The gallium beating heart is an alternative to the mercury beating heart demonstration. In both cases, an electrochemical reaction causes a blob of liquid metal to oscillate like a beating heart. While popular and interesting, the mercury beating heart isn’t often performed because it generates toxic waste. The gallium beating heart offers a safer alternative. Here are two ways of performing the demonstration and a look at the chemistry involved.
Gallium Beating Heart Using an Iron Nail
Since its discovery in 1800 by Alessandro Volta and William Henry, many variations of the original mercury beating heart have come along. The following method for performing the gallium beating heart eliminates both toxic mercury and potassium dichromate. It also uses a smaller amount of materials and a lower concentration of sulfuric acid.
- ~1.5 g gallium metal (piece about 5-7 mm diameter)
- 10 mL 1M to 1.4M sulfuric acid
- 200 mL water
- 250 mL beaker
- vial or test tube
- clean iron nails (fresh iron must be available)
- stand and clamps
- Add about 200 mL of 40–50°C (warm) water to the beaker. This acts as a warm water bath and keeps the gallium liquid.
- Place the gallium and sulfuric acid in the vial and clamp the vial so its base rests within the water bath. You may not need all of the sulfuric acid. Just cover the gallium with 1-2 cm of acid.
- Clamp the nail so its point sits within the vial, near its edge. Proper placement has the nail tip touching the gallium when it flattens into a puddle, but not touching the gallium when it is a round ball. With the nail in position, the gallium heart beats for about half an hour.
The combination of a gallium-indium (GaIn) alloy with a stainless steel wire also works.
Gallium Beating Heart Using Potassium Dichromate
An earlier version of the project simply replaced mercury with gallium. The nail is not required if the ratio between acid and dichromate is just right. But, using a nail ensures success.
- Dilute sulfuric acid (e.g. battery acid or ~6M H2SO4)
- Potassium dichromate
- Petri dish or watch glass
- Warm the gallium in the palm of a gloved hand so it melts. Allow a drop to fall into the glass.
- Cover the gallium with dilute sulfuric acid. The flattened drop rounds into a ball as gallium sulfate forms on the metal surface.
- Add a small amount of potassium dichromate. The gallium relaxes its shape as the sulfate layer is removed and the surface tension of the droplet changes. With the correct proportion of dichromate to sulfuric acid, the drop alternates between shiny round and dull flattened shapes and oscillates like a beating heart. If you don’t see oscillation, add a bit more dichromate until you achieve the effect.
The exact amount of chemicals depends on the scale of your demonstration. For example, using 15 grams of gallium and 50 mL of 6M sulfuric acid, you need 3-4 mL of 0.1 potassium dichromate solution.
The gallium heart beats slowly on its own, but you can use a clean iron nail to enhance the reaction. Touch the tip of the nail to the dull puddle of gallium. It immediately forms the shiny sphere. Fixing the nail in place so it only touches the metal when it pools ensures a beating heart.
While simpler than the reaction that requires iron nail, this method involves potassium dichromate (K2Cr2O7). It’s a common oxidizer in laboratory settings, but it does contain hexavalent chromium, so disposal may be a concern for educational settings.
How the Gallium Beating Heart Works
Gallium is an electron switch acting between the corroding negative anode (the iron nail) and the cathode (the half-reaction occurring on the gallium surface. When the acid oxidizes the iron, the nail surface has an excess of electrons. The electrons transfer to the gallium when the two metals touch. Bubbles of hydrogen gas form.
Fe(s) + 2H+(aq) → Fe2+(aq) + H2(g)
Dichromate oxidizes surface atoms on the gallium droplet and sometimes forms a gallium sulfate coating. The reaction decreases surface tension of the gallium drop so it loses its spherical shape and flattens. When gallium touches the iron the gallium sulfate gains the electrons. The gallium reverts from the compound back into gallium metal, restoring surface tension and returning the metal to its spherical shape.
Ga3+(aq) + Fe(s) → Fe3+(aq) + Ga(s)
The “heart” beats for around 30 minutes. Eventually, the dichromate concentration falls below a minimum value and stops forming a film.
While certainly safer than the mercury beating heart, the gallium beating heart still uses sulfuric acid and possibly potassium dichromate. Wear gloves and eye protection and avoid contact with either chemical. In the case of a sulfuric acid spill or splash, neutralize the area with a weak acid, such as baking soda. Then, rinse thoroughly with water.
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- Lin, Shu-Wai; et al. (1974). “On the Mechanism of Oscillations in the Beating Mercury Heart.” Proceedings of the National Academy of Sciences of the United States of America. 71 (11): 4477–4481. doi:10.1073/pnas.71.11.4477
- Wang, Bingxing, et al. (2022) “A Safer Alternative for the Mercury Beating Heart Demonstration.” J. Chem. Educ.
- Yi, Liting; Wang, Qian; Liu, Jing (2019). “Self-Powered Gallium-Base Liquid-Metal Beating Heart.” J. Phys. Chem. A 123(43): 9268-9273. doi:10.1021/acs.jpca.9b05743
- Yu, Zhenwei, et al. (2018). “Discovery of a Voltage-Stimulated Heartbeat Effect in Droplets of Liquid Gallium.” Physical Review Letters. 121(2). doi:10.1103/PhysRevLett.121.024302