Part 2: Half-cells, reference electrodes and measurement

Explain how an electrode potential is measured and what standard conditions and the salt bridge contribute to a valid comparison.

An isolated half-cell has no directly measurable absolute potential

A half-cell combines a redox couple with an electrical conductor. A metal/ion half-cell uses the metal dipped into its aqueous ions. When both oxidation states are dissolved, such as Fe³⁺/Fe²⁺, an inert platinum electrode provides a conducting surface without appearing in the net reaction.

A voltmeter measures a potential difference between two electrodes, so a reference is needed. The standard hydrogen electrode is assigned 0.00 V. The standard electrode potential E° of a half-cell is its measured potential relative to that reference under standard conditions, using the reduction convention.

Standard electrode data are written with electrons on the left: oxidised form + electrons ⇌ reduced form. A more positive E° indicates a stronger tendency for the reduction relative to the reference under those conditions. The value is an electrical potential, not an amount of energy per mole that scales with coefficients.

Construct the standard hydrogen electrode

The hydrogen electrode uses hydrogen gas at 100 kPa in contact with a platinum surface and aqueous H⁺ of concentration 1.00 mol dm⁻³, at a specified temperature conventionally 298 K for tabulated values. Platinum conducts and provides a surface for the hydrogen equilibrium; it is not the source of H⁺.

Connect it to the comparison half-cell through wires and a high-resistance voltmeter, and join the solutions through a salt bridge containing a suitable inert electrolyte. A gas electrode needs effective contact among gas, electrode and electrolyte, not a gas label beside a disconnected wire.

Read the polarity as well as the magnitude. If the test electrode is the positive terminal against the hydrogen reference, its reduction potential is positive. If the connections are swapped the meter sign reverses; the chemistry has not changed.

Why the circuit needs both electron and ion movement

Electrons travel through the external wire from the electrode where oxidation occurs to the electrode where reduction occurs. The salt bridge completes the internal circuit through ion movement and limits charge build-up. It does not transfer electrons between solutions.

Use ions that do not react materially with the half-cell components. For example, chloride is unsuitable if it precipitates a metal ion in the chosen system. The bridge must make contact with both solutions; metal electrodes must contact their electrolytes. A high-resistance voltmeter draws very little current, limiting changes caused by the measurement.

For a zinc–copper cell, zinc dissolves as Zn²⁺ while Cu²⁺ is reduced and copper deposits. Anions from the bridge compensate for positive charge accumulating in the zinc half-cell; cations compensate for cations consumed in the copper half-cell. This explains the direction of ionic movement instead of treating the bridge as a decorative tube.

Worked example: use reduction potentials consistently

For an illustrative standard pair, Zn²⁺ + 2e⁻ ⇌ Zn has E° = −0.76 V and Cu²⁺ + 2e⁻ ⇌ Cu has E° = +0.34 V. Copper is the reduction electrode and zinc the oxidation electrode in the spontaneous cell. E°cell = E°reduction − E°oxidation = +0.34 −(−0.76) = +1.10 V.

The overall reaction is Zn + Cu²⁺ → Zn²⁺ + Cu. Balance electrons before adding equations, but do not multiply E° by the electron coefficient. If one half-equation needed doubling, its potential would still be the same: voltage is not doubled by rewriting a chemical equation.

A complete PAG8-style measurement records temperature, concentrations, electrode identities, electrolyte contact, bridge and voltmeter polarity. Poorly cleaned electrodes or reacting bridge ions can make a real result differ from a tabulated standard value.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official OCR A mark allocations.

Sources