Single electrode potential and its measurement

Single electrode potential

The potential difference set up between a metal and the solution of its own ions at the metal–solution interface is called the single electrode potential. It arises because a metal M dipped in a solution of Mⁿ⁺ either loses ions to the solution (M → Mⁿ⁺ + ne⁻, so the metal becomes negative) or ions deposit on it (Mⁿ⁺ + ne⁻ → M, so the metal becomes positive), giving an electrical double layer.

Can its absolute value be measured?

No. An electrode potential is a potential difference across one interface. To measure it we must connect a voltmeter, which needs a second electrode and thus a second interface. The voltmeter always gives the difference of two electrode potentials, never one alone.

How the problem is solved

A reference electrode of arbitrarily assigned potential is used: the standard hydrogen electrode (SHE), Pt, H₂ (1 atm) | H⁺ (1 M), whose potential is fixed as 0.00 V. The electrode to be measured is coupled with SHE and the cell emf is measured. This emf is taken as the electrode potential of the unknown electrode on the hydrogen scale.

Example: Zn electrode

Zn | Zn²⁺ (1 M) || H⁺ (1 M) | H₂ (1 atm), Pt

• Anode: Zn → Zn²⁺ + 2e⁻
• Cathode: 2H⁺ + 2e⁻ → H₂
• Measured emf = 0.76 V = E°(cathode) − E°(anode) = 0 − E°(Zn²⁺/Zn)

So E°(Zn²⁺/Zn) = −0.76 V (reduction potential). Similarly, for Cu with SHE, E°(Cu²⁺/Cu) = +0.34 V. The values are therefore relative to SHE, not absolute.