# Is redox potential a valid term? [closed]

I mean doesn't every compound has its own different reduction and oxidation potential ? Or its just one potential that can be compared to understand which one will get reduced and which one will get oxidised ?

I mean doesn't every compound has its own different reduction and oxidation potential ? Or its just one potential that can be compared to understand which one will get reduced and which one will get oxidised ?

A compound has neither. You need to specify the half reaction. For example, take an aqueous solution of $$\ce{Fe^2+}$$ and hydrogen peroxide, among others. What are possible half reactions for $$\ce{Fe^2+}$$?

$$\ce{Fe^2+ -> Fe^3+ + e-}\tag{1}$$ $$\ce{Fe -> Fe^2+ + 2e-}\tag{2}$$

and maybe some more exotic ones. For hydrogen peroxide, you would have e.g.

$$\ce{H2O2 -> O2 + 2e- + 2H+}\tag{3}$$ $$\ce{2H2O -> H2O2 + 2e- + 2H+}\tag{4}$$

Notice that these are all written in the direction of reduction, and are associated with a reduction potential. To see which reactions are feasible, you would compare reduction potentials and come up with a redox reaction combining a half reaction as written with a half reaction (the oxidation half reaction) in the reverse direction. For example, hydrogen peroxide could undergo disproportionation:

$$\ce{2H2O2 -> O2 + 2H2O}$$

This is a slow reaction, but the enzyme catalase (which happens to have an iron(III) bound to its active site) increases the rate substantially, as you can see from the oxygen evolving when you put some hydrogen peroxide on a freshly cut slice of potato.

Is redox potential a valid term?

Yes, under certain circumstances it makes sense to talk of redox potential as a property of an aqueous solution, similar to discussing its pH. For example, if a biological cell contains glutathione at fairly high concentration in the reduced and oxidized form (abbreviated as GSH and GSSG), this will determine the redox state of other substances in the cell. If a strong oxidation agent is introduced, it will react with GSH. If a strong reduction agent is introduced, it will react with GSSG. Like mainining a pH with a buffer, the redox potential of the cell is maintained through the ratio of GSH and GSSG (and we could call it a redox buffer).

The analogy between pH buffer and redox buffer only goes so far. The biggest difference is in the role of water. While water acts as an acid and base at any pH (i.e. whenever acids and bases are added), it does not undergo reduction and oxidation for many redox reactions in aqueous solution. Any pair of acid and base will react (because water provides the "reservoir" of hydrogen and hydroxide ions). Many pairs of possible oxidizing and reducing agents will not react, even when thermodynamically favored because there is no good mechanism for the electrons to be transferred (a question of kinetics).

There is the table of standard electrode potentials. These are values which is relative to the standard hydrogen electrode. They were measured at certain, constant for all, conditions.

To measure actual potentials one can use Nernst equation

$$E_\mathrm{Red} = E_\mathrm{Red}^\circ + \frac{RT}{nF}\ln\frac{a_\mathrm{Red}}{a_\mathrm{Ox}}.$$

Sometimes you just have to compare standard electrode potentials to understand which one is stronger oxidant or reductant.

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Is redox potential a valid term?

Yes it is a valid thermodynamic term. Also oxidation potential is a valid term. These terms differ only by a sign. If a reduction potential of a given half cell is -0.73 V with respect to hydrogen cell. its oxidation potential is +0.73 V. So you have to stick to one term for comparative purposes.

After a century of arguments, electrochemical societies decided to write all the half cells as reduction e.g., $$\ce{Fe^{3+} + e- -> Fe^{2+}}$$ and call the potential of this reaction as the reduction potential with respect to hydrogen half cell which is arbitrarily taken as zero.

• This does not address the question which seems to be shall we say, "reduction (or oxidation) potential" or can we use redox as passpartout? I am oriented to say that is either reduction or oxidation, at least in a pedantic way. – Alchimista Dec 12 '19 at 7:51
• The post title was a decent question. The text of the question does not make any sense. Also, what do you mean by "passpartout"? Only the signs differ between reduction potential and oxidation potential. – M. Farooq Dec 12 '19 at 14:15
• Ok but you have treated the term redox as if it is red... I am not saying you wrote something wrong. Just the question wasn't addressed, while it seems so for the answer by @Karsten Theis. Passpartout in the sense that often we indeed say or hear of redox potential instead of a more correct reduction (or oxidation) potential, especially when a value is then given. – Alchimista Dec 13 '19 at 7:34