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Chemistry LibreTexts

Formaldehyde Clock Reaction-- Red/Green Race


Chemical Concepts Demonstrated

  • Concentration effects on reaction rate
  • Addition reactions across a C=O double bond


The first set of beakers contains varying amounts of solution A and water. The second set of beakers contains varying amounts of solution B and water. One set has green indicator, the other red.
  • Solution A: Formalin (37% formaldehyde) diluted.
  • Solution B: NaHSO3, Na2SO3, and disodium EDTA dissolved in water and diluted.
  • Green Indicator: Thymolphthalein mixed with p-nitrophenol solution.
  • Red Indicator: Phenolphthalein mixed withp-nitrophenol solution.


The clock reaction shown proceeds in a linear fasion.  The beakers on the left change first while the beakers on the right change last.


The beakers change color at different rates because of the different concentrations of starting materials, particularly the materials involved in the rate-limiting step, the production of formaldehyde:

CH2(OH)--> HCHO + H2O

The faster the formaldehyde is produced, the faster the beakers change color.