Standard hydrogen electrode

 

Introduction

The Standard Hydrogen Electrode (SHE) is a reference electrode used to measure the electrode potential of other half-cells. It is assigned a standard electrode potential of 0 V at all temperatures by convention and serves as a baseline for determining the electrochemical potential of other electrodes.

Construction of Standard Hydrogen Electrode

1. Electrode Setup:

   • The SHE consists of a platinum electrode (typically a thin platinum foil or platinum black) that is immersed in a solution containing hydrogen ions (H⁺), typically 1 M HCl or any other acid with a known concentration of H⁺.

2. Hydrogen Gas Supply:

   • Pure hydrogen gas (H₂) is continuously bubbled over the platinum electrode at a constant pressure of 1 atmosphere.

3. Platinum Electrode:

   • The platinum electrode serves as an inert surface for the oxidation and reduction of hydrogen. Platinum is chosen because it is highly resistant to corrosion and has excellent catalytic properties that facilitate the adsorption of hydrogen molecules.

4. Components:

   • H⁺ Ions (1 M solution): This ensures a constant activity of H⁺ ions, which is crucial for maintaining the standard conditions.
   • Platinum Electrode: Provides a surface for electron transfer but does not react in the process.
   • Hydrogen Gas: Bubbled over the electrode at 1 atm pressure, interacting with the electrode surface.

Working Principle of Standard Hydrogen Electrode

1. Electrode Reaction: The SHE operates based on the reversible redox reaction:

   $${\text{H}}_2(g)\rightleftharpoons 2{\text{H}}^{+}(aq)+2e^{-}$$
   • Oxidation: Hydrogen molecules (H₂) lose electrons and produce protons (H⁺).
   • Reduction: Protons (H⁺) gain electrons to form hydrogen gas (H₂).

2. Equilibrium: At the electrode surface, an equilibrium is established between the hydrogen gas and the hydrogen ions in the solution. The electrode potential depends on the concentration of H⁺ ions, the pressure of H₂ gas, and the temperature.

3. Measurement:

   • The SHE acts as a reference half-cell and is paired with another half-cell to form a complete electrochemical cell.
   • The potential difference between the SHE and the other electrode is measured. By convention, the potential of SHE is taken as 0 V, so the potential of the other half-cell can be determined relative to it.

4. Standard Conditions:

   • The standard hydrogen electrode potential is assigned as 0 V under the following conditions:
     • Temperature: 298 K (25°C)
     • The pressure of H₂ gas: 1 atm
     • Concentration of H⁺ ions: 1 M

Applications of SHE

• Reference Electrode: Used as a reference for measuring the electrode potential of other half-cells.
• Standardization: The SHE is essential for the standardization of electrode potentials across different electrochemical cells.
• Determination of Electrode Potential: Helps in calculating the standard reduction potentials of various half-cells.

Advantages of SHE

• It provides a universal reference point for measuring electrochemical potential.
• The platinum electrode is highly resistant to corrosion and ensures accurate, stable readings.

Disadvantages of SHE

• Maintaining 1 atm of hydrogen gas and 1 M concentration of H⁺ ions is difficult.
• It is fragile and not portable, making it less convenient for practical use in field measurements.