Mercury Beating Heart: Electrochemical Modification


When mercury is covered with aqueous solution of sulfuric acid a touch with a point of graphite cathode produces rapid pulsations of the mercury.

Watch glass (with diameter10 cm), Petri dish with diameter 9 cm, plastic pipets, plastic tray, two pencils with free graphite ends, electric cords with crocodile clips, direct current electric power supply with variable voltage 0-12 V, commercial soldering “Helping Hand” with alligator clips designed to hold work and white sheet of paper.

Mercury, sulfuric acid (c = 2 mol/L), sulfur


  • Work in a plastic tray. Place the chemicals in the tray.
  • Place a white sheet of paper in the tray.
  • Place the Petri dish on the paper sheet and on top of it the watch glass.
  • Use the plastic pipet to add 1-2 mL of mercury in the watch glass.
  • Add sulfuric acid to the watch glass to cover the mercury pool.
  • Insert the graphite electrodes into the alligator clips of the “soldering helping hand” (Fig. 1).
  • Connect the electrodes to the direct current power supply.
  • Place the tips of the electrodes in the solution of sulfuric acid and touch the mercury with the on tip.
  • Turn the power on and increase the voltage.
  • Observe the shape changes of the mercury.
  • After the demonstration collect the liquid in a bottle with heavy metal waste.

Fig. 1. Set up

Observation and Discussion
When mercury is covered with aqueous solution of sulfuric acid a touch with a point of graphite cathode produces rapid pulsations of the mercury at Ucritical » 4 V. Further increase of the voltage noticeably increases the beating frequency. In this case, the entire mercury surface is uniformly polarized with an excess of a negative charge (Fig. 2a), which causes uniform changes of the surface tension over the entire mercury surface. The variation of the surface tension is equivalent to the variation occurring at the part of the mercury pool in a contact with the iron nail in the classical version of the experiment.

Figure. 2. A simplified scheme of the structure of the double layer: upon touch of cathode (a) in detached state (b). 

Being negatively polarized, the surface tension increases causes a decrease of the mercury pool surface.  This is an example of the electrocapillary effect.  As a consequence, the mercury pool is detached from the electrode, and the pool relaxes back to the initial position (Fig. 2b).
The electrode reaction occurring at the mercury pool in contact with the cathode is as follows:

2H3O+(aq) + 2e- → 2H2O(l) + H2(g)      (process on cathode)

The overall electrochemical process is in fact electrolysis of water. At a relatively high voltage (» 12 V), the rate of the electrode reaction is markedly increased resulting with a vigorous gas evolution. If working close to the apparatus, one can even experience a problem (irritation of the breathing system) that is due to the formation of aerosol containing sulfuric acid.
Interestingly, under sufficiently high voltage, the oscillation can even be observed without direct contact between the cathode and the mercury surface, if the distance between the mercury pool and the cathode is gradually decreased. In this case pulsations occur mostly in the part of the mercury pool adjacent to the cathode. Being under the influence of the electric field imposed between the electrodes, the mercury pool is nonuniformly polarized, which causes a gradient of the surface tension values and initiates oscillations.
The part of the mercury that is near the cathode has partly positive charge (that’s why it stretches). After being neutralized by the ‘–’charge of negative ions, it springs back (close to oval shape).

Safety Tips
Mercury and its compounds are highly poisonous. Spilled mercury should be collected and saved in a closed labeled container. Sulfur powder may also be used onto the spilled droplets of mercury. In order to prevent mercury spill one should work in a plastic tray. Mercury vapor is especially dangerous, thus in badly ventilated laboratories the spilled mercury may induce chronic mercury poisoning.