Pulsating Electrochemical Reaction: Mercury Beating Heart

Contents

CCA!
A drop of mercury in a watch glass is covered with a solution of potassium chromate in concentrated sulfuric acid. An iron nail is positioned so that it nearly touches the mercury. Eventually, the mercury drop starts to beat rhythmically, like a beating heart.
The mercury drop, iron nail, and aqueous solution of acidic dichromate constitute a galvanic cell in which the mercury serves as the cathode and the iron serves as the anode. There is a voltage difference between the iron and mercury electrodes. During the time when there is no electrical connection between the mercury and the iron, dichromate oxidizes Hg(l ) to Hg2SO4(s ), which forms a layer on the mercury surface. This surface layer reduces the surface tension of the liquid mercury, allowing the mercury drop to flatten. Eventually the mercury contacts the iron nail and the cell is short circuited. Transfer of electrons from the iron to the mercury and thence to the Hg2SO4(s ) surface film reduces the Hg2SO4(s ) back to metallic mercury. This increases the surface tension of the mercury and the drop becomes rounder, causing it to recede from the nail and break the circuit. Then the surface film builds up again as a result of oxidation of mercury by the dichromate.

The equations for these processes are

film 2 Hg(l ) + SO42-(aq ) === Hg2SO4(film ) + 2 e-
formation Cr2O72-(aq ) + 14 H+(aq ) + 6 e- === 2 Cr3+(aq ) + 7 H2O(l )
film Hg2SO4(film ) + 2 e- === 2 Hg(l ) + SO42-(aq )
reduction Fe(s ) === Fe2+(aq ) + 2 e-

The overall reaction is oxidation of iron by dichromate:

Cr2O72-(aq ) + 14 H+(aq ) + 3 Fe(s ) === 2 Cr3+(aq ) + 7 H2O(l ) + 3 Fe2+(aq )

This reaction can occur more rapidly when mediated by the oxidation and reduction of mercury, presumably because a surface coating of hydroxide or oxide on the iron slows the metal's reaction with dichromate ions and hydrogen ions in the aqueous solution. Some of the Gibbs free energy released by this overall reaction serves to drive the physical movement of the mercury drop.

A more detailed discussion of the mechanism of the mercury beating heart is available: Keizer, J.; Rock, P. A.; Lin, Shu-Wai J. Amer. Chem. Soc. 1979, 101, 5637-5649.

Think About It

  1. What is the purpose of the nail in this demonstration?
  2. Write balanced chemical equations for the oxidation of mercury by chromate, and of iron by Hg22+.
  3. Why does oxidation of mercury cause the droplet to flatten? Why does contact with the iron nail reverse the flattening of the droplet?

Safety/Precautions

Poison, poisonous vapors, chemical irritant, carcinogen

Mercury is poisonous. Soluble mercury salts cause corrosive effects on skin and mucous membranes, severe nausea, vomiting, abdominal pain, kidney damage, and death. Mercury is readily absorbed through the respiratory tract, the skin, and the gastrointestinal tract. The concentration of mercury vapor from an open container is above the maximum safe exposure limit. Mercury spills should be cleaned up promptly with approved mercury spill cleaning methods.

Acidic dichromate solutions are human carcinogens. Taken internally chromates and dichromates are corrosive poisons.

The solution used in this demonstration is acidic and therefore corrosive to skin. Gloves should be worn and if any acidic dichromate solution is spilled on the skin, it should be washed off with plenty of water. Spills of large quantities of dichromate require special cleanup.

Journal of Chemical Education is a publication of the Division of Chemical Education, Inc. of the American Chemical Society.