Castner–Kellner process

Castner–Kellner Process

The Castner–Kellner process is an industrial method for the production of sodium hydroxide (NaOH) and chlorine (Cl_) by the electrolysis of sodium chloride (NaCl) solution, commonly known as brine. This process is named after its inventors, Hamilton Castner and Karl Kellner, who developed it in the late 19th century.

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Diagram of the Castner–Kellner process

Process Description

The Castner–Kellner process involves the electrolysis of brine in a cell that uses a mercury cathode. The cell is divided into two compartments by a porous partition. The process can be described in the following steps:

1. Brine Preparation: A concentrated solution of sodium chloride is prepared and fed into the electrolytic cell.
2. Electrolysis: When an electric current is passed through the brine, chlorine gas is liberated at the anode, and sodium ions migrate towards the mercury cathode.
3. Formation of Sodium Amalgam: At the mercury cathode, sodium ions are reduced to form sodium metal, which dissolves in mercury to form a sodium amalgam.
4. Decomposition of Sodium Amalgam: The sodium amalgam is then transferred to a separate chamber where it reacts with water to produce sodium hydroxide and hydrogen gas.

The overall chemical reactions can be summarized as follows:

• At the anode: \( 2 \text{Cl}^- \rightarrow \text{Cl}_2 + 2e^- \)
• At the cathode: \( 2 \text{Na}^+ + 2e^- \rightarrow 2 \text{Na} \)
• Formation of sodium amalgam: \( 2 \text{Na} + \text{Hg} \rightarrow \text{Na}_2\text{Hg} \)
• Decomposition of sodium amalgam: \( \text{Na}_2\text{Hg} + 2 \text{H}_2\text{O} \rightarrow 2 \text{NaOH} + \text{H}_2 + \text{Hg} \)

Advantages and Disadvantages

The Castner–Kellner process has several advantages and disadvantages:

• Advantages:

 * High purity of sodium hydroxide and chlorine produced.
 * Continuous process suitable for large-scale production.

• Disadvantages:

 * Use of mercury poses environmental and health risks.
 * High energy consumption due to the electrolysis process.

Environmental Impact

The use of mercury in the Castner–Kellner process has raised significant environmental concerns. Mercury is a toxic element that can cause severe health problems and environmental damage. As a result, many countries have phased out the use of mercury-based processes in favor of more environmentally friendly alternatives, such as the membrane cell process.

Related Pages

• Electrolysis
• Sodium hydroxide
• Chlorine
• Mercury (element)
• Membrane cell process