What Is Standard Electrode Potential (E°) and How Is It Measured? (HL)

Standard electrode potential, $E^\circ$, is the potential of a reduction half-reaction measured relative to the standard hydrogen electrode under standard conditions. A single electrode potential cannot be measured directly; only the potential difference between two half-cells can be measured.

How It Is Measured

The standard hydrogen electrode (SHE) is assigned a potential of exactly $0.00\text{ V}$ by convention. Its half-equation is:

$$2\text{H}^+(\text{aq}) + 2e^- \rightleftharpoons \text{H}_2(\text{g})$$

It consists of an inert platinum electrode in contact with hydrogen gas and aqueous hydrogen ions. Platinum is chemically inert and provides a conducting surface for electron transfer; it does not appear in the hydrogen half-equation.

To measure an unknown $E^\circ$, its half-cell is connected to the SHE by a salt bridge and an external circuit containing a high-resistance voltmeter. The salt bridge permits ion movement and maintains electrical neutrality, while the voltmeter measures the potential difference with negligible current.

The direction of electron flow identifies whether the unknown half-cell undergoes reduction or oxidation relative to the SHE. Standard electrode potentials are always tabulated for reduction half-equations:

• If the unknown half-cell is the cathode, its $E^\circ$ is positive relative to the SHE.
• If it is the anode, its $E^\circ$ is negative.

A more positive reduction potential indicates a greater tendency for the species on the left of the reduction half-equation to gain electrons. Therefore, it acts as the stronger oxidizing agent under standard conditions.

For example, the copper half-cell has $E^\circ = +0.34\text{ V}$, so $\text{Cu}^{2+}$ is reduced when connected to the SHE. Cell potentials are calculated using:

$$E^\circ_{\text{cell}} = E^\circ_{\text{cathode}} - E^\circ_{\text{anode}}$$

Common misconception: an electrode has an independently measurable absolute potential. It does not; every listed value is relative to the SHE.

Exam Technique

This is AHL content from Reactivity 3.2. State the standard conditions, identify the SHE reference, keep both half-equations written as reductions, and use the correct cathode-minus-anode formula.