Describe the standard hydrogen electrode
Identify the hydrogen half-cell and explain how it provides the zero reference for standard electrode potentials.
Section 24 · Electrochemistry
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 is the conventional E° value assigned to the standard hydrogen electrode?
What you will learn
- State the components of the standard hydrogen electrode
- Write the H⁺/H₂ half-equation
- Explain its assigned potential and reference role
The reference uses hydrogen ions and hydrogen gas
The standard hydrogen electrode uses a platinum electrode in contact with aqueous H⁺ at standard concentration and H₂ gas at standard pressure, at 298 K. Platinum provides an inert conducting surface; it is not consumed in the half-reaction.
The reduction half-equation is 2H⁺(aq) + 2e⁻ ⇌ H₂(g). By convention E° = 0.00 V, so every other standard electrode potential is measured relative to it.
A salt bridge and voltmeter complete the measurement
Connect the hydrogen half-cell to the test half-cell with a salt bridge and a high-resistance voltmeter. The bridge allows ions to move and maintain charge balance while the voltmeter measures the potential difference.
The SHE is a reference system, not a claim that every hydrogen-related reaction has zero potential under all concentrations or pressures. The zero applies to the standard conditions and the stated half-equation.
Pause and explain
Why is platinum used in the hydrogen electrode?
Worked example
Describe the standard hydrogen electrode and state its standard potential.
Show the worked solution
- Use Pt, H₂ gas and aqueous H⁺ under standard conditions at 298 K.
- Write 2H⁺ + 2e⁻ ⇌ H₂.
- State the conventional reference value.
Answer A Pt electrode contacts H₂(g) and standard aqueous H⁺ at 298 K; its reduction half-equation is 2H⁺(aq)+2e⁻⇌H₂(g), and E° is defined as 0.00 V.
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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.
Write the reduction half-equation for the standard hydrogen electrode.
Give me a hint
Hydrogen ions gain electrons to form hydrogen gas.
Compare my reasoning
- Balance two hydrogen ions to form one H₂ molecule.
- Add two electrons to the left.
- Include aqueous and gas states.
2H⁺(aq) + 2e⁻ ⇌ H₂(g).
Look for these in your work
- Atoms balanced
- Charge balanced
- States correct
A student includes Pt(s) as a product in the SHE half-equation. Explain the correction.
Give me a hint
Platinum is the conducting surface, while hydrogen is the species that changes oxidation state.
Compare my reasoning
- The redox couple is H⁺/H₂.
- Pt is an inert electrode and is not consumed.
- Omit it from the half-equation.
The half-equation is 2H⁺(aq)+2e⁻⇌H₂(g); Pt provides a surface but is not a reactant or product.
Look for these in your work
- Redox couple identified
- Electrode role clarified
- Equation corrected
Why does an electrode-potential experiment connect the test half-cell to the SHE instead of measuring the test half-cell alone?
Give me a hint
A voltmeter measures a potential difference between two terminals.
Compare my reasoning
- A single half-cell has no independent absolute potential measurement.
- The SHE supplies a defined reference value of 0.00 V.
- The measured voltage is the test value relative to that reference.
The voltage is a comparison between two half-cells. The SHE provides the defined 0.00 V reference, allowing the test half-cell potential to be determined relative to it.
Look for these in your work
- Relative measurement understood
- Reference value stated
- Measured quantity described
Common mistakes
- Treating Pt as a reactant
- Writing H₂ → 2H⁺ + 2e⁻ when describing the reduction potential
- Forgetting the standard conditions
- Calling the SHE an absolute zero-energy electrode
What can you explain now?
What is the conventional E° value assigned to the standard hydrogen electrode?
Compare with the explanation
0.00 V
It is the reference for the standard potential scale.
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Make it stick
Which species change at the SHE, and why does the inert platinum electrode not appear in the half-equation?
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