The Pumpkinator

Co-Authored by Tom Kuntzleman* and Grazyna Zreda**

*Spring Arbor University, MI and **Tanque Verde HS, AZ

With Halloween right around the corner, we thought we’d share with you a demonstration we’ve developed that’s great for this holiday. The reaction is a modification of an orange-to-blue reversible reaction that makes use of copper, hydrogen peroxide, and tartrate ion (Video 1).^1-3

Video 1: Halloween Chemistry Experiment! Tommy Technetium YouTube Channel. Note that in contrast to what is shown in the video, the reaction works best if sodium carbonate is added to the mixture (dissolving the cream of tartar in the process) prior to hydrogen peroxide.

Drawing a Jack-o-lantern face on the beaker makes this reaction well-suited to perform around Halloween. It’s also a great experiment for National Chemistry Week, which falls very close to Halloween each year.

The mechanism for this reaction is complex, and not entirely understood.^1-3 However, we use the following set of reactions to describe the observations. First, tartrate ion reacts with Cu²⁺ ion in basic solution to form a copper(II)-tartrate complex that is intensely blue in color:⁴

Cu²⁺(aq) + 2 C₄H₄O₆^2- (aq) → Cu^II(C₄H₄O₆)^2-(aq)               Eq. 1

The Cu²⁺-tartrate ion is slowly reduced by peroxide to form an insoluble, orange copper (I) oxide:

2 Cu^II(C₄H₄O₆)^2-(aq) + 2 H₂O₂ → 2 Cu₂O(s) + 2 C₄H₄O₆^2-(aq) +  ½ O₂ + 2 OH^-(aq)        Eq. 2

The reaction tends to stabilize when the copper (I) oxide is formed. However, further addition of peroxide to the copper (I) oxide formed causes it to be rapidly oxidized to copper (II), regenerating the blue color:

H₂O + Cu₂O(s) + H₂O₂ → 2 Cu²⁺(aq) + 4 OH^-(aq)              Eq. 3

Interestingly, the Cu²⁺ in this reaction mixture is not stable, and it slowly relaxes back to the insoluble copper (I) oxide via Equations 1 and 2. Several cycles of the orange-to-blue-to-orange color change can be observed by adding hydrogen peroxide to the mixture any time the orange copper (I) precipitate is observed. Because the color orange is central to this reaction, and because the reaction can be made to oscillate, we have dubbed this reaction the “Pumpkinator”. (Note that oscillating chemical reactions are often given names with an “-ator” suffix, such as the Brusselator and Oregonator).^5-7 It should be noted, however, that the oscillations observed in this experiment are not spontaneous, but instead induced by “pulses” of added hydrogen peroxide.

Notice that we have modified the original experiment so that it can be done solely with items found in grocery, retail, and hardware stores. A solution of 3% hydrogen peroxide can be easily found at pharmacies or grocery stores. Cream of tartar (found in the spice section of grocery stores), which is comprised of potassium hydrogen tartrate, is used as the source of tartrate ion in this experiment. At high pH, the hydrogen tartrate is converted to the tartrate ion. In the “Pumpkinator”, this transition is accomplished by addition of sodium carbonate (found in pool supply stores as pH increaser).

HC₄H₄O₆^-(aq) + CO₃^2-(aq) →  C₄H₄O₆^2-(aq) + HCO₃^-(aq)                Eq. 4

Finally, copper (II) sulfate (found as root killer in hardware stores) is used as the source of copper ions.

As always, let us know how this experiment works if you try it out for yourself. Also drop us a line if you can think of ways to improve the mechanistic description of what’s going on in this experiment.

Happy Experimenting, and Happy Halloween!

References

1. Sherman, M. C.; Weil, D. A Reversible Blue-and-Gold Reaction. J. Chem. Educ., 1991, 68, 1037
2. Toth, Z. A Reversible Blue-and-Gold Reaction. J. Chem. Educ. 1994, 71, 1098.
3. Flinn Scientific, The Reversible Orange and Blue Reaction https://www.flinnsci.ca/api/library/Download/a1dad53d11044d36801cb4325884f6e0
4. Izaki, M.; Koyama, T.; Loon Khoo P.; Shinagawa, T. Light-Irradiated Electrochemical Direct Construction of Cu2O/CuO Bilayers by Switching Cathodic/Anodic Polarization in Copper(II)− Tartrate Complex Aqueous Solution. ACS Omega, 2020, 5, 683-691.
5. Pellitero, M. A.; Lamsfus, C. Á.; Borge, J. The Belousov-Zhabotinskii Reaction: Improving the Oregonator Model with the Arrhenius Equation. J. Chem. Educ. 2013, 90 (1), 82-89.
6. Lozano-Parada, J. H.; Burnham, H.; Martinez, F. M. Pedagogical Approach to the Modeling and Simulation of Oscillating Chemical Systems with Modern Software: The Brusselator Model. J. Chem. Educ. 2018, 95 (5), 758-766.
7. Kuntzleman, T. S.; Kuntzleman, J. T.; Campbell, D. J. A Simple Chemical Oscillator: The “Educator”. J. Chem. Educ. 2022, 99,  3540-3545.