Mercury Cell

The mercury cell, suitable for low-current devices such as hearing aids and watches, consists of a zinc-mercury amalgam as the anode and a paste of HgO and carbon as the cathode. The electrolyte is a paste of KOH and ZnO. The cell maintains a constant voltage of 1.35 V, as the overall reaction does not involve any ions in solution whose concentration changes over time.

1.0What Is A Mercury Cell?

The mercury battery, also known as the mercuric-oxygen battery, mercury cell, or Ruben Mallory, is a primary electrochemical cell. Its operation relies on the interaction between mercuric oxide and zinc electrodes immersed in an alkaline electrolyte. Mercury batteries maintain a nearly constant discharge voltage of 1.35 volts and have a significantly higher capacity compared to similarly sized zinc-carbon batteries.

However, due to mercury's high toxicity and environmental concerns regarding its disposal, the sale of mercury batteries has been restricted in several countries.

2.0Working Principle Of Mercury Cell:

 Components:

• Anode: Zinc-mercury amalgam
• Cathode: Mercuric oxide (HgO) mixed with carbon
• Electrolyte: Paste of potassium hydroxide (KOH) and zinc oxide (ZnO)

Oxidation at the Anode: Zinc from the zinc-mercury amalgam loses electrons (is oxidized) and forms zinc oxide (ZnO), releasing electrons into the external circuit.

Electron Flow: The electrons flow through the external circuit from the anode to the cathode, creating an electric current that can be used to power devices.

Reduction at the Cathode: At the cathode, mercuric oxide (HgO) gains electrons (is reduced) to form mercury (Hg) and hydroxide ions (OH^-). The hydroxide ions remain in the electrolyte and are consumed by the anode reaction, maintaining the overall charge balance in the cell.

3.0Reaction in a Mercury cell

The mercury cell, or the mercury battery, operates through electrochemical reactions between its components. Here's a detailed explanation of the electrochemical reactions:

At the Anode (Oxidation): The zinc in the zinc-mercury amalgam undergoes oxidation.

• Zn (s) +  2OH⁻ (aq)   →  ZnO (s)  +  H₂O (l)  +  2e

At the Cathode (Reduction): The mercuric oxide is reduced.

•  HgO (s) + H₂O (l) + 2e⁻   →   Hg (l)  +  2OH⁻(aq)

Overall Cell Reaction: The overall balanced reaction in the mercury cell is:

•  Zn(s)  +  HgO(s)  →  ZnO(s)  +  Hg(l)

4.0Different Types of Mercury Cells

Mercury cells, also known as mercury batteries, come in various forms tailored for different applications. The primary types include:

Zinc-Mercury Amalgam Cells:

• Characteristics: These cells consist of a zinc-mercury amalgam anode and a mercuric oxide (HgO) cathode with an alkaline electrolyte (KOH and ZnO paste). 

Mercury-Cadmium Cells:

• Application: Characteristics: These cells use cadmium as the anode and mercuric oxide as the cathode. They offer a slightly different voltage profile compared to zinc-mercury cells.

 Comparison between Zinc-Mercuric and Cadmium-Mercuric cells

5.0Advantages of  Mercury Cell

During the electrochemical reaction in a dry cell, zinc is converted into zinc chloride. However, the substance in the small container leaks out due to the porous zinc casing, reducing the cell's lifetime and causing corrosion to the metal. In contrast, mercury cells do not involve ions in the solution during the reaction, contributing to their longevity. Some additional advantages of mercury cells are:

• High Capacity: Mercury cells offer a high capacity relative to their size, making them efficient for various applications.
• Long Shelf Life: They have an impressive shelf life of up to nine to ten years, maintaining their performance over extended periods.
• Constant Voltage Output: Mercury cells provide a constant output voltage of 1.35 volts, ensuring stable performance in devices.
• Economic Production: The production of mercury cells is economical due to the utilization of well-established technologies.
• Purity of Chlorine: In the mercury cell process, chlorine is separated with reduced impurities such as sodium hypochlorite, sodium chloride, and oxygen, enhancing the quality and purity of the chlorine produced.

6.0Disadvantages of  Mercury Cell

• Health Hazards: Mercury vapor inhalation poses severe health risks, damaging organs like the kidneys, nervous system, digestive system, eyes, skin, and immune system. Mercury is highly toxic, even in small amounts.
• High Toxicity: Mercury is extremely poisonous to the human body, even in traces.
• Developmental Risks: Exposure to mercury during pregnancy and early childhood stages can lead to significant developmental issues, impacting cognitive and physical growth.
• Environmental Impact: Mercury contamination has lasting detrimental effects on the environment, especially aquatic ecosystems, due to its toxicity and persistence.
• Availability of Alternatives: Safer and more environmentally sustainable alternatives like zinc-carbon and alkaline batteries are now widely accessible, reducing the reliance on mercury cells.

7.0Major differences between mercury cell and dry cell