Chemically Induced Pulsations of Interfaces: ~he'llercuryBeating Heart David Avnlr Hebrew University of Jerusalem, Jerusalem 91904, Israel
The Phenomenon and Its History When an iron needle just touches a mercury drop submerged in a pool of an-acidic solution of an-oxidizer, it pulsates rapidly (on a 1-Hzscale) in a mode that resemhles a beatine" heart (Fie. 11. Thii is one of a small rlass of ohenomena in which periodic macroscopic patterned motion is induced bv chemical reactions a t interfaces ( I ) . The mercurv beating heart (MBH), which is already a cla&ical example, involves a com~lexnetworkofredox reactions that affect the surface tension of the mercury drop. The experiment is very entertainiua and has been resented in the scientific educational literature as an introductory demonstration for discussion of redox processes (2).From the mechanistic aspect,
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the MBH phenomenon belongs to the large family of electrochemical oscillators. Numerous reports on these heteroeeneous redox oscillating reactions have appeared (3, 4), Tncludina systems in which mercurv is one of the electrodes (4. 5). when mercury is the electride, the oscillations are i'n: duced, among other factors, by the well-known mercury electrocapillary effect (6),namely, by the change in meniscus curvature as a function of the applied voltage. The first report on the MBH oscillator is due to Gabriel Lippmann as early as 1873 (7). However. as r e.~ o r t e dhv ~, ~ i p p m a n nhimself, he was notihe discover& of the phenomenun, hut he learned about it when visitine Wilhelm Kuhne. who was a professor of physiology a t ~ e i d e l b e r g~niversit; and a researcher in electrophysiology. Kiihne's interest in this field hrought him to experiment in the electrochemistry of mercury, which led him later to the discovery of the MBH oscillator. Apparently, Kiihne never published his observations. In a detailed historical study on the BMH phenomenon (8). Hoff et al. have tried to trace earlier reports that may have led Kiihne to his discovery. Effects of various chemicals on the curvature and shape of a mercurv dron were known, but it seems that ~ " h n e w a s e s p e c i a l ldirected ~ by an 1858 report by Carl Adolf Paalzow (91. In that renort. & . Paalzow desciihes in detail geometrical changes that a mercury drop undergoes in the presence of chromatic and sulfuric acid. This, in fact, is the half-cell system necessary to observe the pulsations (see below). ~
The Experiment Prepare the following chemicals (2): mercury, 6 M solution of sulfuricacid, 18 M solution of sulfuric acid, and a 0.1 M solution of K2Cr~07.Work in a ventilated area-Hg vapors are harmful! Be careful while handling the strong acid solutions: wear gogelas! "....
Into a watch glass, 7-10 cm in diameter, place a pool of mercury. -2 cm in dlsmpter. Cover the mercury with 6 M solution of H2S0,. Add 1 mL of the KdhO. solution. Place an iron needle or wlre so
Figure 1. Beating heart (top)and concenhic (bottom)pulsating modes.
that it just touches the mercury drop on the side at its widest circumference along the radius of the watch glass. Connect the needle to an external firm handle. Now add, drop by drop, 18 M HzS01, above the pool of the mercury, until the rhythmic motion suddenly starts. The amount of 18 M HzS04needed may vary from one experiment to the next, in the range 0.5-2.0 mL. The oscillations persist for at least an hour. The common mode of motion is heart-
Volume 66
Number 3 March 1989
211
shaped triangular, although the author also obtained other modes such as pentagons. Concentric contractionsare obtained if the needle is placed higher than the widest circumference (Fig. 1, bottom) or vertical on ton of the drao. Suggestions for further eiperimentation: Measure the pulsating rate with a stroboscope; study the concentration effects on the pulsating rate, explore the effects of replacing some of the chemicals. For instance, rulfuric acid may be replaced by other common acids (e.g., nitricacidJ, and K1Cr20: may be replaced by other strong oxidants ( e x . . KMnO,). I'lav with two needles wuchine t h e d r o ~at various points, and observe the multitude of pulsationpatterns: Figure 2. m e electron-flow scheme of the mercury beatingheart reaction.
The Mechanism
I t has been recognized already in the early papers cited above, that the oscillations originate from the oxidation of the mercury surface by the chromate, followed by reduction of the oxide back to mercury, processes that affect the surface tension and consequently also the geometry of the liquid metal. The most detailed study of the MBH is due to Keizer. Rock. Lin. and Stenschke (10.11) who ~ r o v i d e dnot only a mechaksti'c scheme, but also discAvered a new set of MBH-related systems. Much of the following discussion is based on Keizer e t al.'s work. We start the description of the redox cycle a t an arbitrary point: the situation where the mercury meniscus and the iron-needle tip are slightly separated. At this stage, the Fe undergoes two reactions that &e not involved in th'osci~~atory mechanism and will only briefly be mentioned:
--
Fe + 2Hf
+
+
14H+ 6Fe2+ Cr20;-
+ 6Fe3++ 2Cr3++ 7H20 Fez+ H2(g)
(1) (2)
.
