Red-White-and-Blue Demonstration Sheet

22.6 The Red-White-and-Blue Demonstration

Chemical Concepts Demonstrated: Kinetics, multi-step reactions

Demonstration:

Three solutions are combined in alphabetical order.

Solution A: Na₂SO₃ * H₂O dissolved in water mixed with 2,4,2',4',4"-pentamethoxytriphenyl methanol dissolved in acetone.
Solution B: Cd(NO₃)₂ * H₂O and HCl added to water, shaken, and diluted.
Solution C: NaIO₄ dissolved in water.

Observations:

Solutions A and B produce a red solution. Adding solution C after this turns the solution milky white, then blue after a time.

Explanations (including important chemical equations):

The hydrochloric acid in solution B turns the 2,4,2',4',4"-pentamethoxytriphenyl methanol (an indicator) red. The addition of solution C causes the pH to increase due to the formation of OH^-ions in the IO₄^- oxidation of S₂O₃^2-.

2 S₂O₃^2- (aq) + IO₄^- (aq) + H₂O (l) --> S₄O₆^2- (aq) + IO₃^- (aq) + 2 OH^-(aq)

When the indicator turns colorless again, the milky-white suspension of Cd(IO₄)²⁺ becomes apparent. The further increase in pH triggers another indicator (thymolphthalein), turning the solution blue.