Standard Electrode Potentials: The Hydrogen Half-Cell as a Reference

The Hydrogen Half-Cell: Definition and Setup

The hydrogen half-cell is a simple yet essential electrochemical system. It is represented by the following half-equation:

$${\text{H}}^{+}(aq)+e^{-}\leftrightarrow \frac{1}{2}{\text{H}}_2(g)$$

Key Components of the Hydrogen Half-Cell

• Hydrogen Ions (${\text{H}}^{+}$):
  • These are present in an aqueous solution, typically at a concentration of $1.00{\text{mol dm}}^{-3}$ to meet standard conditions.
• Hydrogen Gas (${\text{H}}_2$):
  • Pure hydrogen gas is bubbled through the solution at a pressure of $1.00\text{atm}$.
• Platinum Electrode:
  • A platinum electrode, often coated with platinum black, serves as an inert surface for the redox reaction.

Standard Conditions for the Hydrogen Half-Cell

To ensure consistency, the hydrogen half-cell operates under standard conditions:

• Concentration of ${\text{H}}^{+}$ ions: $1.00{\text{mol dm}}^{-3}$
• Pressure of ${\text{H}}_2$ gas: $1.00\text{atm}$
• Temperature: $298\text{K}$ (25°C)

Why is the Hydrogen Half-Cell the Reference?

1. The hydrogen half-cell is assigned a standard electrode potential of $E^{\circ }=0.00\text{V}$.
2. This arbitrary but universally accepted value provides a baseline for comparing the electrode potentials of other half-cells.

How the Reference Works

When the hydrogen half-cell is connected to another half-cell in an electrochemical cell:

• The measured cell potential reflects the difference between the standard electrode potential of the other half-cell and the hydrogen half-cell.

Practical Setup of the Hydrogen Half-Cell

The hydrogen half-cell is constructed as follows:

1. A platinum electrode is immersed in a solution containing ${\text{H}}^{+}$ ions at $1.00{\text{mol dm}}^{-3}$.
2. Hydrogen gas is bubbled over the electrode at a pressure of $1.00\text{atm}$.
3. The system is maintained at $298\text{K}$.

Common Misconceptions

Reflection