Catalysing the reaction of sodium thiosulfate and hydrogen peroxide

Hydrogen peroxide oxidises sodium thiosulfate to sulfuric acid. Starting from an alkaline solution, students can observe the resulting pH change via universal indicator which changes from blue to green to yellow to orange-red. Adding an ammonium molybdate catalyst speeds up the colour change.

This introductory demonstration is useful as a starting point for a discussion on catalysts. A white background will help the students to see the colour changes clearly.

Equipment

Apparatus

• Eye protection
• Flasks, 1 dm³, x4
• Measuring cylinder, 100 cm³
• Beaker, 100 cm³

Chemicals

• Sodium thiosulfate-5-water, 8.7 g.
• Sodium ethanoate-3-water (sodium acetate tri-hydrate), 3.8 g or anhydrous sodium ethanoate, 2.3 g
• Sodium hydroxide (CORROSIVE), 0.5 g
• Ammonium molybate(VI) (HARMFUL), 0.08 g
• Hydrogen peroxide, 100 ‘volume’ (HARMFUL), 14 cm³
• Universal indicator solution (HIGHLY FLAMMABLE), a few cm³
• Deionised or distilled water, 1.1 dm³

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout, and consider wearing gloves when handling sodium hydroxide and 100 ‘volume’ hydrogen peroxide.
• Sodium thiosulfate-5-water, Na₂S₂O₃.5H₂O(s) – see CLEAPSS Hazcard HC095A.
• Sodium ethanoate-3-water, CH₃CO₂Na.3H₂O(s) – see CLEAPSS Hazcard HC038a.
• Sodium hydroxide, NaOH(s), (CORROSIVE) – see CLEAPSS Hazcard HC091a. 
• Ammonium molybdate(VI),  (HARMFUL) – see CLEAPSS Hazcard HC009a.
• Hydrogen peroxide, H₂O₂(aq), (HARMFUL) – see CLEAPSS Hazcard HC050 and Recipe Book RB045.

Procedure

Before the demonstration

1. Dissolve the sodium thiosulfate, sodium ethanoate and sodium hydroxide together in deionised or distilled water and make up to 1 dm³. Add sufficient universal indicator solution to give an easily visible blue colour. Pour 225 cm³ of this solution into each of three 1 dm³ flasks labelled ‘Catalyst’, ‘No catalyst’ and ‘Control’, respectively.
2. Make a solution of hydrogen peroxide from 14 cm³ of 100 volume hydrogen peroxide made up to 40 cm³ with deionised or distilled water. Divide this into two 20 cm³ portions.
3. Weigh out 0.08 g of ammonium molybdate(VI).

For the demonstration itself

1. Place the three flasks containing the blue solution on a bench. Add the weighed ammonium molybdate to the one labelled ‘Catalyst’ and swirl.
2. Then add 20 cm³ portions of hydrogen peroxide solution to the flasks marked ‘Catalyst’ and ‘No catalyst’, leaving the third flask as a control for colour comparison.
3. Over three or four minutes, the solution with the catalyst changes from blue through green, yellow and orange to orange-red. The solution without the catalyst follows the same sequence, but more slowly. It will not have reached the same red-orange colour of the first solution after an hour.

Teaching notes

The reaction is:

Na₂S₂O₃(aq) + 4H₂O₂(aq) → Na₂SO₄(aq) + H₂SO₄(aq) + 3H₂O(l)

The sulfuric acid produced by the reaction neutralises the sodium hydroxide (buffered by the sodium ethanoate) and gives the observed colour changes.

If the reaction is done with 20 volume hydrogen peroxide, the reaction is slower than with the diluted 100 volume hydrogen peroxide.