Chlor-alkali (mercury-cell) electrolysis — electrode materials In a mercury-cell process for caustic soda (NaOH) production, the anode and cathode are made respectively of which materials?

Introduction / Context:
In the historical mercury-cell chlor-alkali process, chlorine is generated at the anode and sodium forms an amalgam at the mercury cathode, which is later decomposed to caustic soda.

Given Data / Assumptions:

• Anode material must withstand chlorine evolution in brine.
• Cathode must be liquid mercury moving along the cell trough to form Na–Hg amalgam.

Concept / Approach:
Graphite (or dimensionally stable anodes in modern cells) serves as the anode for chlorine evolution. The cathode is a flowing pool of mercury in which sodium dissolves as an amalgam; later, reacting with water forms NaOH and H2.

Step-by-Step Solution:
Assign anode: graphite (chlorine evolution).Assign cathode: moving mercury (sodium amalgam formation).Thus, the correct pairing is anode = graphite, cathode = moving mercury.

Verification / Alternative check:
Standard cell schematics for the Castner–Kellner mercury cell show graphite anodes and a mercury cathode.

Why Other Options Are Wrong:

• Any choice listing mercury as anode is incorrect for the chlorine-evolving electrode.
• Carbon vs graphite ambiguity: the classical reference is graphite anode; “moving mercury and carbon” in that order misassigns electrodes.

Common Pitfalls:

• Confusing mercury-cell with diaphragm or membrane cells, which use different cathode/anode configurations.

Final Answer:
Graphite and moving mercury