My understanding is that a redox couple is an unordered pair of two conjugate species. So we could speak of the redox couple consisting of $Ag$ and $Ag^+$; depending on whether a given redox couple is undergoing oxidation or reduction, we could then write a half equation involving the species given in the redox couple.

In my textbook, it is given as a convention that all redox couples are expressed in the form Ox/Red. So for instance, it states "at anode containing the $Zn^{2+}/Zn$ redox couple, the oxidation half equation is $Zn \rightarrow Zn^{2+} + 2e^-$". This convention makes sense to me, since it doesn't seem that the couple itself refers to a half equation in a given direction. The order of the half cell line representation, however, does change the order to $Zn | Zn^{2+}$.

However, some texts reverse the order of the written redox couple, which I would interpret as either them writing the couple in the order of the current half equation (which seems wrong) or that they are ignoring the order because it is unimportant.

So I wondered whether anyone could clarify, is a redox couple just a pair of species which exist at a given half cell, or do we need to change the order of the written couple depending on the reaction occurring at that half cell?

  • $\begingroup$ At @EdV but specifically with regards to the notation for redox couples. As in I would say the written couple $Zn^{2+}/Zn$ doesn't correspond to either a reduction or oxidation half equation, it just represents the species present at that half cell. However others might say that the written couple $Zn^{2+}/Zn$ implies the half equation $Zn^{2+} + 2e^- \rightarrow Zn$ whilst $Zn/Zn^{2+}$ implies $Zn \rightarrow Zn^{2+} + 2e^-$. $\endgroup$
    – 13509
    Commented Mar 17, 2020 at 19:51

1 Answer 1


My understanding is that a redox couple is an unordered pair of two conjugate species

This is conceptually perfect and there is no problem when we talk about electrode potentials of half cells because as I had mentioned in your earlier queries, the electrode potential value and its associated sign do not know nor care how you write the half cell.

What is creating confusion in your mind is that you are trying to abbreviate a half cell. Abbreviating a half cell is of no use, because it is creating a problem of ordering.

All we care about the order is when we write a full cell. For that, we must follow the IUPAC recommendation and rules in order to prevent chaos in the published work. There your half-cell is no longer an unordered pair.

I used to remember the IUPAC recommendation with a mnemonic: Write Reduction on the Right.

enter image description here

This has to be parsed as follows: Reduction is on the right, so you must order the copper half cell in such a way that it appears as reduction.

On the left, of course, we have an oxidation half cell and it must be written as such.

Hope that clarifies your confusion.

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    $\begingroup$ +1 I will delete my other two comments and hope this brings closure to this triptych of redox notation queries. $\endgroup$
    – Ed V
    Commented Mar 17, 2020 at 20:15
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    $\begingroup$ But I am glad that the student is thinking and searching historical problems. Hard to find such students these days. $\endgroup$
    – AChem
    Commented Mar 17, 2020 at 20:17
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    $\begingroup$ I agree. But standardization can be extremely helpful: I once read of a situation involving optical effects in polarizers and such, and there were 4 parameters involved. Each could be defined as positive or negative, resulting in 16 possible total sign conventions. It turns out that various authors had used 15 of the 16! I guess the unused one fell out of the ugly tree face first ... $\endgroup$
    – Ed V
    Commented Mar 17, 2020 at 21:05
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    $\begingroup$ Bard and Faulkner are truly outstanding, but, as per my potentiostat answer and comment here, even they can make (or not catch) dinky mistakes. Electrochemistry, unfortunately, is an area where it is surprisingly easy to make little mistakes, so standardization is a real benefit, especially to those of us who have taught parts of it. $\endgroup$
    – Ed V
    Commented Mar 17, 2020 at 21:53
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    $\begingroup$ Bard has crossed mid 80s. So their book may not represent the latest. The authors are notorious for their difficult but respectable book. Anyway, O/R and R/O electrodes are not of our concern. I think R/O or O/R notation is not standard either. It is related to electrolytic cells. $\endgroup$
    – AChem
    Commented Mar 17, 2020 at 21:59

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