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

The Hydrogen Half-Cell: Definition and Setup

1. The hydrogen half-cell is a simple yet essential electrochemical system.
2. 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

1. Hydrogen Ions ($\text{H}^+$):
   1. These are present in an aqueous solution, typically at a concentration of $1.00 \, \text{mol dm}^{-3}$ to meet standard conditions.
2. Hydrogen Gas ($\text{H}_2$):
   1. Pure hydrogen gas is bubbled through the solution at a pressure of $1.00 \, \text{atm}$.
3. Platinum Electrode:
   1. 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:

1. Concentration of $\text{H}^+$ ions: $1.00 \, \text{mol dm}^{-3}$
2. Pressure of $\text{H}_2$ gas: $1.00 \, \text{atm}$
3. 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 $298 \, \text{K}$.