Electrolysis using a microscale Hoffman apparatus

In this experiment, students use a microscale Hoffman apparatus to carry out the electrolysis of sodium sulfate solution. They then consider ways of testing for the gases they have collected.

The practical should take approximately 25 minutes.

Equipment

Apparatus

• Eye protection
• Clamp and stand
• Microscale Hoffman apparatus (see note 3 below)
• 9 volt battery and leads with crocodile clips
• Plastic pipette
• Blu-Tack®
• Beaker, 100 cm³

Chemicals

• Sodium sulfate solution, 0.2 mol dm^–3
• Bromothymol blue indicator

Health, safety and technical notes

1. Read our standard health and safety guidance.
2. Wear eye protection throughout.
3. Check our guidance on apparatus and techniques for microscale chemistry to find out more about setting up microscale Hoffman apparatus.
4. Sodium sulfate, Na₂SO₃(aq), 0.5 mol dm^–3 is of low hazard. See CLEAPSS Hazcard HC098B and CLEAPSS Recipe Book RB107.
5. Bromothymol blue solution is of low hazard. See CLEAPSS Hazcard HC032.

Procedure

1. Set up the Hoffman apparatus in a clamp.
2. Pour approximately 40 cm³ of the sodium sulfate solution into a beaker and add a few drops of bromothymol blue indicator. Note the colour of the solution.
3. Using a pipette carefully fill the electrolysis apparatus with the sodium sulfate solution.
4. Plug the tops of each stem with a small piece of Blu-Tack®.
5. Carefully attach the crocodile clips to the electrodes and record all your observations over the next 15 minutes.
6. Disconnect the leads and try to give explanations for your observations.

Question for students

Can you think of a way of testing for either of the gases that you have collected?

Teaching notes and expected observations

Streams of bubbles are seen at each electrode. The colour of the solution around the cathode gradually turns blue due to the formation of sodium hydroxide. The solution around the anode becomes greenish-yellow.

If the tops of the pipettes are sealed with Blu-Tack® the volume of gas collecting above the cathode (hydrogen) is seen to be greater than that collecting above the anode (oxygen). If left connected for long enough the ratio of the volumes corresponds to the 2:1 ratio of hydrogen to oxygen in water. The shortened pipette (see our guidance on microscale apparatus and techniques) can be used to sample the hydrogen gas and to test it by blowing it into a flame (it ‘pops’).