The mercury beating heart is an
electrochemical
Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outc ...
redox reaction
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
between the elements
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
A double layer (DL, also called an electrical double layer, EDL) is a structure that appears on the surface of an object when it is exposed to a fluid. The object might be a solid particle, a gas bubble, a liquid droplet, or a porous body. The D ...
. It is often used as a classroom demonstration.
The experiment
In the experiment a droplet of mercury is placed in a
watch glass, immersed in an
electrolyte
An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon ...
such as
sulfuric acid
Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular fo ...
which contains an
oxidizing agent
An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or " accepts"/"receives" an electron from a (called the , , or ). In other words, an oxi ...
such as
hydrogen peroxide
Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3 ...
,
potassium permanganate
Potassium permanganate is an inorganic compound with the chemical formula KMnO4. It is a purplish-black crystalline salt, that dissolves in water as K+ and , an intensely pink to purple solution.
Potassium permanganate is widely used in the c ...
, 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 the following: The dichromate oxidizes the mercury, forming a layer of mercury oxide. 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 loses 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.
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
A double layer (DL, also called an electrical double layer, EDL) is a structure that appears on the surface of an object when it is exposed to a fluid. The object might be a solid particle, a gas bubble, a liquid droplet, or a porous body. The D ...
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
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
starts in which iron is oxidized to the
ferric
In chemistry, iron(III) refers to the element iron in its +3 oxidation state. In ionic compounds (salts), such an atom may occur as a separate cation (positive ion) denoted by Fe3+.
The adjective ferric or the prefix ferri- is often used to sp ...
ion. At the same time the oxidizing reagent is spent for instance when hydrogen peroxide together with
hydronium
In chemistry, hydronium (hydroxonium in traditional British English) is the common name for the aqueous cation , the type of oxonium ion produced by protonation of water. It is often viewed as the positive ion present when an Arrhenius acid ...
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
Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension is what allows objects with a higher density than water such as razor blades and insects (e.g. water striders) t ...
is no longer homogeneous resulting in oscillations.
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
A Belousov–Zhabotinsky reaction, or BZ reaction, is one of a class of reactions that serve as a classical example of non-equilibrium thermodynamics, resulting in the establishment of a nonlinear chemical oscillator. The only common element in ...
, 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
Alessandro Giuseppe Antonio Anastasio Volta (, ; 18 February 1745 – 5 March 1827) was an Italian physicist, chemist and lay Catholic who was a pioneer of electricity and power who is credited as the inventor of the electric battery and th ...
and
William Henry. The chemical phenomenon in the form best known today was first described by German chemist
Friedlieb Ferdinand Runge
Friedlieb Ferdinand Runge (8 February 1794 – 25 March 1867) was a German analytical chemist. Runge identified the mydriatic (pupil dilating) effects of belladonna (deadly nightshade) extract, identified caffeine, and discovered the first c ...
, the discoverer of
caffeine
Caffeine is a central nervous system (CNS) stimulant of the methylxanthine chemical classification, class. It is mainly recreational drug use, used recreationally as a Nootropic, cognitive enhancer, increasing alertness and attentional perfor ...
.
Jansen et al.: 200 Jahre „Pulsierendes Quecksilberherz”︁. Zur Geschichte und Theorie eines faszinierenden elektrochemischen Versuchs.
(first published in CHEMKON, 1994). Wiley Online Library.
See also
* Simple classroom demonstrations: Chemical garden
Comparison of chemical gardens grown by NASA scientists on the International Space Station (left) and on the ground (right)
A chemical garden while growing
up Cobalt(II) chloride
upA chemical garden
A chemical garden is a set of complex biol ...
- Barking dog reaction - blue bottle reaction
* Chemical oscillator
A chemical oscillator is a complex mixture of reacting chemical compounds in which the concentration of one or more components exhibits periodic changes. They are a class of reactions that serve as an example of non-equilibrium thermodynamics wit ...
References
External links
*
* {{YouTube, 3WF8zFieuKU, Mercury 'Beating Heart'
Mercury (element)
Electrochemistry
Chemistry classroom experiments
Articles containing video clips