# Electrode potentials vs interfacial potential differences

Between an electrode and an electrolyte there exists an interfacial potential difference.

Many authors give the electrode potential to be the electric potential of the electrode relative to the electrolyte. I have a few problems with this, and was wondering whether anyone could help.

We define the electrode potential of SHE to be 0V. It is impossible to measure the interfacial potential difference, however nevertheless one certainly exists between the reference electrode and the solution. Are we saying that we define the interfacial potential difference of the SHE to be zero, and then have electrode potentials of other couples as interfacial potential differences measured relative to this interfacial potential difference?

To my mind, this cannot be right. Don't we just set our zero reference (i.e. one end of our voltmeter) of electric potential at the electrode of the SHE, and measure electric potentials of other electrodes relative to this?

Sergio Trasatti wrote this regarding electrode potentials,

Equation (4.7) shows that the "electrode potential", as obtained in practice, does not measure the electric potential drop between the bulk of the metal and the bulk of the solution, but it is a more complex quantity which includes both electrical and chemical contributions.

Why do so many authors conflate interfacial potential differences and electrode potentials? Or am I missing something here? Thank you!

• Comments are not for extended discussion; this conversation has been moved to chat. – user7951 Mar 19 '20 at 19:14
• The comment by the author was very useful. " @M.Farooq I have now obtained a copy and have read through the chapter, and it has indeed given me a better understanding (though some parts I couldn't fully appreciate since I haven't formally studied electrochemical energy yet!). On another note, I have found a truly outstanding explanation by Professor Richard G. Compton of Oxford University on "Electrode Potentials", which clears up almost every doubt I had. Pages 18-22 are especially illuminating in the context of my original question. – M. Farooq Mar 20 '20 at 0:51

IUPAC defines the electrode potential of an electrode to be the EMF of a cell in which the electrode in question is on the right hand side of the line representation of the cell and a SHE is on the left hand side.

As such, the electrode potential of the $$M^+/M$$ couple is

$$E^o_{M^+/M} = \phi_{M} - \phi_{R}$$

where $$R$$ refers to the reference electrode, in this case the SHE. This is equivalent to

$$E^o_{M^+/M} = (\phi_{M}-\phi_{S}) - (\phi_{R}-\phi_{S})$$

Where $$\phi_{S}$$ is the potential of the solution. Essentially, this means that a relative electrode potential really means

$$E^o_{M^+/M} = \Delta \phi_{M^+/M} - \Delta \phi_{H^+/H_2}$$

That is, the potential difference of the electrode in question and the solution relative to the potential difference between the SHE electrode and the solution.

It doesn't help the cause that electrochemists refer to potential differences when they use the word potential, however this is just a case of getting to grips with the terminology used in the field!