tested demonstrations Gallium Beating Heart Submined by
James L. Ealy, Jr The Peddie School Hightstown, NJ 08520
Checked by James Niewahner Northern Kentucky University Highland Heights, KY 41076 History of the Discovery There is a wonderful account ofthe discovery of the oscillation of mercury by Lippman and Kuhne in Hoff's paper ( I ) . I wish my discovery was as serendipitous as theirs. However, my discovery occurred because my colleague, Julie Ealy, and I were discussing the possibility of substituting gallium for mercury in some thermometers. Mercury is banned in many schools. For years I have had fleeting thoughts about finding a possible substitute for mercury in the Mercury Beating Heart demonstration. The late Miles Pickering encouraged us constantly to look for safer substitutes for dangerous chemicals in ~orrular demonstrat~ons,such as usmgglyoxal instcad of fokaldehvdc r2, :I,. As the discussion ~rocecded,the infamous light appeared and I suspected gaiium might be that substit& for mercury. "Luck does favor the prepared mind." Immediately, I ordered some gallium. After waiting through two backorders, it finally arrived. I melted it and placed it in a Petri dish containing dilute sulfuric acid. I then added a few drops of a potassium dichromate solution and approached the gallium with a nail. The gallium oscillated! Success! I showed it to all of the students and colleagues I could corner. Since then I have devoted as much time as a high school teacher can to researching and comparing the two oscillating mechanisms (4-8). Phenomena The phenomena associated with the mercury beating heart seems to be consistent with gallium: an iron wire in an acidic solution and two types of reactions--one involving a visible surface film and the second involving no visible surface film. Procedure Place about 15 g of gallium in a glass Petri dish. Add about 50 mL of 6 M sulfuric acid to the dish and place it on a hot plate. Gently warm the gallium on a hot plate until the gallium melts and the solution is about 50 'C. Add 3 4 mL of 0.1 M potassium dichromate solution to the dish. Remove the dish from the hot plate. Place the dish on an overhead projector. The gallium will have one of two appearances a t this stage: it will he a dull, flat puddle or a bright, shiny sphere in the solution. If it is a dull, flat puddle, touch it with the iron nail, and it will immediately form the shiny spherical shape. The oscillations can be induced by the careful positioning of the nail next to the shiny sphere. The nail must be close enough to be touched, but only when the sphere spreads out to form a puddle, evidenced by a small amount of penetration.
edited bv GEORGE L. GILBERT Denison University Granville,OH 43023
Concepts The gallium serves a s an electron switch for the electrons from the corroding negative anode (iron nail) to the cathodic half-cell reaction on the surface of the gallium. The iron of the nail is oxidized by the acid and the surface of the nail gains an excess of electrons. These electrons are transferred to the gallium when they touch.
F4s) + 2Ht(aq)+ Fe2+(aq)+ Hz(g) The dichromate oxidizes the surface atoms ofthe gallium and sometimes forms a coating of gallium sulfate. More study needs to be done on this specific interfacial reaction. 2Ga(s)+ 14HYaq)+ cr20?-(aq) + 2cr3+(aq)+ 2Ga3+(aq)+ 7Hz0(1) The formation of the surface film interferes with the surface tension. The surface tension decreases and the gale flattens out. When the lium loses its s~hericals h a ~ and gallium touches'the iron nai. the clectronr; are transferred to the gallium sulfate. Thc eallium 31 ion rains three electrons and forms gallium metal-returningto its spherical shape by the restoration of surface tension. Fe(s) ~a~+(+ aq ) + Fe3+(aq)+Gab) The oscillations continue for about a half hour. The dichromate concentration decreases to a minimum value, and the film apparently no longer forms.
The potentials for the four principle reactions give a final positive value. The Feo/Ga3+//Fe3+/Gao reaction is negative 6< + 2Ga3++ 2Ga0 4.520 2Fe0 + We2++ 4eK 0.441 2Fe2++ 21Je3++ 2e4.770 -0.849 -0.849 Ga3++ 2Fe0 + 2Fe3++ 2Ga0 The Ga0/C~/Ga3+/Cr3+ reaction is positive.
Two events occur with the gallium that do not occur with the Mercury Beating Heart. First, when the nail touches it should jerk away and form a the surfaceof the spherical shape with a very shiny surface. Holding the nail a&st the gallium t w long causes the gallium 6 become very viscous. This condition resembles the freezing of the gallium. ARer removing the nail, it "pops" back to a spherical shape in a few seconds and begins to oscillate. The Volume 70 Number 6 June 1993
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heat produced by the oxidation and reduction appears to keen the ~-~ eallium melted. Also as the eallium cools to room tempera&e, the beating will slow f&m 3 He at 50 'C to 1 Hz at 30 'C. Accordingly, this is different from that reported in past'studies for mercury (10.11).
