My Heart Beats for Chemistry

I have always wanted to try the mercury beating heart experiment, in which a drop of liquid mercury is made to throb like a beating heart.¹ However, I have never conducted this experiment due to the issues associated with mercury toxicity. I was therefore quite pleased to see an article published in the Journal of Chemical Education that describes how to generate pulsations in a liquid metal droplet.² To do this, a liquid alloy called Galinstan is used in place of mercury. Galinstan is a mixture of 68.5% gallium, 21.5% indium, and 10% tin that melts at -19^oC, so it is a liquid at room temperature. Because it looks and behaves a lot like mercury but has minimal toxicity, it is used as a replacement for mercury in many applications. I was able to successfully follow the procedure outlined in the Journal article that describes how to use Galinstan in the mercury beating heart experiment (Video 1).³

Video 1: Galinstan: An Alternative to Mercury, Tommy Technetium YouTube Channel.

It is easiest to understand this “beating heart” phenomenon by looking at what happens when the Galinstan droplet is touched by an inert metal, like platinum. When this occurs, the surface tension of the droplet changes as it donates electrons to the platinum. These electrons come from the oxidation of gallium, indium, and tin atoms on the surface of the Galinstan:

Ga(l) → Ga³⁺(aq) + 3e^-                           Equation 1

In(l) → In³⁺(aq) + 3e^-                               Equation 2

Sn(l) → Sn²⁺(aq) + 2e^-                            Equation 3

The positive ions produced in these half reactions enter solution and surround the liquid alloy, while the negative electrons remain spread about the surface of the liquid alloy. This sets up an electric double layer around the liquid (Figure 1). The surface tension, and therefore the shape of the liquid droplet, depends upon this distribution of charge on the surface.

Figure 1: Electric double layer on the surface of a Galinstan droplet.

When the droplet is touched with a metal such as platinum, electrons are transferred from the Galinstan to the platinum. The loss of electrons changes the charge distribution around the droplet, which changes both the surface tension and shape of the liquid droplet.

The electrons transferred to the platinum are used to reduce protons into hydrogen gas. This is what forms the bubbles on the metal surface:

2 H⁺(aq) + 2e^- → H₂(g)                          Equation 4

The overall chemical reactions involved in the transfer of electrons are:

2 Ga(l) + 6 H⁺(aq) → 2 Ga³⁺(aq) + 3 H₂(g)                  Equation 5

2 In(l) + 6 H⁺(aq) → 2 In³⁺(aq) + 3 H₂(g)                      Equation 6

Sn (l) + 2 H⁺(aq)  → Sn²⁺(aq) + H₂(g)                          Equation 7

The processes involved if a reactive metal (such as iron) touches the Galinstan are slightly more complex and can be reviewed in the Journal article.² 

This experiment can be connected to the chemistry curriculum in a quantitative way by noting the reduction potentials of the substances involved in Equations 5-7:²

Ga³⁺(aq) + 3e^- → Ga(l)          E⁰ = -0.53 V               Equation 8

In³⁺(aq) + 3e^- → In(l)             E⁰ = -0.34 V                Equation 9

Sn²⁺(aq) + 2e^- → Sn(l)            E⁰ = -0.14 V                Equation 10

2 H⁺(aq) + 2e^-  → H₂(g)         E⁰ = 0 V                       Equation 11

From these reduction potentials, the standard cell potentials of Equations 5-7 can be calculated to be +0.53 V, +0.34 V, and +0.14 V, respectively. In all three cases the positive cell potentials indicate a spontaneous electron transfer from the elements in the liquid alloy to the platinum electrode, as is observed. Students can be challenged to identify the anode (liquid alloy) and cathode (platinum metal) in the overall process and to make these calculations.   

On a personal note, this experiment holds a special place in my heart (pun intended). You see, I suffered a heart attack earlier this year, and I thought it would be appropriate to conduct this “beating heart” demonstration the first time I was back in the lab after being cleared to work again. The experiments presented (Video 1) are what I captured on video during my initial visit to the lab, post-heart attack. As I marveled at the silvery blob palpitating in the beaker, I felt a wave of gratefulness wash over me for both the scientific advancements that helped me survive, and the chemistry experiments that I get to enjoy on a regular basis. Here’s hoping that my heart – and yours – will be beating for chemistry for a long time to come.

Happy Experimenting!

References:

1. Shakhashiri, B. Z. Chemical Demonstrations: A Handbook for Teachers of Chemistry; University of Wisconsin Press, 1992; Vol. 4.
2. Wang, B.; Jiang, X.; Zhang, Y.; Yu, L.; and Zhang, Y. Journal of Chemical Education 2022 99 (2), 1095-1099.
3. Tommy Technetium, Galinstan: An Alternative to Mercury. https://www.youtube.com/watch?v=yRQ000QApWU