Measure standard potentials for metal and ion half-cells
Describe a cell measurement against SHE for a metal in contact with its ions and for ions in different oxidation states.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 24. Treat all tabulated electrode potentials as reduction potentials; state conditions, phases, signs and units. Written practice is self-checked
Before you begin
- Use AS redox half-equations, oxidation states, mole ratios, current and charge.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What electrode is used for Fe³⁺/Fe²⁺ when both forms are aqueous?
What you will learn
- Describe a standard metal/ion half-cell
- Describe a non-metal/ion half-cell with an inert electrode
- Use SHE, a salt bridge and a high-resistance voltmeter to obtain E°
Metal and its ions use the metal as the electrode
For Mⁿ⁺(aq)/M(s), immerse the metal in a standard-concentration solution containing its ions. Connect it to the SHE through a salt bridge and high-resistance voltmeter at 298 K.
For non-metal/ion couples or two oxidation states of one element, use an inert platinum electrode in a solution containing both forms. For example, Fe³⁺(aq)/Fe²⁺(aq) requires Pt because neither aqueous ion is a solid conducting electrode.
Read polarity and write the reduction value consistently
The measured cell voltage gives the test half-cell potential relative to SHE. State which side is positive and whether the test reduction is more or less favourable than H⁺ reduction; report the tabulated half-equation in reduction form.
Use standard concentrations, gas pressures when relevant, and 298 K. The salt bridge completes the ionic path; the high-resistance voltmeter measures voltage with minimal current.
Pause and explain
What does the salt bridge provide in the electrode-potential cell?
Worked example
How would a standard electrode potential be measured for Fe³⁺(aq)/Fe²⁺(aq)?
Show the worked solution
- Put both ions at standard concentration in contact with an inert Pt electrode.
- Connect this half-cell to the SHE with a salt bridge.
- Measure the potential difference with a high-resistance voltmeter at 298 K.
Answer Use a Pt electrode in a solution containing standard concentrations of Fe³⁺ and Fe²⁺, connect it to the SHE by a salt bridge, and measure E with a high-resistance voltmeter at 298 K.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe how to measure E° for Zn²⁺(aq)/Zn(s) relative to SHE.
Give me a hint
Specify the two half-cells, ionic path, voltmeter and conditions.
Compare my reasoning
- Place Zn(s) in standard Zn²⁺(aq).
- Connect to the SHE through a salt bridge.
- Measure voltage with a high-resistance voltmeter at 298 K.
Use Zn(s) in standard Zn²⁺ solution and connect it to the SHE with a salt bridge; measure the potential difference using a high-resistance voltmeter at 298 K.
Look for these in your work
- Metal/ion half-cell correct
- Reference and bridge included
- Measurement conditions stated
Which electrode is appropriate for Cl₂(g)/Cl⁻(aq), and what other half-cell is needed for a standard potential measurement?
Give me a hint
The couple has no solid conducting member, so use an inert surface and a reference.
Compare my reasoning
- Use Pt in contact with Cl₂ gas and Cl⁻ solution.
- Connect to the standard hydrogen electrode.
- Use a salt bridge and voltmeter under standard conditions.
Use an inert Pt electrode in contact with Cl₂(g) and standard Cl⁻(aq), connected by a salt bridge to the SHE; measure the voltage at standard conditions.
Look for these in your work
- Inert electrode selected
- Both couple species included
- Reference measurement described
A method measures Fe³⁺/Fe²⁺ potential with a high-resistance voltmeter but no salt bridge. Explain the missing function.
Give me a hint
Consider what maintains electrical neutrality in the two solutions as charge moves.
Compare my reasoning
- Electrons flow in the external circuit.
- Ions must move between half-cells to prevent charge build-up.
- The salt bridge supplies this ionic path without mixing the solutions directly.
The salt bridge provides ionic continuity and helps maintain charge balance; without it, charge separation would stop sustained cell operation and the voltage measurement would not represent the intended cell.
Look for these in your work
- Charge build-up identified
- Ionic path explained
- Bridge role stated
Common mistakes
- Trying to use an aqueous ion as a solid electrode
- Omitting an inert conductor for a redox couple with no conducting solid
- Using a wire instead of a salt bridge for ionic continuity
- Leaving the conditions unspecified
What can you explain now?
What electrode is used for Fe³⁺/Fe²⁺ when both forms are aqueous?
Compare with the explanation
Inert platinum
Neither aqueous ion supplies a solid conducting electrode.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
What changes between a metal/ion half-cell and an aqueous ion/ion half-cell, and what measurement components are shared?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.