Topic 2 of 7
Measuring standard potentials
Build the reference and the three kinds of test half-cell.
A-Level 9476 (2026-2027)
Measure every half-cell against the same reference
An isolated electrode has no independently measurable absolute potential.
A standard electrode potential, E°, is the potential difference of a half-cell relative to the standard hydrogen electrode, with all species in their standard states and negligible current flowing. Values are quoted for the half-reaction written as a reduction. A more positive value indicates a stronger tendency for that reduction under the stated standard conditions.
For the usual school measurements, aqueous concentrations are 1 mol dm-3, gases have standard pressure 100 kPa, and pure solids or liquids are used in their standard states. Strictly, dissolved species have unit activity; concentration is the course approximation. Temperature must be stated: the Data Booklet values are at 298 K. Standard state is not another name for 273 K gas conditions.
The standard hydrogen electrode (SHE) uses an inert platinum electrode coated with finely divided platinum in contact with H2(g) at 100 kPa and aqueous H+ at 1 mol dm-3, at the stated temperature. Its reversible half-equation is 2H+(aq) + 2e- ⇌ H2(g), and its standard potential is assigned 0.00 V. Platinum provides electrical contact and a catalytic surface; it is not consumed in this equation.
| Type to measure | Test half-cell arrangement | Reduction represented |
|---|---|---|
| Metal with aqueous ions | A clean copper strip in 1 mol dm-3 Cu2+(aq). | Cu2+ + 2e- ⇌ Cu. |
| Non-metal with aqueous ions | Inert platinum contacting Cl2(g) at 100 kPa and 1 mol dm-3 Cl-(aq). | Cl2 + 2e- ⇌ 2Cl-. |
| Two oxidation states of the same element | Inert platinum in a solution with both Fe3+ and Fe2+, each at 1 mol dm-3. | Fe3+ + e- ⇌ Fe2+. |
Keep both half-cells at 298 K when using the tabulated values. A suitable salt bridge, such as one containing KNO3, permits ion movement and prevents charge build-up without rapidly mixing the solutions. The voltmeter draws negligible current so the reading approximates the equilibrium electromotive force. Record which terminal is positive, not only the size of the reading: copper is +0.34 V relative to the SHE, while zinc is -0.76 V.