduction potential. However, some chemists use the perfectly acceptable convention that oxidation reactions are accompanied with oxidation potentials.

I wondered whether anyone could inform me about the most unambiguous or widely used conventions for indicating which type of potential is being employed when written next to a half equation.

For instance, for the oxidation half equation $$Ag \rightarrow Ag^+ + e^-$$, I could write the associated potential in a number of ways:

1. $$E_{Ag^+/Ag} = +0.80V$$ - the electrode potential of the $$Ag^+/Ag$$ couple
2. $$E_{red}(Ag^+/Ag) = +0.80V$$ - the reduction potential of the $$Ag^+/Ag$$ couple
3. $$E_{ox}(Ag^+/Ag) = -0.80V$$ - the oxidation potential of the $$Ag^+/Ag$$ couple
4. Often, just $$E$$, or some other notation!

I think the notation in points 1 through 3 is fairly clear, with the first two applicable to the European convention and the third applicable to the American convention, when written next to the oxidation equation. However I was wondering if anyone more experienced could advise as to what the most common conventions in the field are!

Thank you!

I don't know why your textbook author is confusing and still teaching American & European conventions. It is obsolete now. I will show you how oxidation potentials were quoted by American electrochemists in the 1950s-60s. Have a look at the table Latimer's book: Oxidation States of the Elements and their Potentials in Aqueous Solutions pg 340. This was a very famous book of its time. Latimer and Gibbs had great influence on American science and thermodynamics.

I would say you should stick to only (1). Your number 3 is not correct because you are still writing the cell as reduction. The correct version would be

$$E_{ox}(Ag/Ag^+) = -0.80V$$ - the oxidation potential of the $$Ag/Ag^+$$ couple, so that you are consistent.

Also make sure your instructor is aware of these things (American European conventions, and that the sign of the electrode potential is an electrostatic one, and that electrode potential does not care how the equation is written. You seem to know more than an average PhD student). I will not be surprised, your instructor is not a trained electrochemist, he/she may not be aware of these subtleties.

Also note that the small "o" on E, indicates standard conditions.

• I wonder, why is it that number (3) is incorrect? The oxidation potential is the emf of a cell where the redox couple in question is on the left side of the cell (undergoing oxidation), and is well defined independent of the choice of convention. I personally wouldn't use (3) (like you say, the convention of labelling reactions with oxidation potentials is rarely used anymore), but just for completeness' sake isn't it still a correct formulation? Commented Mar 16, 2020 at 23:37
• @JamesWirth, I have edited the answer.
– ACR
Commented Mar 17, 2020 at 1:30
• Ah, I thought a couple was an unordered pair of species (i.e. $𝐴𝑔/𝐴𝑔^+ \equiv 𝐴𝑔^+/𝐴𝑔$) which is just written in the form 𝑂𝑥/𝑅𝑒𝑑 for consistency. Because then, we could always speak of the electrode potential of the $𝐴𝑔^+/𝐴𝑔$ couple without needing to worry about whether the cell reaction is a reduction or oxidation. That is to say we can always refer to the $X/X^-$ electrode no matter the reaction occurring there. Commented Mar 17, 2020 at 6:31
• So if $Ag^+/Ag$ represents a half cell which might perform two different half reactions, then its electrode potential is the emf if we connect that half cell on the right wrt SHE, the oxidation potential of that couple the emf if we connect that half cell on the left wrt SHE, etc. It actually appears important that the order of the couple does not imply a particular half equation. Commented Mar 17, 2020 at 6:37
• Well, you are right that the electrode potentials do not care how the half cell is written, but there are norms in a given field. People do stick to writing certain conventions. For example if I invent a new symbol for integration, mathematicians will reject my proposal . The notation you are using is not canonical either. Today, $E$ implies a reduction half cell with respect to hydrogen. This has been settled by electrochemists and decided by IUPAC. I am not an electrochemist, but I was lucky to have a fine electrochemistry professor. I have personally studied these conventions for a decade.
– ACR
Commented Mar 17, 2020 at 13:43