Dry cell (plural dry cells) noun part of a battery: a current-generating electric cell that cannot be regenerated
and contains an electrolyte in the form of a paste or within a porous material
to keep it from spilling.
The most common form of primary cell is the Leclanché cell, invented by
the French chemist Georges Leclanché in the 1860s. It is popularly called
a dry cell or flashlight battery. The Leclanché cell in use today is very
similar to the original invention. The electrolyte consists of a mixture of ammonium
chloride and zinc chloride made into a paste. The negative electrode is made of
zinc, as is the outside shell of the cell, and the positive electrode is a carbon
rod surrounded by a mixture of carbon and manganese dioxide. The Leclanché
cell produces about 1.5 V.
Dry Cell Battery
The functional elements of a dry cell battery are the negative terminal (a zinc
can which encloses the battery materials), the positive terminal (the carbon
rod and carbon and manganese dioxide mixture that surrounds the rod), and an
electrolyte paste between the two terminals. The electrolytic paste facilitates
a chemical reaction involving the constituents of both terminals; this reaction
causes a current to flow through a conductor that connects the positive and
In a dry cell the zinc casing serves as the anode and is consumed in the anodic
electrode reaction Zn(s) --> Zn2+ + 2e-; the zinc ion dissolves in the moist
ZnCl2-NH4Cl electrolyte. A carbon rod serves as the cathode, but it is chemically
inert. The cathode electrode reaction, which consumes MnO2, is best written
aMn4+ + 2O2-i + H2O + e- --> aMn3+ + O2- + OH-i + OH-
Where the square brackets indicate the species present in the solid phase at
the cathode. The cathode reaction actually occurs within the solid structure;
the carbon rod serves only to transfer electrons from the external circuit.
The dry cell has a potential difference of about 1.25 V; the zinc electrode
is negative. It is a good source of electrical power and the materials of construction
are relatively cheap. The cell voltage during discharge falls off rather badly
and the dry cell is not a good source of power when a constant voltage is needed.
They are called dry cell because they electrolyte is a paste rather than a
Types of dry cells:
1. Primary cells - They are not rechargeable. The cell will not function once
their chemicals are used up and the cells have to be thrown away. Examples are
zinc-carbon cell, alkaline manganese cell and silver oxide cell.
2. Secondary cells - They are rechargeable (can be recharged) and can be used
again. Example: nickel-cadmium cell.
Note: The lead-acid accumulator (car battery) used in cars is also a secondary
cell but it is not considered as a type of dry cell.
How to make a dry cell
Dry cells are one of the most commonly used household objects. We use dry cells
in watches, torches, transistors, walkmans and even the remote controls of our
TVs. Dry cells provide the necessary electricity required to power these devices.
A normal dry cell is cylindrical in shape made of zinc. A carbon rod passes
through its center and a paste of manganese dioxide and ammonium chloride surrounds
this rod. When the both ends of the cell are z connected to a bulb through a
wire, the bulb glows due to the flow of current. The voltage of such a cell
is about 1.5 volt. Let us now make a dry cell at home.
Material Required · A small carbon plate · A small zinc plate
· Manganese dioxide · Starch powder · Ammonium chloride
· Cotton wool · Copper wire · Two metallic clips ·
One 1.5 volt bulb and one bulb holder
Procedure Make some starch paste by mixing some starch and water and then boiling
it. Add sufficient quantity of manganese dioxide to the starch paste, making
a very thick paste of manganese dioxide. Spread this manganese dioxide paste
evenly on the zinc plate. Now take some cotton wool and flatten it to fit the
shape of the zinc plate. Soak this cotton wool in ammonium chloride solution.
Now add another layer of manganese dioxide paste over the cotton wool. Now put
the carbon plate over this layer of manganese dioxide and your dry cell is ready
to use. To see the dry cell in action connect wires to the two ends of the bulb
holder and connect the other ends of the wires to the carbon and zinc plates
using metallic clips. The bulb begins to glow. The forms of dry cells Dry cell
- acid form: This is the source of power for an ordinary flashlight. Most "flashlight
batteries" produce 1.5 volts. The case of the cell is zinc metal acting
as the anode. At the center of the cell is a stick of graphite for the cathode.
The graphite stick is surrounded by a paste of MnO2 and NH4Cl.
Half-reactions for this cell are: Anode - Zn (s) Zn+2 + 2e - Cathode - 2NH4+
(aq) + 2MnO2 (s) + 2e - Mn2O3 (s) + 2NH3 (aq) + H2O (l) Remember that the term
"battery" refers to two or more cells connected together. The 9-volt
transistor battery is a true battery. It contains six individual 1.5-volt cells
connected in series to produce 9 volts. Dry cell - alkaline form: The NH4Cl
in the "acid form" cell is replaced by KOH and the zinc is in powder
form rather than a solid piece of metal. The graphite cathode is eliminated
and acid corrosion of the container does not occur. The alkaline cell is more
efficient and can be miniaturized to fit more varied applications. In the alkaline
version the half-cell reactions are: Zn + 2 OH- -------> ZnO + H2O + 2e-
2 MnO2 + 2e- + H2O -------> Mn2O3 + 2 OH- The alkaline dry cell lasts much
longer as the zinc anode corrodes less rapidly under basic conditions than under
acidic conditions.Lead storage battery - acid form: The normal "car battery"
consists of six 2-volt cells connected in series to produce 12 volts. Each cell
uses a plate of lead for the anode. The cathode is PbO2 powder formed into a
conducting grid. The electrodes are immersed in dilute H2SO4. Half-reactions
for each cell are: Anode - Pb (s) + SO4 -2 (aq) PbSO4 (s) + 2e - Cathode - PbO2
(s) + SO4 -2 (aq) + 4H + (aq) + 2e - PbSO4 (s) + 2H2O (l) Pb (s) and PbO2 (s)
are converted to PbSO4 (s) while the battery is being used. When the battery
is recharged, the overall reaction proceeds in the reverse direction, restoring
the reactants. This allows the battery to continue to be used. Most car batteries
become useless after 3 to 5 years because side reactions occur that produce
a sludge that interferes with the battery's operation. The amount of charge
in this type of battery can be determined by measuring the density of the electrolyte.
Sulfuric acid has a density greater than water. As the battery operates, sulfuric
acid is consumed, lowering the density of the electrolyte. If the density falls
below 1.2 g/cm3, the battery is ready for recharging. Other types of dry cell
batteries are the silver battery in which silver metal serves as an inert cathode
to support the reduction of silver oxide (Ag2O) and the oxidation of zinc (anode)
in a basic medium. The type of battery commonly used for calculators is the
mercury cell. In this type of battery, HgO serves as the oxidizing agent (cathode)
in a basic medium, while zinc metal serves as the anode. Another type of battery
is the nickel/cadmium battery, in which cadmium metal serves as the anode and
nickel oxide serves as the cathode in an alkaline medium. Unlike the other types
of dry cells described above, the nickel/cadmium cell can be recharged like
the lead-acid battery.