(Fez+ is also produced in reaction 6. below. and Cr.0~2is " also consumed in reaction 3.) The surface'of the mercury drop undergoes oxidation by the chromate t o He+. The two half cells are
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6Hg 6e-
+ 14H' + Cr2072-
6Hg + Cp0;-
+ 14H'
6Hgt
+ 6e-
2CrS++ 7H20 6Hgt
(3a) (3h)
+ ZCP+ + 7H20
(3) We shall return to this six-electron process below. Of the several reactions that Hg+ can undergo--e.g., further oxidation t o the insoluble HgO, which is unstable to acid conditions:
+
-
(4) HgO 2H+ Hg2++ H20 one predominates, i.e., the precipitation of the insoluble salt HgzS04 on the surface of the mercury drop, as a thin film: SO-:
+ 2Hg'
Hg2S0,(s)
(5)
Formation of the film decreases the surface tension, and the mercury flattens out, until i t touches the iron needle. This is a short circuit situation. Since Fe is above Hg in the electrochemical series, it is capable of reducing Hg+ hack to Hg in the two-electron redox reaction: (6) Fe + 2Hgt -Fez+ + 2Hg
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In other words, the film will now dissolve according to Fe Hg2S0,(s) Fez+ 2Hg SO-: (6a) Dissolution of the film increases the surface tension, the curvature is restored, and the Hg is disconnected from the Fe; one cycle has been completed. There are three types of drivingforcefor the oscillations: a redox chemical reaction, surface tension, and gravity. We continue first with the chemistry. The net oxidation-reduction chemical reaction that pro. vides the necessary free energy for the oscillations is obtained by combining reactions 3 and 6:
+
3Fe 212
-
+ Cr,O;- + 14HC
+
+
3Fe2++ 2Cr3++ 7H20
Journal of Chemical Education
or more explicitly
+
3Fe + K2Cr207 7H$O,
SFeSO,
+ Cr,(SO,), + K,(S04)
(la)
where all salts are soluble. In other words, the chemical driving force is a six-electron transfer from the iron to the chromate, mediated by the mercury. The sacrificial components are therefore the iron and the chromate, but not the mercurv. The mercurv acts as a switch for electron transfer. as illusiratedin Figure 2. Notice that the only function of the iron is that of an electron donor. Indeed, the iron needle can be replaced by an inert (Pt) needle, connected t o a power supply. The other driving forces are physical in nature: surface tension, affected by the chemical reactions and by the electric discharge a t the moment of touch (the elect~ocapillary effect, (6)),which in turn dictates the curvature; and the flow of mercurv. -. which is driven bv eravitv. Takine all these forces into account one would perhaps expect concentric ~ulsations.This is indeed the case when the electrode needle is mounted higher than the drop's largest periphery or vertically above the center of the mercury drop (11) (Fig. 1, bottom). The heart-shaped oscillations obtained by the sidetouching eeometrv are due t o the verv different time scales of all theevents involved: the fastest is the electric discharge at the short rircuit (10-6-10-'9): slower are the rates of the chemical reactions;and the slowest is the mercury flow. Consequently, a t the moment of touch, the mercury still has the inertia of flow, and so it continues to flatten out, while a t the point of touch, the curvature starts to increase, dragging with i t the other parts of the drop. T o the best of our knowledge, i t has not been shown why this interplay of forces chooses a triangular geometry as a stable mode of oscillations. Readers interested in the quantitative aspects of this mechanism (voltage changes, surface tension changes, etc.) and in other MBH-related oscillators are referred to ref 11.
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Acknowledgment
I thank S. C. Miiller, Max Planck Institute, Dortmund, for providing Figure 1. He also carried out the photographed experiment. Literature Clted
M.: Vigna-Adler,M. J.Colloidlnforfore Sei. 1983.94.187. (bl Ksi, S.: Muller, S. C. Sci. Form 1985.1.9. , ed.; Alyee, 2. For exampla, Campbell, J. A. In Tpsfed Domorrsfrntionr in C h e m i r t ~6th H. N.: Dutton, F. B. Ed%;J. Chem. Edue.: Ea8ton. PA. 1965:p 143. 3. W o h i a , J. In Modern Aspects of Electroehemisfry; Bwkris, J. O'M. et al., Fds.; Plenum: New York, 1973:Vol.8, ~ 4 7 . 4. Keizer, J.: Sehenon, D. J. Phya. Chem. 1988.84.2025. 5. Treindel, L.;Olerova, A . Elactrochim. A d a 1983.28, 1495. RashdestwensLy, A. P.; Lewis, W. C. McC. Tmw. Fomdqy Sac. 1912.8.220. 6. Graham*. D. C. Chrm.Reu. 1947.41.441. 1. (a) Nakaehe, E.; Dvpeyraf
-. ... ,.... ....., ....,.,. . ...... 9. Paalnow, C. A. Ann. Phys. 1858,104,413. 10. Lin,S.-W.;Keizer,J.;Roek.P.A.;Stensehke,H.Rac.Nol.Acod.Sci. (USA1 1974.71, *A,"
(7)
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,.
11. Keizer, J.;Roek,P.A.:Lin,S.-W.J.+.
Chem.Sw. 1979.101,€437.
