Standard electrode potentials and the hydrogen electrode

Redox revisited (spec 16.1)

Oxidation is loss of electrons or an increase in oxidation number; reduction is gain of electrons or a decrease in oxidation number. An oxidising agent is itself reduced. This applies to s-, p- and d-block elements alike, for example MnO4- (Mn +7) is reduced to Mn2+ (Mn +2).

Standard electrode potential (spec 16.2 to 16.4)

A metal dipping into a solution of its ions sets up an equilibrium such as Zn2+(aq) + 2e- ⇌ Zn(s), and a potential difference. A single potential cannot be measured on its own, so every half-cell is connected to a reference electrode.

The standard electrode potential, E°, is the emf of a cell made of the half-cell and a standard hydrogen electrode, under standard conditions: 298 K, 100 kPa for gases, 1.00 mol dm^-3 for ions.

The standard hydrogen electrode is hydrogen gas at 100 kPa bubbled over a platinum electrode (coated with platinum black) in a solution with [H+] = 1.00 mol dm^-3, such as 1.00 mol dm^-3 HCl. Its E° is defined as 0.00 V.

Measuring E° (spec 16.5)

• Metal / metal ion: the metal dips into 1.00 mol dm^-3 of its ions.
• Non-metal / ion, or ions of one element in two oxidation states (Fe3+ and Fe2+): use an inert platinum electrode in a solution with both species at 1.00 mol dm^-3 (gases at 100 kPa).

Join the half-cells with a salt bridge (filter paper soaked in saturated potassium nitrate solution), which lets ions move to complete the circuit without mixing the solutions. Measure with a high-resistance voltmeter so that almost no current flows and concentrations do not change.

The electrochemical series (spec 16.13)

E° values are also called standard reduction potentials, because half-equations are always written as reductions. Listed in order they form the electrochemical series:

• the more positive E°, the more readily the species on the left gains electrons: a stronger oxidising agent (Cl2 +1.36 V, MnO4-/H+ +1.51 V)
• the more negative E°, the more readily the species on the right loses electrons: a stronger reducing agent (Mg -2.37 V, Zn -0.76 V)

Worked example

Explain why the standard electrode potential of Cu2+/Cu is +0.34 V.

1. When connected to the standard hydrogen electrode under standard conditions, the copper half-cell is the positive terminal.
2. The cell emf is 0.34 V, and E°(H+/H2) is defined as 0.00 V.

Answer: Cu2+ is more easily reduced than H+, so E° is positive: +0.34 V

Check yourself

1. Which set lists the standard conditions for measuring E°?
   • 273 K, 100 kPa, 1.00 mol dm^-3
   • 298 K, 1 atm, 0.100 mol dm^-3
   • 373 K, 100 kPa, 1.00 mol dm^-3
   • 298 K, 100 kPa, 1.00 mol dm^-3
   Show answer

   298 K, 100 kPa, 1.00 mol dm^-3. Room temperature 298 K, standard pressure 100 kPa, ion concentrations 1.00 mol dm^-3.

2. Which is NOT part of a standard hydrogen electrode?
   • Hydrogen gas at 100 kPa
   • A copper electrode
   • A platinum electrode
   • A solution with [H+] = 1.00 mol dm^-3
   Show answer

   A copper electrode. The electrode is platinum, which is inert and catalyses H+/H2 equilibrium.

3. Which electrode is used in a half-cell containing Fe2+(aq) and Fe3+(aq)?
   • Iron
   • Copper
   • Zinc
   • Platinum
   Show answer

   Platinum. No solid metal is involved in Fe3+ + e- ⇌ Fe2+, so an inert conductor is needed.

4. What is the role of the salt bridge?
   • It allows ions to flow and completes the circuit without the solutions mixing
   • It carries electrons between the half-cells
   • It provides H+ ions
   • It acts as a catalyst
   Show answer

   It allows ions to flow and completes the circuit without the solutions mixing. Electrons go through the external wire; ions move through the salt bridge.

5. Why is a high-resistance voltmeter used to measure electrode potentials?
   • To increase the emf
   • To protect the platinum
   • So that almost no current flows and the concentrations stay standard
   • To speed up the reaction
   Show answer

   So that almost no current flows and the concentrations stay standard. With no current, no reaction happens in the cell, so the maximum emf is measured.

6. Use E° values: Cl2/Cl- +1.36 V, Br2/Br- +1.09 V, I2/I- +0.54 V, Fe3+/Fe2+ +0.77 V. Which is the strongest oxidising agent?
   • Cl2
   • Br2
   • I2
   • Fe3+
   Show answer

   Cl2. The most positive E° belongs to the species most easily reduced.

7. Use E° values: Mg2+/Mg -2.37 V, Zn2+/Zn -0.76 V, Fe2+/Fe -0.44 V, Cu2+/Cu +0.34 V. Which is the strongest reducing agent?
   • Zn
   • Fe
   • Mg
   • Cu
   Show answer

   Mg. The most negative E° means the metal loses electrons most readily.

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Sources

• Pearson Edexcel IAL Chemistry (2018) specification
• Pearson Edexcel IAL Chemistry (2018) scheme of work

Last updated 7 November 2026. Spotted a mistake? Tell us.