Catalysis of the reaction between sodium thiosulfate and hydrogen peroxide
to sodium thiosulfate sulfuric acid. Starting from an alkaline solution, the resulting can be followed using pH change which changes from blue to green to yellow to orange-red. Adding an Universal indicator speeds up the colour change. ammonium molybdate catalyst
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.
The teacher will require:
Flasks (1 dm
Measuring cylinder (100 cm
Beaker (100 cm
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 cm 3
Universal indicator solution (HIGHLY FLAMMABLE), a few cm
Deionised or distilled
water, 1.1 dm 3
Refer to Health & Safety and Technical notes section below for additional information.
Health & Safety and Technical notes
Read our standard health & safety guidance
Wear eye protection and consider wearing gloves when handling
sodium hydroxide and 100 'volume' hydrogen peroxide.
Sodium thiosulfate-5-water, Na 2S 2O 3.5H 2O(s) - see CLEAPSS Hazcard.
Sodium ethanoate-3-water, CH 3CO 2Na.3H 2O(s) - see CLEAPSS Hazcard.
Sodium hydroxide, NaOH(s), (CORROSIVE) - see CLEAPSS Hazcard.
Ammonium molybdate(VI), (HARMFUL) - see CLEAPSS Hazcard.
Hydrogen peroxide, H 2O 2(aq), (HARMFUL) - see CLEAPSS Hazcard and Recipe Book
Before the demonstration
a Dissolve the sodium thiosulfate, sodium ethanoate and sodium hydroxide together in deionised or distilled water and make up to 1 dm 3. Add sufficient Universal indicator solution to give an easily visible blue colour. Pour 225 cm 3 of this solution into each of three 1 dm 3 flasks labelled 'Catalyst'. 'No catalyst' and 'Control', respectively.
b Make a solution of hydrogen peroxide from 14 cm 3 of 100 volume hydrogen peroxide made up to 40 cm 3 with deionised or distilled water. Divide this into two 20 cm 3 portions.
c Weigh out 0.08 g of ammonium molybdate(VI).
For the demonstration itself
d Place the three flasks containing the blue solution on a bench. Add the weighed ammonium molybdate to the one labelled ‘Catalyst’ and swirl.
e Then add 20 cm 3 portions of hydrogen peroxide solution to the flasks marked ‘Catalyst’ and ‘No catalyst’, leaving the third flask as a control for colour comparison.
f 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.
The reaction is:
2S 2O 3(aq) + 4H 2O 2(aq) → Na 2SO 4(aq) + H 2SO 4(aq) + 3H 2O(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.
Health & Safety checked, 2016
This Practical Chemistry resource was developed by the Nuffield Foundation and the Royal Society of Chemistry.
© Nuffield Foundation and the Royal Society of Chemistry
Page last updated October 2015