Lesson 2 of 7

Measuring standard potentials

Build the reference and the three kinds of test half-cell.

A-Level 9476 (2026-2027)

I can define standard electrode and standard cell potentials.Syllabus
Syllabus 9476, 12(b). Define standard electrode and standard cell potentials.
I can describe the standard hydrogen electrode.Syllabus
Syllabus 9476, 12(c). Describe the standard hydrogen electrode.
I can describe standard electrode-potential measurements.Syllabus
Syllabus 9476, 12(d). Describe standard electrode-potential measurements.

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.

Connect the test half-cell to the SHE using a salt bridge and high-resistance voltmeter
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.

Check your understanding

A standard half-cell is connected to the SHE. Its terminal is negative and the voltmeter reads 0.25 V. What is its standard electrode potential?

0.00 V, because it is measured against a reference half-cell
+0.25 V, because a voltmeter reading is written as a positive value
-0.25 V, because it is the negative terminal relative to the SHE

Show the answer

-0.25 V, because it is the negative terminal relative to the SHE

E is quoted relative to the SHE, with the sign of the test electrode. A negative terminal means the half-cell gives electrons more readily than the SHE, so its reduction tendency is smaller.