Working Principle of Mercury Cell

The mercury battery (also called mercuric-oxygen batteries, mercury cells, or Ruben Mallory) is a primary electrochemical cell. Mercury batteries are made up by the reaction of mercuric oxide and zinc electrodes within an alkaline electrolyte. The discharge voltage remains almost constant at 1.35 Volts. The capacity is significantly higher than a comparable size zinc-carbon battery. Mercury cells were used as button cells to power watches, hearing aids, cameras, calculators, and in larger sizes for different uses. At a certain point during and following World War II, batteries made from mercury were an extremely popular source of power for electronic devices that could be carried around.

But lately, due to the high concentration of mercury, that is toxic and environmental concerns over the disposal of mercury, the purchase of mercury batteries has been restricted in several nations.

Working Principle Of Mercury Cell

Manufacturers worldwide use mercury in their batteries to avoid the build-up of internal gases, which could cause the battery to expand and leak. In the United States, however, the use of mercury in batteries for consumer use has decreased dramatically.

Mercury cells use either pure mercuric oxide or the combination of mercuric dioxide with manganese dioxide for the cathode. Mercury cells are built with a zinc anode, a mercury oxide cathode, and sodium hydroxide or potassium hydroxide to form the electrolyte. Since mercury oxide is not a conductor, it is suggested that there is a small amount of graphite mixed into it. This stops the formation of mercury into huge droplets. In the discharge process, zinc is converted to zinc oxide, and mercuric oxide is reduced to elemental mercury. A small amount of mercuric oxide is added to the cell to prevent the formation of hydrogen gas when it reaches the end of its lifespan.

Potassium hydroxide, also known as sodium hydroxide, can be used as an electrolyte in a battery. The cells of sodium hydroxide have an almost constant voltage when operating at low discharge currents and are therefore ideal for use in hearing aids, computers, and electronic watches. Potassium hydroxide cells, in turn, offer steady voltage when working at higher voltages which makes them suitable for applications that require large currents, for example, photography cameras equipped with flashes and watches that feature backlights. Potassium hydroxide cells also offer superior performance in lower temperatures.

The electrolyte used in Mercury Cell

The potassium hydroxide and sodium hydroxide can be used to create electrolytes used in mercury cells. The sodium hydroxide cells provide constant voltage even at low discharge currents, making them suitable for hearing aids, calculators, and electronic watches. Potassium hydroxide cells provide steady voltage even at higher currents. They are suitable for applications requiring large current surges, e.g., photographic cameras equipped with flashes and watches with a backlight. The cells made of potassium hydroxide also have superior performance in lower temperatures. The cells of mercury have a long shelf life, ranging from 10-years. The shelf life of mercury cells is ten years.

Alumina mercuric-based and Cadmium

An alternative form made of mercury batteries utilises mercury oxide and Cadmium. It has a lower terminal voltage, about 0.9 Volts, and is less energy-dense. However, it does have its temperature range extended with special designs that can reach as high as 180 C. Since Cadmium is a metal with very low solubility in the electrolyte, these batteries can have longer storage lives. A 12-volt battery like this was once used for household smoke detectors. It was constructed as an array of cells. One cell was smaller than the others, leading to an unusual two-step voltage discharge. As the battery wore out its lifespan, the smaller cell would first discharge and cause the battery’s terminal voltage to decrease by 0.9 Volts. This was a common and consistent way of telling users that their battery required replacement, while bigger capacity batteries helped keep the system functioning normally.

Electrochemical Reactions

In the Mercury cell, the cathode could consist of 100% pure mercury(II) oxide (HgO) or a mix of mercuric oxide and manganese dioxide. Because magnesium oxide (MgO) is not a conductor of electricity, Some graphites can be mixed with this.

At cathodes, mercury oxide reacts.

On presence of water  the Reaction of half cells at cathodes-

HgO + H₂O + 2e⁻ —> Hg + 2OH⁻

The normal potential for the reaction of reduction is +0.0977 V.

Meanwhile, in the anode, when Zn(Hg) reaction with two ions of OH half-cell reaction on the anode-

Zn(Hg) + 2OH⁻ —->ZnO + H₂O + 2e⁻

The general reaction to the battery

Zn + HgO → ZnO + Hg

Conclusion

Environmental issues and economic concerns Mercury vapour inhalation can be harmful to the human body, including organs like the nervous system, kidney, digestive system, eyes, skin, and immune systems. A small amount of mercury is extremely harmful to our human body.

Risky for the development of babies in the uterus and the early stages of their development

Although mercury cells were extremely popular in their time during the 2nd World War, they have been replaced by different dry cells because of their environmental and economic risks. The drawback is that it is contaminated with mercury, which is harmful to the environment. The battery uses the oxidising zinc using mercury(II).