Vanishing Valentine Chemistry Demonstration

Pink Color Change Chem Demo

Here’s a fun chemistry demonstration that’s perfect for Valentine’s Day or for illustrating an oxidation-reduction reaction. The Vanishing Valentine involves shaking a solution and turning it pink. If the pink Valentine solution is left undisturbed, it becomes colorless. You can repeat the color change cycle several times. The color change comes from the oxidation and reduction of resazurin. an indicator that is either pink or colorless depending on its oxidation state.

Vanishing Valentine Materials

The key to the pink color is the resazurin indicator. It is available prepared as a solution or as a powdered dye.

• 100 ml of a 0.133 M dextrose solution (C₆H₁₂O₆)
• 100 ml of a 1.0 M sodium hydroxide solution (NaOH)
• 1 ml of a 0.1% resazurin solution
• a 250-ml or 500 ml Erlenmeyer flask or separatory funnel (resembles a heart)
• stopper for the flask
• dropper or pipette

Prepare the Solutions

You need three chemical solutions for this demonstration:

Dextrose Solution: Dissolve 2.4 g of dextrose in distilled or deionized water to make 100 ml of solution.

Sodium Hydroxide Solution: Prepare the 1.0 M sodium hydroxide solution by dissolving 4.0 g of sodium hydroxide in enough distilled or deionized water to make 100 ml of solution. Add the sodium hydroxide a little at time, stirring constantly. This reaction evolves heat.

Resazurin Solution: Dissolve 0.1 g of resazurin in distilled or deionized water to make 100 ml of solution. The shelf life of resazurin solution is 6-12 months. This solution should be a deep blue color.

Perform the Vanishing Valentine Demonstration

Performing the demonstration is simple:

1. Pour 100 ml of dextrose solution and 100 ml of sodium hydroxide solution into the Erlenmeyer flask or separatory funnel.
2. Add 8 drops of resazurin indicator solution to the flask or funnel.
3. Stopper the solution and swirl the flask to mix the contents. Initially the solution will be blue.
4. Let the solution sit undisturbed. Once the resazurin is fully reduced the solution will become clear or colorless. This takes up to 10 minutes.
5. Swirl or shake the solution to turn it a pink Valentine color.
6. Repeat the clear-pink cycle by allowing the solution to sit and then shaking it again. Once prepared, the solution lasts approximately an hour (depending on temperature and available oxygen in the flask). The pink color becomes less vivid over time.

Vanishing Valentine Chemical Reactions

Dextrose irreversibly reduces resazurin to resorufin. The red resorufin molecule is further reduced (reversibly) to colorless dihydroresorufin. Dihydroresorufin (clear) oxidizes back to resorufin (pink) by swirling or shaking the flask to introduce oxygen from the air into the solution. The reaction is an example of a color-change redox reaction, like the classic blue bottle chemistry demonstration.

Vanishing Valentine Demo Safety

Wear appropriate chemistry lab safety gear when performing this demonstration, such as a lab apron, gloves and safety goggles. While the resazurin and dextrose solutions are not hazardous, sodium hydroxide solutions are caustic and can produce a chemical burn with skin or eye contact.

References

• Dutton, F. B. (1960). “Methylene Blue – Reduction and Oxidation”. Journal of Chemical Education. 37 (12): A799. doi:10.1021/ed037pA799.1
• Engerer, Steven C.; Cook, A. Gilbert (1999). “The Blue Bottle Reaction as a General Chemistry Experiment on Reaction Mechanisms”. Journal of Chemical Education. 76 (11): 1519–1520. doi:10.1021/ed076p1519
• Shakashiri, B. Z. (1989). Chemical Demonstrations: A Handbook for Teachers in Chemistry. Volume 2. University of Wisconsin: Madison, WI.