Mercury beating heart

In electrochemistry, the mercury beating heart is an effect observed in mercury demonstrating the effect of a non-homogeneous electrical double layer ^[1]. It is often used as a classroom demonstration.

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The experiment

In the experiment a droplet of mercury is placed in a watch glass, immersed in an electrolyte such as sulfuric acid which contains an oxidizing agent such as hydrogen peroxide or potassium permanganate. When the mercury is touched by the tip of an iron nail the drop starts to oscillate.

The explanation

An electrical double layer forms between the surface of the mercury droplet and the electrolyte solution. At rest this layer is uniform. When the iron tip is introduced a redox reaction starts in which iron is oxidized to the ferric ion. At the same time the oxidizing reagent is spent for instance when hydrogen peroxide together with hydronium ions is reduced to water. Because the oxidation only takes place in the vicinity of the tip and the reduction process covers the whole droplet surface the surface tension is no longer homogeneous resulting in oscillations.

The mercury beating heart was first observed by Carl Adolf Paalzow in 1858. Jöns Jakob Berzelius is reported to have used electrodes.

See also

• Simple classroom demonstrations: Chemical garden.

References