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{{Short description|Chemical reaction}}
The '''mercury beating heart''' is an [[electrochemistry|electrochemical]] [[redox reaction]] between the elements [[mercury (element)|mercury]], iron and chromium. The reaction causes a blob of mercury in water to oscillate.
The observeable reaction demonstrates an effect of a non-homogeneous [[electrical double layer]]
==The experiment==
[[File:Gallium beating heart.webm|thumb|Mercury beating heart is similar to gallium beating heart (shown on video).]]
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]], [[potassium permanganate]], or potassium dichromate. The tip of an iron nail is positioned almost touching the mercury. If the position of the nail tip is just right, the mercury blob begins to oscillate, changing shape.
==The explanation==
In one variation the mechanism is thought to be as follows:
In one variation the mechanism is thought to be the following: The dichromate oxidizes the mercury, forming a layer of mercury sulfate. In the process the dichromate is reduced to the chromium (III) ion. The oxidized layer on the mercury reduces the surface tension of the blob and the blob flattens out coming in contact with the iron nail. Then the mercury sulfate oxidizes the iron to the iron (II) ion, and in the process is reduced back to metallic mercury. Once there is no oxide coating left on the mercury blob, the surface tension increases and the blob rounds up and looses contact with the nail, to start the process over again.▼
▲
The net reaction is that the dichromate oxidizes the iron. This favorable reaction drives the mercury oxidations/reductions and the oscillations in shape. When the dichromate is all reduced, the reaction stops?▼
▲The net reaction is that the dichromate oxidizes the iron. This favorable reaction drives the mercury oxidations/reductions and the oscillations in shape. When the dichromate is
There may be other mechanisms involved, however. Lin et al. appear to report that the oscillations occur without the presence of the oxidizing agent, though the mercury does not appear to get an oxidizing layer on it and the oscillations are much weaker.
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
| title = On the Mechanism of Oscillations in the
|
| volume = 71
| issue = 11
| pages = 4477–4481
| year = 1974
|
| doi =10.1073/pnas.71.11.4477
| pmid=16592197|bibcode = 1974PNAS...71.4477L
| pmc=433909|doi-access=free }}</ref>
Although this reaction is mediated by changes in surface tension, it is very similar in mechanism to other chemical oscillators such as the [[Belousov–Zhabotinsky reaction]], which has several intermediate redox reactions driven by the oxidation of malate by bromine.
==History==
The mercury beating heart was first observed in the year 1800 by [[Alessandro Volta]] and [[William Henry (chemist)|William Henry]]. The chemical phenomenon in the form best known today was first described by German chemist [[Friedlieb Ferdinand Runge]], the discoverer of [[caffeine]].<ref>{{cite journal | url=https://onlinelibrary.wiley.com/doi/abs/10.1002/ckon.19940010303 | doi=10.1002/ckon.19940010303 | title=200 Jahre "Pulsierendes Quecksilberherz"︁ zur Geschichte und Theorie eines faszinierenden elektrochemischen Versuchs | date=1994 | last1=Möllencamp | first1=Hartwig | last2=Flintjer | first2=Bolko | last3=Jansen | first3=Walter | journal=CHEMKON | volume=1 | issue=3 | pages=117–125 }}</ref>
==See also==
* Simple classroom demonstrations: [[Chemical garden]] - [[Barking dog reaction]] - [[blue bottle reaction]]
* [[Chemical oscillator]]
==References==
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==External links==
* {{YouTube|0hxt6hd-wV0|Pulsating Electrochemical Reaction: Mercury Beating Heart}}
* {{YouTube|3WF8zFieuKU|Mercury 'Beating Heart'}}
[[Category:Mercury (element)]]
[[Category:Electrochemistry]]
[[Category:Chemistry classroom experiments]]
[[Category:Articles containing video clips]]
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