To Bubble or Not to Bubble Demonstrating Boyle's Law
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Gallium metal as well as the nitrate produced no skin injury, and subcutaneous injections of relatively large amounts could be tolerated both by rabbits and rats without evidence of injury, (Sax,N. I.) By comparison, mercury is highly toxic by skin absorption and inhalation of fumes or vapor-TLV 0.05 mgiM3. Gallium is, therefore, relatively harmless. Its vapor pressure is 0.01 mm of Hg pressure, and there is little chance of breathing any vapors. However, prolonged handling of t h e p u r e metal, gallium, o r ingesting it may cause harmful effects. Sulfuric acid is corrosive to a l l body tissues. Inhalation of concentrated vapor may cause serious lung damage. Contact with eyes may result i n total loss of vision. Work i n a fume hood, wear gloves, a rubber apron, and goggles. Potassium dichromate is a strong oxidizing agent that is harmful t o t h e skin, m u c o u ~ m e m b r a n e as n d eyes. Avoid inhalation of the dust. Disposal: An acidic dichromat&hromium (111) solution can be disposed of by the careful addition of 50% excess sodium bisulfite until heat is liberated. Then precipitate with 1M sodium hydroxide solution and dispose of in an approved EPA landfill.
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Literature Cited 1. HofP,H.E.;Ceddea, hA.;Vdpntmtmtmi,M.E.:Powel,T.;Cdiouar.RPS. ConkrB~lL (Houston) 1911.9, Ser 2,117. 2. Eel% J. B: CHEM 13NEWS lW, 295.9.
Elvin Hughes, Jr. and L. H. Holmes, Jr.
Southeastern Louls~anaUniversity Hammond. LA 70402
Safety Precautions
3. Eel% J. B. The Sci. b h . l W l , 58.26.
submmed by
Checked by
Jesse Binford
University of South Florida Tampa, FL 33620 Procedure
Fill a 500-mL graduated cylinder with water. Take a piece of glass tubing (6 mm or 8 mm) about 50 cm long that is graduated in distance units (centimeters, inches, etc.) and connect it to a natural gas outlet with rubber or Tygon tubing. Immerse the other end of the tubing in the water in the 500-mL graduated cylinder. When the gas valve is turned on, the gas pressure will push the water down in the niece of elass tubing. Because the nressure in natural gas iines is not high (geierally about 6 k . of water gauge), the tubing can be put far enough into the water so that the gas will not bubble out. The length of the bubble of gas in the tubine below the surface of the water in the maduated cylinders the gauge pressure of the gas line in&hes (or centimeters) of water. This setup is actually a manometer with a slightly different ~hvsicalamearance. It can be used to measure the of an$hing connected to it that does not exhibit a vressure w a t e r than the number of inches of water in the baduatea cylinder. The height of water available can be increased by using something longer than the graduated cylinder (a gas oollection tube or a piece of 14 mm (or larger diameter) glass tubing that has been closed on one end). The range of pressure that can be measured also can be increased by using a liquid with a higher density than water.
4. Adams,R H. J. Chrm.Educ 1989,IO. 512
Discussion This is a simple demonstration that can be done in intro6. Alyes, H.N. Persadmmmunimtion. 1992. ductory chemistry or physics classes when pressure is in7. Avir, David J. C h m . Edue 1988,66,211. troduced. Many students initally will believe that the gas 8. Campbell. J. A. J. C k m Educ 1967,34 363. will bubble out if the end of the glass tubing is placed on 9. Weaat,R.C.,Editorin-Chief,HondbmkofC~mistryondPhysiu;ChemiemialR"bber the bottom ofthe graduated cylinder. It will, of course, only Company. Cleveland, OH, 48th ed,D86. bubble out if the end of the tubing is near enough to the 10. &im, J.;hk.P.A.;Lin,S:W JAm.Chem.Soc ls?B,IOI,5637. surface so that the gas pressure (absolute)is greater than 11. h,S.-W.;L~,S.-W,:Keiiii,J.;Ro~ok,P.A:Stenstenshhe;H.Phc.Nol.Amd.Sci.1814, 71, 4471. the pressure due to the water and atmospheric pressure. 5. Alysa, H. N. TOPS,J Chrm Edur 1986.43.A971
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Journal of Chemical Education
