The standard hydrogen electrode

1. Definition

• The Standard Hydrogen Electrode (SHE) is used as a reference electrode for measuring standard electrode potentials.
• Its electrode potential is defined as 0.00 V under standard conditions.

2. Structure of the Standard Hydrogen Electrode

• A platinum $(Pt$) electrode is used to provide a surface for hydrogen gas to react.
• Hydrogen gas $(H_2)$ is bubbled through a solution containing 1 $mol·dm⁻³$ of $H⁺$ ions (e.g., $HCl$ solution).
• The electrode is immersed in the solution at a pressure of 1 atm and a temperature of 25°C $$(298 K).$

3. Standard Conditions for SHE

• Temperature: 25°C $(298 K)$
• Pressure: 1 $atm$ $(101.3 kPa)$
• Concentration of $H⁺$ ions: 1 $mol·dm⁻³$

4. Purpose of SHE

• Used as a reference electrode with 0.00 V to measure the $E°$ values of other electrodes.
• Helps determine whether a substance is a strong oxidising agent (positive $E°$ value) or a strong reducing agent (negative $E°$ value).

5. Example: Determining the Standard Electrode Potential of Copper

• Cell setup: A SHE is connected to a $Cu²⁺/Cu$ half-cell.
• Observed voltage: 0.34 V (positive reading).
• Conclusion:
  • Electrons flow from SHE to $Cu,$ meaning $Cu²⁺$ is reduced.
  • $E° (Cu²⁺/Cu) = +:highlight[0.34 V],$ meaning copper is a stronger oxidising agent than hydrogen.
• Cell Notation: $H_2(g) | H^+(aq) || Cu^{2+}(aq) | Cu(s)$

6. Example: Determining the Standard Electrode Potential of Zinc

• Cell setup: A SHE is connected to a $Zn²⁺/Zn$ half-cell.
• Observed voltage: 0.76 V (negative reading).
• Conclusion:
  • Electrons flow from $Zn$ to SHE, meaning $Zn$ is oxidised.
  • $E° (Zn²⁺/Zn) = -:highlight[0.76 V],$ meaning zinc is a stronger reducing agent than hydrogen.
• Cell Notation: $Zn(s) | Zn^{2+}(aq) || H^+(aq) | H_2(g)$

7. Key Takeaways

• SHE is the universal reference electrode with 0.00 V potential.
• More positive $E° →$ Stronger oxidising agent (e.g., $Cu²⁺$).
• More negative $E° →$ Stronger reducing agent (e.g., $Zn$).
• $E°cell = E°cathode - E°anode$ is used to calculate cell potential.