Chlor-Alkali Process Experiment

Na⁺ (to cathode)
Cl⁻ (to anode)
OH⁻ (forms NaOH)
H⁺ (to cathode)

At anode (+)

2Cl⁻(aq) → Cl₂(g) + 2e⁻

Overall reaction

2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂

At cathode (−)

2H⁺(aq) + 2e⁻ → H₂(g)

What this experiment shows

Electrolysis of concentrated sodium chloride solution - brine - is one of the largest industrial processes in the world. It gives three useful products at once: chlorine at the anode, hydrogen at the cathode and sodium hydroxide left in the solution. The reason chlorine appears rather than oxygen is the concentration of the brine: with chloride ions that plentiful, they are discharged in preference to hydroxide ions.

2NaCl + 2H₂O → Cl₂ + H₂ + 2NaOH

What you need

Concentrated sodium chloride solution
Inert electrodes - graphite or platinum
A d.c. supply and a diaphragm or membrane cell
Test tubes for collecting the two gases
A fume cupboard: chlorine is toxic

How it is done

Fill the cell with concentrated brine and put the electrodes in place.
Connect the d.c. supply, anode to positive and cathode to negative.
Collect the gas at each electrode in an inverted tube.
Test the anode gas with damp blue litmus and the cathode gas with a lit splint.
Test the remaining solution with indicator.

What you should see

Anode: a pale green gas that bleaches damp litmus paper - chlorine.
Cathode: a gas that burns with a squeaky pop - hydrogen.
The solution around the cathode becomes alkaline as sodium hydroxide builds up.
The products must be kept apart, or they react together.

Where marks are lost

Saying oxygen is given off at the anode. In concentrated brine it is chlorine; in very dilute solution it would be oxygen.
Mixing up the electrodes. Anions go to the anode; the anode is positive in electrolysis.
Forgetting that sodium hydroxide is a product, not a leftover.
Saying chlorine turns litmus red. It turns it red and then bleaches it white.

Questions students ask

Why chlorine and not oxygen?

The brine is concentrated, so chloride ions greatly outnumber hydroxide ions at the anode and are discharged instead. Dilute the solution and oxygen is produced instead.

Why is sodium not formed at the cathode?

Hydrogen is below sodium in the reactivity series, so hydrogen ions from the water accept the electrons more readily. The sodium ions stay in solution.

What are the three products used for?

Chlorine for bleach, disinfectant and PVC; hydrogen for ammonia and margarine; sodium hydroxide for soap, paper and drain cleaner.

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Written for Cambridge IGCSE Chemistry 0620 and CBSE Class 9-12 by Ajay Shekhawat, founder of MrChemCoach. Run the simulator above, then check yourself against the questions.