# Does the order of a written redox couple matter?

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?

• 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 $Zn \rightarrow Zn^{2+} + 2e^-$. – James Wirth Mar 17 at 19:51

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.

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.