Mercury Cell

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. But Bill Clinton, a President of the United State signed into law “The battery act” on May 13, 1996. The main object of the law was to reduce the toxic and heavy metal in the environment like groundwater, stream and municipal waste. Due to this act, the popularity of the mercury battery reduced.

The Mercury cell is a type of dry cell consisting of 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, non-rechargeable, that is the electric cell produces current by irreversible chemical reactions.

In a mercury cell, mercury compound acts as a cathode where reduction reaction occurs and the zinc compound acts as an anode, where oxidation reaction takes place. Sodium hydroxide or potassium hydroxide used as an electrolyte that ionizes in a molten state to conduct electricity.

Electrochemical reactions

In 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:

HgO+H2O+2eHg+2OHHgO+H_{2}O+2e^{-}\rightarrow Hg+2OH^{-}

The standard potential during the reduction reaction is +0.0977 V.

Half cell reaction at the anode:

Zn(Hg)+2OHZnO+H2O+2eZn(Hg)+2OH^{-}\rightarrow ZnO+H_{2}O+2e^{-}

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 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 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 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, Mercury cell does not involve any ion in the solution during the reactions to change its lifetime.

  • Long shelf time up to 10 years
  • High capacity per size
  • The constant voltage output of 1.35V
  • Inexpensive to produce mercury cell with known technology
  • Using mercury cell chlorine can be separated by reducing the impurities like oxygen, sodium chlorate, sodium hypochlorite.

Limitations

  • Environment issues and economic issues – Inhalation of mercury vapor is harmful to the human body including organs like kidney, nervous system, digestive system, eye, skin and immunity systems. A small amount is very toxic to the human body.
  • Dangerous to the development of kids in utero and early stage of the children.

Conclusion

Even though mercury cells were very popular during the 2nd World War, due to its 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, therefore it has a constant potential of 1.35V.

Question 2: Is Mercury cell rechargeable?

Answer: No

Mercury cell is a primary cell, which is not rechargeable.

Question 3: What is the oxidizing agent in a mercury cell?

Answer: HgO serves as an oxidizing agent and 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 5mg of mercury in a single unit.

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