In the Classroom 1292 Journal of Chemical Education • Vol. 84 No. 8 August 2007 • www.JCE.DivCHED.org Identifying the person who first discovered the mercury beating heart (MBH) is controversial. Berzelius, in his Span- ish edition of Chemistry, stated that this phenomenon was observed for the first time by Ermann and further studied by Herschel’s son and Pfaff (1). However, Avnir (2) stated that Carl Adolf Paalzow (1858) deserves the credit for the discovery. The MBH is a well-known experiment that has been performed under various conditions (3–6). Generally the ex- periment is performed by placing a drop of mercury in a watch glass and covering it with an aqueous solution of sul- furic acid and an oxidizing agent. When the mercury drop is touched with a tip of an iron nail, oscillations start. Previous versions of this experiment are based on varying the oxidiz- ing agent and substituting other metals for the iron. For in- stance, the oxidizing agent could be potassium permanganate (3), potassium chromate (3, 4, 7) or dichromate, or hydro- gen peroxide. The oscillations create different shapes of the mercury drop (oval, equilateral triangle, pentagon, etc.), depending on the experimental conditions. To understand the origin of the oscillations it is important to note the existence of an electric double layer at the interface between the mercury drop and the aqueous electrolyte. The electric double layer is formed as a consequence of the interactions between wa- ter molecules, electrolyte ions, and the mercury surface. The structure of the double layer is related to the distribution of the electric charge over the mercury surface, and it determines the surface tension of the mercury drop. When mercury is immersed in an electrolyte solution, without contact with the iron nail, a double layer with a uniform structure exists, and the surface tension is equal over all points of the mercury surface. Both the structure of the double layer and the sur- face tension are subject to dramatic changes when the mer- cury drop is touched by the tip of an iron nail. For instance, in a system consisting of an aqueous solution of sulfuric acid and hydrogen peroxide as an oxidizing agent, touching the mercury surface with the tip of the iron nail causes the fol- lowing redox reactions: Fe 3+ (aq) + 3e − Fe(s) (1) 4H 2 O(l) H 2 O 2 (aq) + 2H 3 O + (aq) + 2e − (2) Contrary to the misleading interpretations in the litera- ture (4), it is important to stress that these two redox reac- tions should proceed simultaneously to maintain the charge neutrality of each phase at each instant of the experiment. By analogy to the galvanic cell (8), if a part of the mercury drop is in contact with the iron nail, oxidation of iron takes place (eq 1), whereas at the rest of the mercury surface the reduction of hydrogen peroxide occurs (eq 2). Therefore, the behavior of the mercury drop can be, to some extent, ex- plained considering the behavior of a bipolar electrode with a nonuniform polarization over its surface. Consequently, the mercury drop is characterized by nonuniform values of the surface tension. The surface tension gradient is the driving force for oscillations of the mercury drop. Browsing through literature we found that Berzelius de- scribed similar mercury oscillations using two electrodes con- nected to a battery. We propose using two electrodes as an additional version to the well-known experiment—one that can strengthen the offered explanation. Berzelius states (1) that the oscillations did not take place when alkaline solu- tion was used. We show that this is only partly true and in- clude a video clip of MBH in aqueous solution of potassium hydroxide. Equipment and Chemicals • Overhead projector • Watch glass, 10 cm diameter • Petri dish, 9 cm diameter • Pencils with free graphite ends • Electric cords with alligator clips • Power supply with variable dc voltage 0–12 V • “Helping hand” with alligator clips • Paper sheet with a 9 cm hole • Deionized or distilled water • Aqueous solution of sulfuric acid (2 mol L 1 ) • Aqueous solution of potassium hydroxide (2 mol L 1 ) • Aqueous solution of sodium sulfate (10%) All equipment is presented in Figure 1. Mercury Beating Heart: Modifications to the Classical Demonstration W submitted by: Metodija Najdoski,* Valentin Mirceski, and Vladimir M. Petrus ˇevski Institute of Chemistry, Faculty of Natural Sciences and Mathematics, Sts. Cyril and Methodius University, 1000 Skopje, Republic of Macedonia; *metonajd@iunona.pmf.ukim.edu.mk Sani Demiri Private Yahya Kemal College, ul. Aleksandar, Makedonski, No 221000 Skopje, Republic of Macedonia checked by: Daniel Rosenberg Science Center, Harvard University, Cambridge, MA 12138 JCE DigiDemos: Tested Demonstrations edited by Ed Vitz Kutztown University Kutztown, PA 19530