The mercury cell, also called “mercury battery, mercury oxide battery”, is a primary cell, which is a non-rechargeable, non-reusable electrochemical battery. During and after the 2nd World War, mercury batteries were the most powerful power source for many small portable electronic devices like calculators, watches, hearing aids, digital thermometers, toys in the shape of a button or coin and large appliances like walkie-talkies. However, Bill Clinton, the former President of the United States, signed into law “The Battery Act” on May 13, 1996. The main object of the law was to reduce toxic and heavy metals in the environment like groundwater, stream and municipal waste. Due to this act, the popularity of the mercury battery got reduced.
| The mercury cell is a type of dry cell consisting of a zinc anode, mercuric oxide cathode and potassium hydroxide as an electrolyte. |
Working Principle of Mercury Cell
The mercury cell is a type of primary cell which is non-reusable and non-rechargeable, that is, the electric cell produces current by irreversible chemical reactions.
In a mercury cell, the mercury compound acts as a cathode, where a reduction reaction occurs, and the zinc compound acts as an anode, where an oxidation reaction takes place. Sodium hydroxide or potassium hydroxide is used as an electrolyte that ionises in a molten state to conduct electricity.
Electrochemical Reactions
In a mercury cell, the cathode can be pure mercury(II) oxide (HgO) or a mixture of mercuric oxide with manganese dioxide. Since magnesium oxide (MgO) is a non-conductor of electricity, some graphites are mixed with this.
Half-cell reaction at the cathode:
The standard potential during the reduction reaction is +0.0977 V.
Half-cell reaction at the anode:
The overall reaction for the battery:
Zn + HgO → ZnO + Hg
Types of Mercury Cells
There are two varieties of mercury cells. One is a zinc-mercuric oxide cell, and the other is a cadmium-mercuric oxide cell.
| Zinc-Mercuric Cell | Cadmium-Mercuric Cell |
| Cell reaction is Zn + HgO = ZnO + Hg | Cell reaction is Cd + HgO + H2O = Cd(OH2) + Hg |
| long storage life | Stable at high as well as at a low temperature |
| Stable voltage(1.35V). | 0.91V |
| Electrochemical efficiency is 820 mAH/g(Zn), 250 mAH/g(Hg) | Electrochemical efficiency is 480 mAH/g(Cd) |
Advantage of Mercury Cell over Dry Cell
A dry cell during an electrochemical reaction involves the conversion of zinc to zinc chloride, casing zinc porous. Due to the porous casing, a substance in the cell leaks out, corrodes the metal and the lifetime of the cell is reduced. On the other hand, the mercury cell does not involve any ion in the solution during the reactions to change its lifetime.
- Long shelf time of up to 10 years
- High capacity per size
- The constant voltage output of 1.35V
- Inexpensive to produce mercury cells with known technology
- Using a mercury cell, chlorine can be separated by reducing the impurities like oxygen, sodium chlorate, and sodium hypochlorite.
Limitations
- Environment issues and economic issues – Inhalation of mercury vapour is harmful to the human body, including organs like the kidney, nervous system, digestive system, eye, skin and immunity systems. Even a small amount is very toxic to the human body.
- Dangerous to the development of children in utero and early stage of their lives.
Conclusion
Even though mercury cells were very popular during the 2nd World War, due to economic factors and environmental risk factors, they have been replaced by other dry cells.
Solved Questions
Question 1: Why does the cell potential of mercury remain constant?
Answer: Since mercury ion doesn’t involve any ions whose concentration changes, it has a constant potential of 1.35V.
Question 2: Is mercury cell rechargeable?
Answer: No, the mercury cell is a primary cell, which is not rechargeable.
Question 3: What is the oxidising agent in a mercury cell?
Answer: HgO serves as an oxidising agent, and the reaction takes place at the cathode.
Question 4: How much mercury will be present in a mercury cell battery?
Answer: Button cell batteries can contain 5 mg of mercury in a single unit.
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