The Mercury “Beating Heart” Electrochemical Reaction

This chemical reaction “oscillates” much like the Belousov–Zhabotinsky reaction we have studied in class. A mercury droplet is covered with a solution of potassium chromate in dilute sulfuric acid. An iron nail is placed near the mercury, but does not touch the droplet. After some time, the mercury drop starts to beat regularly like a heart. This motion is accomplished via a series of redox reactions. The chromate oxidizes the surface of the mercury droplet, which binds to the surrounding sulfate ions to form a film of mercury I sulfate around the droplet. This film decreases the surface tension of the mercury droplet, causing it to flatten. This flattening results in the mercury droplet coming into contact with the iron nail, which facilitates the transfer of electrons to the mercury, destroying the film of mercury I sulfate as it is reduced to metallic mercury. Once the film is destroyed, the surface tension of the mercury increases and the droplet becomes round once more. The process is then repeated again, starting with the oxidation of mercury and reformation of the mercury I sulfate film. Several variables affect the oscillations, including the distance and difference in electric potential between the iron nail and the droplet of mercury.

More information can be found at:
Keizer, J.; Rock, P. A.; Lin, Shu-Wai J. Amer. Chem. Soc. 1979, 101, 5637-5649.

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