19.4 A Reversible Blue-and-Gold Reaction

Chemical Concept Demonstrated: Reversible oxidation-reduction reactions


  • Rochelle salt (KNaC4H4O6) is dissolved in water.
  • 3% H2O2 is added while stirring the solution.
  • CuSO4 is also added.


    Within about 20 seconds, a blue solution forms, which undergoes a vigorous reaction that liberates gas and raises the temperature of the solution.  Soon, an opaque orange-gold precipitate of Cu2O forms. Additional H2O2 is added to the soltuion, and the metal is then oxidized back to Cu (II) oxidation state. The solution turns from orange to blue as the precipitate disappears.  With time, the orange precipiate of Cu2O forms once again.

Explanations (including important chemical equations):

    The Cu 2+ tartrate ions react to form a blue complex ion, which catalyzes the decompostion of the initial aliquot of H2O2. This raises the temperature of the solution to the point that the Cu 2+ ion oxidizes the tartrate ion and is thereby reduced to Cu+, which precipitates from solution as Cu2O.  The aliquot of H2O2 then oxidizes the copper (I) oxide back to Cu 2+ , which forms a blue complex with the tartrate ion that remains is solution.  Most of the gas evolved in this demonstration is oxygen, with a small amount of CO2.  Only about 2-3% of the tartrate is oxidized to CO2 in each cycle of the reaction.