Primary Cell



      It is seen that the primary cell is that which is required to replace by new one when it ir run down.
       The oldest types of primary cell are simple voltaic cell, Daniell cell, Leclanche cell etc.Some commonly used primary cells are,
1. Dry cell (Zinc- Carbon)      2-Mercury cell
3. Zinc-Chloride cell   4. Lithium cell
5. Alkaline Zinc-mercury oxide cell
1. Dry Zinc-Carbon Cell
      This is most common type of dry cell. It is the type of Leclanche cell.

       The space between the paper and carbon rod is filled with a paste of sal ammoniac, zinc chloride, manganese dioxide and carbon dust.
Note : the past is not dry and if it becomes dry, the cell becomes useless.
       The sal ammoniac acts as an electrolyte, the zinc chloride improves the chemical action and manganese dioxide acts as depolarizer.
      The Fig. 1 shows the cross-section of zinc-carbon dry cell.

       The zinc container is lined with paper to avoid direct reaction of zinc with carbon. The container is sealed with an insulator called pitch which is plastic seal of the cell. The tin plates are used at top and bottom which are positive and negative terminals of the cell respectively. The carbon dust added to the paste used to improve the conduction of the electrolyte. Externally the zinc container is covered with cardboard jacket to avoid any type of leakge.
1.1 Cell Reaction 
      When zinc atoms react with the electrolyte, the electrons are removed from the carbon rod. These electrons accumulate on the zinc electrode. As the electrons are negatively charged, the zinc electrode acts as negative terminal. The carbon rod from where negative charge is lost in the form of electrons, acts as positive terminal.
      The sal ammoniac i.e. ammonium chloride paste reacts with zinc to liberate hydrogen.


      The hydrogen reacts with manganese dioxide as,

Note : Hydrogen is liberated at faster rate while reaction of MnO2 is slow. Due to this, hydrogen accumulates in the form of thin layer on carbon rod. This is called polarization. Due to this, cell e.m.f. goes down if cell is operated for longer time.
1.2 Feature of Cell
1. The e.m.f. of new dry cell is about 1.5 V.
2. The internal resistance is about 0.1 to 0.4 .
3. The capacity of 32 Ah when discharged through 20 resistor till voltage drops to 0.5 V.
4. When not in use for long time, zinc gets attacked by the past slowly and cell becomes useless though not in use.
5. In working condition, voltage drops due to polarization hence used for intermittent service. When disconnected the depolarization occurs to restore the cell e.m.f.
6. Least Expensive.
7. Portable and Convenient to use.
1.3 Applications
      Mostly used to get intermittent service to avoid polarization. The various applications are,
1. Torch lights                                     2. Telephone and telegraph systems
3. Electronic apparatus and toyes     4. Wall clocks
5. Electric bells
6. Radio receivers and many other consumer applications
2. Mercury Cell
      This is another type of a primary cell

      The concentrated solution of potassium hydroxide (KOH) and zinc oxide (ZnO) is used as an electrolyte.
      It is available in cylindrical shape or miniature button shape.
      The Fig. 2 shows the construction of a cylindrical mercury cell.

       The zinc electrode acting as negative terminal is made in the form of a hollow cylinder. The steel casing acts as a positive terminal. The layer of mercury oxide covers the electrolyte which is solution of KOH and ZnO. The cell is sealed with the help of insulating gasket.
2.1 Chemical reaction
The net chemical reaction involved in the cell is,

2.2 Features of the cell
1. The chemical reaction does not evolve any gas hence no polarisation.
2. The cell maintains its e.m.f. for longer time in working condition.
3. The terminal voltage is about 1.2 to 1.3 V.
4. It has long life.
5. It has high ratio of output energy to weight of about 90 – 100 Wh/kg.
6. Costlier than dry cell.
7. It has high energy to volume ratio of about 500 – 600 Wh/L.
8. It has high efficiency.
9. Good resistance to shocks and vibrations.
10. Disposal is difficulty due to presence of poisonous materials inside.
2.3 Applications
      The cells are preferred for providing power to small devices as available in miniature buttons shapes. The various applications are,
1. Hearing aids.
2. Electronic calculators.
3. Electronic clocks.
4. Guided missiles.
5. Medical electronic appliance such as pace makers.6.Audio devices and cameras.

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