A mercury battery (also called mercuric oxide battery, mercury cell, button cell, or Ruben-Mallory) is a non-rechargeable electrochemical battery, a primary cell. Mercury batteries use a reaction between mercuric oxide and zinc electrodes in an alkaline electrolyte. The voltage during discharge remains practically constant at 1.35 volts, and the capacity is much greater than that of a similarly sized zinc-carbon battery. Mercury batteries were used in the shape of button cells for watches, hearing aids, cameras and calculators, and in larger forms for other applications. For a time during and after World War II, batteries made with mercury became a popular power source for portable electronic devices. Due to the content of toxic mercury and environmental concerns about its disposal, the sale of mercury batteries has been banned in many countries. Both ANSI and IEC have withdrawn their standards for mercury batteries.
History The mercury oxide-zinc battery system was known since the 19th century, but did not become widely used until 1942, when Samuel Ruben developed a balanced mercury cell which was useful for military applications such as metal detectors, munitions, and walkie-talkies. The battery system had the advantages of long shelf life (up to 10 years) and steady voltage output. After the Second World War the battery system was widely applied for small electronic devices such as cardiac pacemakers and hearing aids. Mercury oxide batteries were made in a range of sizes from miniature button cells used for hearing aids and electric wrist watches, cylindrical types used for portable electronic apparatus, rectangular batteries used for transistor radios, and large multicell packs used for industrial applications such as radio remote control for overhead crane systems. In the United States, mercury oxide batteries were manufactured by companies including P. R. Mallory and Co Inc, (now Duracell), Union Carbide Corporation (whose former battery division is now called Energizer Holdings), RCA Corporation, and Burgess Battery Company.
Chemistry Mercury batteries use either pure mercury(II) oxide (HgO)—also called mercuric oxide—or a mixture of HgO with manganese dioxide (MnO2) as the cathode. Mercuric oxide is a non-conductor, so some graphite is mixed with it; the graphite also helps prevent collection of mercury into large droplets. The half-reaction at the cathode is:
HgO + H 2 O + 2 e − ⟶ Hg + 2 OH − {\displaystyle {\ce {HgO + H2O + 2e- -> Hg + 2OH-}}}
with a standard potential of +0.0977 V. The anode is made of zinc (Zn) and separated from the cathode with a layer of paper or other porous material soaked with electrolyte; this is known as a salt bridge. Two half-reactions occur at the anode. The first consists of an electrochemical reaction step:
Zn + 4 OH − ⟶ Zn ( OH ) 4 2 − + 2 e − {\displaystyle {\ce {Zn + 4 OH- -> Zn(OH)4^2- + 2e-}}}
followed by the chemical reaction step:
Zn ( OH ) 4 2 − ⟶ ZnO + 2 OH − + H 2 O {\displaystyle {\ce {Zn(OH)4^2- -> ZnO + 2OH- + H2O}}}
yielding an overall anode half-reaction of:
Zn + 2 OH − ⟶ ZnO + H 2 O + 2 e − {\displaystyle {\ce {Zn + 2OH- -> ZnO + H2O + 2e-}}}
The overall reaction for the battery is:
Zn + HgO ⟶ ZnO + Hg {\displaystyle {\ce {Zn + HgO -> ZnO + Hg}}}
In other words, during discharge, zinc is oxidized (loses electrons) to become zinc oxide (ZnO) while the mercuric oxide gets reduced (gains electrons) to form elemental mercury. A little extra mercuric oxide is put into the cell to prevent evolution of hydrogen gas at the end of life.
Electrolyte Sodium hydroxide or potassium hydroxide are used as an electrolyte. Sodium hydroxide cells have nearly constant voltage at low discharge currents, making them ideal for hearing aids, calculators, and electronic watches. Potassium hydroxide cells, in turn, provided constant voltage at higher currents, making them suitable for applications requiring current surges, e.g. photographic cameras with flash, and watches with a backlight. Potassium hydroxide cells also have better performance at lower temperatures. Mercury cells have very long shelf life, up to 10 years.
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