Create an activated complex that needs to be seen to be believed – turn the solution from pink to green, and explore the reaction catalyst involved

This experiment shows young learners a clear and fast way to explore catalysts, turning a cobalt solution different colours under different conditions. 

This practical should take 5 minutes.



  • Eye protection
  • Bunsen burner
  • Tripod
  • Gauze
  • Heat-proof mat
  • Beaker, 250 cm3
  • Thermometer, 0-100 °C
  • Measuring cylinder, 25 cm3
  • Pipette


The quantities given are for one demonstration

  • Potassium sodium tartrate-4-water, 5 g (Rochelle salt, potassium sodium 2,3- dihydroxybutanedioate, KNaC4H4O6.4H2O).
  • Cobalt(II) chloride-6-water, 0.2 g (CoCl2.6H2O).
  • Hydrogen peroxide solution, 20 cm3 of 20 volume (ie approximately 6 %) (H2O2(aq)).
  • Deionised water, 65 cm3.
  • Crushed ice, 200 cm3 (optional).

Health, safety and technical notes

  • Read our standard health and safety guidance.
  • Always wear eye protection.
  • Cobalt(II) chloride-6-water, CoCl2.6H2O (s), is a carcinogen, mutagen, reproductive toxin and a skin & respiratory sensitiser. It is also harmful to the aquatic environment (see CLEAPSS Hazcard HC025). 
  • Hydrogen peroxide solution, 20 vol H2O2 (aq) is a skin/eye irritant (see CLEAPSS azcard HC050).
  • Potassium sodium tartrate-4-water is of low hazard (see CLEAPSS Hazcard HC095a).


Before the demonstration

  • Make a solution of 0.2 g of cobalt chloride-6-water in 5 cm3 of deionised water. 
  • Make a solution of 5 g of Rochelle salt in 60 cm3 of deionised water in a 250 cm3 beaker.

The demonstration

  • Add 20 cm3 of 20 volume hydrogen peroxide to the solution of Rochelle salt and heat the mixture to about 75 °C over a Bunsen burner.
  • There will be a slow evolution of gas showing that the reaction is proceeding.
  • Stirring the solution makes the evolution of gas more obvious. 
  • Now add the cobalt chloride solution to the mixture.
  • Almost immediately, the pink solution will turn green and after a few seconds vigorous evolution of gas starts and the froth will rise almost to the top of the beaker.
  • Within about 30 seconds, the frothing subsides and the pink colour returns.


The green activated complex can be trapped if a sample of the green solution is withdrawn with a dropping pipette and then transferred to a test-tube that is cooled in crushed ice.

The solution remains green for some time. If the reaction is considered to be going too fast for easy observation, carry it out at a lower temperature (although this will make it less easy to see the evolution of CO2 before adding the catalyst).


The basic reaction appears to be:

5H2O2(aq) + C4H4O62–(aq) → 4CO2(g) + 2OH(aq) + 6H2O(l)

The equation may also be written in two parts:

3H2O2(aq) + C4H4O62–(aq) → 2CO2(g) + 2HCOO(aq) + 4H2O(l)

2HCOO(aq) + 2H2O2(aq) → 2CO2(g) + 2H2O(l) + 2OH(aq)

The reaction is catalysed by pink Co2+ ions which are first oxidised to green Co3+ ions (complexed by tartrate ions) and then reduced back to Co2+.

While the majority of the gas evolved is carbon dioxide, oxygen will also be produced from the decomposition of some of the hydrogen peroxide.

The gas mixture will turn limewater milky, but does not extinguish a glowing splint.