Electrolysis of sodium chloride solution

The electrolysis of sodium chloride solution is the experiment where concentration takes centre stage. In the Form 5 Redox Equilibrium chapter it shows that the product at the anode is not fixed: dilute brine gives oxygen, but concentrated brine gives chlorine, because the concentration of chloride ions changes which ion is discharged. Run both concentrations, compare the anode gas, then use the Paper 3 section to state the factors that decide selective discharge.

Aim

To investigate the electrolysis of sodium chloride solution using carbon electrodes, to identify the gases at the cathode and anode, and to study how the concentration of the solution affects the product at the anode.

Apparatus and materials

• Electrolytic cell or U⁻tube with two carbon (graphite) electrodes
• Test tubes to collect the gases
• Battery or d.c. power supply, ammeter, switch and connecting wires
• Dilute sodium chloride solution and concentrated sodium chloride solution
• Wooden splint and damp blue litmus paper
• Safety goggles; access to a fume cupboard

Procedure

1. Pour dilute sodium chloride solution into the cell.
2. Fit the two carbon electrodes and connect them through the switch, ammeter and battery.
3. Invert a test tube of solution over each electrode to collect the gas produced.
4. Close the switch and let the current flow until enough gas is collected.
5. Test the gas from the cathode with a lighted splint, and the gas from the anode with a glowing splint and damp blue litmus paper.
6. Switch off, then repeat steps 1 to 5 using concentrated sodium chloride solution.
7. Record the gas at each electrode for both concentrations in a table.

Expected observations

• Dilute sodium chloride solution, cathode: a gas is collected that pops with a lighted splint, showing hydrogen. Anode: a gas is collected that relights a glowing splint, showing oxygen.
• Concentrated sodium chloride solution, cathode: a gas that pops with a lighted splint, showing hydrogen. Anode: a greenish-yellow, pungent gas that bleaches damp blue litmus paper, showing chlorine.
• The solution around the cathode gradually turns alkaline.

Report the gas tests and colours you observe; do not invent volumes of gas collected.

Inference and conclusion

At the cathode, hydrogen is discharged in preference to sodium in both cases, because sodium is more electropositive and is higher in the electrochemical series, so its ions are harder to discharge; the hydrogen ions from water gain electrons and are reduced. At the anode, the product depends on the concentration of chloride ions. In dilute solution, hydroxide ions are discharged and oxygen is released. In concentrated solution, the high concentration of chloride ions leads to chloride being discharged instead, so chlorine is released. This shows that selective discharge at an electrode depends not only on position in the electrochemical series but also on the concentration of the ions. Reduction at the cathode and oxidation at the anode make the whole process a redox reaction. The alkaline solution left near the cathode is sodium hydroxide, formed as hydrogen ions are removed.

Science process skills (Paper 3 style)

• Hypothesis. “The more concentrated the sodium chloride solution, the more likely chlorine, rather than oxygen, is released at the anode.” State a testable relationship between concentration and product.
• Variables. The manipulated variable is the concentration of the sodium chloride solution; the responding variable is the identity of the gas released at the anode; the controlled variables are the electrodes used, the voltage, the volume of solution and the time.
• Tabulating data. Use columns for the concentration, the cathode gas and its test, and the anode gas and its test.
• Making an inference. From chlorine at the anode only with concentrated solution, infer that a high concentration of chloride ions favours their discharge.
• Operational definition. “Selective discharge is the preferential release of one ion over another at an electrode, shown here by chloride being discharged instead of hydroxide when the solution is made concentrated.”

Safety precautions

• Carry out the experiment in a fume cupboard, because the chlorine released from concentrated solution is toxic and irritates the lungs.
• Keep the cathode gas away from flames while collecting, because hydrogen is flammable and can explode.
• Wear safety goggles, because the electrolyte solutions can splash into the eyes.
• Use only a low-voltage d.c. supply with dry hands, so there is no risk of electric shock.

Common errors

• Testing the anode gas with a lighted splint only and missing the litmus test, so chlorine is not distinguished from oxygen.
• Letting the collected chlorine escape into the room instead of working in a fume cupboard.
• Assuming chlorine is always released at the anode, ignoring that dilute solution gives oxygen.
• Confusing which gas pops and which relights a splint, so hydrogen and oxygen are swapped.
• Using electrodes that are too close together, which short-circuits the cell.

How we help

In our online one-to-one SPM Chemistry lessons, taught in English, our teachers drill the three factors of selective discharge, position in the electrochemical series, concentration and electrode material, using this experiment as the clearest example of the concentration effect. Fees start from RM50 per hour, and a paid one-hour trial lesson lets you see how the anode question is reasoned out before you commit. Because brine electrolysis also underlies the industrial chlor-alkali process elsewhere in SPM Chemistry, understanding it here connects your practical work to a real application and to Paper 2 and Paper 3 questions.